Agric. Rev.. 26 (3) • 173 - 186. 2005

BORON NUTRITION IN CITRUS - CURRENT STATUSAND FUTURE STRATEGIES - A REVIEW

A.K. Srivastava and Shyam Singh

National Research Centre for Citrus.Amravati Road. Nagpur - 440010. India

ABSTRACTGrowing scale of B-deficiency, second to Zn-deficiency, has imparted a greater significance to

B-amendment. An adequate B-amendment ensures not only ample fruit set, but guarantees optimumfruit yield with excellent fruit quality in terms of juice content, ratio between total soluble solids andacidity, and fruit peel colour. A critical analysis of work accrued in the past from worldover usingcitrus as a test crop suggested, has revealed a variety of soil-plant test values, fit to diagnose B­constraints in citrus orchards established under varying edaphological conditions. Large variationin B-application through soil and foliar spray showed that both the methods of fertilization areequally efficient in meeting crop B-requirement. However, the combination of two methods offersa better scope to raise a nutrient constraint free production.

Boron is one of the most importantmicronutrients, requires to be researched moreelaborately to have answer to many evasiveissues associated with B-nutrition. Thisbecomes even more imperative followingemerging B- deficiency from both acidic as wellas alkaline soils irrespective of their mineralogy.Citrus trees in this regard, is no exception. Theresultant widening gap between B-containingfertilizers applied and rpmoved by the plantand fruits annuaily takes an alarming shapewhen a plant enters into reproductive phasefollowing the long juvenility period of citruswhich varies from 4-5 years in budded plantsto as high as 8-9 years in seedling plants.

Next to zinc, boron deficiency is widespread in many soils, especially in northeastIndia and to a lesser extent in other citrus beltslike Marathawada and Vidarbha region ofMaharashra, northwest and south India leadingto low crop yields. Of the 36825 surfacesamples analyzed from length and breadth ofIndia, 33% soil samples were found to bedeficient in available boron. Extensivedeficiency (39-68%) has been recorded in redand lateritic soils and leached acidic soils ofhot semi-arid ecoregion with shallow andmedium black soils of, hot sub-humid ecoregionwith alluvium derived soils, hot sub-humid to

humid (inclusion of perhumid) ecoregion withalluvium derived soils, warm perhumidecoregion with brown and red hill soils andwarm perhumid ecoregion with red and lateriticsoils and highly calcareous soils of hot sub­humid ecoregion with alluvium derived soils andwarm sub-humid to humid (Sakal. 2001;Tiwari, 2002a). These are the combinationsof soil and climate where best quality citrus iscommercially produced in other countries likeBrazil, China, Japan etc. as well.

Role of B-nutrition in maintainingsustained fruit yield and quality are nowincreasingly observed. The essentiality of boronfor higher plants was demonstrated as earlyas in 1910 (Agulhon, 1910). Since then, anumber of reports have appeared ascribingnumerous plant physiological roles to boron.The earlier literature reviewed by Gauch andDugger (1953) and interpreted in the light ofsome of the more evidences. Boron for1']1_scomplexes with certain carbohydrates blItnatural sugar-borate complexes in plants yetto be identified. However, a role of B- inflowering, pollen tube growth, N-metabolism,hormonal activity. and maintenance of Ca insoluble form is well established (Srivastava andSingh, 2003a). It is one such essential nutrientwhich occurs as a non-ionized molecule in soil

174 AGRICULTURAL REVIEWS

solution and plant root absorbs B as boric acidmost efficiently. Alt and Schwarz (1973)observed that B is..§bsorbed as a molecule\andis passively distributed with transpiration streamto different plant organs. An analysis onchronological developments took place invarious aspects of B-nutrition with citrus asperennial test crop, was systematicallyreviewed to harness the potential benefitstowards quality citrus production accruing outof B-fertilization.

Diagnosis of B-ConstraintsA number of methods are frequently

used to identify B-constraints in citrus orchards.These are deficiency symptoms developed onfoliage or fruit, leaf analysis, soil analysis andbiochemical markers. Each one of them hasits own merits or demerits.

Morphological deficiency symptor.1sAs early as Haas and Klotz (1931)

described the boron deficiency symptomsexhibited by experimen,al citrus plantsgrowing under controlled nutrient conditions,characterised by the mature leaves turningthickened and brittle, with downward curlingat right angles to the midrib and are yellowishto bronze in colour. The veins are enlargedand corky: and split on the upper surface.Shoots from multiple buds produce a cabbagehead type of growth. Splits developed in thebark of the internodes through which ambercoloured gum oozed and woody tissues areexposed. A marked reduction of growth andin a few instances, death of the tree may occur.In most instances. the symptoms disappearedand the trees recovered fully in two years' timefollOWing the B-fertilization. Morris (1937.1938) later reported for the first occurrenceof hard fruit disease in citrus in addition ofsplitting and corking of leaf veins. In citrus,boron deficiency usually results in gum-soakedspots in the albedo of the fruit, lumpiness anda hard, dry fruit, prone to drop off V0maturely(Smith and Reuther. 1949).

