EFFECTS OF PHOTO-SELECTIVE NETS ON TOMATO (Solanum lycopersicum)

PLANT GROWTH AND FRUIT QUALITY AT HARVEST

By

PHUSHUDI PETER TINYANE

Submitted in partial fulfillment of the requirements for the degree

MAGISTER TECHNOLOGIAE: AGRICULTURE

In the

Department of Crop Science

FACULTY OF SCIENCES

TSHWANE UNIVERSITY OF TECHNOLOGY

Supervisor: Prof D Sivakumar

Co-supervisor: Prof P Soundy

November 2013

i

DECLARATION

I hereby declare that the work herein submitted as a dissertation for the degree M

Tech: Agriculture, at the Tshwane University of Technology, is the results of my own

investigation and has not previously been submitted to any other institution of higher

education. Work by other authors that served as sources of information have fully

been acknowledged by means of a comprehensive list of references.

_____________________ _______________

Mr P.P TINYANE DATE

ii

DEDICATION

This thesis is dedicated to my late father, Mr Lekgoo Wilson Tinyane, with much love

and appreciation.

iii

AKNOWLEDGEMENTS

First I would like to give thanks to GOD Almighty, for he has provided me strength

and wisdom during the period of my studies.

I am grateful to my supervisor, Prof D Sivakumar, for her leadership, expertise and

excellent contributions throughout the course of my study, and my co-supervisor,

Prof P Soundy, thanks for your assistance.

I am also grateful to Dr M.M Maboko from ARC-Vegetable and Ornamental Plant

Institute, and Mr P Peacock from Hygrotech South Africa for their assistance and

advice during the execution of the experiments in the net houses.

I wish to thank Dr T Reigner from Tshwane University of Technology and Dr M

Beukes from University of Pretoria for providing laboratory training on analyzing

bioactive compounds.

I would like to thank Hygrotech (Pty) Ltd South Africa and SeedCor (Pty) Ltd for

providing fertilisers and seedlings.

iv

The Department of Crop Science (Tshwane University of Technology) and Tshwane

University of Technology‟s Experimental Farm for the equipment and facilities to

undertake this study.

Thanks to my family, especially my late father (Mr L.W Tinyane) who passed away

before completion of this manuscript, my mom (Mrs D Tinyane), my four brothers

(Tseke, Makgoka, Koko and Mothebele) and my sister (Mosima) for their support

and motivation throughout the course of this study.

I am grateful to my colleague, Ms K Selahle and Ms L Ntsoane for their contribution

towards the completion of this study.

Finally and most importantly, I would like to acknowledge the financial assistance

from the National Research Foundation and the Tshwane University of Technology

postgraduate scholarship.

v

LIST OF PUBLICATIONS AND PRESENTATIONS

1 PUBLICATIONS

1.1 Article online

Tinyane, P.P., Sivakumar, D. & Soundy, P. 2013. Influence of photo-selective

netting on fruit quality parameters and bioactive compounds in selected tomato

cultivars. Scientia Horticulturae, 161:340-349.

1.2 Farmers communication-Transfer of technology

Tinyane, P.P., Sivakumar, D., Soundy, P. 2013. Influence of photo-selective netting

on tomato fruit quality. Afgriland, 57 (5).

2 PRESENTATIONS

2.1 International Conference

Postharvest Africa 2012, 7th International CIGR Technical Symposium

Tinyane, P.P., Sivakumar, D. & Soundy, P. Influence of photo-selective netting on

fruit quality parameters and bioactive compounds in tomatoes.

2.2 National Conference

Combined Crops, Soils, Horticulture and Weeds Congress 2013, South Africa.

Tinyane, P.P., Sivakumar, D., Maboko, M.M. & Soundy, P. Influence of photo-

selective netting on tomato plant growth and yield.

vi

ABSTRACT

Tomatoes (Solanum lycopersicum) are rich in carotenoids (especially lycopene, and

β-carotene, a precursor of vitamin A), phenolics (flavonoids) and vitamin C.

However, growth, yield, fruit quality, and nutritional content of tomato plants are

affected not only by genetics (cultivars), but also by environmental conditions,

including abiotic and biotic factors such as pests and diseases. Therefore, a study

was conducted to investigate the influence of photo-selective nets (ChromatiNet™)

(red, pearl and yellow) with 40% shading on the plant growth, yield, and production

of good quality fruits that contain improved bioactive compounds. A commercial

black net with 25% shading was as a control. Three indeterminate tomato cultivars

(AlfaV, Irit, and SCX 248) were planted. The photosynthetically active radiation, air

temperature and relative humidity were monitored throughout the growing period.

The following parameters such as plant height, stem diameter, leaf area, leaf

chlorophyll, flowering, fruit trusses and numbers of fruits per plant were measured

during the growing period. The fruit mass, fruit marketable yield, fruit firmness and

colour, titratable acidity, soluble solids content, and bioactive compounds (ascorbic

acid, lycopene content, β-carotene, total phenols and flavonoids, and total

antioxidant activity) were determined at harvest maturity (pink colour stage). Photo-

selective nets indicated a higher air temperature and a lower relative humidity, while

the black (control) net accumulated a higher photosynthetically active radiation. The

results of the study demonstrated that the two photo-selective nets (red and yellow)

had a great influence on plant height and leaf area, while the stem diameter

measurements were greater in plants grown under the red net. High intensities of

flowering were noted in plants produced under photo-selective red, pearl and yellow

vii

nets, whereas the number of fruits per plant was higher for plants grown under the

pearl and yellow nets. Marketable fruit yields were higher in cultivar AlfaV grown

under the pearl net. Furthermore, no significant differences were noted among the

shade nets with regard to disease and pest incidences. The three cultivars grown

under the black net revealed a lower plant height, less fruit mass, and lower fruit

marketable yields. Principle component analysis demonstrated that cultivar AlfaV

fruits produced under black nets were lower in mass, less firm, and higher in

bioactive compounds but lower in titratable acidity and more intense in red colour.

However, under pearl nets cultivar AlfaV exhibited a higher fruit mass, more firmness

and moderately higher bioactive compounds. Pearl and red photo-selective nets

improved the overall fruit yield and quality parameters such as fruit mass, fruit

firmness and bioactive components in AlfaV and SCX 248.

Keywords: Environmental conditions, bioactive compounds, plant growth, tomatoes,

yield, fruit quality

viii

TABLE OF CONTENTS

PAGE

DECLARATION i

DEDICATION ii

ACKNOWLEDGEMENTS iii

LIST OF PUBLICATIONS AND PRESENTATIONS v

ABSTRACT vi

TABLE OF CONTENTS viii

LIST OF FIGURES xiii

LIST OF TABLES xvii

LIST OF SYMBOLS AND ABREVATIONS xx

CHAPTER ONE

1 GENERAL INTRODUCTION 1

1.1 AIM AND OBJECTIVES 6

1.1.1 Aim 6

1.1.2 Objectives 6

1.2 THESIS STRUCTURE 7

ix

CHAPTER TWO

2 LITERATURE REVIEW 9

2.1 TOMATO (Solanum lycopersicum) 9

2.1.1 Origin, history and uses 9

2.1.2 Nutritional content and health benefits 10

2.1.3 Plant morphology 13

2.1.4 Cultivars 14

2.1.5 The fruit 15

2.1.6 Maturity of the fruit for harvesting 17

2.1.7 Physiological disorders 20

2.1.8 Common tomato diseases 21

2.2 HYDROPONICS/SOILLESS VEGETABLE PRODUCTION 22

2.2.1 Definition 22

2.2.2 Closed and open hydroponic systems 23

2.2.3 Aggregate hydroponic system 24

2.2.4 Nutrient solution 26

2.2.5 Nutritional disorders 28

2.2.6 Disease and insect control in hydroponic systems 29

x

2.3 PHOTO-SELECTIVE NETTING 30

2.3.1 Introduction 30

2.3.2 Plant growth and yield 31

2.3.3 Disease and pest infestation 35

2.3.4 Physico-chemical properties in fruits 39

2.3.5 Bioactive compounds in fruits 42

2.3.5.1 Carotenoids 45

2.3.5.2 Ascorbic acids 48

2.3.5.3 Total phenols and flavonoids 49

2.4 CONCLUSION 53

CHAPTER THREE

3 SUMMARY 55

3.1 INTRODUCTION 56

3.2 MATERIALS AND METHODS 59

3.2.1 Location 59

3.2.2 Treatments and experimental design 59

3.2.3 Planting materials 61

3.2.4 Cultural practices 62

xi

3.2.5 Light interception by nets and microclimatic measurements 64

3.2.6 Plant measurements 64

3.2.7 Harvesting and fruit sampling 65

3.2.8 Statistical analysis 66

3.3 RESULTS AND DISCUSSION 66

3.3.1 PAR and microclimate under the shade nets 66

3.3.2 Plant height 68

3.3.3 Stem diameter 70

3.3.4 Leaf area 72

3.3.5 Leaf chlorophyll 75

3.3.6 Flowering and fruit trusses 77

3.3.7 Number of fruits 79

3.3.8 Fruit mass 83

3.3.9 Marketable yield 84

3.3.10 Disease and pest infestation 85

CHAPTER FOUR

4 SUMMARY 87

4.1 INTRODUCTION 88

xii

4.2 MATERIALS AND METHODS 91

4.2.1 Analysis of physico-chemical properties of tomato

cultivars at harvest maturity stage (pink) 91

4.2.1.1 Colour, firmness, soluble solids content (SSC),

and titratable acidity (TA) 91

4.2.2 Analysis of bioactive compounds of tomato

cultivars at harvest maturity stage (pink) 92

4.2.2.1 Ascorbic acid, lycopene and β-carotene contents 92

4.2.2.2 Total phenol and flavonoid contents 93

4.2.2.3 Antioxidant scavenging activity 94

4.2.3 Statistical analysis 95

4.3 RESULTS AND DISCUSSION 96

4.3.1 Fruit quality parameters and bioactive compounds 96

4.4 CORRELATION ANALYSIS 106

4.5 PCA ANALYSIS 107

CHAPTER FIVE

5 GENERAL CONCLUSIONS 111

REFERENCES 113

xiii

LIST OF FIGURES

PAGE

FIGURE 2.1: Transverse section of a multilocular tomato fruit 16

FIGURE 2.2: Ripening stages of mature tomato fruit, from left to right: green,

breaker, turning, pink, light red, and red 18

FIGURE 2.3: Photo-selective shade nets and the black commercial shade

net 38

FIGURE 3.1: Photographs illustrating yellow photo-selective nets and black

control nets 60

FIGURE 3.2: Photograph illustrating red and pearl photo-selective

nets 60

FIGURE 3.3: Shading effects of different shade nets 67

xiv

FIGURE 3.4: Variations in the plant height of the three tomato

cultivars 69

FIGURE 3.5: Effect of photo-selective nets on the plant height of the three

tomato cultivars 70

FIGURE 3.6: Variations in the stem diameter of the three tomato

cultivars 71

FIGURE 3.7: Effect of photo-selective nets on the stem diameter of the three

cultivars 72

FIGURE 3.8: Variations in the leaf area of the three tomato cultivars 73

FIGURE 3.9: Effect of photo-selective nets on the leaf area of the three

cultivars 74

FIGURE 3.10: The effect of cultivar versus shade net on the leaf area 74

xv

FIGURE 3.11: Photograph illustrating plants grown under the black (control)

net 75

FIGURE 3.12: Photograph illustrating plants grown under the red net 76

FIGURE 3.13: Photograph illustrating plants grown under yellow photo-

selective nets 76

FIGURE 3.14: Photograph illustrating plants grown under the pearl photo-

selective net 77

FIGURE 3.15: Effect of shading on flowering intensity at four weeks after

transplanting 78

FIGURE 3.16: Fruit trusses of the three cultivars planted 79

FIGURE 3.17: Effect of photo-selective nets on the number of fruits per

plant 80

FIGURE 3.18: Number of fruits per plant or net type 81

xvi

FIGURE 3.19: Variations in the fruit mass of the three tomato cultivars 84

FIGURE 3.20: marketable yield of the three cultivars planted under different

shade nets 85

FIGURE 3.21: Cultivars effect on incidences of pests and diseases 86

FIGURE 4.1: Principal Component Analysis (PCA) for physicochemical

properties and bioactive compounds 108

FIGURE 4.2: PCA analysis for the cultivars and shade nets 109

xvii

LIST OF TABLES

PAGE

TABLE 2.1: Tomato (raw) nutritional value per 100 g 12

TABLE 2.2: Terms used to describe tomato colour as an indicator of

ripening 19

TABLE 2.3: Recommended concentration ranges of essential elements

in nutrient solution 27

TABLE 2.4: Tomato fruit pH and TA ranges of different cultivars at red

ripe maturity stage 40

TABLE 2.5: SSC/TA for different cultivars planted in field and protected

environments 42

TABLE 2.6: Lycopene content in tomato varieties (100% ripe

stage), expressed as mg kg-1 on a fresh weight basis 47

xviii

TABLE 2.7: Ascorbic acid content in tomato varieties (100% ripe stage),

expressed as mg 100 g-1 fresh fruit 49

TABLE 3.1: Nutrient composition of the fertiliser applied during tomato

production 62

TABLE 3.2: Nutrient composition of the fertilisers used in the foliar

feeding programme 63

TABLE 3.3: Air temperature (AT), relative humidity (RH) and

photosynthetic active radiation (PAR) measurements

under different photo-selective nets and the control net

during cultivation 67

TABLE 3.4: Cultivar versus shade net interaction on plant height, stem

diameter, leaf chlorophyll, fruit trusses per plant, and

number of fruits per plant 82

xix

TABLE 4.1 A: Effect of photo-selective netting on the physicochemical

properties and bioactive compounds of three tomato

cultivar 97

TABLE 4.1 B: Effect of photo-selective netting on the physicochemical

properties and bioactive compounds of three tomato

cultivar 98

TABLE 4.2: Effect of cultivar and shade net on physical properties

(colour) of tomato fruits 106

TABLE 4.3: Pearson‟s correlation coefficients between PAR or

microclimate conditions (AT or RH) and the fruit

quality parameters or bioactive compounds 107

xx

LIST OF SYMBOLS AND ABBREVIATIONS

% w/v Percentage weight per volume

% Percentage

°C Degree Celsius

µg Microgram

µl Microlitre

a* +a refers to the red colour, while –a refers to the green colour

AA Ascorbic acid

AlCl3 Aluminium chloride

AT Air temperature

b* +b refers to yellow colour, while –b refers to blue colour

B Boron

Ca Calcium

Ca(NO3)2 Calcium nitrate

CEA Controlled environment agriculture

cm Centimeter

CMV Cucumber mosaic virus

CO2 Carbon dioxide

xxi

Cu Copper

cv. Cultivar

DPPH 2, 2-diphenyl-1-picrylhydrazyl

DRI Recommended daily intake

DV Daily value

E East

E Environment

e.g. For instance

etc. And so forth

FAO Food and Agriculture Organisation

Fe Iron

FR Far red

g kg-1 grams per kilogram

g L-1 Grams per litre

G Genotype

g Gram

g-1 FW Per gram fresh weight

GAE Gallic acid equivalents

h Hour

xxii

h° Hue

IU International unit

K Potassium

kcal Kilocalorie

kg m-2 Kilogram per meter squared

kg Kilogram

kg−1 FW Per kilogram fresh weight

KNO3 Potassium nitrate

L* Lightness,

L Litre

LSD Least significant difference

m Meter

M Mole

mg g-1 Milligrams per gram

mg kg−1 FW Milligrams per kilogram fresh weight

mg kg-1 Milligrams per kilogram

mg ml-1 milligrams per millilitre

Mg Magnesium

mg Milligram

xxiii

min Minute

ml Milliliter

mm Millimeter

mM Millimolar

mm2 Millimeter squared

Mo Molybdenum

N Nitrogen

N Normality

n Number

Na2CO3 Sodium carbonate

NaNO2 Sodium nitrite

NaOH Sodium hydroxide

NH4 Ammonium

nm Nanometer

P Phosphorus

PAR Photosynthetically active radiation

PARo Photosynthetically Active Radiation outside the net house

PCA Principal Component Analysis

ppm Parts per million

xxiv

PVY Potato virus Y

R Red

RDA Recommended daily allowance

RH Relative humidity

S South

S Sulfur

SPAR Photosynthetically Active Radiation inside the net house

SSC Soluble solids content

TA Titratable acidity

TF Total flavonoids

TP Total phenolics

TYLCV Tomato yellow leaf curl virus

™ Trade mark

UK United Kingdom

USA United States of America

USDA United States Department of Agriculture

UV Ultra violet

v/v Volume per volume

Zn Zinc

xxv

β Beta

μmol m−2 s−1 Micromole per meter squared per second

1

CHAPTER ONE

1 GENERAL INTRODUCTION

Tomato (Solanum lycopersicum) is the most important and the most popular

vegetable crop in South Africa. The tomato has also been recognised as the most

important source of lycopene, a red coloured carotenoid associated with several

health benefits (Ilahy et al., 2011:588). In addition to lycopene, the tomato contains a

number of flavonoids and phenolic acids that can contribute to a healthy diet (Anza,

Riga & Garbisu, 2006:17). However, the yield and quality of tomatoes is limited due

to varying climatic conditions characterised by high solar radiation and temperatures,

and pests which cause direct feeding damage and often transmit viruses during the

growing periods (Ben-Yakir et al., 2013:249, Higashide, 2009:1874, De Koning,

1989:329). It is a well-known fact that climatic conditions and tomato variety have an

effect on yield and fruit quality (Davies & Hobson, 1981:205, Tshiala & Olwoch,

2010:2945). Tomato is sensitive to temperature in different physiological aspects

(Picken, 1984:1); hence, it also plays a role in determining floral quality. In

comparison to the vegetative parts, the reproductive parts of the tomato plant are

more vulnerable to higher temperatures (32/26 °C day and night) (Sakata et al.,

2000:395). On the other hand, tomatoes grown at higher day temperatures (>24 °C)

as well as at lower night temperatures (<18 °C) were reported to have a negative

effect on fruit quality during harvest and postharvest (Ohio Vegetable Production

Guide, 2010). Aspects relating to loss of fruit quality incurred under excessive

preharvest temperatures include alterations in the colour, shape and texture of the

2

tomato fruits (Zipelevish et al., 2000:431). Furthermore, tomato yields are

determined by the amount of intercepted light (Newton et al., 1999:43) and

assimilate partitioning (Ho, 1996:1239). Temperature significantly affect the

partitioning of assimilates between vegetative and generative parts (De Koning,

1989:329; Adams et al., 2001:869).

At higher temperatures, fruit trusses appear faster (Hurd & Graves, 1985:359;

Adams et al., 2001:869), resulting in more fruits per plant. These fruits will grow at

the expense of vegetative growth, and may also cause a delay in the growth of newly

set fruit or may even lead to flower or fruit abortion (De Koning, 1989:329), as the

newly developed and flowering trusses are weaker sinks than fruiting trusses (Ho &

Hewitt, 1986:201). A loss of firmness and fruit damage that can be noted at light

intensity levels above photosynthetic saturation, particularly under intense light

exposure, have been associated with an increase in the temperature of fruit

(Adegoroye & Jolliffe, 1987:297; Sams, 1999:249).

The lycopene content of tomato fruits is influenced by fruit variety as well as by

cultivation methods and environmental conditions. In the fruit, chlorophyll breaks

down and carotenoids, mostly lycopene, accumulate during the ripening process

(Brandt et al. 2006; Helyes, 1999). Fruit exposure to excessive sunlight was reported

to inhibit lycopene synthesis (Brandt et al., 2006:568). Fruits exposed to strong solar

radiation may have a temperature which is 10 °C higher than that of fruits protected

by leaves. According to Brandt et al. (2006:568), when the temperature of the fruits

exceeds 30 °C, the formation of lycopene is inhibited. Lycopene develops best when

the temperature is between 12 °C and 21 °C (Tomes, 1963:180). Raymundo,

3

Chichester and Simpson (1976:59) reported that a low light intensity results in an

uneven fruit colour due to a decrease in lycopene accumulation, but on the other

hand, direct radiation is too strong and therefore harmful to fruits. The accumulation

of phenolic compounds such as flavonoids and phenylpropanoids is, however,

induced by thermal stress (Helyes, Lugasi & Pek, 2012:703). According to George et

al. (2004:45), the polyphenol level in tomatoes produced at 35 °C is double the

amount of that produced at 25 °C. A higher temperature also favours ascorbic acid

synthesis (Helyes, Lugasi & Pek, 2012:703). Large parts of land in Africa and the

world at large have shown high irradiation; with temperatures having risen by over

0.7 °C in the last 100 years, and 11 of the last 12 years (1995-2006) having been the

warmest on record (Allali et al., 2001:490). As a result, the need for crop cultivation

in protected environments has become essential so as to improve the quality and

yield of agricultural produce by minimising thermal stress on heat sensitive traits.

