Fertilization and fruit setting in date palm: biological and technological challenges

Abstract

Control of pollination and fertilization in date palms is essential for development of high quality fruits. The female flower has three separate carpels. Only a single carpel develops into a fruit, while the others degenerate. When pollination is inefficient, non-fertilized flowers may develop into parthenocarpic fruits, which have no commercial value. The main aim of our research is characterization of fertilization and fruit setting in date palms and assessment of the effect of temperature on these processes. Since date is a very large tree, it is practically impossible to study its reproductive biology under completely controlled conditions, in a greenhouse or phytotron. Therefore, two alternative research approaches have been applied. In vitro assay was developed for culturing of isolated pollinated spikelets in liquid media under fully controlled conditions. Alternatively, the controlled environmental conditions were applied in planta, using specially designed units, assembled on pollinated inflorescences of whole date trees in the orchard. Each technique had specific advantages, as well as technical and biological limitations. Taken together, they complement as an efficient research tool.

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Acta Hortic. 1130. ISHS 2016. DOI 10.17660/ActaHortic.2016.1130.53
XXIX IHC – Proc. Int. Symposia on the Physiology of Perennial Fruit Crops and
Production Systems and Mechanisation, Precision Horticulture and Robotics
Eds.: D.S. Tustin et al.
351
Fertilization and fruit setting in date palm: biological
and technological challenges
Y.Cohen1,F.Slavkovic1,2,D.Birger1,2,A.Greenberg3,A.Sadowsky3,M.Ish‐Shalom1,M.Benita1,
T.  T i c u c h i n s k i 3,Y.Avnat4andR.Kamenetsky5
1Departmentof FruitTreeSciences, VolcaniResearchCenter,P.O.Box6, BetDagan, Israel;2TheRobertH.Smith
FacultyofAgriculture,FoodandEnvironment,The HebrewUniversityof Jerusalem,Israel;3SouthernAravaR&
D, Yotvata, Israel; 4Crystal Vision, Kibbutz Samar, Israel; 5Department of Ornamental Horticulture, Volcani
ResearchCenter,P.O.Box6,BetDagan,Israel.
Abstract
Control of pollination and fertilization in date palms is essential for
development of high quality fruits. The female flower has three separate carpels. Only
a single carpel develops into a fruit, while the others degenerate. When pollination is
inefficient, non-fertilized flowers may develop into parthenocarpic fruits, which have
no commercial value. The main aim of our research is characterization of fertilization
and fruit setting in date palms and assessment of the effect of temperature on these
processes. Since date is a very large tree, it is practically impossible to study its
reproductive biology under completely controlled conditions, in a greenhouse or
phytotron. Therefore, two alternative research approaches have been applied. In vitro
assay was developed for culturing of isolated pollinated spikelets in liquid media
under fully controlled conditions. Alternatively, the controlled environmental
conditions were applied in planta, using specially designed units, assembled on
pollinated inflorescences of whole date trees in the orchard. Each technique had
specific advantages, as well as technical and biological limitations. Taken together,
they complement as an efficient research tool.
Relatively low temperatures (from 8 to 20°C) enhanced formation of
parthenocarpic fruits and reduced normal fruit development. Temperatures also
affected the rate of fruitlet development. Stages of pollen tube growth, fertilization,
carpel development and/or degeneration, and early development of normal and
parthenocarpic fruits were defined and characterized by macro- and microscopic
analyses.
Keywords:Phoenix dactilifera, temperature, controlled environmental conditions, pollen
tubegrowth
INTRODUCTION
Datepalm (Phoenix dactyliferaL.)isaveryimportantfruitcropinaridregionsofthe
MiddleEastandNorthAfrica(ChaoandKrueger,2007;ZaidandDeWet,2002a).Datepalm
isalargedioecioustree,havingfemaleandmaleflowersseparated on different trees. In
order to fertilize female flowers, pollen from male trees must reach the stigma. Under
natural conditions, wind‐mediated pollination occurs in date palms. In commercial
plantations,pollenisharvestedfrommaletreesandthenactivelydistributedonthefemale
treeinflorescences.Overlyhighrateoffruitsetmaycauseexcessive fruit load. Thus,
expensivefruitthinningisrequiredtopreventreductioninfruitsizeandmarketability.On
the other hand, sub‐optimal pollination conditions (usually occurring at cooler
temperatures) may result in non‐efficient fertilization and in low fruit yields. Therefore,
controlofpollinationandfertilizationindatepalmsisessentialforthedevelopmentofhigh
qualityfruitandyield.
Thedatefemaleflowerhasthreeseparatecarpels,eachcontainingasingleovule.After
fertilizationonlyoneofthethreecarpelsdevelopsintoafruitwhiletwoothersdegenerate.
Whenpollinationisinefficient,non‐fertilizedflowersmaydevelopintoparthenocarpicfruits
thathavenocommercialvalue(Reuveni,1986).