Many authors have concluded that thehigh concentration of sugars in the leaves ofboron B-deficient plants, is due to a breakdownof the conduction tissues with a concomitantreduction in translocation. Most of these studieswere conducted on plants showingmorphological symptoms. There is someevidence (Gauch and Dugger. 1953; Mitchellet ai., 1953) that boron deficiency has an effecton the translocation of sugars, and possiblyother organic compounds, before vasculardearrangement occurs. It is reported that borondeficient plants loose their capacity to respondto gravity. This would indicate that there issome relation between boron and theproduction. translocation. or action ofhormones. Mitchell et al. (1953) found that 2,4-D and other growth modifying substancesare more rapidly translocated when boron andsugars are applied. A decrease in thetranslocation of various compounds could beinvolved in the appearance of boron deficiency ,symptoms but, as with hormones. the effectof boron on the translocation of carbohydratesmay be the main effect.

Opitz and Platt (1967) describedsymptoms of B-deficiency in citrus orchardsof central California, USA. Symptoms of Bdeficiency were more prominent during fruitmaturation. when fruit colour changes fromgreen to orange which is delayed in the areashaving gummy granules (Primo et al.. 1969).The symptoms included development ofgummy granules in the fruit albedo andexceptionally coarse leaf venation. Due to B­deficiency, fruits are deshaped. with rough andthick peel and often brown spots are observedin the pulp (Feroughi et al.. 1974). MarshGrapefrUit leaves affected by B-toxicity on theother hand are characterized by scatteredyellow spots on the upper surfaces. brownishresinous gummy spots on the lower surfaceand edge or tip-burn (Cardenas et al.. 1971).Boron toxicity .n lemon leaves is characterized

Vol. 26, No.3, 2005 175

by the typical tip burn. The base of the burnedleaf tip is in a direction at right angles to themidrib. The yellowing that precedes theburning, progresses from the leaf marginsdown toward the midrib. The green area alongthe lateral and midrib vems is the most resistantto the increasing B- concentration (Haas,1950).

Chiu and Chang (1985) described B­deficiency on fruits which consists of smallerfruit size, thicker peel, less juice, and harderthanrqature fruits. Lumpiness in fruits isconsidered a primary symptom of B-deficiency.Sometimes, the fruits may become hard,abnormal in shape and small in size. Older fruitswould be abnormal in shape with very thickalbedo containing gum deposits. Seeds fail todevelop and gum deposits are also foundaround the axis of fruits. Affected fruits arehard, dry and low in sugar and their quality ispoor. The symptoms on fruits are more reliableas hard fruit and dry due to lumps in the rindcaused by gum impregnations (Chiu andChang, 1986; Zekri, 1995). In the orchard,premature wilting of the trees occur in spite ofsufficient moisture in the soil. Subsequently,small water soaked spots or flecks appear onthe leaves which become translucent as theleaves mature. Premature shedding results insevere defoliation, dieback of trees whichassume a bushy upright growth similar to thatof Zn-deficiency. The conspicuous curling ofleaves at right angle to the midrib occurs dueto enlargement, splitting and curling of theveins. The fruits develop gum spots both inthe albedo and flesh. These spots give rise tolumps under the peel which can usually be feltwith hand (Randhawa and Srivastava, 1986).

Terminal necrotic leaves shedprematurely with internodes of terminal shootsshortened, usually rosetting with apicalmeristems blacken and die, coupled withgeneral breakdown of meristimatic tissue, androots turning short, stubby, resulting in

dwarfed and stunted plants. The flowerdevelopment and seed production are usuallyimpaired or lacking. The death of terminalgrowing points of the mc.in stem coupled withthickened leaves with tendency of leaves tocurl downward are noticeable features of B­deficiency (Zekri, 1995).

Leaf analysisAccording to Bould (1984), the leaf

analysis is based on four factors viz., i. leaf isthe principal site for the plant metabolism; ii.changes in the nutrient supply are reflected inthe composition of leaf; iii. the changes aremore pronounced at certain stages ofdevelopment than at the others; and iv. theconcentrations of the nutrient in leaf at aspecific growth stage are related to theperformance of the crop. Leaf analysis is ofthe major importance in a number of wayswhich include: i. an aid to understand theinternal functions of nutrients in the crop, ii.confirms the deficiency detected by visualsymptoms, iii. distinguishes between twonutrients which cause similar deficiency ortoxicity symptoms, iv. identifies the mineralimbalance in the absence of visible symptoms.which is not correctable with the addition of asimple nutrient, v. helps in investigating thetoxicity of the elements, vi. identify theinteractions or antagonism between thenutrients, vii. used for the diagnostic purposesin cases of the simultaneous multiple nutrientdeficiencies and/or toxicities, viii .. ascertains.whether the applied nutrients have entered inthe plant system or not, ix. identifies, whetherthe hidden hunger is causing suboptimal plantperformance, slow growth, and lower yieldand/or quality or not, x. prevents thedeficiencies rather than correcting them afterthey develop, and xi. helps in locating the areasof the incipient deficiencies during thenutritional survey.