Netting has the potential to offer an effective solution, because it (a) provides

protection from environmental hazards (e.g., excessive solar radiation, wind, hail,

flying pests); (b) improves the plant microclimate (thus reducing heat or chill, drought

stresses); (c) moderates rapid climatic stresses; and (d) is cheaper and less energy

consuming than greenhouses.

Photo-selective nets (ChromatiNet™), are light-dispersive shade with potential

benefits for crop production in protected culture. Unlike the common black and white

nets, photo-selective nets have the ability to modify the quality of transmitted light, in

addition to their basic protective function (from hail, wind, pests, or excessive solar

radiation) (Shahak et al., 2004a:144). Photo-selective nets are composed of

semitransparent threads that selectively screen out the defined spectral bands of

4

light, generally transmitted through them in UV or visible spectral ranges, together

with transforming direct light into scattered or diffused light (Shahak, 2004b:610).

Such spectral manipulation can trigger a wide range of physiological responses,

while the scattering improves the penetration of spectrally modified light into the

inner plant canopy, thus increasing the efficiency of light-dependent processes

(Rajapakse & Shahak, 2007:290, Shahak, 2004b:609).

The implementation of net protection using photo-selective technology is gaining

popularity around the world. Photo-selective nets have been tested with various

perennial crops in order to improve their fruit yield and quality, particularly in high

irradiance environments (Jifon & Syvertsen, 2001:177, Oren-Shamir et al., 2001:353,

Pastenes et al., 2003:71, Raveh et al., 2003:365, Shahak et al., 2004b:609, Shahak

et al., 2004a:143). Previous research has shown that photo-selective nets have a

positive effect on fruit yield (in relation to full sun conditions) in some varieties of

Vaccinium corymbosum L., depending on the colour and shade percentage used

(Lobos et al., 2009:465, Retamales et al., 2008:193). In bell pepper cultivars grown

in the Besor (semi-arid) region of Israel under 35% shading, the red, yellow, and

pearl nets were found to markedly increase the productivity and improve the fruit

quality (both pre- and postharvest) compared with the black nets commonly used

(Fallik et al., 2009:37). Growing fruit trees in stressed environments (high irradiances

and temperatures) could reduce their photosynthetic rate, carbohydrate

accumulation and fruit yields (Darnell, 1991:856, Darnell & Birkhold, 1996:1132).

5

In summary, researchers have widely used photo-selective nets as a: biological

control against the key pests of protected crops (the whitefly B. tabaci, and the

aphids M. persicae and M. euphorbiae) and two of the main natural enemies of B.

tabaci (O. laevigatus and A. swirskii) (Legarrea et al., 2012:523); light manipulation

to improve various aspects of fruit tree performance (flowering intensity, fruit setting,

productivity, fruit maturation rate, fruit size, vegetative vigour) (Shahak et al.,

2004b:609, Takeda et al., 2010:134); and a method to improve yields as well as

quality in tomato fruits and sweet peppers (Ilic et al., 2012:90, Kong et al., 2013:290).

However, the effects of light quality on plants are well known and the response of

different species to light management is variable; therefore, it is clearly important to

treat plants with the correct type of light filters (McMahon & Kelly, 1995:203),

especially those with economic benefits.

In the present study, the effects of three photo-selective nets (red, pearl, and yellow)

and a black net (control) on growth, yield and fruit quality of three tomato cultivars

were investigated. A major concern in agricultural productivity is heat stress and pest

infestation which have a direct effect on the product quality and postharvest storage

of fresh produce. High temperatures can increase the capacity of air to absorb water

vapour and consequently generate a higher demand for water. Higher

evapotranspiration indices could lower or deplete the water reservoir in soils,

creating water stress in plants during the dry seasons. It is well documented that

water stress not only reduces crop productivity but also tends to accelerate fruit

ripening (Henson, 2008:384). Exposure to elevated temperatures can cause

morphological, anatomical, physiological, and, ultimately, biochemical changes in

6

plant tissues and can consequently affect the growth and development of different

plant organs. These events can cause drastic reductions in crop yields. Based on

previous positive results of coloured shade nets on plant development, we

investigated the hypothesis that the light quality of the red, pearl and yellow photo-

selective net spectrum, rather than the light intensity (common black net), is

responsible for the major plant development related to the growth parameters, yield

and product quality in tomato cultivars.

1.1 AIM AND OBJECTIVES

1.1.1 Aim

The aim of this project was to determine a suitable photo-selective net

(ChromatiNet™), a suitable cultivar type and the cultivar versus photo-selective net

interaction necessary to improve crop yields, product quality and bioactive

compounds.

1.1.2 Objectives

Firstly, to investigate the effect of photo-selective nets on plant growth parameters

(plant height, leaf area, leaf chlorophyll, stem diameter, flowering intensity, and fruit

7

setting), pest and disease incidence, and yield (number of fruits per plant) in three

selected cultivars.

Secondly, to investigate the effect of shading on fruit quality parameters (fruit mass,

firmness, colour, soluble solids content, and titratable acidity) and health promoting

compounds (ascorbic acid, lycopene content, β-carotene, total phenols and

flavonoids, and antioxidant scavenging activity) in three selected tomato cultivars.

1.2 THESIS STRUCTURE

This thesis is organised into five chapters. The literature review on tomato production

and photo-selective netting is presented in Chapter 2, and covers information on the

crop origin and description of tomato fruit, varieties, production and trade, fruit

structure and nutritional composition, maturity indices for harvesting tomatoes,

physiological disorders and common tomato diseases, hydroponic culture and

conventional production practices, and production under photo-selective nets.

Chapter 3 illustrates the effect of three photo-selective nets (red, pearl and yellow)

(ChromatiNet™) and a black net (control) on plant growth parameters (plant height,

leaf area, stem diameter, leaf chlorophyll and flowering), yield (number of fruits per

plant, fruit trusses, and marketable fruits), PAR and microclimatic conditions, pest

infestation and disease occurrences. Chapter 4 describes the effect of shading on

tomato fruit quality (fruit mass and firmness, colour, soluble solids content, titratable

acidity) and health promoting compounds in tomatoes (lycopene, β-carotene,

ascorbic acid, total phenols and flavonoids, and antioxidant scavenging activities). A

general conclusion is furnished in Chapter 5 and the conclusions obtained from

8

Chapters 3 and 4 are summarised herein. Outcomes of this study were presented at

an international conference (Postharvest Africa 2012, 7th International CIGR

Technical Symposium on “Innovating the Food Value Chain” Postharvest

Technology and Agri-Food Processing Stellenbosch, South Africa, 25-29 November,

2012.) and at a national conference (Combined Crops, Soils, Horticulture and Weeds

Congress, Durban, South Africa, 21-24 January, 2013.) and published in the popular

horticulture journal, Scientia Horticulturae (Tinyane, Sivakumar & Soundy, 2013:340-

349).

9

CHAPTER TWO

2 LITERATURE REVIEW

2.1 TOMATO (Solanum lycopersicum)

Tomatoes are one of the most widely produced and consumed vegetables in the

world. The tomato belongs to the Solanaceae family (also known as the nightshade

family), genus Lycopersicon, subfamily Solanoideae and tribe Solaneae (Taylor,

1986:1). The Solanaceae family includes other well-known important species such

as chilli and bell peppers (Capsicum spp.), potato (Solanum tuberosum), aubergine

(Solanum melongena), tomatillo (Physalis ixocarpa) and tobacco (Nicotiana

tabacum) (Naika et al., 2005:6). Some common names for the tomato of different

regions are: tomate (Spain, France), tomat (Indonesia), faan ke‟e (China), tomati

(West Africa), tomatl (Nahuatl), jitomate (Mexico), pomodoro (Italy), nyanya (Swahili)

(Naika et al., 2005:6).

2.1.1 Origin, history and uses

Tomatoes originated from the South American Andes, specifically in Peru, Bolivia

and Ecuador (Costa & Heuvelink, 2005:2). According to the available literature, wild

plants of Lycopersicon esculentum were more widespread and have been distributed

to other South American regions and into Mexico (Naika et al., 2005:6; Peralta &

10

Spooner, 2001:1902). Archaeological, ethnobotanical and linguistic evidence

suggests that the tomato was domesticated in Mexico (outside its centre of origin)

from where a variety of fruit sizes and colours were selected (Van der Vossen, Nono-

Womdim & Messiaen, 2004). The Spanish brought the cultivated tomatoes to Europe

in the early 16th century (Harvey, Quilley & Beynon, 2002:304). According to Naika et

al. (2005:6), the tomatoes were then introduced to Southern and Eastern Asia, Africa

and the Middle Eastern countries from Europe.

According to Orzolek et al. (2006:1), commercial tomato production was initiated

after the 1860s. More recently, through tomato breeding, many tomato varieties have

been developed and adopted for consumption around the world. Tomatoes have

become one of the most important and widely eaten vegetables in the world today

(Costa & Heuvelink, 2005:ix). Tomatoes are eaten either fresh (salads) or in cooked

form in sauces, soups and meat or in processed forms (purees, juices, ketchup,

canned or dried tomatoes). The worldwide production of tomatoes totalled

159,023,383 tonnes in 2011, with the top producers being China, followed by India,

the United States, Turkey, Egypt and Italy (FAOSTAT, 2013).

2.1.2 Nutritional content and health benefits

Tomatoes can contribute to a healthy, well-balanced diet. They are rich sources of

minerals, vitamins (A, B and C), essential amino acids, sugars and dietary fibres

(Bhowmik et al., 2012:34). Tomatoes also contain high levels of potassium,

11

phosphorus, magnesium and iron (Beecher, 1998:98). They are further regarded as

the most important source of fibre in the human diet (Bhowmik et al., 2012:40). Both

fibre and vitamins possess the ability to neutralise dangerous free radicals and lower

high cholesterol levels (Bhowmik et al., 2012:40; Gahler, Otto & Bohm, 2003:7962).

According to Carr and Frei (1999:1086), Nagy and Smoot (1977:135), and Watada,

Aulenbach and Worthington (1976:856), 60 mg of vitamin C per day is the

recommended daily allowance (RDA) for maintaining good health. Although the

vitamin content of tomatoes differ due to varying cultural practices, cultivars and

postharvest handling practices, Hamner and Maynard (1942), and Bhowmik et al.

(2012:38) reported that tomatoes provide 40% of the daily value (DV) of vitamin C,

15% DV of vitamin A, 8% DV of potassium and 7% of the RDA of iron for women and

10% RDA for men.

Tomatoes are the major source of dietary lycopene. Unlike nutrients in most fresh

produce, tomatoes have greater bioavailability with regard to dietary lycopene after

cooking and processing (Bhowmik et al., 2012:34). Lycopene is a powerful

antioxidant with a high oxygen-radical scavenging and quenching capacity, which

helps in the fight against cancerous cell formation and other kinds of health

complications and diseases (Beecher, 1998:98; Di Mascio, Kaiser & Sies, 1989:533;

Levy et al., 1995:258; Clinton et al., 1996:824). Tomatoes are also rich in antioxidant

micronutrients such as lutein, phytoene, phytofluene, β-carotene, vitamin E and

phenolic compounds (Bhowmik et al., 2012:36). Several of these constituents may,

therefore, contribute to the health-giving properties of tomatoes. They contribute

more to human nutrition than other vegetables because large quantities of tomato

12

products are widely available, relatively cheap and consumed in so many different

ways by people of all ages and cultures.

Table 2.1: Tomato (raw) nutritional value per 100 g

Principle Unit Nutritional Value Per 100 g

Proximate Energy kcal 18 Proteins g 0.88 Total lipid (fat) g 0.20 Carbohydrates g 3.89 Dietary fibre g 1.2 Sugars, total g 2.63 Vitamins Vitamin C mg 13.7 Thiamin mg 0.037 Riboflavin mg 0.019 Niacin mg 0.594 Vitamin B-6 mg 0.080 Folates µg 15 Vitamin A IU 833

Vitamin E (alpha- tocopherol)

mg 0.54

Vitamin K µg 7.9 Electrolytes Sodium mg 5 Potassium mg 237 Minerals Calcium mg 10 Iron mg 0.3 Magnesium mg 11 Phosphorus mg 24

Zinc mg 0.17

Source: USDA National Nutrient data base

13

2.1.3 Plant morphology

The tomato is a perennial plant but is grown as an annual plant. A detailed

description of the growth and development, botanical information, ecology, anatomy

and histogenesis of the tomato is furnished by Van der Vossen, Nono-Womdim and

Messiaen (2004), and Hayward (1938:39). These authors describe the plant as a

branching, herbaceous plant with hairy, weak, trailing stems. The leaves are hairy

and vary in size. The plant bears yellow flowers in clusters. Its fruits are round to

lobed and vary in size and colour ranging from red, pink or yellow when ripe. Flat,

slightly curved, hairy, light brown seeds are produced. The tomato plant has three

different growth tendencies, namely indeterminate, semi-indeterminate and

determinate types (Van der Vossen, Nono-Womdim & Messiaen, 2004; Hayward,

1938:39). Indeterminate and determinate types are entirely different kinds of crops.

The indeterminate varieties are best suited to a long harvest period, because they

keep growing even after flowering. However, under tropical conditions, their growth

is limited by disease and insect attacks. The plants generally have more foliage

which helps to keep the temperature within the crop lower, allowing the fruits to grow

under the shade of the leaves. Shade protects the fruits from sun damage, and

facilitates slow ripening (Van der Vossen, Nono-Womdim, Messiaen, 2004;

Hayward, 1938:39). Slower ripening improves the taste and in particular the

sweetness of the fruits. Indeterminate cultivars have a main stem which grows

upwards indefinitely, reaching more than 10 m per year; therefore, these cultivars

have to be trellised and they are best suited to greenhouse cultivation as well as

hydroponic production (Costa & Heuvelink, 2005:3).

14

Determinate cultivars can grow to a height of 2 m, but they stop growing after

flowering, thereby restricting fruiting. They require less labour; hence, they are

popular in commercial cultivation (Costa & Heuvelink, 2005:3). They can be grown

under field conditions and within greenhouses, have large fruit sets which last only

for a short period and their fruits ripen faster than those of indeterminate cultivars

(Naika et al., 2005:6).

2.1.4 Cultivars

The choice of cultivar to be planted depends on the local conditions and the purpose

of growing them. There are local varieties and value added varieties available to

farmers which result from a continuous process of selection of plants based on

characteristics such as fruit type and size, type of plant, yield, vitality and resistance

to specific diseases and pests, specific market and planting time, and also on factors

related to climate and management (Naika et al., 2005:12). Farmers will most

certainly choose cultivars that perform better under local conditions with minimum

capital input. Tomato breeding companies have also developed F1-hybrids, and the

advantage of these hybrids is that they combine high yield, disease resistance and

other plant and fruit characteristics (Naika et al., 2005:12). Disease resistance is an

added advantage because plants will require less spraying with pesticides. Hybrids

are cost-effective, but farmers will have to purchase new seeds each season.

15

Popular cultivars in South Africa are: cultivars best suited to the fresh market,

including Florodade, Heinz 1370, Karino, Rodade, Fortress, Hytec, Star 9001, Star

9003, Sundance and Zeal; fresh market cultivars with a long shelf life are Baldo,

Blockbuster, Disco, P747 and Shirley; cultivars that are suitable for processing

include HTX 14, Legato, Roma, VF, Rossol, Star 9056F, Sun 6216 and UC 82B;

cultivars for cherry tomatoes are Bamby and Josephine; and lastly, cultivars suited to

protected cultivation are Atletico, Daniella and Gabriella (Department of Agriculture

Forestry and Fisheries, 2011).

2.1.5 The fruit

Tomato fruits develop from the ovary of flowers after fertilisation of the ovules.

During the initial stages of fruit development, cell division and expansion delay fruit

growth. After 2-3 weeks of slow growth, rapid growth commences and the cells

continue to enlarge (Ho & Hewitt, 1986:201). Rapid growth takes 3-5 weeks resulting

in mature green fruits. At the mature green stage, the fruit will accumulate most of its

final weight (Ho & Hewitt, 1986:201). Within two days after the onset of the mature

green stage, the colour of the fruit will begin to change (Ho & Hewitt, 1986:201). The

green pigment will become lighter and a faint yellow-orange colour will appear. As

the orange pigment accumulates on the outside, several metabolic changes will be

occurring on the inside (Grierson & Kader, 1986:241). The pulp of the fruit will

gradually become softer as a result of enzymatic digestion of the cell walls (Grierson

& Kader, 1986:241). A placental tissue which fills much of the locular spaces and the

areas around the ovules will be degraded and assume a gelatinous character

16

(Grierson & Kader, 1986:241). When the fruit is fully matured, an abscission layer

forms between the calyx and the fruit, causing the fruit to detach from the pedicel

(Grierson & Kadar, 1986:241).

The tomato fruit can be bilocular or multilocular (Figure 2.1). The locules (cavities

containing seeds that are derived from carpels) are surrounded by the pericarp. The

pericarp includes the inner wall, columella; the radial wall, septa; and the outer wall.

The pericarp and placenta comprise the fleshy tissue of the tomato. Seeds are

located inside the locules and are enclosed in gelatinous membranes. There are

vascular bundles that surround the outer wall of the pericarp and extend from the

stem to the centre of the tomato, radiating to each seed (Ho & Hewitt, 1986:201).

Figure 2.1: Transverse section of a multilocular tomato fruit

17

2.1.6 Maturity of the fruit for harvesting

Harvesting tomatoes at the correct stage of maturity is very important and plays a

key role in limiting instances of post-harvest damage (Davis & Gardner, 1994:613).

Over-mature fruits are susceptible to bruising and injury and can also rot easily.

Tomatoes, in particular, have a high water content which makes them more

vulnerable to post-harvest losses (Eckert & Ogawa, 1988:433). Fruit maturity can be

assessed by examining the external and internal indicators of maturity. External fruit

maturity indices are based on skin colour, while internal indices are based on seed

development and locular gel formation (Ministry of Fisheries, Crops and Livestock,

New Guyana Marketing Corporation, National Agricultural Research Institute, 2003).

The most widely used tomato maturity index is the skin colour (Batu, 2004:471). The

latter remains green during fruit development on the plant, and as the fruit becomes

mature, distinct changes occur in the external colour which can then be used to

determine harvest maturity. Skin colouration in tomatoes follows a typical sequence

with regards to ripening (Alexander & Grierson, 2002:2047). In red skinned cultivars,

after the mature green stage, the tip of the blossom end will change to a pinkish-

yellow colour, which is referred to as the breaker stage because lycopene begins to

accumulate at this point (Fraser et al., 1994:405). The breaker stage usually occurs

within a day after the mature green stage. The entire fruit then turns pink, followed by

a light red and finally a deep red colour. The ripening stages of mature tomato fruit

are categorised as being green, breaker, turning, pink, light red, and red (Figure 2.2)

as described in Table 2.2.

18

Source: USDA

Figure 2.2: Ripening stages of mature tomato fruit; from left to right: green, breaker,

turning, pink, light red, and red

The internal fruit characteristics used to determine the harvest maturity of green

fruits are seed development and locular jell formation (Kader & Morris, 1976). Mature

green fruits have fully developed tan-coloured seeds whereas the seeds in immature

green fruits are white and not adequately formed. The fruit cavities of mature green

fruits are completely filled with jelly in each of the locules, while immature green fruits

have one or more locules without jelly (Ministry of Fisheries, Crops and Livestock,

New Guyana Marketing Corporation, National Agricultural Research Institute, 2003).

