Effect of pre-flowering irrigation on leaf photosynthesis, whole-tree water use and fruit yield of...

Scientia Horticulturae 102 (2004) 189–211

Effect of pre-flowering irrigation on leafphotosynthesis, whole-tree water use

and fruit yield of mango trees receivingtwo flowering treatments

Alonso Gonzáleza,∗, Ping Lua, Warren Müllerba Darwin Laboratory, CSIRO, Plant Industry, PMB 44, Darwin NT 0822, Australia

b Department of Mathematics and Information Science, CSIRO, Canberra, Australia

Accepted 10 December 2003

Abstract

Two flowering treatments and two irrigation treatments were applied to mango trees of cultivarKensington Pride in a commercial orchard near Darwin, northern Australia. Paclobutrazol (PBZ)was applied as a soil drench. For the mango flowering treatment (MFT) a cincture was cut into eachtree trunk and a length of twine soaked in a solution of morphactin was tied into the cincture. Extrairrigation treatment (XTI) was applied between 30 and 46 days before peak flowering, and normalirrigation (NI) was applied when flowering activity was detected in about 65% of the tree canopy.Leaf light-saturated carbon assimilation (Alsat), stomatal conductance (gs), chlorophyll fluorescenceparameters, whole-tree water use, flowering activity, fruit growth and fruit yield were recorded overthree years.

Alsat, gs, and electron transport rates (ETR) were low at time of flowering regardless of the irrigationor flowering treatment. A significant increase inAlsat, andgs (10–20%) was detected at some datesin trees receiving extra irrigation. ETR of XTI trees was significantly higher than that of NI treesat flowering and during early phase of fruit growth, but not later in the season. In both floweringtreatments whole-tree water use of XTI trees was significantly higher than that of NI trees in thepre-flowering and flowering period but not near the end of the fruiting season. XTI treatment did notchange flowering intensity; however, it caused flowering to be more variable. XTI treatment resultedin larger fruits in PBZ treated trees and increased fruit retention in MFT treated trees. Fruit relativegrowth rate was higher in fruits from XTI trees early in the season but was similar to those from NItrees later in the season. Regression analysis of the proportion of flowering and fruit number/yieldduring the 3 years of the experiment reflected a similar conversion of flowers to fruits in the M-XTIand all PBZ trees, but was lower in the M-NI trees. After 3 years of treatment, XTI resulted in trees

∗ Corresponding author. Tel.:+61-8-8944-8479; fax:+61-8-8947-0052.E-mail addresses:[email protected], [email protected] (A. Gonzalez).

0304-4238/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.scienta.2003.12.011

190 A. Gonzalez et al. / Scientia Horticulturae 102 (2004) 189–211

with larger trunk circumference and higher leaf area index compared to NI trees. The effect of low Cavailability at time of flowering on fruit yield is discussed.© 2004 Elsevier B.V. All rights reserved.

Keywords:Irrigation; Fruit growth; Photosynthesis; Kensington Pride;Mangifera indica; Sap flow; Whole-treewater use

1. Introduction

The mango industry in northern Australia is rapidly growing and currently includes almost1 million trees, mainly of cultivar Kensington Pride (KP). In Australia, average fruit yieldof KP is about 4 t/ha and it is considered to be one of the lowest in the world (Kulkarni,personal communication). Poor and unreliable flowering is one of the factors responsible forlow productivity in this cultivar in the warm tropics (Whiley, 1993; Leonardi et al., 1999).

Studies on floral induction in Kensington Pride have involved the use of paclobutrazol(PBZ) and a combination of girdling, strangulation and morphactin known as the mangoflowering treatment (MFT) (Leonardi et al., 1999; González and Blaikie, 2003), tip pruning(Kulkarni and Hamilton, 1996) and water deficit before flowering (Bally et al., 2000; Luand Chacko, 2000). Despite those efforts (seeKulkarni, 2004for a recent review) a reliablemethod to induce flowering every year is still absent, because the response to floweringtreatments is affected by many factors such as tree size, previous cropping history andprevailing weather conditions.

A recent multi-site study of flowering of Kensington Pride in response to the chemicaltreatments PBZ and MFT highlighted the need for applying flowering treatments to thiscultivar in the warm tropics.Blaikie et al. (2004)found that the flowering activity of con-trol trees (no flowering treatment) was extremely low, that response to PBZ was highlyvariable and that MFT induced massive flowering in the first year of application but wasnot different from control trees in subsequent years. MFT also caused a dramatic reductionin the photosynthetic capacity of treated trees while PBZ trees tended to have higher leafphotosynthetic rates than control trees (González and Blaikie, 2003).

It is generally agreed that a period of quiescence is required for floral induction (Ballyet al., 2000) which is caused by temperatures<16◦C (Schaffer et al., 1994). In the warmtropics temperatures<16◦C are not common, and consequently a dry period is neededto restrain growth and enhance flowering (Chacko, 1986; Lu and Chacko, 2000); however,soil water deficit alone does not guarantee flowering in this cultivar. Although pre-floweringwater deficit might enhance the potential for development of floral structures (Davenportand Nuñez-Elisea, 1997; Lu and Chacko, 2000; Bally et al., 2000) it has a detrimental effecton the photosynthetic activity of the trees at time of flowering, pollination and fruit set (Luand Chacko, 1997; González and Blaikie, 2003), which might negatively affect productivity.

In the northern tropical region of Australia, light-saturated leaf carbon assimilation(Alsat) of Kensington Pride follows a seasonal pattern.Alsat is highest in the wet months(November–March) and lowest during the dry months (June–October) on irrigated trees(González and Blaikie, 2003). If pre-flowering water deficit further reduces CO2 assim-ilation, it is a matter of concern as it is possible that lack of photo-assimilates supply at

A. Gonzalez et al. / Scientia Horticulturae 102 (2004) 189–211 191

flowering is responsible for the low yield observed in this cultivar. Consequently, efforts toovercome the reduction inAlsat at the time of flowering need to be assessed with regard totheir effect on fruit yield.

Currently, there are no reports on the combined effects of different irrigation regimesand the flowering treatments PBZ and MFT on fruit yield of Kensington Pride in the warmtropics. Although inducing soil water deficit before flowering is nearly a standard practicein the warm Australian tropics, the effects on productivity and fruit growth characteristicshave not been explored in detail. In this study we investigated the effects of pre-floweringirrigation on whole-tree water use, leaf photosynthetic characteristics (chlorophyll fluo-rescence and gas exchange), flowering intensity, fruit growth and fruit yield in the mangocultivar Kensington Pride.