With reference to plant analysis, whenfor the purpose of diagnosis a static criterion

176 AGRICULTURAL REVIEWS

is applied, this cannot be final as the level ofthe element varies throughout the cycle andeven during the same day, according to thetime when the sample is taken. Moreover, thephilosophy of conventional leaf analysis wasoriginally based on two starting points. It isbased on the concept of the total content of anutrient at a given moment, as an integral valuein the feeding of the said element closely linkedwith the law of the minimum, and from whichthe existence of a positive link between thesaid content and the yield is deduced (nutritionproduction curve). It is also based on theconcept of equilibrium, according to whichnutrition is not the result of individual actionsof nutrients but of their total quantity and stateof equilibrium.

A number of methods are beingemployed to develop leaf nutrient gUide whichinclude the sand culture technique, cropmodelling after generating the data throughan extensive survey of citrus orchards and fieldexperimentation. However, all these methodsare under constant scrutiny, test, and recurrentuse.

Leaf B standards: A number ofmethods are being employed to develop leafnutrient guide, which principally include thesand culture technique. crop modelling aftergenerating the data through an extensivesurvey of citrus orchards and fieldexperimentation. The first step to develop acomprehensive leaf nutrient guide is thecategorisation of leaf analysis data. A numberof recommendations from across the variouscitrus belts of world have been suggested forvarious diagnostic value of boron (Table 1).

Soil analysisSoil tests are based on the approach

which describes the amount of nutrient in adefinite chemical form viz., i. water soluble, ii.exchangeable, iii. chelated or complexed form.iv. secondary clay minerals or oxides, and v.primary minerals. The available B. therefore.

does not reflect its total content in soil.Chapman (1968) described a number of soilconditions leading to possible B-deficiencywhich consist of i. acid soils regardless of parentmaterial having B excessively leached out. ii.)soils inherently low in B, especially thosederived from igneous and metamorphicsandstone rocks, iii. alkaline soils thoughpotentially rich in total B but low in availableB, iv. laterite soils with lower silica and highFe and Mn, v. acid muck or peat soils, and vi.soils with low clay content. The soil analysismethod rests on the assumptions that rootswould extract nutrients from the soil in amanner comparable to chemical soilextractants, and that there is a simple directrelation between the extractable concentrationof nutrients in the soil and uptake by plants.(Jones et al., 1955). Optimum value of boronare in citrus orchards from across the continenthas been suggested to be 0-50-75 mg kg 1 hotwater soluble B in USA. China and Japan(Bingham and Martin, 1956; Sato et al., 1962;Bingham 1973; Quyang et al., 1984; Swietlik,1996), 1.0 mg kg'! in Egypt (Elseewi arid El­Malky, 1977) and 0.32-0.48 mg kg'! in India(Srivastava and Singh, 2003b). Chapman andKelly (1943) observed that granitic soilcontaining upto 10mg kg'! total B showed B­deficiency.

Biochemical analysisThe variable activity of phenylalanine

ammonia lyase in different plant species wasvariable, thus, indicated differentialsusceptibility of the test plants subjected toboron deficiency (Shkol'nik et al.. 1980). Sakaland Singh (1995) explained the role of B incell differentiation and development,germination and growth of pollen grains. sugartranslocation through formation of sugar­borate complex, movement of growthregulators. and lignin synthesis throughpolymerisation of phenolic compounds.,

Boron acts by forming a strong.

Tab

le1.

Lea

fbo

ron

stan

dard

s(p

pmon

dry

wei

ght

basi

s)as

sugg

este

dfo

rdi

ffer

ent

culti

vars

Citr

usva

riet

yC

ount

ryS

uppl

yle

vel

Sou

rce

Def

icie

ntL

owS

atis

fact

ory

Hig

hE

xces

s

Citr

ussi

nens

isO

sbec

kcv

.Nav

elU

SA<

20

.021

-40

50

-15

016

0-26

0>

27

0C

hapm

an(1

949a

.19

49b)

Citr

ussi

nens

isO

sbec

kcv

.N

avel

Sou

thA

fric

a<

15

.015

-30

30

-10

010

0-25

0D

evill

iers

eet

aJ.

(195

8)C

itrus

sine

nsis

Osb

eck

cv.

