Deficit irrigation and emerging fruit crops as a strategy to save water in Mediterranean semiarid agrosystems

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Contents lists available atScienceDirect

Agricultural

Water

Management

j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / a g w a t

Deﬁcit

irrigation

and

emerging

fruit

crops

as

a

strategy

to

save

water

in

Mediterranean

semiarid

agrosystems

A. Galindo

a,∗,1

_,

_J.

_{Collado-González}

b,1

_,

_I.

_{Gri ˜}

_nán

c,1

_,

_M.

_Corell

d,e,2

_,

_A.

_Centeno

f,2

_,

M.J.

Martín-Palomo

d,e,2

_,

_I.F.

_Girón

e,g,2

_,

_P.

_Rodríguez

h,2

_,

_Z.N.

_Cruz

h,2

_,

_H.

_Memmi

f,2

_,

A.A.

Carbonell-Barrachina

b,2

_,

_F.

_Hernández

c,2

_,

_A.

_Torrecillas

i,2

_,

_A.

_Moriana

d,e,2

_,

D. López-Pérez

f,2

a_Dept._of_Water_Engineering_&_Management,_Faculty_of_Engineering_Technology,_University_of_Twente,_P.O._Box_217,₇₅₀₀_AE_Enschede,_The_Netherlands

b_Department_of_Agrofood_Technology,_Food_Quality_and_Safety_Research_Group,_Universidad_Miguel_Hernández_de_Elche._Ctra._de_Beniel,_km_3,2._E-03312

Orihuela,Alicante,Spain

c_Department_of_Plant_Sciences_and_{Microbiology,}_Plant_Production_and_Technology_Research_Group._Universidad_Miguel_Hernández_de_Elche,_Ctra._de

Beniel,km3,2.E-03312Orihuela,Alicante,Spain

d_Dpto._Ciencias_{Agroforestales,}_ETSIA,_Universidad_de_Sevilla,_Crta_de_Utrera_Km_1,_E-41013_Sevilla,_Spain

e_Unidad_Asociada_al_CSIC_de_Uso_Sostenible_del_Suelo_y_el_Agua_en_la_Agricultura_(US-IRNAS),_Crta_de_Utrera_Km_1,_E-41013,_Sevilla,_Spain

f_Departamento_de_Producción_Vegetal,_Fitotecnia,_ETSIAAB,_Technical_University_of_Madrid,_Ciudad_{Universitaria}_s/n,_E-28040_Madrid,_Spain

g_Instituto_de_Recursos_Naturales_y_{Agrobiología}_(CSIC),_P.O._Box_1052,_E-41080_Sevilla,_Spain

h_Department_of_Physiology_and_{Biochemistry,}_Instituto_Nacional_de_Ciencias_Agrícolas_(INCA)._Ctra._de_Tapaste,_km_3.5,_San_José_de_Las_Lajas,_Mayabeque,

Cuba

i_Centro_de_Edafología_y_Biología_Aplicada_del_Segura_(CSIC),_P.O._Box_164,_E-E-30100_Espinardo,_Murcia,_Spain

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received16May2017

Receivedinrevisedform31July2017

Accepted15August2017

Availableonlinexxx

Keywords: Jujube Loquat

Partialrootdrying

Pistachio Pomegranate

Regulateddeﬁcitirrigation

Sustaineddeﬁcitirrigation

Underutilizedcrops

Waterstress

Waterrelations

a

b

s

t

r

a

c

t

WaterscarcityinMediterraneanclimateareaswillbeprogressivelyaggravatedbyclimatechange, popu-lationincreaseandurban,tourismandindustrialactivities.Toprotectwaterresourcesandtheirintegrity forfutureuseandtoimprovebiodiversity,besidesfollowingadvanceddeficitirrigationstrategiesinfruit cultivation,attentioncouldwellbedirectedtowardswhatareatpresentunderusedplantmaterialsable towithstanddeficitirrigationwithminimumimpactonyieldandfruitquality.Tothisend,thestate oftheartasregardsdeficitirrigationstrategiesandtheresponseofsomeveryinterestingemerging fruitcrops[jujube(ZizyphusjujubaMill.),loquat(EriobotryajaponicaLindl.),pistachio(PistaciaveraL.) andpomegranate(PunicagranatumL.)]arereviewed.Thestrengthsandweaknessesofdeficitirrigation strategiesandthemechanismsdevelopedbytheseemergingfruitcropsinthefaceofwaterstressare discussed.Theresponseofthesecropstodeficitirrigation,withspecialattentionpaidtotheeffecton yieldbutalsoonfruitqualityandhealth-relatedchemicalcompounds,wasanalysedinordertoassess theirsuitabilityforsavingwaterinMediterraneansemiaridagrosystemsandtoanalyzetheirpotential roleasalternativestocurrentlycultivatedfruitcropswithhigherwaterrequirements.Finally,the fac-torsinvolvedinestablishinganidentitybrand(hydroSOS)toprotectfruitsobtainedunderspecificDI conditionsarediscussed.

1. Introduction

Mediterraneanclimatecountriesincludenot onlythose that bordertheMediterraneanSea(fromSpaintoTurkeyandCyprus andfromMoroccotoSyria)butalsootherregionsoftheplanet, includingSouthern California, Chile,South Africa and Southern

∗ Correspondingauthor.

E-mailaddress:agalindoegea@gmail.com(A.Galindo).

1 _These_authors_contributed_equally_to_this_work.

2 _These_authors_contributed_equally_to_this_work.

Australia.Allarecharacterizedbyhotdrysummers,mostlyrainy wintersandpartiallywetspringandautumn.Intheseregion,to ensureregularcropyieldsandfortoreduceinter-annualyield vari-ability,thescarcerainfallhastobesupplementedbyirrigationin ordertoavoidplantwaterdeficits.Indeed,waterscarcityinthese sitesisdestinedtograduallybecomeworsebecausemorefrequent andseveredroughtseventsdrivenbyclimatechange(Collinsetal., 2009).Moreover,asthepopulationincreases,leadingtoan increas-ingexpansionofurban,touristicandindustrialactivities,tension andconflictbetweenwaterusersandpressuresontheenvironment willbeintensified.

http://dx.doi.org/10.1016/j.agwat.2017.08.015

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Consequently,andconsideringthatMediterraneanagrosystems areveryimportantconsumersoffreshwater,itisofparamount importancetoprotectwaterresourcesandtheirintegrityforfuture

use(Katerjietal.,2008).Inthissense,toovercometheproblems

associatedtoaboostinwaterprices,asthediscouragementof farm-ersandultimatelylandabandonment,García-Tejeroetal.(2014)

indicatedthatanalternativecouldbetoprovidecorrectincentives forfarmerstoadoptchangesintheirirrigationmethodsby imple-mentingstrategiesandtoolsforsustainablewatersaving.Among thestrategiesthatcanbeappliedtoattainwatersavingarethe useofimproved,innovativeandprecisedeﬁcitirrigation(DI) man-agementpracticesabletominimizetheimpactoncropyieldand quality(FernandezandTorrecillas,2012).Inaddition,inorderto contributetowatersaving,fruitcultureshouldbedirectedtowards theuseofplantmaterialsthatarelesswater-demandingorableto withstanddeﬁcitirrigationwithminimumimpactonyieldandfruit quality.