19

Table 2.2: Terms used to describe tomato colour as an indicator of ripening

Colour Description Green The tomato surface is completely green. The

shade of green may vary from light to dark

Breaker There is a definite break of colour from green to yellow, with pink or red skin covering not more than 10% of the surface

Turning Tannish-yellow, pink or red colour shows on over 10% but on not more than 30% of the tomato surface

Pink Pink or red colour shows on over 30% but on not more than 90% of the tomato surface

Light red Pinkish-red or red colour shows on over 60% but the red colour covers not more than 90% of the tomato surface

Red More than 90% of the tomato surface, on aggregate, is red

Source: USDA

In practice, tomatoes are harvested at different stages of maturity depending on

market specifications and the planted cultivar. Generally, the average time it takes to

obtain the first harvest after sowing is 3-4 months (Valenzuela, Hamasaki & Hori,

1993). Fruits meant for the fresh market are harvested at the mature green to pale

pink stage, depending on the distance and the time required to market them (Kader

et al., 1977:724). Green tomatoes can continue to ripen after picking, until they are

completely red. A few ripe tomatoes in the harvest will help to speed up the ripening

process. However, fruits harvested at the pre-ripe stages tend to have a lower quality

(lower soluble solids, ascorbic acid, and reduced sugars) than fruits that have

ripened on the plant (Naika et al., 2005:60). In contrast to the fruits for the fresh

market, processed fruits can be harvested when they are fully ripe. Tomatoes that

are used to make pureed products (soup, juice, sauce) are usually left on the tree

20

and picked when they are red and completely ripe (Department of Agriculture

Forestry and Fisheries, 2011).

2.1.7 Physiological disorders

Tomato fruit quality is determined by appearance (colour, shape, size, freedom from

physiological disorders and decay) firmness, dry matter and flavour, and health

benefits (Fleisher et al., 2006:126). The absence of physiological disorders in

tomatoes is very important not only for the sake of their appearance, but also

because other attributes such as the nutritional status and shelf life may be affected

(Masarirambi et al., 2009:123).

Physiological or abiotic disorders are caused by changing environmental conditions

such as the temperature, moisture, unbalanced soil nutrients, inadequate or

excessive soil minerals, extremes in soil pH and poor drainage (Jarvis & Mckeen,

1991; Khavari-Nejad et al., 2009:209). Genetic factors are also involved; thus, there

is a genetic (G) and an environmental (E) interaction (G x E). The physiological

disorders found in tomatoes can be divided into groups, namely: nutrient imbalances

between potassium (K) and nitrogen (N) or magnesium (Mg) (blotchy ripening, grey

wall); amounts or movement of calcium (Ca) into the fruit (blossom-end rot); genetic

sensitivity (green or yellow shoulder) and watering (cracking) (Peet, 2009:151); high

irradiance and low temperature (sunscald, catfacing) (Olson, 2004, Masarirambi et

21

al., 2009:123).These disorders drastically affect the production of tomatoes and

result in severe economic losses.

2.1.8 Common tomato diseases

Tomato diseases are weather dependent and can spread rapidly. The diseases are

caused by parasitic fungi, bacteria, viruses, nematodes and environmental

conditions. Some of these diseases are more prevalent in outdoor tomato production

while others are more prevalent in indoor production. The latter produces a physical

environment which is favourable for the development of pathogens due to the ideal

temperatures, high humidity and restricted air movement. Biotic diseases in

tomatoes that favour these conditions include gray mould, leaf mould and powdery

mildew (Elad et al., 2007:285). Common diseases occurring in outdoor tomato

production are septoria leaf spot, early blight, fusarium wilt, verticillium wilt, late

blight, bacteria spot, bacteria speck, viruses and nematodes (Gleason & Edmunds,

2006). These diseases are spread by insects, splashing water, leafhoppers and

humans. Diseases can, however, be controlled by buying treated seeds, sterilising

the soil before planting, spraying plants with appropriate pesticides, using resistant

cultivars or soilless media and implementing sanitary measures (Jarvis & McKeen,

1991).

22

2.2 HYDROPONICS/SOILLESS VEGETABLE PRODUCTION

2.2.1 Definition

The term “soilless culture” is defined as the cultivation of crops of plants in a nutrient

solution that supplies all the nutrient elements needed for optimum plant growth, with

or without the use of an artificial growing medium (sand, gravel, vermiculite,

rockwool, peat moss, sawdust, coir dust, coconut fibre, etc.) to provide mechanical

support (Gruda, 2009:141). Liquid hydroponic systems have no supporting medium

for the plant roots; whereas, aggregate systems offer a solid medium for support.

Hydroponic systems can be either open (once the nutrient solution is delivered to the

plant roots, it is not reused) or closed (surplus solution is recovered, replenished,

and recycled). Hydroponics are employed in protected cultivation practices; either,

inside greenhouses or under shade nets in order to provide temperature control,

reduce evaporative water loss, provide better control of diseases and pests and to

protect crops against weather elements such as wind and rain.

A major advantage of hydroponics is its isolation of the crops from the soil, which is

often associated with problems such as diseases, salinity or poor structure and

drainage (Abd-Elmoniem et al., 2006:1). Also, in hydroponics, continuous cultivation

is possible, and a more efficient use of water and fertilisers is possible. The principal

disadvantages of hydroponics, relative to conventional open-field practices, are

firstly, its high initial input costs, which will increase if soilless culture is combined

23

with controlled environmental agriculture, and secondly, the energy inputs required

to operate the system. A high level of management skills is also necessary to

prepare the solutions, maintain the pH and EC values, judge and correct the nutrient

deficiency, ensure aeration, and maintain favourable conditions inside the protected

structure. Because of these significantly high input costs, the successful practical

applications (e.g., for food production) of soilless culture are limited to the production

of high economic value crops such as tomato, strawberry, cucumber, and so forth

(Jenson & Collins, 1985:483). Greater emphasis is placed on aggregate (open)

hydroponic system, as described below.

2.2.2 Closed and open hydroponic systems

Hydroponic systems of cultivation are classified according to the method of applying

nutrient solutions to the plant roots, either in a “closed form” where the nutrient

solution is recirculated, or an “open form” where the nutrient solution is not

recirculated (Jensen & Collins, 1985:484). Closed hydroponic systems can be further

divided into two methods; the circulating and non-circulating method. In the

circulating method, the nutrient solution is pumped through the root system and any

excess solution is collected, replenished and reused (Jensen & Collins, 1985:484).

The nutrient solution is monitored for its salt concentration before it is recycled, and

in some cases growers replace the nutrient solution every week with a fresh solution

in order to ensure that sufficient nutrients are available to the plants. In the non-

circulating method, the nutrient solution is not circulated, but used only once. The

nutrient solution is replaced when its concentration declines or its pH or EC changes.

24

Therefore, the nutrient composition in the recirculated nutrient solution is

continuously changing due to the plant uptake and evapotranspiration of water from

the solution (Graves, 1985:1-44). Applying a closed system requires good

management skills as the system is vulnerable to poor water quality, and toxic ions

can accumulate in the water if the entire solution is not changed periodically.

In an open hydroponic system, a solid, inert medium provides support for the plants

and like in closed systems, the nutrient solution is delivered directly to the plant roots

through irrigation. Fertilisers can be provided through the proportioners or can be

mixed with the irrigation water in a large tank. Irrigation is usually programmed using

a timer, and in large installations, solenoid valves are used to allow only a section of

the greenhouse or net house to be irrigated at a time. Compared to the “closed

system”, the “open system” can be managed with less difficulty, nutritional

management problems can be reduced, and incidences of nutritional deficiencies

may also be lessened (Jensen & Collins, 1985:484).

2.2.3 Aggregate hydroponic system

This technique involves inert solid media (sand, gravel, rockwool, peat moss,

sawdust, coconut fibre, coir dust, perlite, vermiculite) which provides support for the

plants. The selected media material must be flexible and crumbly, with a water

holding capacity that can be drained easily. In addition, the media must be free of

toxic substances, pests, disease causing microorganisms, nematodes, and so forth.

25

The chosen media must also be thoroughly sterilised before use (Jensen & Collins,

1985:484). As in liquid systems, the nutrient solution is delivered directly to the plant

roots. Aggregate systems can be closed or open, depending on whether the

recovered excess solution is recirculated through the system or not. Aggregate

hydroponic systems include the bag culture and the rockwool culture. For the

purpose of this manuscript greater emphasis will be placed on the bag culture.

However, the use of horticultural rockwool as the growing medium in open

hydroponic systems is increasing rapidly. Rockwool was first developed as an

acoustical and insulation material made from a mixture of diabase, limestone and

coke which was melted at a high temperature, extruded in small threads, and finally

pressed into lightweight sheets (Thiyagarajon, Umadevi & Ramesh, 2007). In the

bag culture, the growing medium is placed into plastic bags which are placed in lines

on the greenhouse floor, thus avoiding the cost of complex drainage systems. The

bags can be used for at least two years, and can be sterilised by steam much more

easily and cheaply than bare soil. The bags have a black interior and are composed

of UV-resistant polyethylene, which will last for up to two years in controlled

environment agriculture (CEA). The exterior of the bags should be white if they are

placed in desert areas and other regions with high light intensities, so as to reflect

radiation and minimise the heat reaching the growing medium. Conversely, a darker

exterior colour is recommended in low-light latitudes in order to absorb winter heat.

Growing media for use in the bag culture includes peat, vermiculite; a combination of

these materials with perlite is often added to reduce costs. Bags are placed flat on

the greenhouse floor at a normal spacing for vegetable production. It is beneficial,

however, to first cover the entire floor with white polyethylene film so as to maximise

the amount of light reflected back into the plant canopy. Such a covering may also

26

reduce the relative humidity and the incidence of some fungal diseases. The soil in

the bag has to be moistened before planting. The use of the bag culture is largely

dependent on the availability and cost of the growing media (Jensen & Collins,

1985:483; Thiyagarajon, Umadevi & Ramesh, 2007).

2.2.4 Nutrient solution

Unlike soil which stores nutrients, the growing media used in hydroponic systems

have little effect on the nutrition of the plants. The only source of nutrients is the

nutrient solution, and therefore all the nutrients required for plant growth have to be

present in the circulating water of the hydroponic system. There are macro and micro

nutrients that are essential for good plant growth. Macro elements: nitrogen (N),

phosphorous (P), potassium (K), calcium (Ca), magnesium (Mg) and sulphur (S) are

required in high concentrations, whereas micro elements: iron (Fe), boron (B),

manganese (Mn), copper (Cu), zinc (Zn), and molybdenum (Mo) are required in very

low concentrations. These elements must be maintained at appropriate

concentrations since the management of the nutrient solution is the key to success in

a hydroponic culture, (see Table 2.3) (Marr, 1994).

27

Table 2.3: Recommended concentration ranges of essential elements in nutrient

solution

Element Symbol Concentration (ppm) Nitrogen N 200-236 Phosphorus P 60 Potassium K 300 Magnesium Mg 50 Calcium Ca 170-185 Sulphur S 68 Iron Fe 12 Copper Cu 0.1 Zinc Zn 0.1 Manganese Mg 2.0 Boron B 0.3 Molybdenum Mo 0.2 Source: Cooper, 1988

In an “open system” the nutrient solution is not recovered and recycled; as a result it

does not require monitoring and adjustment. Once mixed, it is used until depleted.

Growing media will, however, require constant monitoring, particularly if the irrigation

water is relatively saline or if the hydroponic structures are located in a warm, high

sunlight region. To avoid an accumulation of salt in the growing medium, some

irrigation water can be used to allow a small amount of drainage or “leaching” from

the bags. This drainage should be collected and tested periodically for total dissolved

salts. If the salinity of the drainage water reaches 3,000 ppm or above, the bags

must be leached free by using plain water in the irrigating system (Marr, 1994).

28

2.2.5 Nutritional disorders

Nutritional disorders are plant symptoms that result from either too much or too little

of a specific nutrient element. There are no nutritional disorders unique to

hydroponics or open field cultivation. Plants are more likely to show nutritional

disorders in a closed hydroponic system than in an open system. In a closed system,

levels of impurities or unwanted ions in the recycled liquid or from the chemicals

used can interfere with the balance of the formulation and accumulate to toxic levels

(Thiyagarajon, Umadevi & Ramesh, 2007). Common nutritional disorders in

hydroponics are caused by high levels of ammonium (NH4), zinc toxicity or low levels

of potassium and calcium. Physiological disorders resulting from high levels of

ammonium can be avoided by providing no more than 10% of the required nitrogen

in an ammonium form. Low levels of potassium (less than 100 ppm in the nutrient

solution) can affect tomato acidity and reduce the percentage of high-quality fruit

(Marr, 1994). Low levels of calcium can induce blossom end rot in tomatoes (Taylor

& Locascio, 2004:123) and tip burn in lettuce (Barta & Tibbitts, 2000:294). Zinc

toxicity resulting from dissolution of the elements from galvanised pipes in the

irrigation system can be prevented by using plastic or other non-corrosive materials

suitable for agriculture. At toxic concentrations, ions of heavy metals act as efficient

generators of reactive oxygen species (Luna, Gonzalez & Trippi, 1994:11; Assche &

Clijsters, 1990:195).

29

2.2.6 Disease and insect control in hydroponic systems

Hydroponic systems were initially developed to control soil-borne diseases.

However, diseases that are specific to hydroponics have been reported (Stanghellini

& Rasmussen, 1994:1129). Nevertheless, a soilless culture provides several

advantages such as greater production of crops, reduced energy consumption,

better growth control and independence of soil quality. Sources of pathogens in

hydroponic systems include the following: plant material, growth media, air, sand,

soil, peat, and water from lakes, rivers and wells, and insect pests. Airborne spores

of Fusarium oxysporum f. sp. radicis-lycopersici fungus have been identified as

major sources of fungal attacks on both the roots and the crown of tomato plants

(Rowe et al., 1977:1513). These spores are present in the soil introduced into the

production facilities on the shoes of greenhouse personnel, and in nutrient solution

reservoirs placed at ground level in most greenhouse facilities. To prevent soil borne

pathogens, all soil within the greenhouse should be covered to prevent splash

dispersal or generation of dust which may harbour soil borne pathogens and the

nutrient reservoirs should be covered at all times. Washed river sand has been used

in greenhouses as an aggregate substrate or as a ground cover pathway between

production areas (Stanghellini & Rasmussen, 1994:1131). Washed river sand can be

infested with many plant pathogens and has been identified as the source of

introduction of Pythium aphanidermatum and P. dissotocum into hydroponic systems

(Bates & Stanghellini, 1984:989). Crops affected by Pythium aphanidermatum and P.

dissotocum show extensive root rot, wilt, severe stunting and death. Peat and peat

mixtures are major sources for the introduction of plant pathogens. A survey

conducted by Kim, Faorer and Longenecker (1975:124) revealed that 15 out of 52

30

randomly selected peat products contained Pythium, and all 52 contained Fusarium

(which is a pathogenic fungal inocula). Fungus gnats (Bradysia spp.) and shore flies

(Scatella stagnalis) are common greenhouse insect pests; both their larva and adult

stages have been implicated as vectors of some of the root pathogens (Stanghellini

& Rasmussen, 1994:1132).

2.3 PHOTO-SELECTIVE NETTING

2.3.1 Introduction

Netting is frequently used in agriculture to protect crops from excessive solar

radiation (shade-nets), thereby improving the thermal climate (Kittas et al., 2009:97),

and minimizing environmental stresses (wind and hail) or flying pests (Teitel et al.,

2008:99). The nets commonly used for the shading of ornamental crops and

nurseries are black and offer from 40-80% shading (Shahak et al., 2004a:143). Anti-

hail and insect proof nets are made of clear or white threads or a combination of both

(Shahak et al., 2004a:143). The available netting products are mostly woven, knitted

or embossed, and they may vary in their texture, design, mechanical properties and

durability (Shahak et al., 2004a:143).

Photo-selective nets include “coloured-ColourNets” (red, yellow, green, and blue net

products) as well as “neutral-ColourNets” (pearl, white and grey) absorbing spectral

31

bands shorter or longer than the visible range (Shahak et al., 2008:75). Photo-

selective nets represent a new agro-technological concept which combines physical

protection together with differential filtration of the solar radiation in order to

specifically promote the desired physiological responses that are light regulated

(Shahak et al., 2004a:144). The targeted responses are those that determine the

commercial value of each crop, including yield, product quality and rate of maturation

(Shahak et al., 2004a:143). The nets are based on the incorporation of various

chromatic additives, and light dispersive and reflective elements into the netting

materials during manufacturing.

Photo-selective nets also have an ability to transform direct light into scattered light,

which improves the penetration of light into the inner plant canopy, prevents burning,

offers a moderate cooling effect, and influences pest control (Shahak et al.,

2008:75). Radiation use efficiency increases when the diffuse component of the

incident radiation is enhanced under the shade (Healey et al., 1998:665). Shading of

crops results in a number of changes in both the local microclimate and the crop

activity. Changes in the local microclimate modify the assimilation of carbon dioxide

(CO2) and consequently affect crop growth and development (Kittas et al., 2009:97).

2.3.2 Plant growth and yield

Nettings, regardless of colour, possess the ability to reduce radiation from reaching

the crops underneath them. With a higher shading factor, more radiation will be

32

blocked. Reductions in radiation resulting from the type of netting used affect the

temperatures (air, plant, soil) and the relative humidity (Stamps, 1994:35). However,

besides affecting the amount of radiation, netting can also influence the direction of

the radiation. The ability of the shade nets to influence the radiation direction induces

light penetration into the inner plant canopy. Diffuse light has been demonstrated to

increase radiation use efficiency, the yields and may even affect flowering (timing

and amounts) (Gu et al., 2002:1). Shade netting that maximises light scatterings

without affecting the light spectrum has also been shown to increase branching,

plant compactness, and the number of flowers per plant (Nissim-Levi et al., 2008:9).

Photo-selective nets prevent excess sunlight and retain soil moisture levels for

proper plant growth, which lead to an increase in productivity (yield) of the plants. Air

movement inside the nets is restricted, thus reducing wind damage to the crop and

evaporation of soil moisture. The nets also ensure that the air beneath the shade

cloth stays humid which is of further benefit to the plants.

According to Kasperbauer and Hamilton (1984:967), plants react to changes that

occur in the spectrum of electromagnetic radiation to which they are exposed

through adjustments in their morphology and physiological functions that result in

their adaptation to different environmental conditions. Such adjustments are

facilitated by the phytochromes, cryptochromes and phototropin photoreceptors

which have absorption peaks in the red and blue or ultraviolet regions of the

spectrum (Li et al., 2000:216). Photoreceptors are able to detect variations in light

composition and induce photo-morphogenetic responses that influence growth and

development, morphology, leaf and stem anatomy, distribution of photosynthetic

products, photosynthetic efficiency and chemical composition (Lee et al., 2000:447,

33

Kasperbauer, 1987:353; Lee et al., 1997:4; Schuerger et al., 1997:273; Brown et al.,

1995:810; Kasperbauer & Peaslee, 1973:442; Macedo et al., 2004:517). These light-

regulated signalling events are necessary for normal plant development, and they

ensure that adaptive changes occur in response to environmental changes.

Phytochromes are capable of detecting wavelengths from 300 nm to 800 nm, with a

maximum sensitivity in the red (600 nm to 700 nm with peak absorption near 730

nm) wavelengths of the spectrum (Rajapakse & Shahak, 2007).

Plants produce new and structurally altered leaves as their primary means of

responding to changes in the radiation environment. Costa et al. (2010:349)

conducted an experiment to evaluate the effect of coloured shade nets (blue & red

net, 50% shading) versus full sunlight on the vegetative growth and development of

the medicinal plant Ocimum selloi. Results indicated that shade grown plants were

taller when compared to plants grown in full sunlight; their total dry biomass, roots,

stem and leaf tissue biomasses were lower. Plants exhibited a high degree of

physiological, morphological and anatomical plasticity, as explained by the difference

in total leaf area, specific leaf area, leaf area ratio and leaf weight ratio values

recorded under the different light treatments. The quantity and the quality of light

under shade nets induce optimal plant growth (shoot and stem elongation) in order

to increase light interception and to facilitate photosynthetic processes

(Kasperbauer, 1987:350). These processes result from the translocation of

photosynthates from the leaves and root reserves (Brutnell, 2006:417; Marks &

Simpson, 1999:133). Shade-grown plants also tend to have high inflorescences in

comparison with field grown plants (Costa et al., 2010:352). This is a phenomenon

34

that can be viewed as a survival strategy; whereby under unfavourable conditions

plants opt to minimise the energy spent on biomass production in order to support

reproductive functions upon which the continued existence of the species depend

(Costa et al., 2010:352).