352
Pollination and fertilization processes are limited by various environmental factors.
Foremost,datepalmsflowerwhentheshadetemperatureincreasesto18°C,andfruitset
occurswhentemperatureishigherthan 25°C. Theeffectivetemperatureduringtheperiod
frompollinationtofruitripeningofthedatepalmrangesfrom21to27°C(ZaidandDeWet,
2002b).However,temperaturesin some arid regions vary drasticallyonadailybasis with
amplitudereachingmorethan20°C;asaresult,theefficiencyofpollination,fertilizationand
consequentfruitsetisoftenbelowoptimum.
Studyoffloweringandpollinationmechanismsandtheirregulationbyenvironmental
conditions (especially temperature) will provide tools for the optimization of fertilization
and fruit setting in date palms. However, the date is a very largetreeandthestudyofits
reproductivebiologyundercontrolledconditionsisachallengingtask.Inthisresearch,two
alternative approaches have been applied. First, an in vitro assay was developed for
culturingisolatedpollinatedspikeletsin liquidmediaunderfullycontrolledenvironmental
conditions. Second, the controlled environmental conditions were applied in planta, using
speciallydesignedunits,assembledon pollinated inflorescencesofwholedatetreesin the
orchard.Eachtechniquehasspecificadvantages,aswellastechnicalandbiological
limitations.Takentogether,theycomplementas efficientresearchtoolsthatallowin‐depth
studiesofthereproductiveprocessindatepalm.
MATERIALS AND METHODS
Plant material
Date palm (Phoenix dactylifera)‘Medjoul’,grownattheexperimentalorchardsof
SouthernAravaR&D,Yotvata,andCanarypalm(Phoenix canariensis)growninthecampusof
Agricultural Research Organization, the Volcani Center in Bet Dagan,wereusedinthis
research.Bothspeciesarecloserelativesandpossesssimilarreproductivemechanismsbut
varyintheirannual cycleandfloweringseason. Datepalmsflowerduring arelativelyshort
season in March‐April, while canary palms flower in October. Therefore, we used canary
palmtocompsecondlementour main researchtoobtainplantmaterialbeyondthedateof
floweringseason.Forthelaboratoryinvitrostudiestheinflorescences of date palm and
canarypalm,enclosedinthespathe,weredeliveredtothelaboratoryinFebruary‐Aprilof
2013andOctober2013,respectively.Forinplantastudies,weusedtenyearoldintactdate
palmtrees(‘Medjoul’),growninanorchardatSouthernAravaR&D,Yotvata,Israel.
In vitro pollination assay
Singlespikeletscarryingflowerswerecuttoapproximately15cm,pollinated with a
small paintbrush with pollen collected during the previous season and stored at ‐20°C.
Spikeletswerethenplacedintubeswith10mLofliquidmediaorinagarmedia.Toprolong
“vaselife”andtoincreasesurvivalperiodoftheflowers,severalchemicalswereaddedtothe
media:TOG6(GADOTAgro),TOG6+2%sucrose,“Longlife”(GADOTAgro)wereaddedto
theliquidmedia.Additionallysurvivalwastestedonsolidagar plates supplemented with
3%sucrose,Murashige&Skoog(MS)medium(GetterM0222),casein hydrolysate (Getter
YB‐C1301), plant agar (0.8%, Getter YM‐P1001) and active charcoal (0.25%, Getter YB‐
C1302).pHwassetto5.7(Table1).
To delaysenescence, spikeletsweretreatedwith ethylene inhibitors: incubated with
500 ppb 1‐methylcyclopropene (1‐MCP) in sealed glass chambers at 20°C, or with 0.2%
silverthiosulfate(STS)pulse‐treatedfor4hat20°C.
Sampleswereincubatedatgrowthchamberswithconstanttemperatureof15,20,25
and30°C,and12‐hphotoperiod.Viabilityofspikeletsandindividual flowers was visually
evaluated, using a six‐point scale, where 5 is completely viable and 0 being dead/most
contaminated. To assess the viability we used the following parameters: (a) stigma
browning, (b) spikelet browning, (c) flower browning, (d) spikelet drying, (e) flower
abscissionand(f)senescence,Inparallel,sampleswerecollectedformicroscopicanalysisat
severaltimepointsduring14dayspostpollination.