Inthislastrespect,itisimportanttoconsiderthatinhuman history, 40–100,000plantspecies have been regularlyused for food,ﬁber and for industrial, cultural and medicinal purposes. Today,atleast7000cultivatedspeciesareinusearoundtheworld. However,inrecentcenturies,agriculturalsystemshavepromoted thecultivationof avery limitednumber ofcropspecies.While thesehavebeenthefocusofattentionofcommerceandscientiﬁc researchworld-wide,manycropshavebeenrelegatedtothe sta-tusofneglectedorunderutilizedcropspecies,andlargelyignored

(Padulosi et al., 2001; Chivenge et al., 2015).In addition, this

reductioninthenumberofcropspeciesusedforfoodproduction throughouttheworldhasadirecteffectonbiodiversity,whichis fundamentalfor ecosystem functioning,sustainableagricultural production and theattainmentof foodand nutritionalsecurity

(ToledoandBurlingame,2006;ChappellandLaValle,2011).

There-fore,toimprovenotonlybiodiversitybutalsotosavingwaterand henceprotectingtheintegrityofwater resourcesforthefuture, itisnecessarythediversificationofproductionandconsumption habits,includingtheuseofabroaderrangeofplantspecies,in par-ticularthosecurrentlyidentifiedasunderutilizedandneedinga lowinputofsyntheticfertilisers,pesticidesandwater.Thisoption hastobecompatiblewiththeconsolidationofthecultivationof otherMediterraneantraditional crops,suchasolive,almondor grapevine,whicharelowwaterdemandingandprofitablecrops.In thissense,insomecountries,duringrecentdecadestherehasbeen acertaininterestindiversifyingfruittreeproductionby cultivat-ingspecieswithunder-exploitedpotential.Amongtheseemerging cropsmanyarecharacterizedbytheirattractivefruitsand health-relatedqualities,sothattheymayattractconsumerattentionand contributetoproducerprofitability

Forthesereasons,theaimofthisreviewwastopresentthestate oftheartofdeficitirrigationstrategiesandtheresponsetothemof someveryinterestingemergingfruitcrops[jujube(Zizyphusjujuba Mill.),loquat(EriobotryajaponicaLindl.),pistachio(PistaciaveraL.) andpomegranate(PunicagranatumL.)].Tothisend,the follow-ingaspectswereconsidered:(i)thestrengthsandweaknessesof deficitirrigationstrategies,(ii)themechanismsdevelopedbythese emergingfruitcropstoconfrontwaterstress,and(iii)theresponse ofthesecropstodeficitirrigation,payingspecialattentionnotonly totheeffectonyieldbut alsototheeffectonfruitquality and health-relatedchemicalcompounds.

2. Deﬁcitirrigation.Conceptsandstrategies

Tocopewithwaterscarcity, Mediterraneanagrosystems are increasinglylookingtomoreefﬁcienttechnologicalinnovationand irrigationmanagementapproaches. In thisrespect,many coun-trieshaveshiftedfromirrigating crops inorder tosatisfytheir

evapotranspirationrequirements(ETc)orfullirrigation(FI),the conventionalnormwhichseekstomaximizecropyieldperunitof land,todeficitirrigation(DI)strategies,whichinvolvereducingthe amountofwaterprovidedtothecropduringthegrowingseasonby thesoilmoisturestock,rainfallandirrigationtoalevelbelowthat neededformaximumplantgrowth.InmostofcasesDIinducesa gradualwaterdeficit,duedepletionofsoilwaterreserves, accom-paniedbyareductioninharvestableyields,especiallyinsoilswith asignificantlylowwaterstoragecapacity.

Whenwaterscarcityis theconsequenceofuncontrolled fac-torsandwatersupplyisnotguaranteed,farmersfinditdifficultto scheduleanyreasonableDIstrategy.Incontrast,ifgrowershavea guaranteedwatersupplyfortheircropsduringthegrowingseason, itispossibletoimprovewaterproductivity(WP)bydrawingup DIstrategiesbasedonscientificprinciples,attemptingtoproduce near-maximumyieldsevenifcropsareprovidedwithlesswater thantheywouldotherwiseuse(maintainingcropconsumptiveuse belowitspotentialrate).Inotherwords,improvingthemarketable yieldperunitofwaterusedratherthanattainingmaximumyields

(Kijneetal.,2003;Zhang,2003)Complementaryadvantagesofthe

sameincludeareductionofnutrientlossfromtherootzoneand adecreaseinexcessivevegetativevigour,accompaniedbyalower riskofcropdiseaseslinkedtohighhumidity(GoodwinandBoland,

2002;Ünlüetal.,2006)(Table1).However,thereisashortageof

researchintotheriskofsoilsalinizationasaconsequenceofany decreaseintheleachingofsaltsandtheuseoflowqualityirrigation water(Bolandetal.,1996;Kamanetal.,2006)(Table2).

ThreemainDIstrategiescanbementioned;sustaineddeficit irrigation(SDI),inwhichirrigationwaterusedatanymoment dur-ingtheseasonisbelowthecropevapotranspiration(ETc)demand, andtwoothers,bothbasedonphysiologicalaspectsoftheresponse ofplantstowaterdeficit−regulateddeficitirrigation(RDI)and partialroot-zonedrying(PRD)(Fig.1).

2.1. Sustaineddeﬁcitirrigation(SDI)

Attheendof1970s,trialsapplyingirrigationwateramounts belowtheETcdemandbutatveryfrequentintervalstookplace withencouragingresults.Calleddeﬁcithigh-frequencyirrigation (DHFI),this strategyproved unsuccessfulwhenlittlewater was storedinthesoil.ItwasonlypossibletouseDHFIandobtain max-imumyieldswhenETcwasreachedthroughthecombinationof irrigationwaterappliedandsoilwaterdepletion(Fereresetal.,

1978).

Infact,theDHFIstrategyisverysimilartoSDI(Fig.1),whichis basedontheideaofallottingthewaterdeﬁcituniformlyoverthe wholefruitseason,thusavoidingtheoccurrenceofseriousplant waterdeﬁcitatanycropstagethatmightaffectmarketableyield orfruitquality,ordistributingtheirrigationwaterproportionally toirrigationrequirementsthroughouttheseason.