Normally, shaded plants have low photosynthetic rates. Shading increases flower

and fruit abortions, and reduces fruit set. The quantity of fruit produced is directly

influenced by the number of flowers, flower abortions and fruit setting. However,

covering the plants with ColourNets has been demonstrated to be beneficial in that

they allow for the high transmission of red light which is essential for chlorophyll (663

nm), photosynthesis and phytochrome (666 nm, 730 nm) (Solomakhin & Blanke,

2008:211). Shahak et al. (2004:146a) found that even though the total

photosynthetically active radiation (PAR) intensity was reduced by about 30%, the

photosynthetic rates of exposed leaves of apple trees were improved during most of

the day by the nets, parallel with stomatal conductance, and leaf cooling. The

highest photosynthetic rates were obtained under the red net (55%). The daily time

course of photosynthesis differed between the colour nets; common morning and

afternoon peaks were recorded for the red and black (48%) nets, while there was no

apparent mid-day drop in the grey (46%), pearl (54%) and blue (52%) nets. Similarly,

the mid-day stem water potential was better under the red net, relative to the un-

netted control, while being the best under the pearl, grey and blue nets. This

indicates that the ColourNet approach can be adopted as a common practice, and

presents a great opportunity to promote netting of crops which are not yet grown

under nets.

35

Applying net houses under high sunlight intensities is more beneficial when

compared with lower intensities both on a daily and seasonal basis. In the summer

seasons, PAR (400 nm to 700 nm) that is transmitted through net covered structures

usually fulfils crop growth requirements because the outside incidence of PAR is

high. Shading in winter or in a cool and cloudy environment can be more deleterious

due to low sunlight intensity. In a study conducted in England, 23% shading was

sufficient to reduce tomato yield by 20% (Cockshull, Graves & Cave, 1992:11).

Under intense sunlight in Spain, shading increased the marketable yield of tomatoes

by 10% (Lorenzo et al., 2003:181). Fruit set in Hermosa peaches was increased by

two treatments of netting compared with a no-net control; 30% red net enhanced the

quality of fruit set, and fruit size was also greater under the nets, except under blue-

coloured nets (Shahak et al., 2004b:612). Results to the contrary were reported by

Gent (2007:514) and Sandri et al. (2003:642).

2.3.3 Disease and pest infestation

Insect pests are a major cause for the reduction in the quantity and quality of crop

plant products. The most important ones under protected cultivation include aphids,

whiteflies and thrips. Aphids and whiteflies feed by suckling fluids directly from the

phloem vessels of plants, and thrips feed by breaking the epidermal cells of plants

and sucking their contents. These pests cause injuries to plant tissue by the

penetration of their mouthparts which can cause scars and often serve as ports of

entry for bacterial and fungal pathogens. They also contaminate the surface of their

host plants with their sticky sweet excrements (honeydew) that serve as a growing

36

substrate for fungi. Hence the majority of plant viruses are transmitted by these

insect pests (Hogenhout et al., 2009:115). Insect-borne viral diseases often cause

substantial economic damage to growers of agricultural crops worldwide. To protect

the crop plants from these pests, growers usually apply insecticides. Frequent

applications of insecticides, however, create health hazards for workers, consumers,

and the environment. Moreover, frequent applications of insecticides often induce

resistance in the treated pest populations. Therefore, alternative methods for

protecting crop plants from pests are constantly being sought (Ben-Yakir et al.,

2013). These pests use reflected sunlight as optical cues for finding hosts (Ben-

Yakir et al., 2012:609). They are known to have receptors for UV light (peak

sensitivity at 360 nm) and for green-yellow light (peak sensitivity at 520 nm to 540

nm) (Ben-Yakir et al., 2012:609). The latter colour induces landing and favours the

settling (arresting) of these pests (Shahak et al., 2008:78). This attraction to colour

can be enhanced by plant odour over a short distance. While, high levels of reflected

sunlight (glare) deter these insects from landing, at temperatures between 20 °C and

25 °C, these insects complete their life cycle in 1-4 weeks and can build up high

populations on plants (Ben-Yakir et al., 2013). Adult pests disperse to adjacent

plants by walking or by short flights. Some of the adults migrate on long flight, aided

by the wind, to colonise new areas (Byrne, 1999:310).

Traditionally, black shading nets have been used in the protected cultivation of

plants. However, coloured (photo-selective) shading nets are currently being

developed for improving crop production in addition to their protective function. In a

study conducted by Ben-Yakir et al. (2008:205) and Shahak et al. (2009:79), yellow

37

and pearl nets protected crops from aphids and whiteflies, but not from thrips. The

effects of coloured shading nets on the infestation of aphids and whiteflies and the

incidence of viral diseases transmitted by these insects were studied by Ben-Yakir et

al. (2012:249) from 2006 to 2010. The studies were conducted in the semi-arid

Besor region in southern Israel. Plants were grown in “walk-in” tunnels that were

covered by various coloured nets of 35% shading capacity. The nets had large holes

that permitted free passage of sucking pests of 1-2 mm in length (Figure 2.3). Ben-

Yakir et al. (2008:205) found that whiteflies landed on the yellow net 20 to 40 times

more often than on the other nets. Despite that, the infestation level of aphids and

whiteflies in the tunnels covered by either the yellow or pearl nets were consistently

two to threefold lower than in tunnels covered by the black or red nets. The reduction

in pests leads to a similar reduction in the incidences of viral diseases transmitted by

them. When the incidence of the cucumber mosaic virus (CMV) in peppers grown

under the black or red nets ranged between 35% and 89%, it was two to tenfold

lower under the yellow or pearl nets (Ben-Yakir et al., 2012:249). Similarly, when the

incidence of the necrotic strain of the potato virus Y (PVY) in tomatoes grown under

black or red nets ranged between 42% and 50%, it was two to threefold lower under

the yellow or pearl nets (Ben-Yakir et al., 2012:249). Also, when the incidence of the

tomato yellow leaf curl virus (TYLCV) in tomatoes grown under the black or red nets

ranged between 15% and 50%, it was two to fourfold lower under the yellow or pearl

nets (Ben-Yakir et al., 2012:249).

The mechanism by which yellow and pearl nets provide protection against aphids

and whiteflies is unclear, but it is believed that the optical properties of these nets,

38

light reflection and arrestment play a major role in this protection (Ben-Yakir et al.,

2012:255; Shahak et al., 2008:78). The sunlight transmission and scattering

characteristics of these nets were reported by Shahak et al. (2004a:143), and

Rajapakse and Shahak (2007). Covering crops with shading nets may interfere with

the ability of flying pests to see the host plants under the nets and to discern the

plants from their background. Since the threads of the light coloured nets are more

translucent than the black threads, light coloured nets have a higher thread density

than black nets of the same shading capacity. Therefore, the light coloured nets

probably block the view and hide the plants to a greater extent than do the black

ones. However, the study conducted by Ben-Yakir et al. (2012:253) indicated that

the red net did not provide any protection from pests, even though its thread density

was about twice as thick as that of the yellow and pearl nets.

Figure 2.3: Photo-selective shade nets and the black commercial shade net

39

2.3.4 Physico-chemical properties in fruits

Fruit and vegetable consumption is growing globally. At the same time, consumers

are becoming increasingly conscious of the quality of the fruit and vegetables that

they consume. Satisfying consumer demands and assuring the markets of fresh fruit

and vegetables, therefore, necessitates optimum quality in produce in terms of their

state of ripeness and organoleptic quality. Sweetness and firmness are important

components of fresh fruit quality, and they provide a good indication of the state of

fruit ripeness and the potential shelf life. Firmness affects the susceptibility of fruit to

physical damage and storability. Titratable acidity (TA) in fruits is used along with

total soluble sugar content (SSC) (°Brix) as an indicator of maturity (Gonzalez-

Cebrino et al., 2011:444). Acids tend to decrease with fruit maturity while sugar

content increases; therefore, the SSC/TA ratio is often used as an index of maturity

(Caliman et al., 2010:77). High acid levels are associated with lower pH values and

vice-versa. They also play a significant role in the taste, colour and microbial stability

of the juice; they prevent the growth of microorganisms that are harmful to the

conservation of the fruit (Caliman et al., 2010:76).

Fruit pH increases with maturity and generally has an inverse relationship with

titratable acidity (Gould, 1992) (Table 2.4). Sugars and acids and their interactions

are important to the sweetness, sourness and overall flavour intensity of fruits

(Debruyn, Garretsen & Kooistra, 1971:241). Fructose and citric acid were found to

be more important to the sweetness and sourness than the glucose and malic acid in

tomatoes (Kader, 1986:212). High sugar and relatively high acid levels are essential

40

for the best flavours in fruits (Kader et al., 1978:742; Mencarelli & Saltveit,

1988:742). High acid and low sugar levels produce a sour taste, while high sugar

and low acid levels result in a bland taste (Kader, 1986:212). When both sugar and

acid levels are low, they result in tasteless fruits (Kader, 1986:212).

The growth and development of plants under a protected environment depend on the

intensity, quality and duration of solar radiation (Cockshull, Graves & Cave,

1992:11). The normal growth and development of crops occur when the amount of

radiation received exceeds the trophic limit (Beckmann et al., 2006:86). Climatic

variables such as air temperature, relative humidity and solar radiation are modified

by shade structures, thereby influencing the growth, development and production of

plants. Fruit growth and development depends on the translocation rates of the

carbohydrate supply, and the sink strength in plants (Lowell, 1986). Temperature

and light may influence these processes at any stage of the development of the fruit

(Marsh, Richardson & Macrae, 1999:443), because the accumulation of

carbohydrates depends largely on the photosynthetic activity in the plant.

Table 2.4: Tomato Fruit pH and TA ranges of different cultivars at red ripe maturity

stage

Tomato plant genotype pH Citric acid (%) Agriset 4.21bc (0.05) 0.57a (0.03) FL7692B 4.25ab (0.09) 0.44c (0.01) FL7692D (og) 4.34a (0.06) 0.38d (0.05) Suncoast (og) 4.19bc (0.02) 0.50b (0.04) Equinox 4.09d (0.10) 0.57a (0.03) 97E212S (rin/+) 4.13cd (0.08) 0.45b (0.02) FL7655 4.16cd (0.06) 0.53b (0.04) Solar Set 3.19e (0.04) 0.53ab (0.01)

Source: Thompson et al., 2000:719

41

In a previous study regarding the quality of tomatoes grown under a protected

environment and under field conditions (Caliman et al., 2010:75), it was found that

the field produced fruits were more acidic (greater TA) than those produced in a

protected environment. The lower acidity levels of fruits grown in the protected

environment may be due to the lower photosynthetic activity of the plants in that

environment as well as the lower carbohydrate accumulation in the fruits (Caliman et

al., 2010:77). In the same study, Caliman et al. (2010:77) also found that the

SSC/TA ratios were greater in fruits produced in the field than those produced in

protected environments (Table 2.5). Similar results were reported by Zoran &

Milenkovic (2012:25); field produced fruits were more acidic (TA-0.37%) than those

produced in a protected environment (0.34%).

Environmental effects on fruit acidity is a complex matter and many studies favour

the hypothesis that organic acids are produced from stored carbohydrates in the fruit

itself, although some of these acids may be translocated from the leaves and roots to

the fruit (Bertin et al., 2000:749). According to Mahakun, Leeper and Burns

(1979:241), genetic factors are the determinants of major acid content in tomato fruit,

with a great variation occurring between genotypes (Stevens & Rick, 1986:34).

When evaluating the hybrid „Carmen‟, Loures (2001:31) found fruit titratable acidity

(% citric acid) levels of 0.46% and 0.49% under greenhouse and field conditions

respectively. Sakiyama (1968:67) reported that titratable acidity is increased by high

air temperatures, but unaffected by shading.

42

The SSC content is mostly composed of sucrose as well as reducing sugars (Ho &

Hewitt, 1986:201). Thus, any factor that alters a photosynthetic activity will affect the

glucose and fructose accumulation in the fruit, thus altering the SSC content

(Caliman et al., 2010:77). A lower SSC content was reported by Tombesi,

Antognozzi and Palliotti (1993:88), Dussi et al. (2005:256), and Richard et al.

(1991:386) for plants grown under protected cultivation as opposed to field

cultivation. These authors attribute this phenomenon to the reduced light which the

plants received, leading to poor carbohydrate accumulation in the fruit. Reduced light

conditions are also likely to affect fruit firmness, as light is necessary for cell wall

formation. „Fuerte‟ avocados exposed to direct sunlight (32 °C) were 2.5 times firmer

than those positioned on the shaded side (20 °C) of the tree. Changes in cell wall

composition, cell number and cell turgor properties are influenced by temperature

and sunlight exposure (Woolf et al., 2000:370).

Table 2.5: SSC/TA for different cultivars planted in field and protected environments

Environment Tomato plant genotype BGH-320 Carmen Santa Clara Total average Protected 10.33 12.17 13.72 12.07b Field 13.59 17.07 19.81 16.82a Total average 11.96b 14.62a 16.76a Source: Caliman et al., 2010:78

2.3.5 Bioactive compounds in fruits

Fruit and vegetables are rich in phytochemicals (Olajire & Azeez, 2011:022).

Phytochemicals mediate the interaction of plants with their environments; thus,

43

functioning as feeding deterrents, pollination attractants, protective compounds

against pathogens or various abiotic stresses, antioxidants, or signalling molecules

(Kennedy & Wightman, 2011:33). Maintaining a high level of phytochemicals in fruit

and vegetables is therefore desirable. A high concentration of antioxidants in

horticultural produce is also believed to improve its storability and reduce its rate of

deterioration (Bergquist, 2006). These functional compounds can consist of a wide

range of substances such as lycopene, carotenoids, ascorbic acid, phenolic acids,

flavonoids, and other polyphenols such as glucosinolates, allylic sulphides,

isothiocyanates, dietary fibres, phytosterols and monoterpenes (Schreiner &

Huyskens-Keli, 2006:268; Lister, 1999:18; Kris-Etherton et al., 2002:71).

Antioxidants protect living cells against the harmful effects of free radicals and other

reactive oxygen species (Bergquist, 2006). Some of these phytochemicals such as

flavonoids and other phenolics, and carotenoids are marked by a broad spectrum of

health promoting functions (Heinonen, Lehtonen & Hopia, 1998:25). The vitamins C

(ascorbic acid) and E (tocopherols and tocotrienols) and some enzymes are also

part of the antioxidant defence system (Lurie, 2003:131). Free radicals are mostly

highly reactive and unstable due to their unpaired valence electrons (Bergquist,

2006). In the human body, an overproduction of free radicals may be triggered by

radiation, pollutants, cigarette smoke, alcohol, exercise and stress (Bergquist, 2006).

Free radicals have been found to be involved in ageing, several forms of cancer,

heart disease, cardiovascular disease, Alzheimer‟s disease, inflammatory conditions,

rheumatism, insulin resistance and cataracts (Halliwell & Gutteridge, 1989; Houstis,

Rosen & Lander, 2006:944), and increasing the antioxidant intake therefore

purportedly decreases the risk of contracting these diseases (Berger, 2005:172).

Bioactive compounds are also believed to affect the storability of the fruit and

44

vegetables that contain them. Reactive oxygen species are known to be involved in

leaf senescence and the antioxidant defence system most likely plays an important

role in determining the onset and rate of senescence (Philosoph-Hadas et al.,

1994:2376; Meir et al., 1995:1813; Hodges & Forney, 2003:930). Bergquist (2006)

opines that a product with a high concentration of antioxidants is well protected

against oxidation and may therefore retain its quality longer. Thus, increasing the

concentrations of the presumed beneficial compounds in fruit and vegetables may

not only have positive health effects for consumers, but may also extend the shelf

lives and increase their stress tolerance, leading to lower postharvest losses in

produce. Furthermore, since a high antioxidant concentration may be associated

with a fresh appearance, the freshness of a fruit or vegetable may to some extent be

a marker of its food value regarding its content of bioactive compounds.

The concentrations of bioactive compounds in fruit and vegetables are clearly not

constant and are affected by many pre- and postharvest factors which include

genetic variations, climatic conditions, fertilisers, harvesting, storage conditions,

temperature management and relative humidity. Some bioactive compounds are

common among fruit and vegetables but may vary in concentration between species

or cultivars (Lee & Kader, 2000:207). Genetic factors strongly influence the content

of bioactive compounds in fruit and vegetables. Variations between cultivars of the

same species may also be extensive (Mercadante & Rodriguez-Amaya, 1991:1094;

DuPont et al., 2000:3957; Howard et al., 2002:5891). Climatic factors such as light

and temperature are known to regulate not only plant growth and development, but

also the biosynthesis of both primary and secondary metabolites (Hemm et al.,

2004:765; Liu et al., 2002:581). Cultural practices, including nutrient and water

45

availability, influence these compounds, as do soil type variations in growing sites

(Weston & Barth, 1997:812). Variation in concentrations of bioactive compounds

resulting from the maturity stage are more evident in fruit ripening, particularly when

the carotenoid or flavonoid content provide the colour of the ripe fruit (Kalt, 2005:11).

The timing of the harvest during the day is also important, as there may be large

variations in the concentrations during the day, possibly related to the water content

(when concentrations are given on a fresh weight basis) or the light intensity

(Mozafar, 1994; Veit et al., 1996:478). Temperature management is therefore crucial

for maintaining the quality of harvested fruit and vegetables, both in terms of

appearance and biochemical composition (Wills et al., 1998). Ascorbic acid rapidly

decreases during storage in many plant products, whereas carotenoids and

flavonoids appear to be more stable (Kalt, 2005:11).

2.3.5.1 Carotenoids

Carotenoids are yellow, orange or red pigments that are responsible for the colour of

many fruits, vegetables and flowers. Carotenoids are tetraterpenoids, that is, they

are built on a 5-carbon isoprenoid unit and are hydrophobic. There are two groups of

carotenoid; the hydrocarbon carotenes and the oxygenated xanthophylls. Carotenes,

such as α- and β-carotene and lycopene, are predominantly orange or red-orange

pigments, whereas xanthophylls such as lutein, violaxanthin, zeaxanthin,

antheraxanthin and neoxanthin are mainly yellow. Carotenoids have important

functions in photosynthesis. Some of them are accessory pigments that absorb light

energy and transfer it to chlorophyll, which can use the energy for photosynthesis

46

(Bergquist, 2006). β-carotene and lutein are prevalent in photo-systems I and II

respectively (Demmig-Adams, Gilmore & Adams, 1996:403). The xanthophylls

violaxanthin, antheraxanthin and zeaxanthin are involved in photoprotection through

the xanthophyll cycle (Demmig-Adams, Gilmore & Adams, 1996:403). Carotenoids

are able to absorb energy that may otherwise lead to singlet oxygen formation from

excited chlorophyll molecules (Bergquist, 2006). Carotenoids may also scavenge

any singlet oxygen which forms during photosynthesis (Halliwell & Gutteridge, 1989).

Lycopene, like other carotenoids such as lutein and β-carotene, plays an important

role as a free-radical scavenger in humans and plants (Jones & Porter, 1999:295). In

plants, lycopene provides antioxidant protection for photosystem events and appears

in chromoplasts during ripening, and it is further thought to improve the attraction of

the fruit, its consumption and seed dispersal by herbivores (Collins, Perkins-Veazie

& Roberts, 2006:1135). In humans, lycopene exhibits antioxidant activities (Di-

Mascio, Kaiser & Sies, 1989:523), suppresses cell proliferation (Levy et al.,

1995:257) and interferes with the growth of cancer cells (Clinton et al., 1996:823). In

tomatoes, lycopene is responsible for the deep red colour in fully ripe fruit. The

colour of the fruit forms an important property from the consumer‟s point of view; it is

the first quality attribute that stimulates them to purchase, consume and enjoy the

fruit. The lycopene content also adds a very important quality attribute to tomato

processing (Barrett & Anthon, 2008:132); it constitutes 80% to 90% of the total

pigments present in mature processing tomatoes (Shi & Le Maguer, 2000:293).