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Table1. Effectofculturemediaandtemperatureonisolatedspikeletsviability andflower
abscission. Spikelet sections were incubated at different temperatures, there
viabilityandflowerabscission wasestimatedat1, 5 and9daysaftersetup(DAS)
usingasix‐pointscale,where5iscompletelyviableand0being dead/most
contaminated. Means with different letters in columns are significantly different
(P≤0.05)accordingtotheTukey‐KramerHSDtest.
Culture media Overall vitality Abscission
1 DAS 5 DAS 9 DAS 1 DAS 5 DAS 9 DAS
TOG 5 a 4.8 a 1.5 ab 5 a 4.8 ab 2.8 abc
TOG + 2% sucrose 5 a 5 a 4.5 cd 5 a 4.5 ab 4.6 bc
LongLife 3.8 b 2 bc 0 a 5 a 2.6 a 0 a
DDW 4.3 ab 3.5 abc 0 a 4.9 a 4.6 ab 0 a
MS agar + 3% sucrose 4.9 a 3.5 abc 0 a 5 a 3.8 ab 2.3 abc
In planta pollination and fruit setting assay in temperature-controlled units
Controlledenvironmentalconditionswereappliedinplanta, usingspeciallydesigned
units,assembledonpollinatedinflorescencesofwholedatetreesintheorchardofSouthern
Arava R&D, Yotvata. Temperature was controlled by Peltier elements, capable of rapid
heating or cooling the environment. Ventilators were employed to remove excessive heat
within the units. Three sinusoidal temperature regimes were applied with a
maximal/minimaltemperaturesof20/8°C,25/12°Cand 32/18°C, mimickingextremecool,
average,andextremewarmconditionsatSouthernArava,Israelduringtheflowering
season.Thehighesttemperaturelevelwassettoapproximately2:00pm,andtheminimum
level at 5:00 am. Pollinated bunches, exposed to open‐air temperatures, were used as
control.Inflorescences/bunchesweresampledapproximatelytentimesinMarch‐May2013,
withinfirstsixweeksfrompollination.
Pollen tube elongation in vitro and on the stigma
DatepalmpollengrainswerecollectedfromYotvataorchardin2012and2013.Pollen
wasgerminatedinvitroatdifferenttemperaturesfor3hinasolutionof 10%sucroseand
500mgL‐1boricacid(Bernestein,2004).Pollengrainswerevisualizedunderamicroscope
(MZFLIII,LEICA),photographed(Nikon DS‐Fi1) and theirtubelengthwasmeasuredusing
theNIS‐ElementsBR3.1Program.
Analysisof pollentubeelongationin stigmaswasadapted fromReuvenietal.(1986)
and Cohen et al. (2004). Prior to histological evaluation, FAA fixed flowers were washed
three times in double distilled water (DDW): 100% ethanol (1:1), and then five times in
DDW.Usingstereoscope,stigmaswereseparatedandclearedwith10MNaOHfor2hand
washed in DDW fivetimes. Stigmas were stained with aniline blue (0.4% in 0.35% K3PO4
solution) and examined under fluorescence microscope (MZFLIII, LEICA) with a UV
excitationfilterset(340‐380/400/425nm).
RESULTS AND DISCUSSION
Pollen germination in vitro
Pollen germination in vitro was significantly affected by temperature. At lower
temperaturesitwasslowerandpollen tubeelongationwasretardedascomparedwith 25‐
30°C(datanotshown).Wearguethatthisassaydidnotfullyrepresenttheeventsoccurring
within the stigma and carpel of the flower in vivo. Therefore, for the further study of the
effect of environmental conditions on fertilization and fruit setting in date palm two
additionalapproacheswere used: (1)invitropollinationof flowersoncutspikeletsunder
controlled conditions and (2) in planta pollination, fertilization and fruit setting in
temperature‐controlledunits,placedontheintactinflorescencesofthewholetree.