The objectives of this study were to examine whether reducing the period of water stressbefore flowering results in: (1) significantly higher photosynthetic rates at time of floweringand (2) better fruit yield.

2. Materials and methods

2.1. Study site and plant material

The experiment was conducted between December 1999 and November 2002 on 5-year-old (in 1999) grafted mango trees of cv. Kensington Pride in a commercial orchard locatednear Darwin (12.42◦S, 130.88◦E) in northern Australia. KP scions were grafted onto KProotstocks planted at a density of 100 trees ha−1. The Darwin area is characterised by distinctwet and dry seasons with an annual average rainfall of 1670 mm per year, mostly fromOctober to March. Temperatures are warm throughout the year, with the lowest temperaturesrecorded in July. Flowering normally takes place in the dry season, commencing in late Mayand continuing through August. Fruit harvest peaks in October.

Trees were managed following normal commercial practices. In this experiment, treeswere pruned and fertigated in December and fertigated again at the end of the wet season(April). Irrigation was applied as described in the irrigation treatments below.

2.2. Experimental treatments and design

The trial was a split-plot design, with duplicates of the main plots. The irrigation treatmentwas applied to the main plots and the flowering treatments were applied to the subplots.Trees receiving different irrigation treatment were separated by two rows of guard trees.Each subplot contained five trees. The order of the subplots on the first main plot wasreversed on the second main plot. Due to the higher variation observed between subplotsthan between plots the experiment was analysed as a completely randomised design.

Two irrigation treatments were applied. For the pre-flowering irrigation (XTI) water wasapplied beginning between 45 and 30 days before the expected date of flowering using threeunder-tree sprinklers delivering approximately 90 l/h three times a week (1260 l per week).The normal irrigation (NI) treatment started when approximately 65% of the terminals inthe canopy of most trees in the farm had commenced flowering. The NI trees were irrigated


Table 1Summary of relative available water content (RAWC) in the top 50 cm of the soil layer and total soil water storage(TSWS, mm) in the soil profile down to 1.2 ma

Year Treatment Start of irrigation (DOY) RAWC (%) TSWS (mm)

2000 P-XTI 129 50.5 176P-NI 175 1.2 139M-XTI 129 84.2 183M-NI 175 8.9 149

2001 P-XTI 148 23.4 146P-NI 177 0.7 135M-XTI 148 13.7 136M-NI 177 0.4 135

2002 P-XTI 148 12.3 142P-NI 187 0 134M-XTI 148 7.24 132M-NI 187 0.3 135

a Trees were treated with two flowering treatments, MFT and PBZ and two irrigation treatments, normalirrigation and pre-flowering irrigation. M-NI: MFT-normal irrigation; M-XTI: MFT, pre-flowering irrigation;P-NI: PBZ-normal irrigation; P-XTI: PBZ, pre-flowering irrigation.

using one under-tree sprinkler which delivered 60 l/h (840 l per week). After NI treatmentstarted water was applied to all trees in the farm aiming to top up the soil water storageand then all trees were irrigated until the end of the fruiting season. The amount of waterapplied to the tree was under the control of the farmer, and consequently varied from yearto year. Details of applied irrigation are presented inTable 1.

Two chemical treatments to induce flowering, PBZ and MFT were first applied in De-cember 1999 as described byGonzález and Blaikie (2003). The MFT treatment was appliedby cincturing the bark and tying a length of twine previously soaked into a solution contain-ing 0.5% a.i. of morphactin (Maintain CF125, Uniroyal Chemical, USA) above the graft.The typical dose of morphactin applied with MFT was about 11 mg a.i. per tree. MFT wasonly applied once in December 1999. PBZ was applied after the post-harvest pruning bydrenching the soil around the trunk of each tree at a typical rate of about 5–7.5 g a.i. pertree (González and Blaikie, 2003).

PBZ trees were treated with PBZ in 1999–2001. MFT trees were treated with PBZ in2001 and 2002.

2.3. Sap flow and whole-tree water use

Xylem sap flow was measured in four trees per flowering× irrigation treatment (16 treesin total) using Granier’s heat dissipation sensors (Granier, 1985, 1987). Trees were spacedthroughout both replications. Each sensor consisted of one pair of probes with the distal2 cm as the sensing part (seeLu et al., 2000for full description of the method). Xylem sapflux density (l dm−2 h−1), expressed on a sapwood area basis, was calculated from the sapflow signals according toGranier (1985)with specific correction for mango according toLu et al. (2000).


2.4. Soil water measurements

Volumetric soil water contents were measured within the canopy drip-line on three ofthe four trees equipped with sap flow sensors in each treatment by a CS615 soil moisturesensor probe (Campbell Scientific, Logan, USA) inserted into the top 30 cm soil. Also, aneutron probe (Model 503 DR, CPN, USA) with one aluminium access tube per samplingtree installed to a depth of 1.2 m during the wet season 1999–2000 was used to detectvolumetric soil water data. These tubes were sited at a distance of 1 m from both of thetree base and the sprinkler(s) and were 20 cm from the CS615 probe. Measurements weremade at 10 cm intervals from 10 to 60, and at 20 cm intervals from 60 to 120 cm. Data werecollected weekly during the dry season and monthly during the wet season. In situ calibrationof the neutron meter was carried out at three times during the wet season 2001–2002 anddry season 2002 covering a range of the soil water content.

Volumetric soil water content (VSWC, %) was directly calculated from the calibrationfor the CS615 probes or from in situ calibration for the neutron meter.

Relative available water content (%) was calculated as

RAWC(%) = VSWCactual− VSWCmin

VSWCmax − VSWCmin

where VSWCactualis the actual VSWC, VSWCmin the minimum VSWC observed in NI trees(it was used for XTI trees of the same flowering treatment) and VSWCmax the maximumVSWC observed in each of the NI and XTI treatments.