Val

enci

aU

SA<

20

.021

-40

50-1

5016

0-26

0>

270

Reu

ther

etaJ

.(1

958)

~S

wee

tor

ange

(Citr

ussi

nens

isO

sbec

k)A

ustr

alia

<2

020

-30

31-1

2913

0-25

0>

250

Gal

lasc

han

dPf

eile

r(1

988)

Citr

ussi

nens

isO

sbec

kcv

.V

alen

cia

Bra

zil

<3

53

6-1

00

>1

50

Qua

ggio

etal

.(1

996)

~ Q

Sw

eet

oran

ge(C

itrus

sine

nsis

Osb

eck)

Bra

zil

35-6

8Pr

imo

eta!

.(1

969)

ZS

wee

tor

ange

(Citr

ussi

nens

isO

sbec

k)C

hina

0.1

617

-19

20-5

960

-99

Kot

oet

al.

(199

0)0

Citr

usre

ticul

ata

Bla

nco

cv.

Cle

men

tine

Italy

<3

030

-60

60-8

58

5-1

40

>14

0D

etto

riet

al.

(199

6)5-'-

'C

itrus

sine

nsis

Osb

eck

cv.

San

guin

eIta

ly<

20

20-3

73

7-5

555

-90

>90

Det

tori

etal

.(1

996)

~ 0

Citr

ussi

nens

isO

sbec

kcv

.M

aoro

clat

Italy

<30

30-6

060

-85

85-1

40>

140

Det

tori

etal

.(1

996)

0 ()l

Citr

ussi

nens

isO

sbec

kcv

.V

alen

cia

USA

<1

515

-40

50

-20

020

0-25

0>

260

Cha

pman

(196

0a,

1960

b)C

itrus

sine

nsis

Osb

eck

cv.

Val

enci

aU

SA<

20

20-3

53

6-1

00

101-

200

>26

0Sm

ith(1

966)

Citr

ussi

nens

isO

sbec

kcv

.V

alen

cia

USA

<21

21-3

031

-100

101-

260

>26

0E

mbl

eton

eta!

.(1

973)

Citr

ussi

nens

isO

sbec

kcv

.V

alen

cia

USA

<2

021

-35

36-1

0010

1-20

0>

250

Koo

eta!

.(1

984)

Citr

ussi

nens

isO

sbec

kM

osam

biIn

dia

24

.9-3

56

Sriv

asta

vaan

dS

ingh

(200

3b)

..... --.J

--.J

178 AGRICLII TURAL REVIEWS

positive electrostatic change in the membranethrough the uptake of an electron 100 senfrom a donor (Probably a sulfahydryl containingcompound) which is perturbed by actions oflight. gravity, and phytohormones. Generatedpositive charge would control the passage ofions through pores of the cell membrane toregulate pinnule movement. The positivecharge could attract and orient negativelycharged molecules such as nucleic acid, andthereby, initiate, facilitate or control certain vitalreactions involved in the cell division, cellelongation and flowering (Tanada, 1995).

Deficiency of B in browning planttissues is thought to be related to theaccumulation of polyphenolic compounds withits involvement in synthesis of cell wallcomponents. Since, B combines with 6phosphogluconic acid to form an enzyme­inhibited complex, it regulates the phenolsynthesis and thus, prevents necrosis death ofB-deficient leaves. Boron toxicity symptomsconsist of irregular areas of yellowish orangetint between the veins along margin and thetips of the leaves as described by Kelly andBrown (1928). Some tip burn and defoliationare also observed.

Distribution and Availability of Boron in SoilThe total B content in most of the soils

has been observed to vary from 20 to 200 mgkg'! (Berger and Pratt, 1963) with a smallproportion existing in available form (Bakerand Mortensen, 1966). The main source of Bin well drained soils is the tourmaline containing3-4 per cent B especially in acid rocks anddispersed in silicate minerals and becomeavailable fairly slowly (N0rrish, 1975). As earlyas Wheatstone et ai. (1942) analysing the totalB concentration in major citrus soil types inUSA reported that the content varied from 4to 133 mg kg'! with lowest values concentratedto basic igneous rocks viz., granite, gneiss, andschists and soils derived from shales, limestone,and sedimentary rocks had high B content.

Very little is known about the mineral forms ofB in soils (Lindsay, 1972).

Three possible mechanisms arecommonly considered for the chemicaltransformation of B in soil viz., anionexchange, chemical precipitation, andmolecular adsorption(Wilson et ai.. 1951). Animportant mechanism governing the Bsolubility in soils is the adsorption of B on Fe:>and AI-oxides (Sims and Bingham, 1968).Availability of B has shown a negativecorrelation with soil pH bey~nd 6:5. While,below the soil pH 6.5. the trend is still notclear. Increasing soil pH imparted an increasein monolayer adsorption and decrease inbonding energy for kaolinite andmontmortillonite minerals with slight increasefor illitic minerals. On a weight basis, illiteabsorbed highest B. Over a soil normal pHrange, montmorillonite adsorbed appreciableB while kaolinite absorbed least. The relativeamount of reversibility of adsorbed boron is acomplex function of the drying and rewettingcycles, pH and type of exchangeable cations(Keren and Gaust, 1981). Boron adsorptionby both physical and chemical mixtures(hydroxy aluminium and montmorillonite) wasgreater than by montmorillonite alone, withthe physical mixture and adsorbing the largestamount of boron (Keren and Gaust. 1983).