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In vitro pollination and fertilization on isolated spikelets
Creating controlled environment conditions for fruit bearing indatepalmsisa
challengingtask.Wecalibratedaninvitroassay,inwhichisolatedspikeletswillhavealong
enough “vase life”, thus enabling fertilization and fruit settinginvitro.Severalparameters
werecalibratedtoreachthebestconditions.Fivegrowthmediaweretestedtoprolong“vase
life”andtoincreasesurvivalperiodoftheflowers.Thelongest“vase‐life”ofinflorescences
wasrecordedatTOG6andTOG6+2%sucrosegrowthmedia(Table1).Furtherexperiments
werelaterperformedusingthisgrowthmedia.
InOctober2013,cutspikeletsofCanarypalm,weretreatedwithethyleneinhibitors,
1‐MCPorSTS.Bothtreatmentsextended“vaselife”oftheflowersandspikelets.Theflowers
werehealthierandlessflowershaddropped3‐10DAP(Figure1), and lower fungal
contamination was observed (data not shown). Pollinated spikeletstreatedby1‐MCPand
STSremainedviable13DAP,whilethecontrolflowershaddried(Figure1).However,these
treatmentswerenotabletosupport invitrodevelopment oftheflowersandfruitsetafter
pollination.
0
1
2
3
4
5
024681012
Grades
Days after setup
MCP
STS
Control
0
1
2
3
4
5
024681012
Grades
Days after setup
1-MCP
control
1-MCP
control
STS
ABC
D

Figure1. Effectofethyleneinhibitorssilverthiosulfate(STS)(A)and1‐methylcyclopropene
(1‐MCP) (B) on viability (C) and flower abscission (D) on isolated spikelets.
Pollinatedspikeletsectionswereincubatedtwoweeksat25°C.Theirviabilityand
flower abscission was estimated using a six‐point scale, where 5 is completely
viableand0beingdead/sheddedflower.Datarepresentmeans±standarderrors.
Weconcludedthatunderourexperimentalconditionsfastflowersenescencemightbe
caused by fungal or bacterial contamination, ethylene emission, hormonal imbalance or
otherfactors.Inanyevent,flowersenescencewasfasterthanfruitletdevelopment,andeven
ifthepollinationwassuccessful, wewerenotableto followtheprocessoffertilizationand
fruitletdevelopment.
Temperature effect on pollen tube growth on stigma of in vitro pollinated flowers
Pollen tube elongation was estimated on stigmas of the flowers, pollinated on cut
spikelets and cultured in vitro (Table 2). Pollen tube elongation was observed under all
temperatureregimesinthefirstdayafterpollination.Thehighestpollengerminationrate
was observed at 30°C, followed by 25 and 20°C. Even at 15°C moderate germination on