2.5. Leaf photosynthetic characteristics

2.5.1. Gas exchangeSix trees from each flowering× irrigation treatment were selected for monitoring leaf

gas exchange as inGonzález and Blaikie (2003). In 2000 and 2001, 10 leaves per treewere tagged in December or January and gas exchange was recorded on these leaves eitherevery two weeks or monthly using a portable open IRGA (Licor 6400, Li-Cor, Lincoln,NE) at light-saturating conditions (1200�mol m−2 s−1) using an attached light source.Leaves were selected from the last terminal on the branch and were distributed around theperimeter of the canopy. In 2002, leaves were randomly selected from the eastern side ofthe tree during the fruit development period. Measurements of light-saturating CO2 assim-ilation rate (Alsat, �mol CO2 m−2 s−1) and stomatal conductance (gs, mol H2O m−2 s−1)taken in the morning during the pre-flowering period through harvest each year will bepresented.

2.5.2. Chlorophyll fluorescenceChlorophyll fluorescence parameters were recorded from May to September 2002 on

leaves similar to those used for gas exchange. All measurements of chlorophyll fluores-cence were performed in situ with a portable PAM-2000 fluorometer (Walz, Effeltrich,Germany) equipped with a leaf clip holder (Model 2030-B) in sunlit leaves of all trees.One single measurement per leaf and 20 leaves per tree were sampled every time. The rateof photosynthetic electron transport (ETR,�mol e m−2 s−1) was calculated according to


Genty et al. (1989):

ETR = 0.5φpsIIξ PPFD

where PPFD is the photosynthetically active photon flux density (�mol m−2 s−1), ξ the leafabsorptance, andφpsII, the effective quantum yield of PSII, was computed as(F ′

m−Fs)/F′m.

ξ (=0.785) was calculated from the average leaf chlorophyll content with an equation gener-ated for leaf absorptance and chlorophyll content for mango leaves using a spectroradiometerwith an integrating sphere (Licor 1800-12S, Li-Cor, Lincoln, NE).φpsII measures the pro-portion of light absorbed by chlorophyll associated with PSII that is used in photochemistry,and is an indication of overall photosynthesis in vivo (Genty et al., 1989). WhenφpsII andETRs were compared, PPFD was used as a covariate in the analysis.

2.6. Assessment of phenology and fruit yield

Phenological observations of flowering activity were made at irregular intervals duringeach flowering season. In 2000 and 2001, flowering was visually assessed by determiningthe proportion (%) of terminals on the canopy that were actively producing flowers. Thismethod of appraisal related well to quantitative assessment of tagged branches (Gonzálezand Blaikie, 2003).

In 2002, 40 tagged terminals per tree were evaluated weekly for floral or vegetativeactivity, fruit set, fruit retention and fruit growth during this season.

Fruit yield (kg fruit per tree) was calculated from the number of fruits on each tree andthe average fruit weight. The total number of fruits on each tree was counted 3 weeksbefore commercial harvesting, and within a week after fruit counts, the average fruit sizewas estimated by measuring the size of every fruit enclosed within 1 m2 quadrant that wasrandomly positioned at six different places around the perimeter of the canopy. This methodsampled between 15 and 25% of the total fruits on each tree.

Fruit dimensions were converted into fruit weight by allometry. Length (L) of the fruitwas measured from base to the apex and breadth (SS) at its widest part, near the shoulderof the fruit. Thickness of the fruit (CC) was also measured at its widest part, near theshoulder at the right angle to breadth. All the measurements were made in millimetres withan electronic calliper.

The allometric relationship between fruit size and fruit fresh weight was developed bydestructively harvesting 92 fruits in year 2000 and 120 fruits in year 2002 representingvarious sizes and all flowering× irrigation treatments. Fresh weight (FW) was estimatedfrom fruit dimension using the equation:

FW = 10−3.225+1.389 log(CC)+0.958 log(L)+0.626 log(SS), r2 = 0.998

Leaf area indexes of the tree were estimated in April 2000 (MFT trees only) and from all treesat the end of the third year in November 2002 using a LAI-2000 (Li-Cor, Lincoln, NE). Leafarea index (LAI) was defined as the surface area of all leaves and branches per unit of groundarea. LAI was measured by taking four readings below the canopy at four cardinal points.A view restrictor, covering 315◦ of the sensor’s lens was used. Measurements were takenbetween 0600 and 0800 h on a cloudy day. Trunk circumference was measured every year at


about 50 cm above ground. Average canopy diameter for each tree was calculated by mea-suring the distance from the trunk to the edge of the canopy on at least four sides of the tree.

2.7. Fruit growth

Each week, tagged fruits were monitored for growth by recording length, breadth andthickness and fresh weight was estimated using the equation presented above. In 2000, atotal of 30 fruits per irrigation× flowering treatment were tagged to monitor fruit growth(totaln = 120). This number was increased to 60 (totaln = 240) in 2001 and to 140 (totaln = 560) in 2002. Analysis of fruit growth rates was performed only on fruits that remainedon the tree until the end of the season and reached commercial size.

2.8. Statistical analysis

For all measurements of gas exchange, and chlorophyll fluorescence on each samplingdate the observations on individual leaves per tree were averaged to calculate a singlevalue per tree. Regression analyses were performed on these tree means for each variableseparately. In these analyses, the effects of irrigation, flowering treatment and time of day,and their interactions were investigated. The fitted models were used to calculate meanvalues for each treatment at each sampling, adjusted to time 1100 h. Stomatal conductance(gs), and proportion of flowering were log transformed before analysis using log[(%x +5)/(105− %x)], wherex is the variable to be transformed; all other measurements did notrequire transformation. Tree water use, soil water content and fruit relative growth rateswere compared among treatments on each date. All statistical analyses were performedusing GenStat for Windows (2000) (VSN International, Oxford, UK).

3. Results

3.1. Soil water status

In 2000, pre-flowering irrigation started 46 days before the start of normal irrigation(Table 1). At time of flowering (day of year: DOY 175) volumetric soil water content in thetop 50 cm of the soil profile was significantly higher in XTI than in NI (Fig. 1a). Within the NIand XTI treatments, throughout the whole pre-flowering period VSWC around MFT treeswas significantly higher than around PBZ trees, but P-NI and M-NI became similar whenthe soil water deficit developed towards flowering time. After the start of the NI treatment onDOY 174, soil water content increased in all treatments and differences between irrigationand flowering treatments disappeared within 2 weeks (Fig. 1a).