2.2. Regulateddeﬁcitirrigation(RDI)

RDIworksonthepremisethattranspirationismoresensitive towaterdeficitthanphotosynthesisandfruitgrowth,andwater deficit-inducedroot-sourcedchemicalsignalslikeABA.Thus,fruit treescopewithareducedwatersupplybyreducingtranspiration (stomataregulationorreducingleafsurfaceareathrough reduc-ing leaf growth) (Wilkinsonand Hartung, 2009). In this sense, fruittree sensitivitytowater deficit is notconstant duringthe wholegrowingseason,andawaterdeficitduringparticularperiods maybenefitWPbyincreasingirrigationwatersavings, minimiz-ingoreliminatingnegativeimpactsonyieldandcroprevenueand evenimprovingharvestquality(Chalmersetal.,1981;McCarthy

etal.,2002;Domingoetal.,1996)(Table1).Therefore,whenaRDI

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drought-Table1

Keyadvantagesofdeficitirrigation(DI)strategies:sustaineddeficitirrigation(SDI),regulateddeficitirrigation(RDI)andpartialrootdrying(PRD)withanon-exhaustivelist

ofreferences.

DIstrategy Advantage References

SDI,RDIandPRD

Maximizethewateruseefﬁciencyandwaterproductivity(WP) Liuetal.(2006a);Liuetal.(2006b);Saeedetal. (2008);GeertsandRaes(2009);Ahmadietal. (2010).

Minimumimpactsonyieldscanbeachievedwhenprecisiontoolsareusedto

managemildDI

García-Orellanaetal.(2007);Ortu ˜noetal. (2009)

Reducesnutrientlossfromtherootzone,improvinggroundwaterqualityand

loweringfertilizerneedsontheﬁeld.

Ünlüetal.(2006);GoodwinandBoland(2002)

Decreasetheriskofcropdiseaseslinkedtohighhumidity GoodwinandBoland(2002);

RDI

Improveswatersavingsandevenharvestquality Chalmersetal.(1981);McCarthyetal.(2002)

Reducesexcessivevegetativevigour GoodwinandBoland(2002).

Canbescheduledusingonlytrunkdiametersensors Conejeroetal.(2011);Girónetal.(2015).

PRD

Itcanbeoperatedinfurrowordrip-irrigatedcrops Grimesetal.(1968);SamadiandSepaskhah

(1984).

Despiteareductioninstomatalconductance,cropsmaintainafavourable

waterstatus

Santosetal.(2003);KangandZhang(2004)

Thequantityandqualityoftheharvestcanbeimprovedasaconsequenceof

carbohydratespartitioningbetweenthedifferentplantorgans

KangandZhang(2004)

Table2

Keyconstraintsofdeficitirrigation(DI)strategies:sustaineddeficitirrigation(SDI),regulateddeficitirrigation(RDI)andpartialrootdrying(PRD)withanon-exhaustivelist

ofreferences.

DIstrategy Constraint References

SDI,RDIandPRD

Atalltimesitisessentialtoaccesstoaminimumquantityofwater,below whichDIhasnosigniﬁcantbeneﬁcialeffect

Zhang(2003)

Shortageofresearchonsoilsalinizationrisksasaconsequenceofthedecrease

ofleachingofsaltsandtheuseoflowqualityirrigationwater.

Cropssustainsomedegreeofwaterdeﬁcitandsomeyieldreductionexcept

whensoilwaterdepletionsupplementsirrigationtoreachingETc

Fereresetal.(1978);Costaetal.(2007)

SDI Yielddecreaseisduemainlytodecreaseinfruitweight CastelandBuj(1990)

RDI

Themaintenanceofplantwaterstatuswithinnarrowlimitsofwaterdeﬁcit

duringnon-criticalphenologicalperiods.Suddenchangeinevaporative

demandrisksseverelossesofyieldandfruitquality

Jones(2004)

Newandmoreprecisecriteriafordeﬁningwaterdeﬁcitareneeded,because

criteriabasedonETccanhaveunpredictableﬁnaleffectontherhythmof

waterdeﬁcitdevelopmentacrossarangeofdifferentgrowingconditions

(species,weather,soildepth,fruitload,rootstock).

Shackeletal.(1997);Marsaletal.(2008)

Irrigationmanagementinheavyanddeepsoilsbecausesoilwaterdepletion

andreﬁllcantakeplacetooslowly

Gironaetal.(1993)

Scarcityofdetailedstudiestoknowtheeffectofwaterdeﬁcitonbud

development

Naoretal.(2005);Marsaletal.(2008)

PRD Do_fornot_eachexist_rootdeﬁnite_systemsolid_sidecriteriaondeﬁningtheoptimumtimingofirrigation Saeedetal.(2008)

Itisnotpossibletohaveabsolutecontrolofrootdryingunderﬁeldconditions

andhydraulicredistributionfromdeepertoshallowerrootsmaypreventthe

clearresultsthatcanbeobtainedinpottedplants

Bravdo(2005)

sensitivephenological stages (criticalperiods) offruittrees and irrigationislimitedorevenunnecessaryifrainfallprovidesa min-imumsupplyofwaterduringthedrought-tolerantphenological stages(non-criticalperiods)(Chalmersetal.,1981;Mitchelland

Chalmers,1982;GeertsandRaes,2009)(Fig.1).

Stonefruitgrowthfollowsadouble-sigmoidalpatternwithtwo periodsofrapidgrowthseparatedbyaperiodduringwhichlittleor noexpansivegrowthoccurs.Theﬁrstgrowthperiod,stageI,isdue tocelldivisionandcellexpansion;stageIIistheperiodinwhich scleriﬁcationofthefruitendocarptakesplaceandfruitgrowthis extremelyslowornull,andstageIIIisthesecondperiodoffruit growth,whichisrapidduetotheexpansionofexistingcellsand extendsfromtheonsetofthissecondgrowthperioduntil matu-rity.PomeandCitrusfruitsshowonlyaphaseofrapidfruitgrowth (single-sigmoidalpattern),whichtakesplaceaftertheinitialperiod ofcelldivisionandminimalexpansion,andisduemainlytoacell expansionprocesseventhoughsomecelldivisionmayalsotake placeatthebeginning(Rodríguezetal.,2017).

Instonefruittrees,twocriticalperiodshavebeenidentified. Thefirstonecorrespondstothesecondrapidfruitgrowthperiod (stageIII),whendroughtstressinducesareductioninyielddue tothesmallerfruitsizeatharvest,andthesecondcriticalperiod istheearlypostharvestperiod,whendroughtstressaffectsflower budinductionand/orthefloraldifferentiationprocessesthatoccur atthistime.Thisleadstoalowergerminationpotentialinthepollen ofthenextbloomandencouragesyoungfruittodropinthe follow-ingseason(Uriu,1964;Ruiz-Sánchezetal.,1999;Torrecillasetal., 2000).InotherPrunusspecies,suchasalmond(Prunusdulcis(Mill.) D.A.Webb),floweringandrapidvegetativeandfruitgrowthstages (stagesIIandIII)andpostharvest(stageV)havebeenreportedas criticalperiodsbecausewaterdeficitaffectsyield(Goldhamerand

Smith,1995;GoldhamerandViveros,2000;García-Tejeroetal.,

2017).