355
stigmawasobserved.However,pollentubeelongationandpenetrationtotheupperpart of
thecarpelwasmuchslowerat15°Ccomparedwithhighertemperatures. Although no
morphological damage was observedwithinthefirstdaysaftergermination, under our
experimentalconditionsfurthergrowth of thepollentubeswasrestricted andfertilization
didnotoccurunderanytemperatureregime.Inasimilarexperiment, performed with
isolatedspikeletsofadifferentdatecultivar,‘Barhee’,pollentubeelongationwasdetected
withinthecarpelanduptotheovulewithin3‐7daysafterpollination(Cohenetal.,2004).
Wesuggestthatinvitrothe‘Medjuol’flowerswiltrelativelyfastduetointensesenescence
andcontaminationprocesses,andthatfurthercalibrationisrequiredfortheoptimizationof
invitroconditionsforfertilizationexperiments.
Table2. Effectsoftemperature onpollengerminationon stigmaofflowersofcutspikelets
pollinatedinvitro.Pollengerminationwasestimatedonafive‐pointscale,where0
=nogermination;4= high germination. Analysiswasperformed on atleasteight
stigmas per treatment. Statistical analysis includes samples from different
temperaturetreatmentsandDAPs.Meansaresignificantlydifferent (P≤0.05)
accordingtotheTukey‐KramerHSDtest.
Temperature, day/night 1 DAP 3 DAP 7 DAP
15°C 1.95±0.29 bcd 1.45±0.29 c 1.62±0.29 d
20°C 1.37±0.29 d 1.75±0.29 bcd 1.68±0.36 bcd
25°C 1.5±0.29 d 1.91±0.29 bcd 1.35±0.38 d
30°C 3.41±0.29 a 3.08±0.29 ab 3.00±0.29 abc
Effect of environmental conditions on fertilization and fruit setting in date palm in
planta
Inordertostudythetemperatureeffectsonthefertilizationandfruitsettinginplanta,
temperature controlled units were designed (Figure 2). Following pollination, date
inflorescenceswereenclosedinthetemperaturecontrolled units on the treesunderthree
temperatureregimes,forfiveweeksinMarch‐May2013.Warmerconditions resulted in
much bigger fruitlets. However, temperaturedid not affect the spikelet elongation and the
distancesbetweenthefruitletsonthespikeletsdidnotvarysignificantly.Atfiveweeksafter
pollination (WAP) there was no significant difference in fruitlet abscission. Under cooler
conditions(20/8°C)increasedlevelsofparthenocarpicfruitsweredetectedcomparedtothe
warmtemperatureregimes(32/18°C)andthecontrol(datanotshown). The fruitlet
developmentandweightat5WAPwashigherastemperatureincreased(Table3).

Figure2. Schematic representation (A) and picture (B) of temperature controlled units
assembledoninflorescencesoffruitbearingdatepalmsintheorchard.

356
Table3. Effect of temperature on fruitlet weight and density in pollinated date palm
bunches, incubated in temperature‐controlled units, 5 and 10 weeks after
pollination(WAP).Experimentswereperformedonfourdifferent bunches per
treatment. Analysis was performed on at least 10 representativefruitsfrom5
spikeletsfrom each bunch.Meansaresignificantly different(P≤0.05)accordingto
theTukey‐KramerHSDtest.
Temperature,
day/night
Fruitlet density 5 WAP
(cm-1 spikelet)
Single fruitlet weight
5 WAP (g)
Single fruitlet weight
10 WAP (g)
20/8°C 1.5 0.91 c 1.48±0.14 c
25/12°C 1.9 1.22 bc 1.78±0.12 c
32/18°C 1.6 3.12 a 3.76±0.14 a
Control 1.5 1.89 b 2.98±0.12 b
Temperature effect on pollen tube growth in planta
Pollengrainsgerminated in planta in all temperatureregimes(Tab le  4). H owever, a t
lowertemperatures(20/8°C)pollengerminationwasnotdetectedat16h afterpollination
andwasalsodelayedat3DAPcomparedwith 25/12,32/18andcontrolplants.Additional
differences in pollen tube elongation rate have been recorded. At moderate temperatures
(25/12°C),pollentubesweresignificantlyshorterat16hafter pollination than at higher
temperatures(32/18°C).At7DAPgrowthofpollentubeswasobservedinallstigmas.Since
thetemperatureconditionswerechangingduringthe daywithineachtreatment,weargue
thatinthelowertemperaturetreatments,activepollen growthhadprobablyoccurredonly
atthehoursofhighesttemperatureduringtheday.Forfertilization,  pollen tubes have to
extendfromthestigmathroughthecarpelstotheovules.Wesuggest that under lower
temperaturesaconsiderablefractionofthepollentubesdidnot reach the ovule, thus
preventing an efficient fertilization.In this experiment, large amo untso fpollen have been
applied. However, the increased occurrence of parthenocarpic fruitsunderlower
temperatures (20/8°C) implies that the reduced elongation of pollen tubes eliminated
efficientfertilization.Thisfactmightalsoexplainreducedfruit settingandyields occurring
incommercialplantationsundercoolerconditions.
Table4. Effect of temperature on pollen germination on stigma in pollinated date palm
bunches, incubated in temperature‐controlled units 1‐7 days after pollination
(DAP). Pollen germination was estimated on a five‐point scale, where 0 = no
germination; 4 = high germination. Analysis was performed on atleasteight
stigmas per treatment. Statistical analysis included samples from different
temperaturesandDAPs.Meansaresignificantlydifferent(P≤0.05)accordingtothe
Tukey‐KramerHSDtest.
Temperature, day/night 0.6 DAP 3 DAP 7 DAP
20/8°C 0 b 0.79 b 2.06 a
25/12°C 0.15 b 2.58 ab 1.80 a
32/18°C 1.33 a 2.88 a 2.06 a
Control 1.33 a 1.88 ab 1.80 a
The presented study demonstrates that temperature regime significantly affects
pollination, fertilization and fruit setting in palms. When temperatures are relatively low,
reductioninfruitsettingmightbecausedbyslowelongationof pollen tubes. Microscopic
analyses are currently being performed to elucidate the environmental effects at different
stages of fertilization and early fruit development in date palms.Wehopethatfurther
calibration of environmental conditions in vitro will allow for betterunderstanding of the 
mechanismsoffruitsettingsindatepalms.
Two systems for evaluation of the fertilization process in palms were developed to
complement each other. Each system had specific advantages, as well as technical and