In 2001, pre-flowering irrigation was applied about 30 days before normal irrigation(Table 1). On DOY 148, VSWC around XTI trees was 0.082 (23% relative available watercontent, RAWC) and 0.078 (14% RAWC) for PBZ and MFT trees, respectively, comparedto 0.107 (51% RAWC) and 0.117 (84% RAWC) for the same trees in 2000 (Fig. 1b). HigherVSWC levels occurred in the XTI treatment from DOY 149 until DOY 177, when the NItreatment started.


DOY

100 120 140 160 180 200 220 240 260 280 300

VS

WC

in t

he

top

50

cm (

%)

0.06

0.08

0.10

0.12

0.14

VS

WC

in t

he

top

50

cm (

%)

0.06

0.08

0.10

0.12

0.14(a)

(b)

NIXTI

XTI NI

bore failure

P-XTIP-NIM-XTIM-NI

Fig. 1. Volumetric soil water content in the top 50 cm of soil around mango trees (cv. Kensington Pride) receivingtwo flowering treatments (MFT, PBZ) and two irrigation treatments, normal irrigation and pre-flowering irrigation;seeSection 2for description of the treatments: (a) 2000 and (b) 2001. Each symbol represents the mean of atleast four trees and bars represent standard error of the mean. Arrows indicate when the NI and XTI treatmentsstarted.

3.2. Whole-tree water use

In 2000, before any irrigation was applied, PBZ trees used about 60% more water thanMFT trees (Fig. 2a). After the start XTI treatment (DOY 128), irrigated trees maintainedsignificantly higher levels of sap flow than that of the trees in the NI treatment. By DOY171, sap flow on M-NI trees was 65% of M-XTI trees, whereas sap flow of P-NI trees was


Wat

er u

se (

L d

ay-1

tre

e-1)

10

20

30

40

50

60

70

DOY

100 120 140 160 180 200 220 240 260 280 300

Wat

er u

se (

L d

ay-1

tre

e-1)

0

10

20

30

40

50

60

(a)

(b)

NIXTI

XTI NI

bore failure

P-XTIP-NIM-XTIM-NI

Fig. 2. Whole-tree water daily use (l per day per tree) of mango trees (cv. Kensington Pride) receiving two floweringtreatments (MFT, PBZ) and two irrigation treatments, normal irrigation and pre-flowering irrigation; seeSection 2for description of the treatments: (a) 2000 and (b) 2001. Each symbol represents the mean of at least four treesand bars represent standard error of the mean. Arrows indicate when the NI and XTI treatments started.

51% of P-XTI trees. After the start of the normal irrigation, NI trees used about 20% lesswater than XTI trees for the rest of the season.

In 2001, XTI trees used more water than NI trees from DOY 152 to 171 (Fig. 2b). Afterthe recharge of the soil profile by DOY 199, M-XTI and M-NI trees used similar amount ofwater until around DOY 211 but P-NI trees used less water than P-XTI trees until the end of


the season. From DOY 214 until harvest, it seemed that NI trees used less water than XTItrees, however these differences were not reflected in relative soil water content (Fig. 1b)because of greater uptake at depths below 50 cm.

3.3. Leaf gas exchange and chlorophyll fluorescence

3.3.1. CO2 assimilationNI trees had significantly lowerAlsat rates than XTI trees on DOY 149, 179 and 254 of

2000, and on DOY 267 of 2001 but not on other dates (Fig. 3). In 2000, MFT trees hadlowerAlsat than PBZ trees on all dates except on DOY 254 when reduction ofAlsaton P-XTItrees was observed. In 2001 MFT trees showed significantly higherAlsat than PBZ trees onDOY 143, 243 and 267 (Fig. 3).

In 2002, gas exchange measurements were made on DOY 146, 199 and 211 on six PBZtrees (three trees from each irrigation treatment). XTI treatment did not induce a significantincrease onAlsat until DOY 211(P = 0.057).

3.3.2. Stomatal conductanceStomatal conductance (gs) was significantly lower on NI trees than on XTI trees on DOY

149, 179 and 254 of 2000, and on DOY 143 of 2001 but not on other dates (data not shown).MFT had lowergs than PBZ trees on all dates except on DOY 149 and 179 of 2000

and on DOY 143 of 2001. After DOY 254 of 2000 when an interaction irrigation×

DOY

Als

at (µ

mo

l m-2

s-1

)

0

2

4

6

8

10

M-NIM-XTIP-NIP-XTI

2000 2001

120 150 180 210 240 270 120 150 180 210 240 270

Fig. 3. Light-saturating CO2 assimilation rate (Alsat, �mol CO2 m−2 s−1) of tagged leaves of cv. Kensington Pridetreated with flowering treatments, MFT or PBZ during 2000 and 2001 season and receiving normal irrigation orpre-flowering irrigation. Measurements were made between 0900 and 1300 h and each point is the estimated valueat 1100 h of three or five trees and 10 leaves per tree. Bars represent the standard error of the mean.


DOY

172 199 240 267

Rel

ativ

e E

TR

(µm

ol m

-2 s

-1)

20

30

40

50

60

70

80M-NIM-XTIP-NIP-XTI

Fig. 4. Calculated electron transport rate (�mol e m−2 s−1) of sunlit leaves measured in 2002 in mango trees ofcultivar KP receiving normal irrigation or extra irrigation and two flowering treatments, MFT, PBZ. Data werecollected between 900 and 1200 h. Each point represent the average of 20 leaves per tree and between 5 and 10trees per treatment.

flowering treatment was observed, all MFT showed highergs than PBZ trees. The in-teraction of flowering× irrigation treatment was due to reducedgs in P-XTI trees comparedto NI-P trees (data not shown). There were no significant differences ings in 2002, buton DOY 211 the difference was nearly significant (NI= 0.057 mol H2O m−2 s−1; XTI =0.075 mol H2O m−2 s−1, P < 0.058).

3.3.3. Chlorophyll fluorescenceOn DOY 172 of 2002, there was no effect of irrigation treatment on chlorophyll fluores-

cence of PBZ trees (Fig. 4). On DOY 199 and 240 ETR andφpsII were significantly higherin XTI trees than NI trees, but differences disappeared by DOY 267 (Fig. 4).