InpomeandCitrusfruitsrapidfruitgrowthcanbeconsidered asacommoncriticalperiod.InanexperimentinFinolemon(Citrus limon(L.)Burm.ﬁl.)treesoverfourseasons,Domingoetal.,(1996)

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Fig.1. Graphicpatternoffullirrigation(FI),sustaineddeﬁcitirrigation(SDI),regulateddeﬁcitirrigation(RDI)andpartialrootdrying(PRD)strategiesinfruittrees.

growthphase,whenwaterdeficitcausesadelayinattaining mar-ketablefruitsize,whereasmoderatewaterdeficitappliedduring flowering-fruitset-fruitcelldivisionperiodisnotcriticalinterms ofyield.Infact,theeffectofwaterdeficitappliedduringthislast phenologicalperiod onyield isrelatednot onlywiththewater deficitlevelachievedbutalsowiththeplantspecies.InSalustiana orangetrees(Citrussinensis(L.)Osbeck)onsourorangerootstock (Citrusaurantium L.),Castel and Buj(1990)attained a decrease inyieldofonly4%,whereasGinestarandCastel(1996)observed thatClementinadeNules(CitrusclementinaHortexTan)on Car-rizocitrange(CitrussinensisOsb.×Poncirustrifoliata(L.)Raf.)were extremelysensitivetowaterrestrictions(yielddecrease)during thisperiod.

In very early maturing fruit trees, witha very short period fromfruitsettoharvestandaverylongpost-harvestphenological period,deﬁcitirrigationshouldbeappliedonlyduringthe post-harvestperiodeventhoughavoidingaffectbudinductionandﬂoral differentiationprocesses(Torrecillasetal.,2000;Conejeroetal.,

2011).

Takingintoconsiderationthattheeffectsofwaterdeficitdepend notonlyonthetimingbutalsoonthedurationandmagnitudeofthe same,theplantwaterstatusduringnon-criticalperiodshastobe maintainedwithincertainlevelsofwaterdeficitinordertoprevent amoderate,potentiallybeneficial,droughtstressfrombecoming toosevereandendinginreducedyield(Table2)(Johnsonetal.,

1992;KangandZhang,2004).Inthissense,problemshavebeen

foundinmaintainingacertainlevelofplantwaterdeﬁcitbecause, whenlow amounts ofirrigationwater areapplied,adverse sit-uationssuchasasuddenincreaseintemperaturemayresultin severelossesofyieldand quality,(Table2)(Jones,2004).Other problemshavebeenfoundwhenapplyingRDIinheavyanddeep soilsbecausesoilwaterdepletionandreﬁllfrequentlytaketoolong

(Table2)(Gironaetal.,1993).Under this situation,thesuccess

ofRDIdependsstronglyontheappropriateuseofmicroirrigation techniquesandsensorsabletoproviderealtimeinformationon soilandplantwaterstatus(Dichioetal.,2007;Ortu ˜noetal.,2009). Inrecentyears,theuseofplant-basedwaterstatusindicators hasbecomeverypopularforplanningmorepreciseirrigation pro-grammes,becauseitisrecognizedthatthetreeitselfisthebest indicatorofitswaterstatus(Table1)(Shackeletal.,1997;

García-Orellanaetal.,2007;FernandezandCuevas,2010).Inthissense,

sensorslikelinearvariabledisplacementtransducers(LVDTs)are abletomeasuredailytrunkdiameterﬂuctuations(TDF)withgreat precision,generatingsensitiveparameterswhichstrongly corre-latewithestablishedplantwaterstatusparameters(Fernandezand

Cuevas,2010;Ortu ˜noetal.,2010).Themostcommonand

use-fulTDFparametersfortheirrigationschedulingofwoodycrops are maximum daily trunk shrinkage (MDS) and trunk growth rate(TGR)(Ortu ˜noetal.,2010;Morianaetal.,2013).Moreover, theoperationaladvantagesofTDFmeasurementsin adulttrees, suchasthepossibilityofconnectingremotelyoperatedirrigation automaticdevices, andtheabilitytorapidlyadjustschedulesin response tothedailysignal, makethem very suitabletools for preciseRDIscheduling(Conejeroetal.,2011;Girónetal.,2015). 2.3. Partialrootdrying(PRD)

This DI strategy, which has also been called partial root-zoneirrigation,canbeappliedthroughalternatefurrowirrigation

(Grimesetal.,1968)andbysurfaceandsubsurfacedripirrigation

(Table1)(SamadiandSepaskhah,1984),andisbasedon

irrigat-ingonlyonepartoftherootzone,leavinganotherparttodrytoa certainsoilwatercontentbeforerewettingbyshiftingirrigationto

thedryside(DryandLoveys,1998;SepaskhahandAhmadi,2010)

(Fig.1).

Thestrategyisbasedintheideathat,inPRD,rootssensesoil drying,triggeringthesynthesisoftheplanthormoneabscisicacid (ABA),whichreducesleafexpansionand stomatalconductance, while,simultaneously,theroots ofthewatered side ofthesoil absorbsufﬁcientwatertomaintainafavourableplantwater

sta-tus(Table1)(Liuetal.,2006a;Zegbeetal.,2006;Ahmadietal.,

2010).Inaddition,othercomplementaryphysiologicalresponses toPRDcanfavourstomatalclosuresuchaslowercytokinelevels

(Stolletal.,2000;Daviesetal.,2005)andhigherxylempH(Davies

andZhang,1991;Stolletal.,2000).Otherresultsingrapevine(Vitis

viniferaL.)indicatedthatPRDmayalsoincreaserootgrowth(Dry

etal.,2000)

Currently,nodeﬁnitivesolidcriteriaexistfordecidingthe opti-mumtimingofirrigationforeachside(Table2),probablydueto thediversityoffactorsinvolved,suchasevaporativedemand,soil

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characteristics,soilwaterstatusatanyprecisemoment,crop phe-nologicalstage,etc.,anyofwhichmaydeterminetheplantresponse towettingordryingofeachsideofroots(Saeedetal.,2008).Inthis sense,thetimewhensoilwaterextractionfromthedrysideis neg-ligiblehasbeenproposedastheoptimumtimetoswitchwetting fromtheirrigatedrootsidetothenon-irrigatedside(Kriedmann

andGoodwin,2003).Also,thethresholdsoilwatercontentatwhich

themaximumxylemABAconcentrationisproducedwasproposed

byLiuetal.(2008)asacriterionforswitchingirrigation.

SomeauthorsshowedthatcropsunderPRDgavebetteryields thanthesamecropsunderDIwhenthesameamountofwateris applied.ThisresultedinhigherWPandevenbetterfruitquality

(KriedmannandGoodwin,2003;KangandZhang,2004;Liuetal.,

2006a,b).However,Wakrimetal.(2005)reportednosigniﬁcant

differenceinwateruseefﬁciencies(WUE)betweenPRDandDI, butasubstantialincreaseinWUEwhenPRDwascomparedwith FI.