Environmental conditions such as temperature and light, and cultivar type (Table 2.6)

were reported to affect lycopene biosynthesis in plant materials (Dumas et al.,

2003:369; Martinez-Valverde et al., 2002:323). Tomatoes exposed to direct sunlight

accumulate high pulp temperatures which inhibit lycopene synthesis; hence the fruits

47

often develop poor colour under such conditions (McCollum, 1954:182). Fruits

exposed to strong radiation may have a temperature of 10 °C higher than that of

fruits grown in shaded areas. The temperature of the fruit (>32 °C) suppresses

lycopene synthesis completely, but not β-carotene (Dumas et al., 2003:371). Tomes

(1963:180) reported that lycopene develops best when the temperature is between

12 °C and 21 °C. Low light intensity results in an uneven fruit colour due to reduced

lycopene accumulation, but intense direct radiation on fruits can be harmful

(Raymundo, Chichester & Simpson, 1976:59). According to Adegoroye and Jolliffe

(1987:297), lycopene synthesis is inhibited when green fruit is exposed to ∼2990

μmol m−2 s−1 for 1.5 to 4 h. However, fruit exposed to direct sunlight during its

developmental stages accumulate higher carotene levels than shaded fruit

(McCollum, 1954:182). Cabibel and Ferry (1980:27) reported that fruits grown under

a glass or plastic tunnel yielded a lower β-carotene content than those in the open

field. Similar results were obtained by Dorais, Papadopoulos and Gosselin

(2001:239).

Table 2.6: Lycopene content in tomato varieties (100% ripe stage), expressed as mg

kg-1 on a fresh weight basis

Tomato plant variety Lycopene

Rambo 31.97±0.54c Senior 32.24±0.55c Ramillete 31.49±1.17c Liso 18.60±1.00e Pera 63.37±3.21a Canario 49.44±2.73b Durina 64.98±5.12a Daniella 36.32±3.52cd Remate 42.96±3.29bc Source: Martinez-Valverde et al., 2002:325

48

2.3.5.2 Ascorbic Acid

Ascorbic acid (AA) or vitamin C, was first discovered in citrus in 1928, and is a water

soluble antioxidant (Lester, 2006:59). It is the most important vitamin in fruit and

vegetables with regard to human nutrition. More than 90% of the vitamin C in human

diets is supplied by fruit and vegetables (Lee & Kader, 2000:207). Vitamin C is

required for the prevention of scurvy, the maintenance of healthy skin, gums and

blood vessels (Lee & Kader, 2000:208), the maintenance of a healthy immune

system, the reduction of colds by preventing secondary viral or bacterial infections,

and the prevention of cardiovascular diseases and some forms of cancer (Eichholzer

et al., 2001:5). AA is an important cofactor for some plant enzymes, including

violaxanthin de-epoxidase, and is involved in photo-protection through the

xanthophyll cycle and in photosynthesis as an electron donor to photosystem II

(Smirnoff, 2000:1455). Furthermore, ascorbic acid has been implicated in the control

of cell expansion and division in plants (Smirnoff, 1996:661).

Light and average temperatures have a strong influence on the chemical

composition of horticultural crops. Light is not essential for the synthesis of AA in

plants (Caliman et al., 2010:79). However, the amount and intensity of light during

the growing period exert a great influence on the amount of AA formed. AA is

synthesised from products of photosynthesis-produced (carbohydrates) in plants

(Lee & Kader, 2000:210). Thus, those parts of the fruit that are exposed to maximum

sunlight will contain higher amounts of vitamin C than those that are inside or shaded

on the same plant (Table 2.7). In general, plants exposed to lower light intensities

49

during growth and development, yield a lower AA content in their plant tissues (Lee

& Kader, 2000:210; Kalt, 2005:15). Makus and Lester (2002:23) reported a lower AA

content in mustard greens planted under a 50% shade environment rather than in full

sunlight. Similar results were reported by Lopez-Andreu et al. (1986:387) and Brown

(1954:342). In a study conducted by Sayre, Robinson and Wishnetsky (1953:381),

vine ripened tomato fruit grown at low versus medium, or high day or night

temperatures presented altered concentrations of AA. Their results suggest that the

AA content declined with increasing temperatures. Gautier et al. (2008:1241)

reported similar results, where a temperature increase from 27 °C to 32 °C reduced

the AA content of cherry tomato plants in greenhouse conditions. Elevated

temperatures and increased solar exposure of fruits are known to cause ascorbic

acid degradation (Torres, Andrews & Davies, 2006:1933).

Table 2.7: Ascorbic acid content in tomato varieties (100% ripe stage), expressed as

mg 100 g-1 fresh fruit

Environment Tomato plant genotype BGH-320 Carmen Santa Clara Total average Protected 12.70 12.03 13.76 12.83b Field 15.23 14.17 21.66 17.02a Total average 13.96b 13.10b 17.71a Source: Caliman et al., 2010:78

2.3.5.3 Total Phenols and Flavonoids

Flavonoids and phenolics are the most important groups of secondary metabolites

and bioactive compounds in plants. They contribute to the sensory qualities (colour,

50

flavour, taste) of fresh fruit, vegetables and their products. In addition, many phenolic

compounds have antioxidative, anticarcinogenic, antimicrobial, antiallergic,

antimutagenic and anti-inflammatory activities (Kim, Jeond & Lee, 2003:321).

Flavonoids constitute a large family of polyphenols synthesised in plants; they have

been labelled as high level natural antioxidants because of their abilities to scavenge

free radicals and quench active oxygen with the inhibition of enzyme activity (Clifford

& Cuppett, 2000:1063). High contents of natural phenols and flavonoids are found in

green tea, fruit and vegetables, while some amounts of phenolics are present in red

wine and coffee (Ghasemzadeh & Ghasemzadeh, 2011:2436). Epidemiological

studies have found associations between a high risk of diseases and a low intake of

fruit and vegetables or bioactive compounds (Knekt et al., 1996:478). Besides

flavonoids, chlorogenic acids and related compounds are the main phenolic

compounds found in tomatoes (Slimestad & Verheul, 2009:1255). Chlorogenic acids

possess a number of beneficial health properties related to their potent antioxidant

activity as well as their hepatoprotective, hypoglycemic and antiviral activities (Farah

& Donangelo, 2006:23). These acids may also be responsible for a somewhat

astringent taste in tomatoes (Walker, 1962:363; De Bruyn, Garresten & Kooistra,

1971:214).

Flavonoids occurring in tomatoes include: rutin, naringenin and quercitrin,

kaempferol, aglycones, quercetin, myricetin, and chalconaringenin (Slimestad &

Verheul, 2009:1257); they are found in abundance in the leaves, fruit skins, stem

epidermis, cuticle of the fruit and in canned tomato paste. Phenolic acids present in

the tomato skin, pulp and canned pastes are: hydroxycinnamic acids, ferulic, caffeic

and chlorogenic acid, p-coumaric acid, sinapic acid, vanillic and salicylic acids, 5-p-

51

coumaroylquinic acid, p-coumaroylglucoside and feruloylglucoside, 4- and 3-

caffeoylquinic acids (Walker, 1962:363; Wardale, 1973:1523; Fleuriet & Macheix,

1976:407; Schmidtlein & Herrmann, 1975:213; El Khatib et al., 1974:23). The total

level of tomato phenolics varies significantly both with respect to the ripening stage

and the location within the fruit (Senter, Horvart & Forbus, 1988:639; Slimestad &

Verheul, 2005a:3114; Slimestad & Verheul, 2005b:7251). The highest concentration

has been found in the epidermal and placental tissues. Stewart et al. (2000:2663)

measured the distribution of flavonols (sum of free and conjugated kaempferol and

quercetin respectively) in Spanish cherry tomatoes. Each fruit was found to contain

25.3 mg kg−1 FW of flavonols. Its peel or epidermis was found to contain 143.3 mg

kg−1, the flesh 0.12 mg kg−1, whereas the seeds were found to hold 1.5 mg kg−1 FW

of flavonols; thus, more than 98% of total flavonols occurred in the skin. Of these,

conjugated quercetin, primarily rutin, was the main class of flavonols, with a content

of more than 94% of total flavonols in the tomatoes (Slimestad & Verheul,

2009:1260).

The concentration of bioactive compounds in fruit and vegetables can be influenced

by environmental conditions such as light intensity and temperature (both in terms of

total or average temperature and the extremes during the growth period)

(Ghasemzadeh, Jaafar & Rahmat, 2010:4540). Stewart et al. (2000:2663) indicated

that the choice of cultivar was a major factor contributing to the total content of

phenolics in tomatoes when grown under similar environmental conditions. Two

normal-sized field grown tomatoes, cvs Bond and Havanera, grown alongside each

other in Spain contained 10.9 and 6.6 mg flavonol kg−1 FW. Since 98% of the total

52

flavonols occur in the skin (Slimestad & Verheul, 2009:1264), tomato types with

different fruit sizes, and thus different skin-volume ratio or different skin colours, are

expected to present a different flavonol content. The total flavonol content of cv.

Favorita, a red cherry tomato, was found to be 21.5 mg kg−1 FW, twice as high as the

cv. Bond when grown under the same conditions. This findings were confirmed by

Willcox, Catignani and Lazarus (2003:1) who found the total flavonoid content in

normal sized tomatoes to be 5 mg kg−1 and 30 mg kg−1 FW in cherry tomatoes.

Mineral nutrition can have a major influence on phenolic accumulation (Slimestad &

Verheul, 2009:1266); a limited nitrogen supply was found to be associated with

higher levels of phenolics in the plant (Parr & Bolwell, 2000:985; Lea et al.,

2007:1245). Increases in flavonoids during nitrogen starvation could be due to

increased deamination of phenylalanine (Slimestad & Verheul, 2009:1266). Under

such conditions ammonia would be recycled for protein synthesis and cinnamic acid

can constitute a substrate for the flavonoid metabolic pathway (Slimestad & Verheul,

2009:1266).

Phenolic biosynthesis requires light and is also enhanced by light (Ghasemzadeh et

al., 2010:3887), and flavonoid formation is absolutely light dependent; its

biosynthetic rate is related to light intensity and density (Xie & Wang, 2006:51).

However, different plants have a different response to light intensity changes and the

resulting total flavonoids (TF) and total phenolics (TP). Previous studies indicated

that changes in light intensity were able to change the production of TF and TP in

herbs (Graham, 1998:135). Chan et al. (2008:477) reported a much higher

concentration of flavones and flavonols in the leaves of vegetables that were

53

exposed to shade. This finding is in agreement with Ghasemzadeh et al.

(2010:3889), who indicated that the TF content of Zingiber officinale Roscoe can be

increased by reducing the light intensity under the glasshouse shade. Bergquist et al.

(2007:2464) also indicated that the use of shade netting is acceptable for the

production of baby spinach in relation to flavonoid concentration and composition.

The authors postulate that the increased TF content with increasing shade could be

associated with significantly higher leaf chlorophyll and carotenoid contents under

lower light levels. A light intensity of 790 µmol m-2 s-1 yielded a higher TP content in

ginger crops (Z. officinale) planted in the glasshouse shade (Ghasemzadeh et al.,

2010:3888). Similar results were reported by Maršić et al. (2011:190) in tomato fruit

grown inside tunnels. Phenols and flavonoids are major contributors of antioxidant

activity in fruit and vegetables, thus it is suggested that increased phenols and

flavonoid components in shaded plants are related to lower temperatures under such

conditions (Ghasemzadeh et al., 2010:3887). However, Wilkens, Spoerke and

Stamp (1996:247) reported high soluble phenol contents in plants grown under high

light rather than in low light conditions.

2.4 CONCLUSION

This literature review addressed information on the production of tomatoes –

comparing open field and in-house production methods, disease occurrence and

pest infestation, marketable fruit yields and fruit quality at harvest, and lastly, health

promoting compounds. The production of high economic value crops such as

tomatoes and lettuce is occasionally limited to greenhouse structures or normal

54

netting materials, with no additives. However, crop production under greenhouse

conditions carries high input costs, which amongst others include electricity; whereas

under normal netting materials crop yields are lower due to the microclimatic

conditions, high disease and pest infestation, resulting in poor quality produce

(unmarketable fruits). Environmental conditions play a major role in crop production

and determine the types of microorganisms and diseases that are likely to affect

crops. As a result, farmers are constantly seeking alternative methods for improving

yields and the quality of produce, with lower input costs and minimal application of

pesticides. Consumers are also more aware of what they eat, thus demanding high

quality crops with greater health promoting compounds. The application of photo-

selective netting is gaining popularity around the world; it is a technology that can be

used as an alternative to improve crop yields and quality, reduce the application of

pesticides, lengthen the shelf life of produce and ensure sustainable crop production

even in areas were agriculture is not practised due to adverse environmental

conditions.

55

CHAPTER THREE

EFFECT OF PHOTO-SELECTIVE NETS ON TOMATO PLANT GROWTH AND

YIELD

3 SUMMARY

This experiment was conducted to determine the effect of three photo-selective nets

(40% shading – red, pearl and yellow) on the growth parameters and yield of the

tomato (Solanum lycopersicum). The commercial black net (25% shading) was

included for the sake of comparison (control). The variables measured were plant

height, leaf area, stem diameter, flowering, fruit trusses, number of fruits, fruit mass,

leaf chlorophyll content, incidence of bacterial and fungal diseases, pest infestation

and microclimatic assessment [air temperature (AT), relative humidity (RH) and

photosynthetic active radiation (PAR)]. Plant height and leaf area measurements

were significantly high for plants produced under the red and yellow nets, while stem

diameter measurements were greater for plants produced under the red nets. The

flowering intensity was high for plants grown under the red, yellow and pearl nets,

while the number of fruits per plant was higher under the pearl and yellow nets.

Photo-selective nets exhibited higher AT and lower RH compared with the black

(control) nets, whereas the PAR was higher under the black (control) nets with 25%

shading. Of the three cultivars (cv.), cv. SCX 248 showed a significantly higher plant

height and number of fruits, whereas cv. AlfaV revealed a larger fruit mass and leaf

56

area and cv. Irit exhibited a significantly thicker stem diameter. The marketable yield

was higher in cv. AlfaV under the pearl nets, while cv. SCX 248 and Irit showed a

higher marketable yield under the red nets. No significant differences were observed

in the leaf chlorophyll content; either in the three cultivars or between the different

shade nets. Furthermore, the shade nets exerted no significant effect on disease and

pest incidences as was noted in cv. SCX 248 which was highly affected by white

flies. Bacterial leaf speck and spot was also more prevalent on cvs SCX 248 and Irit.

Incidences of early blight were observed to be low during this study in all three

cultivars; however, cvs AlfaV and SCX 248 indicated a higher incidence of early

blight. All three cultivars grown under the black (control) nets exhibited less plant

height, less fruit mass, and a lower fruit marketable yield. This experiment

demonstrated that pearl and yellow nets are best suited to produce a substantial

number of tomatoes with a greater mass. Cultivar versus net interaction was clearly

observed in this study. Cultivar AlfaV under the red nets revealed a higher plant

height, fruit trusses and number of fruits.

3.1 INTRODUCTION

Good agricultural practices in combination with conducive environmental conditions

are vital for sustainable crop productions. Tomato yields and quality are adversely

affected by varying climatic conditions which include high solar radiation and

temperature, followed by pests which cause direct feeding damage and often

transmit viruses (Ben-Yakir et al., 2013:249, Higashide, 2009:1878, De Koning,

1989:329). The optimum temperature for tomato production ranges from 21 °C to 25

57

°C with an average monthly minimum temperature greater than 18 °C and a monthly

maximum temperature of 27 °C (Haque et al., 1999; Araki et al., 2000). Fruit setting

is optimal between 18 °C and 20 °C (De Koning, 1994).

Temperatures that fall below 16 °C can cause flower abscission, whereas

temperatures above 30 °C can induce fruit cracking and blotchy ripening. Elevating

temperatures can also increase the rate of fruit growth, consequently hastening

maturity and leading to a reduction in the mean weight of the tomato fruits (Hurd &

Graves, 1985:359; Sawhney & Polowick, 1985:1031). However, Marcelis and Baan

Hofman-Eijer (1993:321), demonstrated that the effect of temperature on the growth

of cucumbers was dependent on the availability of photosynthetic assimilate. It is

well known that netting can provide sustainable crop production under severe

environmental conditions because of its ability to (a) provide protection from

environmental hazards (e.g., excessive solar radiation, wind, hail, flying pests); (b)

improve plant microclimates (thus reducing heat or chill, drought stresses, etc.); (c)

moderate rapid climatic stresses; and (d) be cheaper and less energy consuming

than greenhouses. According to Polysack Plastic Industries, Ltd (Nir-Yitzhak, Sufa,

Israel), photo-selective nets (ChromatiNetTM) are developed to modify the spectrum

of filtered light, enhance the content of scattered light, and affect the thermal

components of light in the infrared region, thereby enhancing desirable physiological

responses such as yield, quality and maturation rates, and plant compactness and

its morphology (Shahak et al., 2004a:143).

58

According to Shahak et al. (2004b:610), nets offer varying mixes of natural,

unmodified light and spectrally modified, diffused light depending on the chromatic

additives of the plastic and the density of the knitting design. McMahon and Kelly

(1990:209) reported that plants grown under grey ChromatiNetTM exhibit an increase

in their number of shoots and buds as well as a noticeable increase in their

branching. Red ChromatiNetTM reduces the spectrum of blue, green, and yellow light

and increases the R and FR light spectrum (Shahak et al., 2004a:145). McElhannon

(2007:44) reported that the effect of red ChromatiNet™ on the light spectrum yields

plants with a larger surface area, healthier darker green foliage, and longer and

thicker stems. Furthermore, Polysack claims that red ChromatiNetTM produces earlier

flowering plants without decreasing their flower quality (Oren-Shamir et al.,

2001:353). According to Shahak et al. (2004a:145), pearl ChromatiNetTM has the

ability to scatter incoming light, thus allowing the light to better penetrate the plant

canopy, resulting in an increased photosynthetic efficiency. The latter accelerates

plant growth, increases the number of secondary branches and improves the overall

plant quality (Oren-Shamir et al., 2001:353). Therefore, the objective of this

investigation was to compare different coloured photo-selective nets (red, pearl and

yellow) and the commercially used black net on three indeterminate tomato cultivars

(AlfaV, Irit and SCX 248) planted during the 2011 to 2012 growing season. The

comparison was based on plant growth parameters (plant height, leaf area, stem

diameter, leaf chlorophyll, fresh fruit weight and total fruit marketable yield, flowering

and fruit trusses) and incidences of pests and diseases.

59

3.2 MATERIALS AND METHODS

3.2.1 Location

The study was carried out in a net tunnel (length 12 m & height 5 m) at the

Experimental farm of Tshwane University of Technology, Bon-Accord, Pretoria North

(25° 37‟ S and 28° 12‟ E latitude and longitude respectively, at an altitude of 1173 m

above sea level) during the 2011 to 2012 growing season. The tunnels were covered

with red, pearl and yellow photo-selective nets (ChromatiNetTM) and a black control

net, which is used by most commercial farmers in the country.

3.2.2 Treatments and experimental design

Three coloured photo-selective nets (pearl, red, and yellow) with 40% shading each

and a black (25% shading) traditional net, which is used commercially in South Africa

for the production of fruit and vegetables, provided the required shading materials

and were installed as permanent structures. The photo-selective nets

(ChromatiNetTM) were manufactured by Polysack Plastics Industries in Israel. They

are unique in that they modify the non-visible spectrum and enhance light scattering.

The experiment was laid out in a completely randomised design, with three replicate

nets assigned to each of the four treatments (red, pearl, yellow and black net).

60

Figure 3.1: Photographs illustrating yellow photo-selective nets and black control

nets

Figure 3.2: Photograph illustrating red and pearl photo-selective nets

61

3.2.3 Planting materials

Three indeterminate tomato cultivars, namely, AlfaV, Irit and SCX 248 were

transplanted at six weeks old during the 2011 to 2012 growing season. The

seedlings were obtained from SeedCor (Pty, Ltd) South Africa. They were

transplanted into 5 L black plastic bags using coir-sand as a growing medium (the

planting density was 2.88 plants m-2). The physical properties of the coir medium

were moisture (13.30%), pH (0.059), bulk density (0.0619 g ml-1), water holding

capacity (71.7%) and air porosity (6.93%). Irrigation was applied through drippers;

one dripper per plant which was controlled by a computerised irrigation system and

had a discharge rate of 33.3 ml min-1 at four hour intervals, four times a day.