3.4. Flowering intensity

In 2000, MFT trees had uniform and higher level of flowering than PBZ trees (MFT=92%, PBZ= 56%,P < 0.001). There was not effect of irrigation, but the interaction offlowering×irrigation was significant(P = 0.041), as P-XTI trees flowered less than M-XTIand M-NI trees, but similar to P-NI trees. Flowering in P-XTI trees was characterised byhaving a high coefficient of variation (cv. P-XTI= 82.9; cv. P-NI= 23.9), as some treesdid not flower while others flowered close to 100%.

Flowering intensity in 2001 and 2002 was lower than in year 2000 in all trees. Nostatistically significant effects of flowering treatment or irrigation were detected in 2001 or


2002 in the intensity of flowering. However, higher variation was observed in MFT treescompared to year 2000, regardless of the irrigation treatment, and the highest coefficient ofvariation was observed in XTI trees (cv. XTI= 62.3; cv. NI = 41.9). The proportion ofcanopy flowering was around 42% in 2001 and 48% in 2002.

3.5. Foliage density and tree growth

The MFT treatment resulted in severe reduction in foliage density in the first year oftreatment. On DOY 135 of year 2000 leaf area index of MFT was 1.48 (S.D. = 0.61n = 8)and average trunk circumference across all treatments was 55 cm(S.D. = 2.16, n = 7). Bythe end of the experiment, XTI trees had significantly higher LAI (XTI= 3.7, NI = 2.9,P < 0.016) and larger tree trunk circumference (XTI= 79.3 cm, NI= 71.6 cm,P < 0.005)than NI trees. Average LAI of PBZ tended to be higher than LAI of MFT trees (MFT 3.1,PBZ 3.4,P = 0.061). No effect of irrigation or flowering treatment was observed in canopydiameter at the end of the experiment, perhaps due to tree pruning after harvest every year.

3.6. Fruit growth and fruit retention

Across all 3 years of the experiment overall fruit retention was low. The number of fruitsthat remained attached and reached commercial size was 19 out of 120 in 2000, 89 out of240 in 2001 and 78 out of 560 in 2002. The effect of irrigation was very slim as 22.4%of tagged fruits from XTI and 18.5% NI trees remained in the tree. From all tagged fruits,22.4% of fruits tagged in MFT trees and 19.5% in PBZ trees reached commercial size(Table 2).

3.6.1. Fruit growthIn 2000, reliable assessment of fruit growth rates was not possible due to high fruit drop. In

2001, analysis performed on fruits that completed the season and reached commercial sizeindicated that XTI fruits were 32% bigger than NI fruits at time of tagging (NI= 24.9 gversus XTI= 36.6 g,P = 0.022) but difference was 18% at harvest (NI= 415.2 g versusXTI = 508.2 g,P = 0.018) (Fig. 5). MFT fruits were 29% bigger than PBZ fruits (MFT22.6 g versus PBZ 16.2 g,P = 0.03; 29% difference) at time of tagging, but difference was

Table 2Proportion of tagged fruits retained until the end of the season from mango trees cv. Kensington Pride treatedwith two flowering treatments, MFT and PBZ and two irrigation treatments, normal irrigation and pre-floweringirrigation (n is the number of tagged fruits)a

Year 2000 2001 2002

n % n % n %

M-NI 30 26.7 60 33.3 140 13.6M-XTI 30 13.3 60 51.7 140 20.7P-NI 30 6.7 50 20.0 126 11.1P-XTI 30 10.0 59 47.5 140 11.4

a M-NI: MFT-normal irrigation; M-XTI: MFT, pre-flowering irrigation; P-NI: PBZ-normal irrigation; P-XTI:PBZ, pre-flowering irrigation.


DOY

200 220 240 260 280 300

Fre

sh W

eig

ht

(g)

0

100

200

300

400

500

600

Fre

sh W

eig

ht

(g)

0

100

200

300

400

500

600

700

M-NIM-XTIP-NIP-XTI

(a)

(b)

Fig. 5. Fruit growth in 2001 (a) and 2002 (b) of mango trees of cultivar KP receiving normal irrigation or extrairrigation and two flowering treatments, MFT, PBZ. Only fruits that were tagged early in the season and remainedin the tree until the end of the season were used. Each point is the average of a variable number of fruits (>5 and<10). Bars represent the standard error of the mean.

17% at harvest (MFT 507.7 g versus PBZ 441.3 g,P = 0.031) (Fig. 5). On all dates, NIfruits had higher relative growth rates than XTI fruits, but there were not significant effectof flowering treatment (Fig. 5).

In 2002, fruits were tagged on several dates on panicles that were tagged at the beginningof flowering. Fruits were grouped into age classes before the analysis of relative fruit growthrate was performed at various dates. XTI fruits grew faster than the NI fruits when fruits wereless than 10 days old (NI= 0.178 g per day versus XTI= 0.247 g per day,P = 0.032)


and between 50 and 60 days old (NI= 0.104 g per day versus XTI= 0.149 g per day,P = 0.048). The NI fruits grew faster than the XTI fruits when fruits were both between 10and 20 days old, and between 70 and 80 days old. PBZ fruits grew faster than MFT fruits(PBZ = 0.042 g per day versus MFT 0.0314 g per day,P = 0.014) only when fruits werebetween 70 and 80 days old. No significant effect of irrigation or flowering treatment wasfound in the relative growth rate of fruits on other dates.

3.7. Fruit yield

3.7.1. Fruit numberFruit number was positively related to intensity of flowering. Significant effects of flow-

ering and irrigation treatments were observed in 2000 only. In the first year of treatment,MFT trees produced more fruits per tree than PBZ trees (152.3 versus 69.4,P < 0.001),and NI trees had significantly more fruits than XTI trees(XTI = 99.2, NI = 122.2, P =0.019), reflecting differences in level of flowering observed between the two flowering treat-ments. In 2001 and 2002 no significant effects of either irrigation or flowering treatmentwere detected on fruit number, and the average number of fruit per tree was 50.5 over the2 years.

3.7.2. Fruit size at harvestSignificant effects of irrigation, flowering and their interaction were detected in 2001 and

2002 but not in 2000 (Table 3).In 2001, average fruit size of XTI trees was bigger than fruits from the NI trees (XTI=

255 g versus NI= 162 g,P < 0.001) and MFT fruits were bigger than PBZ fruits (MFT=222 g versus PBZ= 194.1 g,P < 0.001). In 2002, fruits from XTI trees were bigger thanfruits from NI trees (XTI= 269 g versus NI= 230 g,P < 0.057), fruits from MFT treeswere bigger than fruits from PBZ trees (MFT= 273 g versus PBZ= 226,P < 0.002) andfruits of the P-XTI trees were significantly bigger than fruits of the P-NI trees (P-XTI=260.6 g, NI = 191.4 g,P = 0.031).