Soil texture has shown to exert astrong influence on the adsorption anddesorption behaviour of B in soil. with finetextured soils have better available B thancoarse textured soils (Wear and Patterson.1962; Hingston. 1964). However, movementof B is less rapid in heavy textured soils due toincreased fixation by clay particles (Berger,1949; Bigger and Fireman, 1960; Gupta,1968; Reisenaur et ai., 1973). In acid soils.very little adsorption of B takes place on themineral fraction at low pH due to organicmatter which serves as a main source of B. Itis suggested that complex formation with

VoL 26. No.3. 2005 179

dihydroxy compounds in soil organic matter isan important mechanism for B adsorption.Boron in organic matter is largely released inavailable form through the actIon of microbes(Berger and Pratt. 1963). The influence oforganic matter on the availability of B in soilsis further amplified by increase in the pH andclay content (Hobbs and Bertramson, 1949).

Distribution of Boron within PlantSoil pH is an important soil properties

affecting the availability of B in soil and plants.The available B becomes less and less availablewith increasing soil pH especially beyond soilpH 6.5 (Martens, 1968). Soil Ca level isanother factor which influences available Bcontent and this is the reason that Ca/B ratiois sometimes observed as an indicator of Bstatus (Gupta, 1979) since Ca has a specificphysiological role in reducing the B uptake byplants. Boron is moved away from veins intothe marginal and interveinal areas. Boron isnot translocated to the older leaves from theleaves it is accumulates first since it is notcarried from the leaf in the phloem and phloemmembranes are largely Impermeable to boron.The concentration of boron has been reportedto be highest in the marginal necrotic areas ofthe lemon leaf from a plant to which excessboron was applied and lowest at the base ofthe mid-vein (Oertli, 1960). Such a situationcan be explained by assuming that boron ispassively carried within the plant through thetranspiration stream. Distribution of boron inleaves is reflected directly in the appearanceof boron toxicity symptoms. The highest boronconcentrations were found in necrotic tissuesfollowed by the yellow leaf areas.

As early as Eaton (1944) reported thattoxic amounts of boron. concentrated inmarginal and interveinal areas near th~ tip andmost of the B contained in the leaf tissue isremoved when the juice is squeezed from theleaves (Eaton, 1944). De Villiers and Beyers(1961) observed a response of B fertilization

which contained as much as 50 ppm in leavesand found evidence that water soluble B mayserve a better index than total B content. Acritical concentration of 15 ppm water solubleB was further suggested. B toxicity in citrusgrowing soils of marmine origin in Australia.

According to Oertli (1960) thedistribution of boron in rough lemon is non­uniform. Concentrations can be found it areanear the top a hundred times higher tha. 'l-}atin the petiole. Symptoms of disorders in boronnutrition are reflected in the boron distributionin the tissue under the conditions of high boronconcentrations. The lowest boronconcentrations were found in the midrib andthe petiole. Such a distribution is not uniquefor sufficient cause of boron deficiencyconditions having scanty B supply. B movesalong cells in the petiole which are potentiallyor actually deficient into the most distant areaof the leaf. The magnitude of boronconcentration at any point on the leaf ",an beexplained by considering two physicalproperties of the system viz., the evaporationof water (solutes getting more concentratedupon evaporation of water as C = cvIVwhere C and C stand for conce~tration ofsolute as boron upon water evaporated to leavethe volume V and original, concentration of

asolute having original volume V, respectively)and the largely irreversible, uniform flow ofwater from the base to tip (transpiration streamis proportional to the percentage of leaf areathat may be substituted for volume as C = CAlA where C is the concentration of s~lute as

a aboron at the line where there is A area fromthere to the tip of leaf) of leaf 3 is hy'pothesizedto be carried out through transpiration stream(Kohl and Oertli, 1961).

Boron is more concentrated in thoseareas of leaf considered sinks for water in whichthe mass flow of transpiration stream ends inthese areas. For those leaves with complexvenation (unifoliately compound and pinnately

180 AGRICULTURAL REVIEWS

reticulate) like citrus, margins and eveninterveinal leaf areas may be such end pointsas sinks. The above explanation stands trueunder an adequate or excess supply of B butunder the conditions having lower B supply,such explanation might not be so accurate.