3. Emergingfruitcropsresponsetodeﬁcitirrigation 3.1. Jujube(ZizyphusjujubaMill.)

Jujubetree(familyRhamnaceae)isnativetoChina,where it hasbeencultivatedformorethan5000years,andto neighbour-ingareasofMongoliaandtheCentralAsianRepublics.Withtime, itscultivationhasspreadtootherregionsoftheworld,including toMediterraneancountries.Jujubefruitisanintegralpartofthe cultureandwayoflifeofmillionsofpeopleandhasalsobecome importantformanyregionsoftheworldfollowingitsintroduction

(Azam-Alietal.,2006);indeed,itcanbeconsideredaso-called

func-tionalfood,sinceithasnutritionalaswellasmedicinaluses(Choi etal.,2011).Nevertheless,untilnowjujubehasbeenconsideredof minorimportanceand,fromaresearchanddevelopmentpointof view,ithasreceivedlittleattentionfrommostgovernments.

Jujubeisabletowithstandseveredroughtduringthegrowing season(Fig.2A)andtotolerateverylowwintertemperatures dur-ingitsdormancy(Dahiyaetal.,1981;MingandSun,1986;Ming andSun, 1986). Inthis sense,jujube treesareable tomaintain leafturgorunderseverewaterdeﬁcit(stem<−3.0MPa),

essen-tiallybydevelopingtwocomplementarymechanisms–leafactive osmoregulation(stresstolerancemechanism)andthecontrolof waterlossviatranspiration(stressavoidancemechanism),while allowingsubstantialgas exchangerates and, asa consequence, goodleafproductivity(Maetal.,2007;Cruzetal.,2012;Galindo etal.,2016).Thegradualrecoveryof leafconductanceafter re-wateringpreviously stressedplantscanalsobeconsideredasa mechanism for promoting leaf rehydration (Cruz et al., 2012). Moreover,thehighleafrelativeapoplasticwatercontent(RWCa)

levelsandthepossibilityofincreasingtheaccumulationofwater intheapoplasminresponsetowaterstresssupportsasteeper gra-dientinthewaterpotentialbetweentheleafandsoil(Cruzetal.,

2012).

Galindoetal.(2016)showedthatincontrastwiththeaxiomthat

expansivecellgrowthrequiresthepresenceofcellturgor,nodirect relationbetweenturgorand growthrateexistsinjujube fruits. Thiscouldbeduetoanenhancementofacellelasticity mecha-nism(elasticadjustment),whichwouldmaintainfruitturgoreven atseverewaterstresslevelsbyreducingfruitcellsize,ortothefact thatjujubefruitgrowthdependsonfruitgrowth-effectiveturgor ratherthanjustonturgorpressure.Theseauthorsalsoreportedthat duringmostofthefruitripeningstagewatercanenterthefruits viathephloemratherthanviathexylem.Thiscouldberelated withtheincreaseinsensitivitytodroughtduringthisphenological period,whenmoderateandseverewaterdeﬁcitsinducea signif-icantreduction intotal marketablefruityield(numberof fruits

and/oraveragefruitweight).Incontrastwiththislastidea,Cuietal.

(2008,2009)concludedthatthejujubefruitmaturationstageisthe

optimalstagetoimplementwaterdeficitstrategiesandthatwhile waterdeficitduringthefruitgrowthslightlyreducedthegrowth rate,re-wateringhadanover-compensatoryeffect,thusreducing thenegativeinfluenceonfruitsize.

Thesameauthors(Cuietal.,2008,2009)mentionedthe rel-ativelylow waterrequirementsofaround360mmand showed thatjujubefruitmaturationcanbeadvancedandthefruityield and quality enhanced if appropriate RDI is applied at certain growthstages (budbursttoleaﬁngandfruit maturation).Also,

Gao etal.(2014)showedthat jujubefruit respondedpositively

to irrigation practices, the concentration of some taste-related (e.g. glucose, fructose, TSS and malic acid) and health-related (e.g.catechinandepicatechin)compoundsbeinggenerallymuch higherindripirrigatedfruits.Inthissensetoo,Collado-González

et al.(2013)demonstrated that water deﬁcitdidnotaffect the

tendencyofprocyanidinstoself-aggregatebutincreasedthe con-tentofprocyanidinsoflowmolecularmass(Table3),improving theirpotentialbioavailabilityandpossiblephysiologicaleffectson humanhealth.Theprocyanidincontentoffruitfromwell-watered treesincreasedduringdomesticcoldstorage,whereasthefruits fromtreessufferingseverewaterstresslostsomeoftheir procyani-dincontent.Moreover,in asubsequentpaper,Collado-González

etal.(2014)pointedtoacertainproportionalityintheresponse

ofjujube fruitstomoderate andseveredeﬁcit irrigationduring fruitmaturation.So,whenplantswereexposedtomoderatewater deﬁcit(stemfrom−1.40to−2.28MPa)duringthisphenological

periodtherewasnochangeinfruitsize,moisturecontent, ﬁrm-ness,orfruitpeelandﬂeshcolourcomparedwithfullyirrigated trees.Onlywhenamoreseverewaterstress(stem from−1.40

to−3.14MPa)wasreached,thereweresignificantincreasesinthe sucroseandarabinosecontentsmeasured(Table3).Inaddition,the responseoffruitaminoacidstowaterdeficitwasnotassensitive asexpected,sincetherewasnodirectrelationshipwiththe magni-tudeofthewaterdeficit.However,thedecreaseinfruitasparagine contentas aresult ofseverer waterdeficit isa positive aspect, becausethisaminoacidisthemajorprecursorofacrylamide,a potentiallytoxiccompoundformedduringtheheat-processingof someplantfoods.However,severewaterdeficitproducedsmaller fruit,withalowermoisturecontentandyield,accompaniedby changesinfirmnessandpeelandfleshcolour.

3.2. Loquat(EriobotryajaponicaLindl.)

Loquatisasubtropicalevergreentreethatbelongstothefamily Rosaceae,subtribePyrinae(formerlysubfamilyMaloideae)(Potter etal.,2007).SomeofthecommonnamesofloquatincludeJapanese plum,Japanesemedlar,Malteseplum,etc.Itisconsidered indige-noustosoutheasternChinaandpossiblysouthernJapan,becauseit issaidtohavebeencultivatedthereforover1000years.Actually, morethan30countriesinsubtropicalandmild-temperateregions oftheworldarecultivatingselectionsofloquatcultivarsperformed duringthe19thcentury(Fengetal.,2007;Ferreresetal.,2009;He

etal.,2011).

Itisimportanttopointoutthatloquatischaracterizedbyan unusualphenologythatmakesitdifferentofthetraditional tem-peratefruitcrops.Itbloomsinautumnonapicalpaniclesformedon currentyearwood,developingfruitsduringwinterandripeningin earlyspring(Fig.2B).Moreover,thisfruitasotherpomespresents asigmoidalpatternoffruitgrowth(Dennis,1988;Cuevasetal., 2003)andarrivesatmarketsbeforeanyotherspringfruit(Cuevas

etal.,2007a;HuesoandCuevas,2008).