A set of 72 plants per cultivar were planted under each replicate net. Each treatment

and block consisted of eight double rows of 36 plants each. The spacing between

the plants within a double row was 50 cm x 50 cm while the spacing between the

rows was 1 m. To minimise the effect of position within the nets, each cultivar was

replicated three times in a Latin square layout. The nutrient solution for irrigation was

mixed in a 5 000 L tank as indicated in Table 3.1; the pH of the solution was

maintained within a range of 5.6 to 6.0 (nitric acid was used to adjust the pH). The

hydroponic® fertiliser was supplied by Hygrotech (Pty, Ltd) Pretoria, South Africa.

The nutritional composition of the hydroponic® fertiliser was [N 68 g kg-1,P 42 g kg-1,

K 208 g kg-1, Mg 30 g kg-1 , S 64 g kg-1, Fe 1254 g kg-1, Mn 299 g kg-1, Zn 149 g kg-1,

Cu 22 g kg-1 , B 373 g kg-1, Mo 37 g kg-1] and SOLU-CAL Ca (NO3)2 [N 117 g kg-1,

Ca 166 g kg-1].

62

Table 3.1: Nutrient composition of the fertiliser applied during tomato production

Period Composition (% w/v)

Transplant to 1st flower truss 0.05 [1.39 kg hydroponics® and, 1.11 kg Ca(NO3)2]

1st flower truss to 3rd flower truss 0.08 [(2.4 kg hydroponics® and, 1.6 kg Ca(NO3)2]

3rd flower truss to end 0.12 [3.125 kg hydroponics® and, 2.095 kg Ca(NO3)2, 0.78 kg KNO3]

Hygrotech Sustainable Solutions (Pty) Ltd

3.2.4 Cultural practices

Foliar feed comprising of nitrospray plus, calmabon liquid, hygroboost flo, hyper

feed, millerplex, asco-gro, sporekill and nu-film P was occasionally applied to all the

treatments to alleviate nutrient deficiencies. The foliar feed recipe was dissolved in a

16 L spraying tank of water and mixed thoroughly before application. The nutrient

composition of the fertilisers used in the foliar feeding programme is presented in

Table 3.2.

63

Table 3.2: Nutrient composition of the fertilisers used in the foliar feeding programme

Fertilizer Active ingredients Concentration

Calmabon liquid Nitrogen 88 g kg-1

Calcium 89 g kg-1

Magnesium 17 g kg-1

Boron 1526 mg kg-1

Molybdenum 1517 mg kg-1

Asco-gro Nitrogen 2 g kg-1

Phosphate 2 g kg-1

Potash 3 g kg-1

Chelated iron 1340 mg kg-1

Chelated calcium 1160 mg kg-1

Chelated manganese 110 mg kg-1

Molybdenum 30 mg kg-1

Hyperfeed Nitrogen 164 g kg-1

Potassium 274 g kg-1

Iron 1.0 g kg-1

Zinc 0.5 g kg-1

Boron 1.2 g kg-1

Phosphate 55 g kg-1

Magnesium 0.9 g kg-1

Copper 0.5 g kg-1

Molybdenum 0.075 g kg-1

Nitrospray plus

Nitrogen

165 g kg-1

Phosphorus 68 g kg-1

Potassium 23 g kg-1

Iron 756 mg kg-1

Manganese 383 mg kg-1

Zinc 383 mg kg-1

Copper 383 mg kg-1

Boron 196 mg kg-1

Molybdenum 6.3 mg kg-1

Cytokinins 0.75 mg kg-1

Sporekill Didecyldimethylammonium chloride 120 g L-1

Nu-film Poly-1-p-Methane 875 g L-1

According to the product labels as manufactured by Hygrotech Sustainable Solutions (PTY) LTD

The application of foliar feed was guided by the development of the plants from the

transplanting stage up to the harvest period, as well as when the plants showed

signs of nutrient deficiency. Insect pests and diseases were controlled chemically.

64

Furthermore, the tomato plants were supported by twisting trellis twine around the

main stem and fixing it to a stay wire. Pruning was also done regularly; lateral

branches, suckers and auxiliary branches were cut and pinched off in order to retain

a single stem.

3.2.5 Light interception by nets and microclimatic measurements

PAR (400±700.nm) outside and under the nets was measured weekly using a

Ceptometer AccuPAR model LP-80 (Decagon Devices Ltd, USA). All measurements

were carried out at 12.00 h on clear days. The relative shading provided by each net

for the PAR ranges, was determined as SPAR=100 x (1-PAR/PARo) where "o"

corresponds to the solar radiation measured outside the net house. Light intensities,

in PAR, were obtained by integration over the respective wavelength ranges of the

solar radiation spectra (Oren-shamir et al., 2001:353). Air temperature and relative

humidity were also monitored and the data were collected by Tinytag T/RH data

loggers (Gemini data loggers Ltd, UK). These data loggers were placed above the

tree canopy and protected from direct solar radiation, rain or sprays.

3.2.6 Plant measurements

Growth parameters such as plant height, stem diameter, number of fruit trusses,

flowering intensity, leaf area, leaf chlorophyll and incidences of pests and diseases

were recorded on a weekly basis for five weeks. Plant growth measurements were

65

taken from 15 plants per cultivar in each shade net replicate. Plant heights were

measured in centimeters (cm), leaf areas were measured in millimeters squared

(mm2) and stem diameters were recorded in millimeters (mm) using a measuring

tape. The number of fruits and flower sets were counted from 15 selected plants per

cultivar from each shade net replicate. Incidences of pests and diseases were

recorded from the affected leaves of 15 selected plants per cultivar in each shade

net replicate. Leaf chlorophyll was measured non-destructively using a SPAD 502

chlorophyll meter (SPAD units) (Konica Minolta, Japan). Individual fruit masses from

selected plants were also recorded.

3.2.7 Harvesting and fruit sampling

Tomato fruits produced from the three cultivars on the Experimental Farm were

harvested at the mature pink (45-53 h °) stage, 26 weeks after planting. Harvesting

was carried out manually in the early morning. The total marketable yield was (kg m-

2) determined by fruits that were free from diseases, disorders, injuries or

deformation and uniform in size and colour. These tomatoes (n=85 per cultivar per

replicate of photo-selective nets) were then weighed and the average fresh weight

was expressed in g per cultivar per photo-selective net. The fresh fruit mass (g) was

thus determined after harvesting.

66

3.2.8 Statistical analysis

The experiment was analysed as a (4) nets versus (3) cultivar factorial design in a

completely randomised design. All data were analysed with the aid of the Genstat

program, and the least significant difference (LSD) was calculated at the 5% level of

significance.

3.3 RESULTS AND DISCUSSION

3.3.1 PAR and microclimate under the shade nets

The PAR was higher (average 1564.356 μmol m-2 s-1) in the open field than under

the different photo-selective nets as indicated in Table 3.3 below. The PAR was

higher under the black nets (control), and lower under the red nets. The photo-

selective nets, supplied by Polysack Plastics Industries (Pty, Ltd), Israel, provided

40% shading from PAR. However, the average shading effect throughout the

production period varied from ~42% to 48% and is illustrated in Figure 3.3.

67

Table 3.3: Air temperature (AT), relative humidity (RH) and photosynthetic active

radiation (PAR) measurements under different photo-selective nets and the control

net during cultivation

Means followed by the same letter within the column are significantly different at 5% level of probability

Figure 3.3: Shading effects of different shade nets

Means in each bar with the same type of letter are not significantly different at 5% level of probability

The shading effects of the photo-selective nets were significantly (p≤0.05) higher

than that of the black control nets (Figure 3.3). The black plastic threads are opaque;

thus, the light entering through the holes in the nets is transmitted. However, the

photo-selective shade nets are knitted more densely to achieve the shading effect,

and according to Oren-Shamir et al. (2001:353), a major fraction of sunlight actually

d

bcab

a

0

10

20

30

40

50

60

Black Pearl Yellow Red

% S

ha

din

g e

ffe

ct

Shade Type Air Temperature (°C)

Relative Humidity (%)

Photosynthetic Active Radiation (μmol m-2s-1)

Pearl 42.18±1.78a 25.41±4.86c 827.56±69.89b Yellow 35.88±2.33b 35.12±11.73b 851.81±52.75b Red 35.43±2.25b 40.00±12.14b 744.13±69.05b Black (control) 30.27±1.52c 60.78±16.22a 1339.33±86.09a

LSD (p≤0.05) 5.16 9.71 107.69

68

passes through the plastic threads and is then selectively filtered. It can therefore be

concluded that the shading effect under red nets may be due to the densely knitted

pattern of the threads. The average air temperature and the RH during the 2011 to

2012 growing season were significantly higher and lower respectively under the

photo-selective nets than under the black nets (control). This could be due to the

more densely knitted pattern of the threads in the photo-selective nets which

negatively affected the ventilation. Significant correlations were also observed

between air temperature and RH (r=0.787, p<0.001) and PAR (r=0.650, p<0.001).

On the other hand, the pearl nets provided the most stable microclimatic conditions

throughout the study as they had the least coefficient of variation for AT and RH,

while the yellow nets were noted to have the least ability to stabilise the fluctuation of

the environmental factors measured.

3.3.2 Plant height

There were significant differences (p≤0.05) in the plant height of the three cultivars

planted under the different shade nets (Figure 3.4). A greater plant height was

attained by cv. SCX 248 compared with cvs. AlfaV and Irit (Figure 3.4). The height of

plants under the black control net was significantly (p≤0.05) lower whereas the plant

height was higher under the yellow and red photo-selective nets (Figure 3.5).The

greater plant height under the photo-selective nets might have been brought about

by a lower light intensity inside these nets which caused the plants to grow taller

(shade avoidance) in order to increase their light interception and to facilitate the

photosynthetic processes (Kasperbauer, 1987:353). The interaction of cv. versus

69

shade net was clearly observed in this study; cv. AlfaV and cv. SCX 248 produced

under the red and the yellow nets respectively, exhibited a significantly (p≤0.05)

higher plant height as indicated in Table 3.4.

Figure 3.4: Variations in the plant height of the three tomato cultivars

Means in each bar with the same type of letter are not significantly different at 5% level of probability

a

b

b

124

125

126

127

128

129

130

131

SCX 248 AlfaV Irit

Pla

nt

he

igh

t (c

m)

Cultivar type

70

Figure 3.5: Effect of photo-selective nets on the plant height of the three tomato

cultivars

Means in each bar with the same type of letter are not significantly different at 5% level of probability

3.3.3 Stem diameter

Cultivar Irit had a significantly (p≤0.05) thicker stem diameter than the other two

cultivars, SCX 248 and AlfaV (Figure 3.6). However, the stem diameter

measurements were significantly (p≤0.05) lower for plants produced under the pearl

nets and higher for those produced under the red nets (Figure 3.7). Cultivar SCX 248

grown under the pearl nets exhibited a lower measurement in stem diameter, while

cv. Irit grown under the red nets showed significantly (p≤0.05) higher stem diameter

measurements (Table 3.4). However, according to Costa et al. (2010:352), no

significant difference was observed in the stem diameters and petiole lengths of

Ocimum selloi grown under red or blue shade (ChromatiNetTM 50%) in comparison

with the full sunlight treatments. The reason for the observed lower measurements in

a a

b

c

105

110

115

120

125

130

135

140

Red Yellow Pearl Black

Pla

nt

he

igh

t (c

m)

Net type

71

stem diameter for plants grown under photo-selective nets in this study can be

explained by the shade avoidance and photoreceptor, phytochrome, which is located

in the meristematic tissue of the shoot tip (Solomakhin & Blanke, 2008:217). Under

shading, plants redirect photo-assimilates for shoot elongation and away from

structures dedicated to resource acquisition and to storage, at the expense of leaf

development (Brutnell, 2006:417).

Figure 3.6: Variations in the stem diameter of the three tomato cultivars

Means in each bar with the same type of letter are not significantly different at 5% level of probability

a

b

c

32

32.5

33

33.5

34

34.5

35

35.5

36

36.5

Irit AlfaV SCX 248

Ste

m d

iam

ete

r (m

m)

Cultivar type

72

Figure 3.7: Effect of photo-selective nets on the stem diameter of the three cultivars

Means in each bar with the same type of letter are not significantly different at 5% level of probability

3.3.4 Leaf area

It is evident from this study that, amongst the three cultivars, cv. AlfaV showed a

significantly (p≤0.05) higher measurement in leaf area than cv. Irit or SCX 248

(Figure 3.8). Plants produced under the black and pearl nets exhibited a significantly

(p≤0.05) lower measurement in leaf area as illustrated in Figure 3.9. However, plants

grown under the yellow nets had significantly (p≤0.05) higher leaf areas (Figure 3.9).

A significant interaction between cv. and shade net on leaf area was observed. The

leaf area of cv. SCX 248 was significantly larger (p≤0.05) under the red nets (Figure

3.10), whereas, cv. AlfaV indicated a significantly (p≤0.05) larger leaf area under the

pearl, yellow and black nets as illustrated in Figure 3.10. On the other hand, cv. Irit

grown under yellow nets showed a significantly (p≤0.05) larger leaf area (Figure

a

b b

c

32.5

33

33.5

34

34.5

35

35.5

Red Yellow Black Pearl

Ste

m d

iam

ete

r (m

m)

Net type

73

3.10). Leaves grown in strong light conditions are generally smaller and thicker and

contain less chlorophyll per unit leaf mass (Gratani, Covone & Larcher, 2006:549).

On the other hand, plants grown in the shade tend to have a larger leaf area (as

observed in the photo-selective nets, Figure 3.9), because cells expand more under

low light intensities in order to receive light for photosynthesis (Boardman,

1977:355).

Figure 3.8: Variations in the leaf area of the three tomato cultivars

Means in each bar with the same type of letter are not significantly different at 5% level of probability

a

bb

700

750

800

850

900

950

AlfaV Irit SCX 248

Le

af

are

a (

mm

2)

Cultivar type

74

Figure 3.9: Effect of photo-selective nets on the leaf area of the three cultivars

Means in each bar with the same type of letter are not significantly different at 5% level of probability

Figure 3.10: The effect of cultivar versus shade net on the leaf area

Means in each bar with the same type of letter are not significantly different at 5% level of probability

b

c

a

d

740

760

780

800

820

840

860

880

900

red pearl yellow black

Le

af

are

a (

mm

2)

Net type

b

a a a

c c

a

c

a

c cd

0

200

400

600

800

1000

1200

Red Pearl Yellow Black

Le

af

are

a (

mm

2)

Net type

AlfaV

Irit

SCX 248

75

3.3.5 Leaf chlorophyll

No significant differences were observed in the leaf chlorophyll content; either in the

three cultivars or between the different shade nets (Table 3.4). Leaf chlorophyll

synthesis depends on the genotype and the light transmitted by the shade nets

during leaf growth (Solomakhin & Blanke, 2008:217). The results of this study

contradict the findings of Solomakhin and Blanke (2008:217). According to

Solomakhin and Blanke (2008:217), the leaf chlorophyll content was higher in apple

cvs Fuji and Pinova produced under coloured hail nets with a light transmission

spectra ranging from 300 to 800 nm.

Figure 3.11: Photograph illustrating plants grown under the black (control) net

76

Figure 3.12: Photograph illustrating plants grown under the red net

Figure 3.13: Photograph illustrating plants grown under yellow photo-selective nets

77

Figure 3.14: Photograph illustrating plants grown under the pearl photo-selective net

3.3.6 Flowering and fruit trusses

At four weeks, after transplanting the plants, the flowering intensity was higher under

the red, pearl and yellow photo-selective nets compared with the black control net

(Figure 3.15). Interaction of cv. versus shade net and cultivar effect failed to show

any significant effect on flowering. Of the three cultivars, SCX 248 indicated a

significantly (p≤0.05) high number of fruit trusses (Figure 3.16) under the black,

yellow and pearl nets (Table 3.4). Cultivar Irit exhibited a smaller number of fruit

trusses per plant under the red nets as opposed to the other shade nets (Table 3.4).

The photo-selective nets revealed no significant effect on the number of fruit trusses.

This appears to contradict Shahak et al. (2008:66), who demonstrated that fruit

78

setting is one of the photo-selective responses resulting from the high flowering

intensity induced by an optimum microclimate under the nets. Zoltán and Helyes

(2004:1671) reported that truss appearance and flowering rates are affected by

temperature. The interaction of cv. versus shade net on fruit trusses was evident in

cv. AlfaV grown under the red nets (Table 3.4), where high numbers of fruit trusses

were obtained.

Figure 3.15: Effect of shading on flowering intensity at four weeks after transplanting

Means in each bar with the same type of letter are not significantly different at 5% level of probability

abc

bc bc

cdef

2.64

2.66

2.68

2.7

2.72

2.74

2.76

2.78

2.8

2.82

Red Yellow Pearl Black

Flo

we

r tr

us

se

s/p

lan

t

Net type

79

Figure 3.16: Fruit trusses of the three cultivars planted

Means in each bar with the same type of letter are not significantly different at 5% level of probability

3.3.7 Number of fruits

Cultivar SCX 248 produced a significantly (p≤0.05) higher number of fruits per plant

under the pearl, yellow, and black (control) nets (Table 3.4). The high number of

fruits produced by cultivar SCX 248 (Figure 3.17) might be due to its greater plant

height (Figure 3.4) and smaller trunk diameter (Figure 3.7), because tall plants are

more likely to produce more flowers and fruit trusses (Susko & Lovet-Doust,

2000:1398). A significantly (p≤0.05) higher number of fruits per plant was recorded

under the pearl and yellow nets (Figure 3.18). The higher number of fruits produced

under the photo-selective nets (pearl and yellow) might be due to the modification of

the light quality by the photo-selective nets which promote fruit setting and fruitlet

survival (Shahak et al., 2008:66). According to Shahak et al. (2008:77), this might

have resulted from either the higher content of scattered or diffuse light under the

a

b

b

1.8

1.9

2

2.1

2.2

2.3

2.4

2.5

SCX 248 AlfaV Irit

Fru

it t

rus

se

s/p

lan

t

Cultivar type

80

photo-selective nets or from the modified spectral composition of the light or both. A

lower number of fruits per plant was produced under the red and the black control

nets (Figure 3.18). The interaction of cv. versus shade net was shown by cultivar

AlfaV produced under the red nets, where a significantly (p≤0.05) higher number of

fruit trusses and fruits per plant were produced (Table 3.4).