357
biologicallimitations.Theinvitroapproachfacilitatedadirectstudyoftheinflorescencesat
constanttemperaturesinartificialconditions.However, major limitations in this approach
weretheshort“vaselife”ofthecutinflorescences,contamination, absence of leaves and
hencedisturbanceofhormonalandenvironmentalstimuli,aswellasfastflowersenescence,
probablyduetoethylenereleaseincutinflorescences.Nevertheless,thisapproachallowed
ustofocusonindividualflowersandcarefullyexaminethestigmasandcarpelsinvitro.
Thesizeofmaturedatepalmtreespreventsstudiesincompletely controlled
conditions,i.e.,inaphytotron.Ourstudyofthepalmsinvivo in controlled temperature
units, actually presents a beneficial “modular phytotron” approach.Inthiscase,the
inflorescenceremainsthe integral partofthewhole tree, and itshormonalandnutritional
balance is intact. However, in these experiments, environmental conditions were modified
only in the inflorescences, while temperature effects on the other plant organs were not
modified. We areaware of the limitations of this approach: by enclosing inflorescences in
environmentalcontrolled units,thetreetrunk, leavesand rootsystemsarestillexposedto
the outdoor temperatures and are not controlled. Hence, one of the drawbacks is the
disregardofanyhormonalandotherenvironmentalsignalssuch aslightandhumiditythat
affecttheplantandcanbetransportedfromorgantoorgan.
CONCLUSIONS
Theexperimentsperformedinthecontrolledenvironmentunitspresentthebeneficial
potentialofthisapproachforothersystemsinhorticulture.Theyareapplicablenotonlyto
study fertilization of date palms, but to study the effects of environmental conditions
(temperature,humidity,light) onanylarge tree. Thecontrolledunits can providemodified
conditionsaroundanytreeorgan‐ aninflorescence,fruitbunch,leafortreesection.Useof
severalunitscanalsocreatevariousenvironmentalconditionson thesameindividualtree.
Such units can function as “modular phytotron” to facilitate studies on the whole‐plant
physiologyinfruittrees.
ACKNOWLEDGEMENTS
ThestudywasfundedbygrantsfromtheIsraeliministryofagriculture,byIsraeliDate
Grower’sBoardandbytheJCACharitableFoundation.Wewishto thank Dr.S. Philosoph‐
HadasandDr.S.Meirforhelpfulsuggestions.
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