Researchonmechanismsdeveloped byloquatplanttoresist droughtisveryscarce,mainlyatplantwaterrelationslevels. Diur-naland seasonal gasexchange values in loquatplants respond

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Fig.2. Seasonalpatternoffruitandshootgrowthofjujube(Z.jujuba,cvGrandedeAlbatera)(A),loquat(E.japonica,cvAlgerie)(B),pistachio(P.vera,cvKerman)(C)and

pomegranate(P.granatum,cvMollardeElche)(D)plantsinthesoutheast(A,BandD)andcentral(C)Spainconditions.Sources:Hernándezetal.(2015),Cuevasetal.(2007a),

Memmietal.(2016b)andMelgarejoetal.(1997),respectively.

tochangesin plantwaterstatus and tochanges in evaporative demand,showingminimumvaluesinsummer.Moreover,the diur-naltrendofphotosyntheticrateinloquat,atleastduringautumn andwinter,wascharacterizedbyadouble-pickedcurve,suggesting thepredominanceofgenotypeovertheenvironmentalfactorson theloquatgasexchangebehaviour(Stellfeldtetal.,2011),because thislastbehaviourdivergesfromthatindicatedforwoody Mediter-raneanvegetation,which ischaracterizedbya maximumvalue inthemorning,decliningtowardsmidday,andremainingmore orlessconstantafterward.Recently,Zhangetal.(2015)observed someloquatdroughtstresstolerancemechanisms:i)theincrease inchlorophyllcontent,whichcanenhancesphotosynthesisunder waterdeficit,ii)theincreaseinthecontentofsolublesugarsand prolineofroots,whichincreasedtheosmoticadjustmentandthe favourablewaterpotentialgradientforwaterintotheroots,iii)the increaseintheABAcontentofleaves,whichinducedthestomata closingandimprovedthewater-useefficiency,andiv)theincrease inthelevelsofantioxidantenzymeactivitiesmainlyatleaflevel. Theabilityofloquatplantstodevelopleafactiveosmoregulation wasearliersuggestedbyGarcía-Legazetal.(2005)whostudied theeffectofsalinity onthewater relationsof loquatplantson twodifferentrootstocks. Luoetal. (2007)studiedtheresponse oftwo differentloquatcultivars towaterdeficitand concluded that‘ChanghongNo.3’,themorewaterdeficitresistantcultivar, respondstowaterdeficitwithahigherincreaseinstomataldensity andreducingstomatalsizethan‘Jiefangzhong’cultivar.Inaddition,

in‘Jiefangzhong’cultivar,leafphotosyntheticpigment concentra-tionsdecreaseinresponsetodroughtstress,whilein‘Changhong No.3’ theconcentrations ofphotosyntheticpigmentsincreased markedlyunderlightdroughtstress.

HuesoandCuevas(2008)estimatedrelativelyhighloquatwater

needsof around724mm,and demonstratedobservingthelong termresponseofthiscroptopostharvestRDIthatthiscropcan beconsideredasamodel forthecontinuousapplicationof RDI strategies,mainlyfortheeconomicbeneﬁtsofsavingwater dur-ingsummer,increasingfruitsizeandgradingandfruitvalueand grossrevenuewithoutaffectingyield(HuesoandCuevas,2008;

Cuevasetal.,2009).ThispositiveresponseofloquattoRDIisbased

intwo mainfacts;theclearseparation betweenvegetative and reproductivegrowth,allowingtheapplicationofpostharvestRDI withoutaffectingfruit growth,and theimprovingoffruitvalue whenpostharvestRDIisappliedbecauseimportantadvancing har-vesttime inthenextseasoncanbeachieved.Inthis sense,the mostproﬁtableRDIstrategyisthecompletesuppressionof water-ingfromaroundonemonthaftertheendofpreviousharvest(early June)uptoreachastemvaluecirca–2.2MPa(8–9weeks),because

donot altertheformationofthefloralorgans andincreasethe advancementofbloomnextseason(Cuevasetal.,2007a,b,2009, 2012),whileprolongingthewaterdeficitperiodduringone addi-tionalmonth(August)mayimpairflowerdevelopmentinloquat

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Table3

Effectofdifferentdeficitirrigation(DI)strategies(SDI,sustaineddeficitirrigation;RDI,regulateddeficitirrigation)onhealth-relatedcompoundscontent(↑,increased;↓,

decreased;≈,noaffected)intheedibleportionofjujube,pistachioandpomegranatefruitswithanon-exhaustivelistofreferences.

FruitCrop DIstrategy Compound Responseto

moderatewater deﬁcit Responseto severewater deﬁcit References Jujube(Ziziphus jujubaMill.) SDI/RDI Epicatechin ↑ ↑ Collado-Gonzálezetal. (2013)

TotalBtypeprocyanidins ↑ ↑

Self-aggregated procyanidins ≈ ≈ SDI/RDI VitaminC ≈ ↑ Collado-Gonzálezetal. (2014)

Sugars Sucroseorarabinose ↑ ↑

Glucose ↑ ↑

Organicacids Malicoroxalic ↑ ↑

Citric ≈ ↓

Proline ≈ ↑

Asparagine ≈ ↓

Otheraminoacids Nouniform

behaviour

Nouniform

behaviour

SDI Flavonoids Epicatechinorcatechin ↓ ↓

Gaoetal.(2014)

Procyanidins ≈ ≈

Rutin ↓ ↑

Quercetin ↑ ≈

Totalphenoliccompounds ≈ ≈

Sugars(sucrose,glucoseor

fructose)

↓ ↓

Organicacids(malic,

succinicorcitric)

↓ ↓

Ascorbicacid ≈ ≈

Pistachio(Pistacia

veraL.)

RDI Fattyacids Oleicorpalmitic ≈ ≈

Carbonell-Barrachinaetal. (2014)

Linoleic ↑ ↑

Volatilecompounds Aldehydes ↓ ↑

Pyrazinesandterpenes ↑ ↓

Pomegranate(Punica

granatumL.)

Anthocyanins ≈ ↓

Menaetal. (2013)

SDI Phenoliccompounds ↓ ↓

Punicalagin ↓ ↓

Ellagicacid ≈ ≈

DuetothefactthatresearchonloquatresponsetoRDIhasbeen alwaysfocussesonfruitearlinessduetoitsenormousimportance inloquatpriceandcommercializationandthehigh susceptibil-ityofmatureloquatfruitstomechanicaldamageduringharvest andpostharvest handling,tothebestofourknowledge,do not existpublicationsontheeffectofdeﬁcitirrigationonloquat qual-ity.Sinceloquatis anon-climactericfruit,prematurepickingis inadvisablebecausefruitsareexcessivelyacidandtaste unpalat-abletoconsumers.Thus, researchhasbeenfocussedtostablish fruitmaturityindicesinordertooptimizeharvesting.Pinillosetal.