3.7.3. Fruit yieldIn 2000, fruit yield (kg per tree) was significantly affected by flowering treatment and

irrigation treatment. NI trees had significantly higher yield than XTI trees (XTI= 31.7 kg,NI = 39.7 kg, P < 0.05), and MFT trees had higher fruit yield than PBZ trees (MFT=48.4 kg per tree, PZB= 23.3 kg per tree,P < 0.05).

In 2001 and 2002 fruit yield was lower than observed in 2000. In 2001, there was no effectof flowering treatment or irrigation but there was a significant interaction of irrigation×treatment(P = 0.023) as the M-NI trees (4.7 kg per tree) yielded much lesser than M-XTItrees (15.6 kg per tree), whereas the yields were similar for P-XTI and P-NI trees (average11.3 kg per tree). In 2002, there was not significant effect of flowering treatment but averagefruit yield was 15.3 and 11.9 kg per tree for XTI and NI trees, respectively(P = 0.067).

3.7.4. Effect of flowering intensity on fruit number and yieldThe effect of irrigation on fruit number and fruit yield was further explored by looking

at the relationships between flowering intensity and fruit number (Fig. 6), and flowering

A.G

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tal./S

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tiaH

orticu

ltura

e1

02

(20

04

)1

89

–2

11

203

Table 3Average fruit weight (g) from fruits measured from six quadrants of 1 m2 each around the perimeter of the treea

Year F from ANOVA Observed means

Flowering Irrigation F× I M-NI M-XTI P-NI P-XTI

2000 2.13 nsb 0.26 ns 0.0 ns 321 a (n = 236) 310 a (n = 215) 338 a (n = 151) 330 a (n = 102)2001 6.94∗∗ 15.68∗∗∗ 4.48∗ 150 c (n = 32) 295 a (n = 76) 173 b (n = 20) 215 a (n = 80)

2002 9.62∗∗ 3.64† 4.67∗ 269 a (n = 89) 277 a (n = 115) 191 b (n = 90) 261 a (n = 86)

a n is the number of fruits measured. Within a row numbers followed by the same letter were not statistically significant at 5% (LSD test). M-NI: MFT-normalirrigation; M-XTI: MFT, pre-flowering irrigation; P-NI: PBZ-normal irrigation; P-XTI: PBZ, pre-flowering irrigation.

b Non-significant.∗ Significant at 5% level.∗∗ Significant at 1% level.∗∗∗ Significant at 0.1% level.† P = 0.057.


Fru

it N

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0

40

80

120

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200

M-NIM-XTIM-NI 2000M-XTI 2000

% Flowering

0 20 40 60 80 100

Fru

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0

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160

P-NIP-XTI

(a)

(b)

Fig. 6. Relationship between proportion of flowering and total fruit number per tree at commercial harvest timefor: (a) MFT treated trees receiving normal irrigation (solid line,y = 0.699x (S.E. = 0.074),r2 = 0.814∗∗) orextra irrigation (dotted line,y = 1.221x (S.E. = 0.055),r2 = 0.869∗∗). Data for 2000 were not included in theregression lines and (b) PBZ treated trees receiving similar irrigation treatment in years 2000–2002 (y = 1.221x(S.E. = 0.055),r2 = 0.869∗∗).

intensity and fruit yield (Fig. 7) over the 3 years of the experiment. Data from MFT treesin 2000 were excluded from the regression line because variability in flowering intensitywas low. For each flowering irrigation treatment, the linear regression between floweringintensity and fruit number was highly significant. The slopes of the linear relationships for


Fru

it y

ield

(kg

tre

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M-NIM-XTIM-NI 2000M-XTI 2000

% Flowering

0 20 40 60 80 100

Fru

it y

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(kg

tre

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30

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50

60P-NIP-XTI

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(b)

Fig. 7. Relationship between proportion of flowering and fruit yield (kg fruit per tree) at commercial harvest timefor: (a) MFT treated trees receiving normal irrigation (solid line,y = 0.160x (S.E. = 0.025),r2 = 0.652∗∗) orextra irrigation (dotted line,y = 0.345x (S.E. = 0.017),r2 = 0.851∗∗). Data for 2000 were not included in theregression lines and (b) PBZ treated trees receiving similar irrigation treatments in years 2000–2002 (y = 0.345x(S.E. = 0.017),r2 = 0.851∗∗).

P-XTI, M-XTI and P-NI did not differ and were higher than the slope of the relationshipfor M-NI (Figs. 6a and 7a). The proportion of flowers that ended up in fruits at the end ofthe season was higher in XTI trees from MFT and PBZ trees than in M-NI trees. A similartrend was observed when the linear regression between flowering intensity and fruit yieldwas analysed (Fig. 7).


4. Discussion

The effect of pre-flowering irrigation on leaf gas exchange, whole-tree water use, flower-ing and fruit yield of mango trees treated with PBZ and MFT was investigated on a 3-yearfield trial. In our experimental design, we deliberately excluded control trees because previ-ous research indicated that lack of chemical or physical treatment to induce flowering in thiscultivar in the warm tropics more likely results in vegetative growth (González and Blaikie,2003; Blaikie et al., 2004). Furthermore, in this experiment we did not attempt to sepa-rate the effects of the individual components of the MFT treatment, naming strangulation(twine), girdling and morphactins. Instead, we aimed to investigate the effect of reducingwater stress before flowering on MFT treated trees and analyse its effect on fruit yield.

4.1. Whole-tree water use

In 2000, the availability of extra water around MFT trees did not result in higher wateruse by MFT trees. The lower water use in MFT trees (Fig. 2a) is likely the result of bothsmaller leaf area index, reduced leaf gas exchange as reported byGonzález and Blaikie(2003)and reduced root growth in the first year of treatment (Blaikie et al., 2004).