Figure 3.17: Effect of photo-selective nets on the number of fruits per plant

Means in each bar with the same type of letter are not significantly different at 5% level of probability

a

b b

0

2

4

6

8

10

12

14

SCX 248 AlfaV Irit

Nu

mb

er

of

fru

its

/pla

nt

Cultivar type

81

Figure 3.18: Number of fruits per plant or net type

Means in each bar with the same type of letter are not significantly different at 5% level of probability

a a

b

b

9

9.5

10

10.5

11

11.5

Yellow Pearl Black Red

Nu

mb

er

of

fru

its

/pla

nt

Net type

82

Table 3.4: Cultivar versus shade net interaction on plant height, stem diameter, leaf chlorophyll, fruit trusses per plant, and number

of fruits per plant

Cultivar (cv) Shade type Plant height (cm)

Stem diameter (mm)

Leaf chlorophyll (SPAD units)

Fruit trusses/plant

Number of fruits/plant

Fruit mass (g/fruit)

AlfaV x Pearl 122.2±22ef 33.99±2cdef 52.83±3.3 2.1±0.7cd 9.6±3cde 108.8±18.4gh Yellow 133.3±17bc 33.93±3def 52.18±2.7 2.0±0.8cd 9.3±4de 113.7±14.5h Red 138.4±23a 33.78±2ef 51.83±3.3 2.4±0.9b 11.2±4.b 94.0±21.7abc Black (control) 114.6±19h 35.04±3bc 53.83±1.6 1.9±0.9cd 8.9±4de 103.6±11.7de Irit x Pearl 124.2±22e 35.03±1bcd 53.28±3.1 2.1±0.9cd 9.8±4cd 100.0±19.2cd Yellow 130.4±22cd 35.62±3b 51.76±2.3 2.1±0.9c 10.4±5bc 93.8±13.5abc Red 132.6±26bc 37.37±3a 51.50±3.2 1.9±1.1d 8.7±5e 96.4±20.4bcd Black (control) 118.5±19g 35.78±3b 52.30±1.5 2.0±0.8cd 9.0±3de 101.4±13.3d SCX 248 x Pearl 127.8±21d 31.49±2g 52.78±4.9 2.5±0.7ab 13.1±4a 91.2±15.7ab Yellow 137.5±18a 34.46±3cde 52.87±2.7 2.4±0.9ab 13.1±4a 91.5±7.9ab Red 127.8±21b 34.84±2bcde 51.86±3.7 2.0±0.9cd 9.4±4de 97.2±15.9bcd Black (control) 119.7±17fg 33.02±2f 53.71±1.4 2.6±0.9a 12.3±4a 88.9±11.1a

LSD (p≤0.05) 3.337 1.1067 1.325 0.1926 0.9850 11.68 Means followed by the same letter within the column are not significantly different at 5% level of probability

83

3.3.8 Fruit mass

The fresh mass per fruit ranged from 88.9 g to 114 g (Table 3.4) and the differences

observed in the fresh mass were more significant (p≤0.05) due to their genotype

(cultivar) rather than their environmental (shading) differences. Cultivar AlfaV

exhibited a significantly (p≤0.05) higher fruit mass under shading than did cv. SCX

248 or Irit (Figure 3.19). Average fruit mass was not significantly affected by any of

the shade nets (data not shown). There were however, cultivar differences in fruit

mass. For instance, a high fruit mass was recorded for cultivar AlfaV grown under

the pearl and the yellow nets (Table 3.4). The higher fruit mass obtained by AlfaV

produced under the photo-selective pearl and yellow nets might also be due to the

suitable microclimate provided by the nets which induced a change in the

microclimate around the fruit surface (decrease of 5-6 °C) and an increase in RH,

leading to a reduction in transpiration during fruit growth (Tombesi, Antagnozzi &

Palliotti, 1993:88). Shahak et al. (2004b:612) also suggest that the larger fruit size

obtained from plants grown under the photo-selective nets might be due to reduced

water stress in the nets as well as the ability of the nets to limit environmental

stresses which may constrain canopy assimilation metabolism. On the other hand,

the fruit mass of cv. SCX 248 improved under the red nets and cv. Irit also exhibited

a higher fruit mass under the pearl and the black nets (Table 3.4).

84

Figure 3.19: Variations in the fruit mass of the three tomato cultivars

Means in each bar with the same type of letter are not significantly different at 5% level of probability

3.3.9 Marketable yield

Tomato production under photo-selective shade nets has been shown to increase

the marketable yield and to protect the fruit from physiological disorders such as

sunscald injury (El-Gizawy et al., 1992:349; El-Aidy & El-Afry, 1983:2), blossom end

rot and cracked skin (Lorenzo et al., 2003:181). In the present study, the marketable

yield was higher for cv. AlfaV under the pearl nets, whereas cv. Irit and SCX 248

showed a higher marketable yield under the red nets. All three tomato cultivars

showed a lower production of marketable fruits under the commercially used black

nets (Figure 3.20).

a

b

b

85

90

95

100

105

110

alfaV Irit SCX 248

Fru

it m

as

s (

g)

Cultivar type

85

Figure 3.20: Marketable yield of the three cultivars planted under different shade

nets

Means in each bar with the same type of letter are not significantly different at 5% level of probability

3.3.10 Disease and pest infestation

The use of shade netting had no significant effect on disease and pest infestation

(data not shown). Furthermore, there was no cultivar versus net interaction observed

in terms of diseases and pests. Of the three cultivars, cv. SCX 248 was highly

affected by white flies, while cvs. SCX 248 and Irit revealed higher incidences of

bacterial speck and spot on the leaves. Although the incidences of early blight were

low during this study in all three cultivars, cvs. AlfaV and SCX 248 were observed to

show higher early blight incidences as illustrated in Figure 3.21. Elad et al.

(2007:285) reported that bacterial and fungal diseases tend to spread more rapidly in

c b ba

ed

c c

gf

c

f

0

10

20

30

40

50

60

70

Black Yellow Red Pearl

Ma

rke

tab

le y

eild

(k

g/m

2)

AlfaV Irit SCX248

86

shade environments than in full sunlight, and the condition could be accelerated in

moist areas.

Figure 3.21: Cultivar effect on incidences of pests and diseases

c

b a

b

a

b

a

a

ab

0

5

10

15

20

25

30

35

40

45

White fly Bacteria (speck/spot)

Early blightNu

mb

er

of

lea

ve

s a

ffe

cte

d

Pest and disease

AlfaV

Irit

SCX 248

Means in each bar with the same type of letter are not significantly different at 5% level of probability

87

CHAPTER FOUR

INFLUENCE OF PHOTO-SELECTIVE NETTINGS ON PHYSICO-CHEMICAL

PROPERTIES AND BIOACTIVE COMPOUNDS IN TOMATO CULTIVARS

4 SUMMARY

The aim of this study was to verify the influence of photo-selective nettings on fruit

quality and nutritional properties of tomato cultivars. Three types of photo-selective

nets (red, yellow and pearl with 40% shading) were compared with commercial black

net (25% shading) for fruit quality parameters [firmness, soluble solids concentration,

titratable acidity, CIE-Lab colour parameters (L*, a*, b*)] bioactive compounds

(ascorbic acid, lycopene, β-carotene, total phenols and flavonoids) and total

antioxidant activity in three tomato cultivars (AlfaV, Irit and SCX 248) at harvest

maturity stage (pink). Principle component analysis illustrated cv. AlfaV fruits under

black nets were lower in mass, less firm, higher in bioactive compounds but lower in

TA and intense in red colour. However, under pearl nets cv. AlfaV showed higher

fruit mass, firmness and moderately higher bioactive compounds. SCX 248 fruits

under red nets were moderate in size, firmness, and bioactive compounds in

comparison to the other nets. Cv. Irit fruit under all net types were lower in mass,

less firm, lower in SSC, higher in TA while the black nets increased their bioactive

compounds. Significant correlations were observed between bioactive compounds

and the air temperature and photosynthetic active radiation. Pearl and red photo-

selective nets improved the overall fruit quality; fruit mass, fruit firmness and

88

bioactive components in AlfaV and SCX 248 respectively and can be further

implemented within protected cultivation practices.

4.1 INTRODUCTION

Tomatoes (Solanum lycopersicum) are a rich source of carotenoids (especially

lycopene, β-carotene - a precursor of vitamin A), phenolics (flavonoids), vitamin C

and trace amounts of vitamin E (Betancourt, Stevenson & Kader, 1977:721; Khachik

et al., 2002:845; Vinson et al., 1998:3630). Lycopene is responsible for the red

colour in tomatoes. A 100 g tomato was reported to provide 20-40% of the U.S

recommended daily intake (DRI) for vitamin A and C (Betancourt, Stevens & Kader,

1977:721). In ripened tomatoes, rutin [quercetin 3-O-rutinoside; quercetin3-(6-

rhamnosylglucoside)] has been reported to be a major flavonoid compound (Davies

& Hobson, 1981:205; Slimestad & Verheul, 2009:1255). Lycopene, carotenoids and

flavonoids have been known to show protective effects against cancers and

cardiovascular diseases (Rao & Agarwal, 2000:563; Levy & Sharoni, 2004:49;

Andersen & Markham, 2006). Lycopene, carotenoids and flavonoids act as strong

antioxidants that protect cells from reactive oxygen species (Spencer et al.,

2005:1005).

Tomato fruit quality for fresh consumption is determined by size, colour, firmness,

flavour, aroma and nutritional properties. The reducing sugars (glucose and fructose)

and organic acids (citric and malic acids) are responsible for the sweet-sour taste of

tomatoes. Tomato flavour is also linked to the ratio of reducing sugars to organic

acids (Bucheli et al., 1999:659) and aroma volatiles. Antioxidants and minerals were

89

shown to vary according to the cultivar and crop management practices (Dorais,

2007:200). The health promoting compounds in tomatoes are also influenced by the

maturity stage at harvest (Helyes & Lugasi, 2006:183). Moreover, interaction effects

were reported with respect to the bioactive compounds and cultivars, environmental

factors (light and temperature) and crop management practices (Dorais, 2007:200).

Photo-selective nets contain spectral filters with differential light scattering properties

and altered proportions of red/far-red waveband (R/FR) ratio (Fletcher et al.,

2005:303; Shahak et al., 2008:75). The actual functions of colour shade net depend

on chromatic additives to the plastic and the knitting design (Shahak, 2012). The

physiological responses linked to light quality include fruit-set, size, weight, colour,

and harvest time (Shahak et al., 2004:143; Rajapakse & Shahak, 2007:290).

Use of photo selective shade netting decreases the light quantity and also alters light

quality to a varying extent, and causing a change in thermal climate (Elad et al.,

2007:285; Shahak et al., 2008:75). The biosynthesis of lycopene is affected by air

temperature and sunlight. Exposure of fruits to excessive sunlight was reported to

inhibit the synthesis of lycopene (Brandt et al., 2006:568). Brandt et al. (2003:269),

stated that the growing methods such as water supply, open field or greenhouse

conditions affected the lycopene content in tomatoes and the lycopene content

varied between cultivar types (Daniela, cherry tomato and Delfine F1).

The optimum temperature for lycopene synthesis is between 22 °C and 25 °C

(Dumas et al., 2003:369; Lumpkin, 2005). Higher temperatures were reported to

reduce the vitamin C content in fruit and vegetable crops (McKeon et al.,

2006:1031). Also, growing tomato without shading under slight water stress and

90

strong light intensity predicated increased sugar content and antioxidant compounds.

Bertin et al. (2000:741) reported higher sugar content in tomato fruits grown during

summer with increased sweetness in tomatoes. On the other hand, antioxidants in

fruit and vegetable crops can also be altered by exposure to high temperatures

during the growing season. Effect of photo-selective netting on lycopene content was

reported by Gomez et al. (2001:1101) and Lopez et al. (2007:121). Ilić et al.

(2012:90) reported the effect of photo-selective netting on lycopene and β-carotene

contents in tomato cv. Vedeta. With the consumer and grower‟s concern about fresh

produce quality, the quality and quantity values of radiation and microclimate

parameters under the photo-selective netting technology should be correlated with

crop performance and produce quality (Stamps, 2009:239).

However, little information is available on the influence of photo-selective shade

netting on overall fruit quality parameters and bioactive compounds at harvest in

different tomato cultivars. On the other hand, the use of shading in horticulture

production is becoming popular due to the increase in temperature during summer.

Therefore, the objective of our study was to investigate the effect of different colour

photo-selective netting in comparison to the commercially used black net on (a).

overall quality parameters [colour, soluble solids content, titratable acidity and

firmness] (b). bioactive compounds [ascorbic acid, total phenols, flavonoids,

lycopene, β-carotene contents] and antioxidant scavenging activity at harvest in

three commercial tomato cultivars grown in South Africa.

91

4.2 MATERIALS AND METHODS

The physico-chemical parameters and the bioactive compounds were determined

from the freshly harvested tomatoes mentioned in chapter 3 that were used to

determine the fresh weight. After harvesting, tomatoes were transported to the

Department of Crop Sciences Postharvest laboratory Tshwane University of

Technology, at 25 °C within 30 min for evaluation of fruit mass, fruit colour, firmness,

soluble solids content, titratable acidity, ascorbic acid, lycopene, β-carotene, total

phenol, flavonoid contents and antioxidant scavenging activity.

4.2.1. Analysis of physico-chemical properties of tomato cultivars at harvest

maturity stage (pink)

4.2.1.1 Colour, firmness, soluble solids content (SSC), and titratable acidity

(TA)

Fruit colour was objectively measured with a Minolta CR-400 chromameter (Minolta,

Osaka, Japan) at four equatorial points on tomato fruit surface. The chromameter

was calibrated with a standard white tile. In the CIE colour system, positive a* values

describe the intensity of red colour, positive b* values describe the intensity of yellow

colour and the L* value describes lightness (black=0, white=100). A destructive

deformation test was used to evaluate fruit firmness by loading the tomatoes on a

penetrometer (T.R. Turoni Srl. Italy). For firmness measurement, fruit sample was

92

placed stationary on a penetrometer stand and the compressive force (kg) required

for 5 mm deformation of the fruit was recorded. Each fruit was cut in to pieces and

homogenised in a conventional blender (Braun, Safeway, UK) in order to obtain the

fruit juice. Thereafter, the fruit juice was filtered using a Whatman No 4 filter paper

and the filtrate was used to determine the SSC and TA. The SSC content of the fruit

was determined by using a pocket refractometer (Atago Co., Tokyo, Japan) and

expressed as % (Javanmardia & Kubota, 2006:153). The TA was determined by

titrating 10 ml of juice with 0.1 N NaOH, using phenolphthalein as an indicator and

expressed as citric acid % (Mazumdar & Majumder, 2003).

4.2.2 Analysis of bioactive compounds of tomato cultivars at harvest maturity

stage (pink)

4.2.2.1 Ascorbic acid, Lycopene and β-carotene contents

The ascorbic acid content was determined according to AOAC (2000) and expressed

as mg per 100 g. Lycopene and β-carotene from tomato cultivars were extracted in a

mixture of acetone:n-hexane (4:6) and centrifuged at 3000 × g for 5 min at 4 °C.

Thereafter, optical density of the supernatant was determined at 663 nm, 645 nm,

505 nm and 453 nm using a Microplate Reader (Zenyth 200rt UK-Biochrom Ltd). The

acetone:n-hexane (4:6) mixture was used as blank. The lycopene and β-carotene

contents were determined according to Nagata and Yamashita (1992:925) using the

following equations:

93

Lycopene (μg g FW−1) = −0.0458A663 + 0.204A645 + 0.372A505 − 0.0806A453

β-carotene (μg g FW−1) = 0.216A663 − 1.220A645+ 0.304A505 − 0.452 A453

The A663, A645, A505 and A453 are the absorbance at 663, 645, 505 and 453 nm,

respectively. According to Nagata and Yamashita (1992:925), these equations

enable the simultaneous determination of lycopene and β-carotene in the presence

of chlorophylls. The assays were carried out in triplicate.

4.2.2.2 Total phenol and flavonoid contents

Total phenolic content was determined using the modified Folin-Ciocalteau method

(Singleton et al., 1999:152; Luthria, Mukkhopadhyay & Krizek et al., 2006:774).

Tomato sample (0.2 g) was homogenised with 2 mL acetone: water (1:1 v/v) for 1 h

at 25 °C. A 9 µL aliquot of tomato extract was mixed with 109 µL of Folin-Ciocalteau

reagent. After 3 min of equilibrium time at 25 °C, 180 µL of (7.5% w/v) Na2CO3

solution was added to the extract. The solutions were mixed and allowed to stand for

5 min at 50 °C and after cooling to 25 °C the absorbance was measured at 760 nm

(Zenyth 200rt Microplate Reader UK-Biochrom Ltd). Total phenolic compounds were

calculated using a standard curve of gallic acid and expressed as mg of gallic acid

equivalents (GAE) 100 g-1 FW.

In order to determine the flavonoid content, tomato sample (0.3 g) was homogenised

with 2 mL of methanol. The tomato extract (1.7 mL) was incubated for 30 min at 25

94

°C, shaking occasionally, and subsequently the extract was centrifuged at 6000 × g

rpm for 10 min and the supernatants were taken for analysis. The flavonoid content

was determined as described by Zhishen et al. (1999:556) on triplicate aliquots of

the homogenous juice. The assay was carried out by pipetting 112.5 µL of distilled

water into a well of microplate followed by the addition of 12.5 µL of methanolic

extract, and 7.5 µL of 5% NaNO2 was added. After 5 min, 15 µL of 10% AlCl3 was

added and finally 50 µL of 1 M NaOH was added after 6 min. The absorbance was

read at 510 nm using a Microplate Reader (Zenyth 200rt Microplate Reader UK-

Biochrom Ltd). Flavonoid content was calculated using a standard curve of rutin and

expressed as mg of rutin equivalents 100 g-1 FW.

4.2.2.3 Antioxidant scavenging activity

The 2, 2-diphenyl-1-picrylhydrazyl (DPPH) method was used to determine the free-

radical scavenging activity. For DPPH, tomato fruits were homogenised in methanol:

water (60:40), and centrifuged at 6000 × g for 10 min. After centrifugation, the

extracts were diluted with extraction solvent in order to obtain solutions of 40, 60, 80

and 100 mg ml-1. Solution (0.01 mg ml-1) of gallic acid was prepared immediately

before the analysis and was used as a positive control. The capacity to scavenge the

“stable” free radical DPPH was monitored according to du Toit et al. (2001:65).

About 210 µL aliquots of 0.04 mM 1, 1-Diphenyl-2-picrylhydrazyl prepared in

methanol was mixed with 23 µL of the test sample in a 96 well microplate. The

control samples contained all the reagents except the extract or positive control

antioxidant. The reaction mixture was left at 25 °C for 60 min. The absorbance (Abs)

was measured spectrophotometrically at 515 nm (Zenyth 200rt Microplate Reader,

95

UK-Biochrom Ltd). The results were expressed as EC 50 (sample required to reduce

the absorbance of the radical by 50%) in mg of gallic acid equivalent per gram of

fruit.

4.2.3 Statistical analysis

Experimental data were subjected to two-way analysis of variance to determine the

effect of cultivar and photo-selective netting on tomato fruit quality parameters and

bioactive compounds in three tomato cultivars. The experiments were repeated over

time, with four harvests. To compare the effect of different photo-selective netting on

the different attributes of the three cultivars, Fischer‟s least significant difference

(LSD) procedures were applied at a significant level of 5%.

Pearson‟s correlation coefficients were calculated to determine the strength of the

linear relationships between the PAR or air temperature and fruit quality parameters

(SSC, TA, firmness, fruit mass) or bioactive compounds (total phenols, flavonoids,

ascorbic acid content, lycopene, β-carotene) or antioxidant scavenging activity for all

three cultivars. Principal component analysis (PCA) was used to reduce the number

of variables in the data matrix and to select the most discriminating parameters. On

the other hand, principal components can show interrelationships between the

variables (loading plot) and detect sample patterns, groupings, similarities or

differences (score plot). The statistical package STSG Statistica for Windows,

version 6.0 (Stat soft Inc., Tulsa, UK) was used.

96

4.3 RESULTS AND DISCUSSION

4.3.1 Fruit quality parameters and bioactive compounds

Table 4.1 A and B shows the physicochemical properties and bioactive compounds

of the tomato fruits grown under different photo-selective nets. The soluble solids

concentration (SSC) ranged between 3.59 and 4.40%. Among the three cultivars,

AlfaV resulted in the highest SSC followed by Irit and SCX 248 resulted in the lowest

SSC. These values are similar to those reported by Lumpkins (2005), Gupta et al.

(2011:167) and Ilić et al. (2012:90). All the three cultivars grown under black nets

had the highest SSC. The data (Table 4.3) shows a strong influence of higher light

conditions (PAR) on SSC (r=0.812, p<0.01). SSC has been was reported to

decrease during cooler days (Aldrich et al., 2010:2548).