(2011)suggestedthatateverypickingdate,onlythosefruitswith

askincolourthatcorrespondstoaminimumTSSandTSS/TA val-uesshouldbeharvested,especiallyattheearliestharvestsofthe season.Also,aTSS/TAof0.7waspreviouslyproposedasminimum valueforharvest(Pinillosetal.,2007).Recently,Ca ˜neteetal.(2015)

showedtheconsumerspreferenceforlightorangeskinfruitsrather thanfullyripeonesduetotheirgreaterﬁrmness,fewerskindefects andbetterbalancebetweensweetnessandacidity,andproposed harvestingloquatfruitswithaminimumvalueofTSSof10Brixand aTSS/TAratiocloseto1.0toguaranteeeatingqualityandconsumer satisfaction.

3.3. Pistachio(PistaciaveraL.)

PistachiotreeisnativetowesternAsiaandAsiaMinor,from SyriatotheCaucasusandAfghanistan.Theyarementionedinthe OldTestament.ArchaeologicalevidencefromTurkeyindicatesthat thenutswerebeingusedforfoodasearlyas7000B.C.Pistachiois amemberoftheAnacardiaceaeorcashewfamily,andistheonly commerciallyediblenutamongtheelevenspecies inthegenus

Pistaciaandbyfarthemosteconomicallyimportant.Thepistachio wasintroducedintoItalyfromSyriaearlyintheﬁrstcenturyA.D., and subsequently itscultivationspread tootherMediterranean countries.

Pistachiotreesareconsideredoneofthemostdrought resis-tantfruitspecies,becausetheycansurviveunderextremedrought conditions.Spiegel-Royetal.(1977)observedthatunderdesert conditions pistachio trees were able to differentiate sufﬁcient ﬂowerbudstoprovideanappreciableyieldandthatrootswere uniformlyspread downtoadepthof2.40 mevenifsoil mois-tureinallthehorizonswasbelow thepermanentwiltingpoint ofsoil.Relatedwiththislastcharacteristic,someauthors,e.g.Lin

etal.(1984)andGermana(1997),suggestedthatpistachiodrought

resistancemainlydependsonextensiverootsystemdevelopment, because,despiteitcommonlybeingthoughaxerophyte,itdoes notpresentthemorphologicalcharacteristicofsuchintheleaves, showing,instead,highvaluesofnetphotosynthesis(Pn)andleaf conductance(gleaf).Furthermore,theleavescanbeconsideredas

isolateralssincetheirupperandlowerpagesarestructurally simi-lar,withalmostidenticalstomataldensityandconductance.Also,

Kanberetal.(1993)showedthatrootactivityisconﬁnedto

shal-lowersoildepthsinshortintervalirrigationconditions.Another singularityofpistachiotreesisthatbothyieldandthewaterstress levelregulatetheflowerbuddropthatoccursbeforethe begin-ningofkernelgrowth.So,thefollowingyear’spistachioyieldcan decreaseconsiderablyasaconsequenceofahigherpercentageof flowerbudsdroppedi)inyearsofhigheryieldorii)whenasevere waterdeficitduringfruitstageIItakesplace(Pérez-Lópezetal.,

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Pistachioplants exposed to water stressalso develop stress avoidanceandstresstolerancemechanisms.Asregardtheﬁrstsort ofmechanism,duringpistachiofruitstagesIandII(Fig.2C),when thesoilwatercontentisquitehighandtheevaporativedemand oftheatmosphereislow,theseplantsshowhigherPnandgleaf

values.Incontrast,duringfruitstageIII,atwhichtheevaporative demandoftheatmosphereis higher,thepistachioplantsshow lowerPnandgleafvalues(netal.,2011,b;Memmietal.,2016a,b).

Whenplantsareunderwaterdeﬁcit,gleafvaluesdecreaseinorder

tolimitwaterlossthroughtranspiration,andatverypronounced levelsofwaterdeﬁcit,thedailypatternofgleafismodiﬁed,showing

maximumvaluesintheearlymorninganddecreasinggradually, whereasPnvaluesremainfairlyconstantuntilsunsetbecausethis parameterisless sensitivetowaterdeﬁcit thangleaf (D.

Pérez-López,unpublisheddata).Inthisrespect,Behboudianetal.(1986)

establishedthatpistachioplantsareabletocontinuetheir photo-syntheticactivityevenwhenleafreachesextremelylowvalues

of−5.0to−6.0MPa.Moreover,thiscrophasanoutstanding capa-bilityforleafthermoregulation,evenatseverwaterstresslevels, becausepistachiocanopiescantranspirewateratratesfarhigher thanthosenormallyfoundinmesophytes,andareabletorapidly compensatewaterlosseswithoutshowingvisiblestresscondition symptoms(Germana,1997).Inaddition,whenpreviouslywater stressedplantsarere-watered,thegradualandslowrecoveryof theplantwaterstatusobservedcanbeconsideredasamechanism forpromotingleafrehydration(Memmietal.,2016b).Asregardthe developmentofstresstolerancemechanisms,Gijónetal.(2011)

identiﬁedchangesintheleafbulkmodulusofelasticityduringpit hardening(stageII) andactive osmoticadjustmentatany phe-nologicalperiod.Similarly,Behboudianetal.(1986)showedthat pistachioplantsataleafvalueof−6.0MPaexhibitedveryhighp

values(3.0MPa).

Pistachio’swater relations are signiﬁcantlyaffectedby root-stock. According to Gijón et al. (2010), the hybrid from crossbreedingP.atlanticaDesf._×P.veraL.maybethebest root-stockforadequatelyirrigatedpistachiossinceitinducesthehighest leafconductanceandvigour,whereasinrainfedordeﬁcitirrigated conditions,P.terebinthusmightbeagoodchoiceforitsdrought tol-erance,asitisabletomaintainagreaterleafareathannon-stressed plantswithlowerstemandgleafvalues.However,incontrastwith

theseresultsandthewidespreadbelief,Memmietal.(2016b) sug-gested thatP.atlantica could bea suitable rootstockfor deﬁcit irrigatedplants.

Becauseofitsreputationforbeingveryresistanttowaterstress, pistachioismainlycultivatedworldwideunderrain-fedconditions. Despitethegoodcropperformanceunderthesedrylandconditions, thereisacleartendencytoincreasetheareadedicatedto irriga-tionbecausethebeneﬁtsderivedfromirrigationinthiscropare probablyhigherthaninothercrops.Irrigationincreasesyield,nut sizeandsplitting,reducesthealternatebearingpatternand inci-denceofblanknuts,buthasnoeffectonthehulltokernelratio

(Monastra etal.,1998;AkandAgackesen,2006).Sedaghatiand

Alipour(2006)suggestedthatearlyhull splitting,aprocessthat

decreasesthequalityoftheyieldbecausethekernel isexposed toinvasionbyfungiandinsects,isrelatedwithplantwaterstatus fromlateApriltoearlyJune.However,Gijónetal.(2009)suggested thatearlysplittingincidenceisnotrelatedtoplantwaterstatusbut totemperaturesbelow13◦C.