Interaction of B with Other NutrientsChapman and Vanselow (1955) were

the pioneers in establishing that liberal Napplications has beneficial effects in alleviatingthe accumulation of excess B in citrus.Nitrogen is the most important macronutrientwhich has a major significance in B uptake(Jones et al., 1963). Later, Jones et al. (1963)observed that under conditions of high Bconcentration, application of N depressed theconcentration in sweet orange leaves from 860to 696 ppm. Other studies have suggested CalB ratio as an indication of B status in plants.Other nutrients such as P and K have also beenshown to have influence on uptake of B butthe results are inconclusive. Bingham et al.(1958) observed that high phosphatecOLcentration decreased the absorption of B.

Boron SourcesA number of boron containing

fertilizers were described by Gupta (1979)which comprise of borax (Na

2B40 T 10HP) ­

11(Xl B, boric acid (H3BO) - 17% B, boron

frits (Na2B

4Hp) - 10 to 17% B, sodium

tetraborate as borate - 46 (Na2B40 75HP) ­14<){) B, borate-65 (Na

2B

40

7) - 20(76 B, sodium

pentaborate (Na 2B40 T 5HzO + Na2BlQOI6'1OHP) - 20 to 21% Band ulexite (NaCaBs0 9 .

8HP) - 9.4 % B. Berger and Pratt (1963)earlier described the other kinds of B containingfertilizers which include borated gypsum,calcium carbonate, superphosphate, calciumnitrate and various mixed fertilizers.

ManagementAs early as Smith (1956) observed that

application of borax to oranges and grapefruitsfailed to affect the yield but its increased leafcontent, induced biennial bearing and increased

leaf size in some species. Boron deficiency isnot a serious problem in citrus growing areasof our country except those on acidic soils likethose of Assam and South India (Chowdhuryand Dutta, 19["0).

Soil applicationOn young mandarin orchards on

Chernozem soil, boron at 3 kg hal was foundbest as far as increase in yield is considered(Kechakmadze, 1987). Abnormal fruits ofcitrus in Tunghsi orchard, at Taiwan wereconsidered to be mainly due to B-deficiencyin the soil. Soil application of boric acid 40­120 g tree I on medium textured soil andfoliar spray of 0.3%l boric salt solution wereeffective but B being applied every year, led toexcess boron accumulation in the soil (Chiuand Chang, 1983). Clementine trees on sourorange, Troyer citrange and Poncirus trifoliatarootstocks with Mo-deficiency showed a poorgrowth and production of, yellow spots onleaves, leaf burn, c;n accumulat~n of nitrate­N in leaves, low nitrate reductase activity,and very low leaf Mo concentration. The effectsof Mo-deficiency were markedly aggravated bysulphate application (Cassin et al., 1983).

In the case of B, larger (50 g B asborax tree- I year- l) one-time applicationsprovided potentially harmful and imprecisedelivery to trees and elevated leaf and fruit Blevels beyond the acceptable level of 20 ppm.Delivery through fertigation can preciselycontrol soil solution levels, but effectiveness ofthe applied nutrient varied between schedulesinitiated in spring vs. fall. When a reduced rate(25 g B as borax tree- I year-I) was applied inthe spring season, the resulting pulse of soilsolution B was evident as elevatedmeasurements of fruit B concentration abovethe 20 ppm level. A fall season application(same rate as spring application) was able tomaintain tree B status without endangering fruitB concentrations (Stauffer and Sulewski,2002).

Cultivar type

VoL 26. No.3. 200S

Table 2. Soil application of boron followed worldwide

Dose Source

181

Mandarin

OrangeSatsuma and Natsudaidai mandarinCoorg mandarinMandarinCoorg mandarin

Eureka lemon

SWl'el orangeMandarin

3 kg hal40-120 g boric acid tree'37 kg borax ha I

37.5-75 g borax tree'lkg borax ha '3 kg B ha'0.5 kg borax tree!3 9 B tree I

100 g borax tree'!40-120 g boric acid tree'3 kg B hal3-9 kg B hal

Kechakmadze (1987)Chiu and Chang (1983)Sam Portch (2002)Sato et al. (1962)Srivastava et al. (1977)Chanturiya (1972)Govinda Rao and Reddy (1957)Egorashvi/i etal (1991)Khalidy et al. (1 ;66)Chiu and Chang (1985)Talakvardze (1977)Mdinaradze and Kechakmadze (1982\

In Coorg mandarin, its applicationimproved the girth, height and tree spread(Srivastava et aI., 1977). Supplying 5-20 g Btree'! prevented the occurrence of deficiencysymptoms and increased both fruit size andproportion of the pulp. Without B, theproportion of pulp/peel on a fresh weight was1.0:1.6-2.0 and compared to 1.0:0.6-0.7 withB (Feroughi et aI., 1974). Singh and Singh(1976) also observed increased shoot growth,total leaf area and fruit yield in Kagzi lime(Citrus aurantjfoIia Swingle) besides total solublesolids, total sugars, and acidity. The juicecontent, total soluble solids total sugars andacidity also improved. Boron or Mn (ratesunspecified) applied to mandarin trees growingunder clean cultivation and receiving N, P, Kincreased the yield by 5 to 9 %. Applied totrees receiving green manure plus NPK, B orMn increased the yield by 6 to 11 %J and whenapplied to trees receiving lime plus NPK, by15-20%. B or Mn also increased fruit sugarcontent (by 1-2%) and fruit acidity anddecreased ascrobic acid contenHKodya, 1980).A number of other recommendations havebeen suggested (Table 2) according to natureof cultivars.