In 2001 whole-tree water use followed the pattern of soil available water (Fig. 2b),and differences in whole-tree water use among the flowering irrigation treatments wereless than in 2000, perhaps reflecting less differences in canopy structure between MFTand PBZ trees in the second year of the experiment as well as higher leaf gas exchangein MFT trees in year 2 after treatment as reported byGonzález and Blaikie (2003)andobserved in this experiment, and probably better root functioning after the first year oftreatment.Blaikie et al. (2004)described less root growth in MFT trees during the first year oftreatment.

4.2. Flowering

Irrigation between 30 and 45 days before flowering did not significantly reduce flower-ing intensity in any of the flowering treatments. However, XTI trees showed higher level ofvariability on flowering than NI trees. In this study, all trees were subjected to water deficitbefore flowering, but stress was less severe and shortened by applying irrigation earlier(XTI treatment) than the normal practice of waiting until 65% of the canopy surface area isflowering (NI treatment). Our results are consistent with those reported bySingh and Ram(1996), who found that beginning irrigation 30 days before flowering did not reduce flower-ing on mango trees growing in the tropics. In contrast,Lu and Chacko (2000)reported that inthe tropical Darwin region, 72% versus 6% of the terminals flowered in pre-flowering waterstressed and non-stressed Kensington Pride trees, respectively. Similarly, in the sub-tropicsBally et al. (2000)reported that pre-flowering water stressed trees of Kensington Prideflowered 36% more than non-stressed trees. However, cool temperatures during the floralinduction period might have promoted flowering in non-stressed trees in the sub-tropics.It is important to note that none of the previous studies on the effect of water deficit onflowering intensity of Kensington Pride applied chemical treatment to induce flowering.In this study, shortening the period of water deficit before flowering did not decrease the


intensity of flowering, more likely because of the positive effect of the chemical treatments,MFT and PBZ, on promoting flowering.

MFT promoted high flowering activity in the first year of treatment but was similar toPBZ trees in subsequent years.Blaikie et al. (2004)reported that MFT treatment resultedin high level of flowering activity in the first year of treatment, but from the second yearonwards MFT behaved similarly to control trees receiving no flowering treatment; however,in their study MFT treatment was applied only in the first year and trees did not receiveany extra flowering treatment in the following years. In this study flowering of MFT andPBZ trees was similar after the second year of treatment because the farmer applied PBZto all trees in the second and third year of the experiment. The high level of variability inflowering of trees without flowering treatment or in response to PBZ is still a major concernfor the industry and has been previously reported (Leonardi et al., 1999; Kulkarni, 2004;González and Blaikie, 2003; Blaikie et al., 2004). Unfortunately, not effective alternativeshave been identified to overcome this problem.

4.3. C assimilation at leaf and canopy levels

In this study, we aimed to minimise the reduction ofAlsat andgs during the dry seasonby reducing the extent of water stress at flowering time. We found that beginning irrigation30–46 days before flowering did not eliminate the decrease ofAlsat andgs at flowering(Figs. 3 and 4). AlthoughAlsat increased in response to irrigation treatments, the incrementwas not substantial enough to bringAlsat rates to the high values observed in the wet monthsas reported byGonzález and Blaikie (2003), suggesting that other factors besides wateravailability play an important role.

In 2002, we observed that ETR followed a trend similar to that ofAlsat, and both reachedthe lowest value near flowering time in XTI and NI trees (Fig. 4). A parallel increase inETR andAlsat of more than 100% from DOY 199 to DOY 267 suggests that, before and atflowering, C assimilation is not only limited by stomatal aperture but that there seems to bedown regulation of photosynthesis at this time. Reduction in leaf photosynthesis of mangoleaves on the flowering terminals was observed in some Indian cultivars (Shivashankara andMathai, 2000) and in cv. IRFA growing in the French Reunion island (Urban et al., 2003).This is an interesting observation that requires further characterisation as to why such alarge drop of photosynthetic capacity at time of flowering occurs, and very importantly, howcommon is this trend within the mango germplasm. We have observed that during the dryseason, even on well-watered trees in the Darwin region, low stomatal conductance prevailsmost of the day in response to high VPD values.Flexas et al. (1999)reported that ETRvalues declined after prolonged stomatal closure (<40 mmol H2O m−2 s−1) under severewater deficit. Whether prolonged low stomatal conductance, due to high VPD, results indown regulation of photosynthesis and reduced ETR values in irrigated mango trees iscurrently under investigation.

Differences in instantaneous measurements ofAlsatandgs between XTI and NI trees weremarginal in most of the dates measured. One possible reason for the lack of substantial dif-ferences between irrigation treatments is the effect of high vapour pressure deficit (VPD)on stomatal aperture.Alsat is highly correlated togs, which in turn is highly responsive toVPD in this cultivar (González and Blaikie, 2003). Stomatal conductance remains very low


at VPD> 2.5 kPa (González and Blaikie, 2003) and in this experiment the bulk of our mea-surements were taken after 9:30 a.m. when VPD> 2.1 kPa, and consequently differencesin leaf CO2 assimilation between irrigation treatments were marginally detected becauseVPD effects prevailed. We recently found that differences between irrigation treatmentscould be more easily detected if gas exchange measurements were done before VPD inducestomatal closure on XTI trees (González and Lu, unpublished results).

Besides the effect of high VPD on stomatal closure, it is possible that root signals werepartially responsible for stomatal closure in this cultivar. Although the area wetted by thethree under-tree sprinklers of the XTI treatment was larger than that of the one under-treesprinkler of the NI treatment, roots of the mango trees are known to extend beyond thecanopy drip-line especially during the wet season when the entire orchard is fully wet(Lu, unpublished results). During the dry months roots beyond the irrigated zone mightsurvive, sensing dry soil, and sending signals (possibly ABA) resulting in stomatal closure(Davies et al., 2002). Further experiments where the area beyond the canopy drip-line isfully irrigated during the dry months might help in the understanding of the role of thosesignals, if they exist in mango.