97

Net type SSC (%) TA (%) Ascorbic acid (mg 100 g FW

-1)

Total antioxidant activity (mg GAE g

-1)

Total phenol content (mg GAE 100 g FW

-1)

Lycopene (µg g FW

-1)

β-carotene (µg g FW

-1)

Flavonoids (mg rutin 100 g FW

-1)

Red 3.99a 0.46c 17.79c 0.107ab 46.80b 14.36b 0.1372b 88.6 Yellow 3.968a 0.51ab 18.12c 0.092b 35.91c 12.92bc 0.1198b 94.5 Pearl 3.928a 0.46c 19.84b 0.121a 46.90b 11.77c 0.1350b 85.1 Black 4.333b 0.51a 21.98a 0.121a 54.53a 17.64a 0.1664a 77.4

p<F <.001 <.001 <.001 <.001 <.001 <.001 <.001 0.132 LSD 0.135 0.029 1.14 0.016 6.15 1.86 0.017 14.50

Cultivar type

AlfaV 4.269a 0.4323b 18.82a 0.1216a 46.4 14.52b 0.1551a 101.24a SCX 248 3.733b 0.4494b 16.99c 0.1026b 47.0 10.37c 0.1067b 94.32a Irit 4.163a 0.5941a 22.50a 0.1062b 44.7 17.63a 0.1571a 63.62b

p<F <.001 <.001 <.001 0.015 0.671 <.001 <.001 <.001 LSD 0.117 0.025 0.986 0.014 5.33 1.61 0.015 12.56

Means followed by the same letter within the column are not significantly different at 0.001 level of probability

Table 4.1 A: Effect of photo-selective netting on the physicochemical properties and bioactive compounds of three tomato

cultivars

98

Net type SSC (%) TA (%) Ascorbic acid (mg 100 g FW

-1)

Total antioxidant activity (mg GAE g

-1)

Total phenol content (mg GAE 100 g FW

-

1)

Lycopene (µg g FW

-1)

β-carotene (µg g FW

-1)

Flavonoids (mg rutin 100 g FW

-1)

Cv x net AlfaV x Red 3.88cd 0.45de 15.06d 0.089de 51.56b 13.00def 0.14bc 91.29abcd AlfaV x Yellow 4.5a 0.36g 16.97cd 0.102cde 32.97c 10.44fg 0.13bcd 114.36ab AlfaV x Pearl 4.08bc 0.43ef 20.84b 0.146ab 45.52b 13.98cde 0.15bc 108.58abc AlfaV x Black 4.60a 0.44def 22.40b 0.150a 55.53a 20.66b 0.20a 90.71bcd SCX 248 x Red 3.89cd 0.36g 17.77c 0.122bc 44.64b 16.69c 0.14bc 91.62abcd SCX 248 x Yellow

3.66de 0.57bc 16.59cd 0.080e 36.83c 11.39ef 0.10de 116.35a

SCX 248 x Pearl 3.59e 0.37g 16.38cd 0.106cde 53.32a 6.19h 0.10ab 85.20cde SCX 248 x Black

3.80de 0.48d 17.20c 0.102cde 53.32a 7.23gh 0.08e 84.10cde

Irit x Red 4.2b 0.63a 20.54b 0.109cd 44.2b 13.4def 0.12cd 83.0de Irit x Yellow 3.78de 0.55c 20.81b 0.093de 37.93c 16.93c 0.13bcd 52.72f Irit x Pearl 4.11bc 0.58abc 22.3b 0.11cdc 41.88b 15.15cd 0.16b 61.51ef Irit x Black 4.55a 0.61ab 26.34a 0.112cd 54.75a 25.05a 0.22a 57.25f

p<F <.001 <.001 <.001 <.001 0.298 <.001 <.001 0.026 LSD 0.2338 0.05077 1.971 0.02716 10.66 3.226 0.03069 25.11 Means followed by the same letter within the column are not significantly different at 0.001 level of probability

Table 4.1 B: Effect of photo-selective netting on the physicochemical properties and bioactive compounds of three

tomato cultivars

99

A cultivar x net interaction was observed whereby, fruits of cvs. AlfaV under yellow (35.8

°C) and black nets (30.27 °C) and Irit under black nets (30.27 °C) had significantly

higher SSC values while SCX 248 under pearl nets (42.1 °C) had significantly lower

SSC. It has been shown that some cultivars have the genetic background to have high

SSC and that solar radiation and temperature can affect SSC levels. Temperature (~30

°C) has previously been shown to influence SSC content of the fruits (Walker & Ho,

1977:825). High temperatures (40 °C) could lead to sink competition as reported in

cherry tomatoes, promoting fruit evapotranspiration and higher sugar levels. However

the sugar concentration is frequently reduced due to the sink competition caused by the

increased respiration during higher temperatures (Gautier et al., 2005:1241).

Among the cultivars, cv. Irit showed significantly higher TA than the other two cultivars

and the black nets gave higher TA in all fruits, which shows that the cooler temperatures

(under black nets) favoured the accumulation of acids than the higher temperatures

(yellow nets) (Aldrich et al., 2010:2548). A moderate correlation was noted between

temperature and TA (r=-0.62). Different observations were reported regarding the

shading effect and titratable acidity in tomato fruit. According to El-Gizaway et al.

(1992:349), increasing shading levels from 35% to 63% increased the titratable acidity

in tomato fruit while Riga et al. (2008:158) reported that shading tomato plants by 50%

did not affect the concentration of titratable acidity. However, significant cultivar x net

interaction was observed as shown in Table 4.1 B. The TA was significantly higher in

cv. Irit produced under red net whereas AlfaV and SCX 248 under yellow and red nets

respectively showed significantly lower TA.

100

According to Davis and Hobson (1981:205) the SSC can be considered as the easiest,

cheapest and quickest measure linked to the sugar content especially during

commercial marketing and the authors suggested that it could be used as the primary

criteria for ranking the fruit quality of commercial varieties.

The bioactive compounds; total phenols, ascorbic acid, lycopene, β-carotene and

flavonoids responsible for the antioxidant properties need to be evaluated in order to

determine the tomato fruit quality (Raffo et al., 2006:11; Rosales et al., 2006:1545). In

our study the total phenolic compounds were significantly higher in fruits harvested

under the black nets. The cultivar difference was not significant and cultivar x net

interaction was not observed with respect to total phenolic compounds. The

accumulation of total phenolic compounds in tomato fruits was moderately correlated to

the PAR (r=0.56, p<0.01) (Table 4.3). The total phenolic compounds in tomatoes were

observed to decrease with increasing temperatures during the production period.

Reduction of total phenolic compounds in tomatoes during cooler period was reported

by Aldrich et al. (2010:2548).

The lycopene content was significantly higher in tomatoes grown under the black nets

whereas the tomatoes grown under photo-selective nets had lower lycopene content

(Table 4.1 A). The observed variation was due to air temperature and light quality with

lycopene content in tomatoes affected by higher temperatures (r= -0.90, p<0.0001) and

PAR (r=0.66, p<0.01) (Table 4.3). However, excessive sunlight was reported to inhibit

101

the synthesis of lycopene (Brandt et al., 2006:568). According to Helyes et al.

(2007:927), fruit surface temperature was a more accurate predictor of fruit lycopene

content than air temperature. In our study the pearl net had higher air temperature

within the net with lower PAR while the black nets had lower air temperature and higher

PAR, due to the knitting patterns of the nets and could have increased the surface fruit

temperature within the pearl net and affected the lycopene content in SCX 248

tomatoes (Table 4.1 B). On the other hand tomatoes grown under red net showed

higher lycopene contents than tomatoes grown under the pearl net (Table 4.1 A).

Similar observations were reported by Lopez et al. (2007:121) and Ilić et al. (2012:90).

The observed differences between the lycopene content in tomatoes under the red and

pearl nets could be due to the stimulation of lycopene accumulation in tomatoes under

the red net due to the slight variation in red light as explained by Ilić et al. (2012:90) that

lycopene synthesis is mediated by phytochromes. Among all the cultivars, Irit showed

significantly higher lycopene content (Table 4.1 A). The lycopene contents of our

tomatoes were lower than those reported for newly bred tomatoes reported by Gupta et

al. (2011:167) and cv. Vedeta (Ilić et al., 2012:90); the values are however, higher than

that reported for cherry (Rosale et al., 2006:1545) and are within the range reported for

Savoura tomatoes (Bui et al., 2010:830). Cultivar x net interaction was significant

(p<0.001) for AlfaV and Irit tomatoes produced under the black net and SCX 248 under

the red net which had significantly (p<0.001) higher lycopene content (Table 4.1 B). It is

interesting to note that the SCX 248 tomatoes produced under the black and pearl nets

produced significantly (p<0.001) lower lycopene content (Table 4.1 B). This confirms the

102

influence of higher temperature (pearl net) and PAR (black net) on lycopene

biosynthesis.

The β-carotene content in tomatoes grown under the black net was significantly

(p<0.001) higher than for the tomatoes obtained from the other nets (Table 4.1 A). β-

carotene biosynthesis in tomato is influenced by the temperature (r= -0.73, p<0.0001)

and PAR (r=0.72, p<0.01). Temperatures over 30-35 °C and strong solar radiation was

reported to inhibit lycopene biosynthesis and stimulate the oxidation of lycopene to β-

carotene (Dumas et al., 2003:369). According to Gautier et al. (2005:1241), β-carotene

degradation increases from 35 to 40 °C. In cherry tomatoes high solar radiation and

temperature (36 °C) was shown to reduce the lycopene and β-carotene contents in the

exocarp (Rosales et al., 2006:1545). However, according to our investigations; cvs.

AlfaV and Irit had higher β-carotene content than cv. SCX 248 (Table 4.1 A). Significant

cultivar versus net interaction was found. Cultivars AlfaV and Irit under the black nets

and SCX 248 under red net produced tomatoes with significantly (p<0.001) higher β-

carotene content whereas cv. SCX 248 tomatoes grown under pearl, black and yellow

nets had significantly lower β-carotene content (Table 4.1 A). However, significantly

lower β-carotene content in SCX 248 tomatoes under the black nets might have been

due to faster degradation. Based on these observations, it appears that the red nets can

improve lycopene and β-carotene contents in tomato cultivars (genotypes) that are

genetically predisposed to produce lower lycopene and β-carotene contents (e.g. SCX

248).

103

Flavonoids are beneficial in many ways and act as antioxidants, antiproliferative or

antibacterial agents (Harborne & Williams, 2000:481). Flavonoid content (mg of rutin

equivalents 100 g-1 FW) was significantly (p<0.001) higher in cv. AlfaV and lower in Irit

(Table 4.1 A). The PAR was negatively correlated to the flavonoid content in our

investigation (r=-0.55, p<0.01) (Table 4.3) and according to the reports of Cen and

Bornman (1990:1481) the PAR could promote flavonoid synthesis. Significant cultivar x

net interaction was observed in our study with respect to flavonoid content. Cultivars

AlfaV and SCX 248 under yellow nets had significantly (p<0.001) higher flavonoid

content (Table 4.1 B). In cherry tomatoes a temperature increase from 27 to 32 °C was

reported to increase the rutin and caffeic acid (Gautier at al., 2008:1241). According to

our results during the fruit production period the average temperature and PAR under

the yellow and red nets were around 35 °C and 851.81 μmol m-2s-1 respectively and

could have stimulated the biosynthesis of flavonoids. On the other hand, cv. Irit under

black and yellow nets had lower flavonoid content (Table 4.1 B). However, the red net

helped to improve flavonoid content in cv. Irit which directly shows the influence of PAR

(744.13 μmol m-2s-1) on flavonoid content (Table 4.1 B). SCX 248 had relatively low

flavonoid content under the black and pearl nets (Table 4.1 B). Our findings with respect

to different cultivars and shade nets show that the influence of PAR and light quality

influences flavonoid content.

Ascorbic acid content was shown to increase in the presence of higher light intensity

during the production period in fresh produce (Lee & Kader, 2000:207) and this explains

the higher ascorbic acid content obtained in tomatoes produced under the black nets

104

(Table 4.1 A). Also, a strong correlation was detected between the ascorbic content and

PAR during our study (r=0.808, p<0.0001) (Table 4.3). Cultivars Irit and SCX 248

showed significantly (p<0.001) higher and lower ascorbic content respectively (Table

4.1 A). Cultivar versus net interaction was detected in our study. Cultivar Irit under black

net produced significantly (p<0.001) higher ascorbic acid content while cv. Irit under red

net produced significantly (p<0.001) lower ascorbic acid content (Table 4.1 A).

Our investigation showed that total phenols, β-carotene and ascorbic acid acted as

antioxidants and moderately correlated to the antioxidant scavenging activity.

Antioxidant scavenging activity was higher in tomatoes produced under the black and

pearl nets (Table 4.1 A). Among the cultivars the antioxidant property was higher in

AlfaV. Cultivar versus net interaction was evident from our study. AlfaV grown under

black nets produced significantly (p<0.001) higher antioxidant activity while SCX 248

under yellow net gave significantly (p<0.001) lower antioxidant activity (Table 4.1 B).

Table 4.2 shows the mean values of CIE-Lab colour parameters of tomato fruits. Black

and yellow nets favoured the increase of colour coordinate a* indicating more reddish

fruits whereas pearl and red nets had significantly lower colour coordinate a* and

slightly less reddish in colour. The a* value showed a moderate correlation with the PAR

(r=0.54, p<0.001). Higher temperature was reported to affect colour development in

tomatoes (r=0.56, p<0.001) (López Camelo & Gómez, 2004:534) and fruit remained

yellow in colour as a result of inhibition of lycopene synthesis and the accumulation of

105

yellow/orange carotenoids (Tijskens & Evelo, 1994:85). On the other hand cvs. Irit and

AlfaV showed reddish fruits than cv. SCX 248. Significant cultivar x net interaction was

found with Irit and AlfaV cultivars under black net producing more reddish fruit whereas

yellow net gave the highest redness in SCX 248 tomato fruit. When red colour was

tended to increase, a decrease in L* value was obtained in tomatoes by Messina et al.

(2012:1). On this basis tomatoes grown under black, yellow and red nets showed higher

L* values and were low in a* indicating less intense red colour (r= -0.45, p<0.1),

although, fruits were harvested at pink stage. The intensity of red colour pigment in

tomato is determined by relative composition of lycopene and chlorophyll while

yellowness depends on the β-carotene content. The intensity of the yellow colour was

shown by higher b*. There was a highly significant correlation between a* and lycopene

content (r=0.72, p<0.01). These findings suggest that light quality and temperature

influence colour pigment synthesis in tomato. AlfaV and Irit tomatoes grown under black

nets had higher a* and lower b* coordinate values and showed intense red colour (Table

4.2).

106

Table 4.2: Effect of cultivar and shade net on physical properties (colour) of tomato

fruits

Cultivar Shading Color

L* a* b*

AlfaV Red 41.7 ± 2.5cd 19.3 ± 4.7d 22.6± 2.8de

Black 42.6 ± 1.5de 23.4 ± 3.7gh 23.3 ± 2.6e

Yellow 41.6 ± 2.3c 21.7 ± 3.1ef 23.5 ± 3.3e

Pearl 43.0 ± 1.4ef 21.3 ± 2.1ef 22.1 ± 2.4cde

Irit Red 41.5 ± 2.3c 20.3 ± 2.6de 17.1 ± 2.6a

Black 39.7 ± 2.8b 23.8 ± 3.7h 22.2 ± 3.7cde

Yellow 40.5 ± 2.6b 22.0 ± 3.6fg 20.8 ± 5.4c

Pearl 41.8 ± 1.3cd 20.2 ± 4.3de 19.1 ± 2.4b

SCX 248 Red 38.7 ± 2.8a 13.4 ± 3.1c 27.7 ± 4.2h

Black 44.5 ± 1.7g 11.1 ± 4.8b 21.6 ± 2.3cd

Yellow 43.6 ± 2.2f 14.1 ± 5.5c 23.7 ± 5.4g

Pearl 48.2 ± 1.9h 9.0 ± 3.0a 17.9 ± 2.4a Means followed by the same letter within the column are not significantly different at 5% level of probability

4.4 CORRELATION ANALYSIS

When all three cultivars were considered over the 2011 and 2012 seasons and growing

conditions (photo-selective nets and control black net). The SSC, colour coordinate a*,

lycopene content, β-carotene and ascorbic acid were positively correlated with the PAR

while the PAR was negatively correlated to the firmness at harvest. Air temperature had

a negative correlation with SSC, TA, β-carotene, lycopene and ascorbic acid. Relative

humidity was positively correlated to SSC, TA, β-carotene, lycopene and negatively

correlated to fruit firmness (Table 4.3).

107

Table 4.3: Pearson‟s correlation coefficients between PAR or microclimate conditions

(AT or RH) and the fruit quality parameters or bioactive compounds

Correlation results include means from three cultivars, grown under four different shade nettings during 2011 and 2012

growing season. n.s, not significant; asterisks indicate significance at *p<0.05; **p<0.01; ***p<0.0001

4.5 PCA ANALYSIS

PCA was carried out to investigate the data structure, in order to establish a cultivar and

net classification based on the obtained data including the physicochemical properties

and bioactive compounds. With respect to Eigen values >1, two principal components

were obtained with their factor loading shown in Figure 4.1. Eighty seven percent of the

original variance in the data set of fruit properties (PC1 21.07% and PC2 43.93%) was

explained by the first two principal components. The two PCs explained 65% of the x-

variables selecting thirteen parameters, including total phenols, flavonoids, β-carotene,

lycopene, ascorbic acid, antioxidant activity, L* , a*, b* , fruit mass, firmness, SSC and

TA. Total phenols, β-carotene, lycopene, ascorbic acid, antioxidant activity L*, a* were

Parameter Air Temperature PAR RH

a 0.63** 0.645** 0.49 ns b -0.69** 0.234 ns 0.64** Soluble solids concentration

-0.835** 0.812** 0.730**

Titratable acidity -0.623** 0.407 ns 0.589** Firmness 0.783** -0.726** -0.762** Fruit weight 0.152ns 0.325 ns -0.388 ns β-Carotene -0.73*** 0.727** 0.53* Lycopene -0.893*** 0.660** 0.703** Total phenols -0.42 ns 0.56** 0.188 ns Flavonoids 0.21 ns -0.556** -0.38 ns Ascorbic acid -0.549* 0.808*** 0.256 ns

108

mainly accounted for with PC1 while fruit mass, firmness, b* and flavonoids were mainly

accounted for with PC2 as shown in Figure 4.1.

Figure 4.1: Principal Component Analysis (PCA) for physicochemical properties and

bioactive compounds

109

Figure 4.2: PCA analysis for the cultivars and shade nets

Results of the PCA analysis are shown in Figure 4.1 and 4.2. As illustrated in Figure

4.1, bioactive compounds such as β-carotene (r=0.91), lycopene (r=0.90), ascorbic acid

content (r=0.82) antioxidant activity (r=0.54) and quality parameters: a* (r=0.92), L*,

SSC (r=0.86), on PC1 and the quality parameters: firmness (r=0.80) and fruit mass

(r=0.65), b*, TA (r=-0.72) and bioactive compounds: flavonoids (r=0.81) and total

phenols (r=-0.36), on PC2 helped to classify the cultivars and nets in Figure 4.2. Based

on this analysis it is evident that cv. AlfaV fruits produced under black nets were lower in

mass and less firm with higher concentration of bioactive compounds, higher in SSC,

110

less acidic, intense in red colour. However, cv. AlfaV fruits under pearl nets showed

greater fruit mass, firmness and were moderately rich in bioactive compounds. SCX 248

fruits under red nets were moderate in size, firmness, and bioactive compounds in

comparison to the other nets. Cultivar Irit under all net types was small, less firm, had

low SSC and more acidic while black nets enabled to increase bioactive compounds.

Overall, the total yield related to marketable fruits was higher in AlfaV produced under

pearl net. Marketable fruits assessments included fruits that were free from pest attack

and diseases.

111

CHAPTER FIVE

5 GENERAL CONCLUSIONS

The selection of a tomato variety that can yield a high number of fruits with better quality

as well as a colour net that can provide conducive growing conditions can both benefit

growers that are currently experiencing lower yields under protected cultivation and in

open fields. Pearl net shading can be recommended to improve marketable fruit yields

of cv. AlfaV, while red net shading can be recommended for cvs SCX 248 and Irit.

Photo-selective netting in general, however, did not show any influence on the

incidences of disease or pest infestation during the growing season. However, plant

morphological parameters, fruit trusses, and numbers of fruit per plant were influenced

by cultivar versus photo-selective net interactions. Flowering, fruit set and fruit

production were also enhanced in cvs AlfaV and SCX 248 produced under red nets.

Cultivar AlfaV exhibited a greater fruit mass when produced under shade nets, which

offers great potential for satisfying consumer demand.

The data presented and discussed in chapter four provide useful information on the

environmental changes related to colour shade nets on fruit quality parameters and

bioactive compounds in tomatoes. Cultivar differences were shown to affect bioactive

compounds, antioxidant activity and fruit quality in tomatoes. Thus, the proper selection

of tomato cultivars that are nutritionally superior can benefit growers and retailers who

cater to health conscious consumers who in turn value fruit size (mass), firmness,

112

colour, as well as health benefits being the most important parameters. Based on the

current study, cv. AlfaV was the best cultivar with respect to fruit quality and bioactive

compounds, followed by cv. Irit. Photo-selective pearl nets can be recommended to

improve the overall fruit quality, fruit mass, firmness and bioactive components in cv.

AlfaV. On the other hand, photo-selective red nets can be recommended for cv. Irit with

respect to quality and bioactive compounds and can furthermore be implemented in

protected cultivation.

113

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