Besides deficiency, ex:ess of B is alsoa serious problem in citrus orchards. Soilapplication of borax to Natsudaidal orchardtrees increased the B content of leaves and

fruit for 4-5 years whereas the effects of foliarspraying were noticeable for only 2 years.Boron application increased fruit and seedweights, improved seed development andlowered the total sugar content and sugar/acid ratio of fruits (Ishihara et aI., 1965). Treesof sweet orange and mandarin on trifoliateorange collapsed due to toxicity as leaf Bcontent increased to 395 ppm (Mahmood,1971). Randhawa and Srivastava (1986)observed that effect on tree vigour, .. yield andfruit quality was not significant under mildtoxicity but significant reduction in growth andyield was observed under severe toxicity. Evencomplete defoliation has been observed undersuch condition. Munshi et aI. (1979) attributedgranulation of sweet orange to B-toxicity.According to, Xiao Min et aI. (1996) fieldsurveys and sample analysis showed that fruitquality was significantly influenced by the levelsof Band Mo in the soil in the Nanteng Mijumandarin orchards of south China.

Epigean applicationFoliar application of nutrients

including B is useful under condition wherenutrient uptake from soil is restricted due toadverse soil conditions. Foliar spray with boricacid and soil application of borax has beenfound effective in correcting B deficiency.Earlier studies (Chowdhury and Dutta, 1950;Aiyappa et aI., 1968) have shown the noted

182 AGRICULTURAL REVIEWS

Table 3. Foliar spray recommendations of boron adopted worldwide

Cultivar type

Coorg mandarinKinnow mandarinKagzi lime

Eureka lemon

Jincheng orangeSweet orange

,Jiaogan mandarinNagpur mandarin

Dose

0.2% borax0.2'}" borax0.4c)·" borax0.3% borax03% B125g borax 100 litre'] water or125g polybar 100 litre] water0.2'}" H,BO,0.6'}" B0,3% boric acid0.2% borax0.2% B

Source

Srivastava et a1. (1977)Singh and Misra (1980)Singh and Singh (1976)Singh et a1. (1990)Singh et a1. (1984)Khalidy et al. (1966)

Nan (1996)Rai and Tewari (1988). Rai et al. (1988)Chiu and Chang (1986)Wang (1999)Tayde and Ingle (2000)

beneficial effects 6f foliar application of B. Aconcentration of 0.2% borax was foundeffective in Coorg mandarin (Srivastava et al..1977) and Kinnow r. ;andarin (Singh and Misra.1980) but application at higher rates (0.4 %)proved better in Kagzi lime (Singh and Singh.1976). Reitz et al. (1972) recommended soilapplication of boron (BP3) equivalent to 1/100 of nitrogen requirement. A singleapplication of 1 kg of borax per tree of Coorgmandarin was sufficient for 2-4 years(Srivastava et al.. 1977). Chanturiya (1972)found that 3 kg B tree l was effective inmandarin plantation but its efficiency decreasedwith simultaneous application with Ca.However. poor response to soil application ofborax at the rate of 0.5 kg plant'l was observedin south India by Govinda Rao and Reddy(1957). Khalidy et al. (1966) compared soilapplication with fohar spray and drip spray inEureka lemon which revealed that foliar sprayof 125 g borax per 100 litre of water or 125g polybar per 100 litre of water, proved betterthan soil application of 100 g borax tree'l.Other recommendations emerged from variouscitrus growing belts are summerised further(Table 3) indicating a large variation amongstcitrus cultivars.

Eight-year-old Jincheng orange treesgrowing on a purple. clay soil (pH 4.3-4.7) inChongaing, China were sprayed with H3B03

(0.2%), ZnS04

(0.1 %) or MgS04

(0.2%) at theend of March (pre-anthesis), in Mid - April (fullbloom) and Mid - May (young fruitdevelopment). Pollen culture showed thatpollen tube elongation was 17.26. 7.85 and23.97% greater in the B. Zn and Mgtreatments. respectively, than in the control,thus favoring fertilization and improving in fruitset. Foliar application of B gave the optimumresults, improvement in fruit coloration. fruityield, fruit size and total sugars and reducedthe total acids. Zinc sulfate further increasedthe sucrose content of the fruit and improvedthe smoothness of the rind (Nan, 1996).Reviewing the efficiency of B when appliedthrough various methods, Tiwari (2002b)reported that soil application of B holds morepromise than foliar application.