XTI trees used more water than NI trees (Fig. 2), regardless of the flowering treatment,throughout the whole fruiting season. Consequently, XTI trees may have assimilated sig-nificantly more carbon than the NI trees. The effect of extra irrigation on whole-tree wateruse was detected in MFT and PBZ treated trees in 2000 and 2001 (Fig. 2), and possibly in2002 too as higherAlsat, andgs in leaves of XTI trees were observed. Although whole-treewater use was not measured in 2002, sap flow is an integrated continuous measurementof whole-plant water use (Wullschleger et al., 2000), which in turn has a good correlationwith canopy transpiration and stomatal conductance (Lu et al., 2003) and consequentlywhole-tree C assimilation. Data of tree growth indicate that XTI grew more than NI trees.In 2000, tree trunk diameter was quite uniform across all different flowering irrigation treat-ments reflecting homogeneous tree size at the start of the experiment. However, at the endof the experiment, XTI trees had larger LAI, larger trunk circumference, and slightly largercanopy diameter than NI trees. Because XTI did not carry excess fruits compared to NI trees,the extra C assimilated by XTI trees was partitioned to tree growth rather than to fruit load.

4.4. Fruit yield

This study provides evidence for an effect of pre-flowering irrigation on fruit retention(Fig. 6andTable 2) and fruit yield without decrease in intensity of flowering. On MFT treatedtrees in 2001 and 2002, the linear relationships between the proportion of the canopy thatflowered and both fruit number and fruit yield had steeper slopes for the M-XTI trees thanthe M-NI trees (Figs. 6a and 7a). As a result, M-XTI trees that flowered at a given intensityproduced more fruits and greater yield than M-NI trees that flowered at similar intensity.The reduction in fruit retention on the M-NI trees in 2001 and 2002 may have resulted fromfactors such as lower carbon availability at the time of flowering as indicated by lower leafphotosynthetic parameters (Figs. 3 and 4), lower LAI, lower whole-tree water use (Fig. 2),and possibly low tree carbohydrate reserves because of high fruit yield observed in year2000. Higher leaf photosynthesis and higher whole-tree water use throughout the seasonmight have played a role in enhancing the conversion of flowers to fruits in M-XTI trees. A


simulation model on peach trees indicated that fruit growth and C reserves are more affectedby water stress at low leaf/fruit ratio than at high leaf/fruit ratio (Ben Mimoun et al., 1999).

In the MFT trees, high flowering and high fruit yield was observed in the first year oftreatment, despite the low photosynthetic activity in the months preceding flowering and atflowering time. However, in the first year partitioning of MFT trees was altered by girdling,as sugars accumulate in the top of girdled trees (González, unpublished results;Li et al.,2003) and might have influenced flowering intensity. High demand for photo-assimilates atthe time of flowering and vegetative growth has been found in several mango cultivars. Forinstance, in cultivar Nan Dok Mai starch levels were reduced during vegetative flush andflowering activity (Phavaphutanon et al., 2000). In cultivar sensation, stored starch levelswere drastically depleted during spring and summer when vegetative growth, floweringand fruit growth occurred (Davie et al., 2000). Kensington Pride grew more at 35◦C/25◦Cday/night temperatures, at the expense of accumulating reserves (Whiley et al., 1989).

In 2000, the irrigation and flowering treatments had no effect on the mean size of harvestedfruits (Table 3). In 2001, XTI trees produced larger fruits than NI trees and the differencewas greater on MFT trees than PBZ trees. In 2002, P-XTI trees produced larger fruits thanP-NI trees, but fruits on MFT trees were not different in size from those on P-NI trees. Therewas no relationship between fruit number and fruit size on any of the flowering× irrigationtreatments over the three years of the experiment perhaps due to the low fruit load in trees inthis experiment.Bally et al. (2002)reported that Kensington Pride trees of about 6–7 yearscarried close to 200 fruits per tree (80–90 kg per tree) in the sub-tropical region of Australia,which is more than three fold the fruit load observed in some years in this experiment.

The flowering behaviour differed from year to year and might have influenced fruitsize at harvest. In 2000, flowering occurred over a shorter period with a distinct peak ofmaximum activity, while in 2001 and 2002 there was an extended flowering period with nodistinct peak. Fruit size measured at time of commercial harvest represents a sample of thepopulation of fruits in the tree. This fruit population is composed of several fruit cohorts,which are related to extent of the flowering season. In other words, the commercial harvestincludes fruits that set early in the season as well as those that set late in the season. NI treesflowered about a week earlier than XTI trees, and, because fruit retention was generallylower on NI trees than XTI trees (Table 2), it is possible that in the NI treatment, earlyflowers had a lower chance of setting fruits than flowers that opened after irrigation wasinitiated. Variation in fruit size showed that the trees were carrying both near fully maturefruits and a big proportion of immature fruits. Higher coefficient of variation in fruit weightwas observed in 2001 and 2002 in NI trees than XTI trees, perhaps reflecting the extendedflowering season in these 2 years. As a consequence, the mean fruit size was smaller thanfruits from the XTI treatment because the fruit population of the NI trees might be enrichedwith younger fruits at time of harvest.

5. Conclusions

Our results indicate that pre-flowering irrigation increased the overall photosyntheticactivity of the tree at time of flowering. Although the effect measured at the leaf levelwas small andAlsat rates were well below the rates observed in the wet season, significant


effects were detected at the whole-tree level as measured by whole-tree sap flow. The effectof extra-irrigation resulted in more fruits per tree at similar level of flowering in the MFTtrees in years 2001 and 2002. In all treatment–year combination, except in MFT trees inthe first year of treatment, pre-flowering irrigation resulted in more variable flowering inXTI trees. We argue that the lower photosynthetic activity, and perhaps C reserves, in M-NItrees at time of flowering reduced fruit set and retention in these trees. The relationshipamong intensity of flowering and fruit number was similar for all PBZ trees and M-XTItrees reflecting a similar conversion of flowers to fruits among these trees. It remains tobe seen what would be the effect of pre-flowering irrigation on fruit retention on treeshaving intensive flowering. Intense flowering is highly costly in terms of C use and nutrientallocation to those structures. Farmers should exercise caution when imposing prolongedwater stress to induce flowering, as a penalty on fruit set and retention is likely to occur ontrees where C reserves are depleted and the tree photosynthetic capacity is reduced.

Acknowledgements

Special acknowledgements are offered to Alan Niscioli, Jon Schatz and other technicalofficers of the Darwin Laboratory for their assistance in collecting field data. The cooperationand willingness of the mango farmers Mr. Alan and Mark Buckley is highly appreciatedsince without their help this experiment would not have been possible. Thanks to Dr. YaffaGrossman for helpful discussion during the analysis of this experiment. Thanks to S. Blaikiefor helpful comments on the manuscript.

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