Future StrategiesThe futuristic strategies should involve

growing of superior nutrient efficient plantshaving high photosynthetic efficiency anddiverse adaptability. The follOWing strategiesare further suggested looking into the futurerequirements to harness the potential benefitsout B-nutrition.

i. Soil is a basic resource that must be healthyfor rest of the ecosystems to remain diverseand productive. Human induced soil changesand their effects on human lives and e<;:ologicalenvironment have received extensive attention.

Vol. 26. No.3, 2005 183

Improving soil quality is one of the mostimportant core problems for sustaining theglobal biosph.zre (Smith et al., 1994). Moreresearch is needed to delineate the critical limitsof different forms of soil-B beyond whichquality of soil environment is severely andirretrievably jeopardized.

ii. Developing the efficient means ofsupplying all the nutrient, and judginginteraction of B with other nutrientsconsidering the successful intensive Citricultureon the dominant deep, vI'?lI drained, acidinfertile red and yellow Qxisols.

iii. The organic matter level in the soil isimportant to help to maintain an activepopulation of organisms in soil to promoteorganic matter mineralization, besidesstabilizing a favourable physical condition ofthe soil (Lee, 1991; Stork and Eggleton, 1992)and promote the absorption of nutrients bythe plant roots (Chen and Aviad, 1990). Theseeffects of soil organic matter imply that thelevel of organic matter may be taken as anindicator of this sustainability of soilmanagement system. One of the majoradvantages to be derived from long termfertility trials, enable soil organic matterchanges vis-a-vis soil available pool of nutrientsincluding B to be monitored. Norms foravailable - B in relation to differentialproductivity levels need to be established fordifferent soils to be used as an index ofsustainability.

iv. Evaluation of long term effect of B­containing fertilizers, is required for ecologicalsustainability, with B-uptake characterizationunder different rootstock-scion combination ismandatory to understand the behaviour of Bfrom soil to plant continum.

v. Subsoil modification can be considered asa viable option, if tillage operations are to bemade effective for several years with respectto maintaining long term fertility. Strategiesto improve subsoil fertility equally applicableB like nutrient, may include : i. mechanical

means of placing nutrients deeper in theprofile, ii. using nutrient sources of lower orhigher mobility, iii. using deep rooted legumesto fix nitrogen at depths, and iv. selection ofplant species and genotypes better suited toacquiring nutrients from subsoils (Grahamet al., 1992).

vi. There is a need to identify/evolveanalytical methods for the determination of Bin alkaline and acidic soils. Besides, effects needto be made for refining and/or standardisingthe soil test methods for the micronutrients,whose soil and plant tests data do notsupplement each other (Nayyar, 1999). Thisshould also address towards evaluating B­toxicity. Studies on relative tolerance of citrusto various forms of Al vis-a-vis dynamics of Bin Alfisols and Oxisols would further help tounravel many of unsolved principles of B­management of these citrus growing soil ordersfor improved quality production.

vii. Monitoring the genesis of B and othernutrient deficiencies in different citrus beltsbelonging to various agroecological regions isessential for forecasting potentialmicronutrients problems, and develop modelsfor different soil-crop situations. Determinationof.available Band Mo status of soils in differentagro-ecological region and identification ofareas where Band Mo fertilizers are to beincluded in the fertilizers schedule. Equalemphasis requires to be laid on characterisationof soil-solution phase equilibria. Basic studiesrelating to transformation and interaction withother soil solid solution phase equilibriumcoupled with intensification of research onmovement of micronutrients in soil and plantsystem as affected by soil conditions andclimatic parameters are necessary tounderstand the processes controlling availabilityof micronutrients to plants in order to improvethe technologies for their efficientmanagement.

viii. Emphasis on methods of improving themobility of B under soil treated with

184 AGRICULTURAL REVIEWS

amendments needs a complete reorientationespecially while addressing the subsoil acidityto sustain the ameliorative effect on yield andquality of citrus cultivar.

ix. Studies in the past have shown a greatdeal of variation in response of B-fertilizationwhen considered over time of fertilization, dueto difference in meteorological conditions. InIndia, citrus is grown in a completely diverseedaphological conditions., right from arid-semi!arid conditions of Punjab and Haryana to yearround extremely humid conditions of north­east India. These conditions warrant adifferential B-use-efficiency. Therefore, theschedule of B-fertilization needs to worked outthrough long term experiments using both

basin method irrigation and controlled deliverysystem through fertigation.

x. Developing the leaf B standards forimportant commercial citrus cultivars grownin India in order to raise B-constraint freeproduction.

xi. Introduction of site specific practices withB as key micronutrient into citrus orchards bycombining the use of a number of informationtechnologies such as application of remotesensing, geographical information system,global monitoring system, and variable rateapplication technology. Role of these tools asa part of precision citriculture requires to befurther explored and consequentlystrengthened.

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