Pistachio’sirrigationwaterrequirementsarequitehigh, vary-ingfrom547to600mmwhencalculatedaccordingtoMemmietal.

(2016b)orKermaniandSalehi(2006)to842–1000mmwhen

cal-culatedaccordingtoTestietal.(2008)orGoldhamer(1995).Taking intoaccountthatwaterisascarceresourceandinfutureonlythe mostefﬁcientagriculturalsystemswillreceiveinputsofirrigation water(Fereresetal.,2003),studiesintooptimizingpistachiodeﬁcit irrigationstrategiesare inprogress.For example,Memmiet al.

(2014)studiedthepistachioresponsetodifferentlevelsofwater

deficitandtimeofapplication,concludingthatirrigationwhen ker-nelweightisincreasing(stageIII)resultsinahigherfruitsizethan whenthesameamountofirrigationwaterisdistributedbetween stagesI(rapidnutgrowth)andIII.Moreover,theseauthorsshowed thatshellhardening(stageII)startswhenthefruitreachesits max-imumexternaldiameterandfinishesashorttimebeforethekernel reachitsfinalweight,bothprocessesbeingsimultaneousattheend ofhardeningandbeginningofkernelgrowth.

Gijónetal.(2009)showedthatSDIprovidedat50and65%of

thefullyirrigatedtreesduringthegrowingseasonreducedtotal yieldandkernelsize,eventhoughdifferencesinkerneldryweight wereunaffected.Memmietal.(2016b)showedthatRDIduring stageIIorpostharvestdoesnotreduceyieldeventhoughitmay reducetreevegetativegrowth.Theseauthorsalsoindicatedthat fullirrigationandRDIinpistachiotreesgrowinginshallowsoilscan besuccessfullyscheduledusingstemmeasurements.Hence,RDI

usingastemthresholdvalueof−1.5MPaduringstageIIinduced

similaryieldandproductionvaluestofullirrigatedtrees,whereas astemthresholdvalueof−2.0MParesultedinanextensivedelay

intherecoveryofgleafvalues,withconcomitantnegativeeffectson

long-termpistachioproduction.Guerreroetal.(2005)studiedthe recoveryofpistachiowaterrelationsunderRDIandconcludedthat inordertoavoidanyadverseeffectofwaterdeﬁcitduringstage III,irrigationshouldbeincreasedtowardtheendofstageIIorbe clearlyhigherthan100%ETcfromthebeginningofstageIII.

Pérez-Lópezetal.(2017)showedthatstagesIandIIIarecritical

becausewaterdeﬁcitreducesthequantityandqualityoftheyield. However,theeffectsofdifferentwaterstresslevelsateachstage havenotbeensufﬁcientlystudied.Inthissense,RDItrees(receiving 50%ofthewaterreceivedbycontroltreesduringstagesIandII,and thesameamountofwaterascontroltreesduringstageIII)provided asimilartotalyieldandpercentageofsplitnutsasfullirrigated treesanddidnotshowanalternatebearingpattern,eventhough theyreceivedaround20%lesswater(Gijónetal.,2009).

Okayand Sevin (2011a,b)studied theeffectof irrigation on

somepistachiofruitcharacteristicsandconcludedthatdifferences amongcultivarsweremoresigniﬁcantundernon-irrigated con-ditions. Irrigation increased kernel weight but did not have a signiﬁcantimpact on shell and kernel colours (Guerreroet al.,

2005).Carbonell-Barrachinaetal.(2014)showedthatthemore

severethewaterstresslevelachievedduringstageII,theharder andcrunchiertheresultingpistachios.

Thekernelfattyacidcontentofpistachioisalsoaffectedbyplant waterstatus(OkayandSevin,2011a),theoleicacidcontent increas-ingandthelinoleicacidcontentdecreasinginfruitsofwellirrigated trees.Incontrast,Carbonell-Barrachinaetal.(2014)indicatedthat thefattyacidproﬁleofpistachiosisdominatedbythreemain com-pounds:oleicacid(∼50%),linoleicacid(∼33%),andpalmiticacid (∼13%)and showed(Table3)thatmoderate RDIduringstageII signiﬁcantlyincreasedtheoilcontentofthenuts,whereasmore severeRDIreducedtheoilcontent,inducinginbothcasesa signif-icantincreaseinthecontentoflinoleicacid,whichisanessential fattyacidforhumans.TheseauthorsalsostudiedtheeffectofRDIon pistachiovolatilecompoundsandconcludedthatsevereRDI dur-ingstageIIincreasedthecontentsofaldehydes(associatedwith greenandvegetablenotes)andreducethoseofpyrazines(nutand toastednotes)andterpenes(citricnotes)(Table3).

AdescriptiveanalysisofpistachiosshowedthatmoderateRDI duringstageIIleadstoanintense“greenpistachio”colour, accom-paniedbyhigherintensitiesofnuttyandpistachionotesinharder, crunchier nuts with a longer aftertaste. Also, an international consumerstudyabouttheopinionofEuropeanconsumerson pis-tachiosgrownunderRDIindicatedthatthekernelsresultingfrom moderateRDIappliedduringstageIIobtainedahigherintensitiesof characteristicsensoryattributesandagreaterlevelofsatisfaction

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amonginternationalconsumersthankernelsfromFItreesorfrom thoseexposedtosevereRDIduringstageII(Carbonell-Barrachina

etal.,2014;Noguera-Artiagaetal.,2016).

3.4. Pomegranate(PunicagranatumL.)

Pomegranate,oneoftheoldestknownediblefruitsandoneof thesevenkindsoffruitmentionedintheBible,ismainlygrown insemi-aridmild-temperatetosubtropicalclimates(Blumenfeld etal.,2000).ThisspeciesandPunicaprotopunicaarethetwospecies thatmakeupthePunicaceaefamily.P.granatumisbelievedtobe anativetothesouthernCaspianbelt(Iran)andnorthernTurkey, whereasP.protopunicaisgenerallyacceptedasbeingendemicof theSocotraIsland(Yemen)(Janick,2007).

Pomegranate is considered to be a drought-resistant crop becauseit supportsheatandthrivesinaridandsemiaridareas, evenunderdesertconditions(Aserietal.,2008),themechanisms developed by this crop to confront water stress being mainly stressavoidanceandstresstolerance(Rodríguezetal.,2012).More precisely,fromthebeginningofwaterdeﬁcitconditions,leaf con-ductancedecreasesinordertocontrolwaterlossviatranspiration andtoavoid leafturgorloss(stressavoidancemechanism)and whenseverwaterstresslevelsarereached,activeosmotic adjust-mentistriggered,contributingtothemaintenanceofleafturgor (stresstolerancemechanism).Otherdroughttolerance character-isticscommonlyseeninxeromorphicplantscanbealsoobserved inpomegranate,suchasahighrelativeapoplasticwatercontent (42–58%),whichwouldcontributetotheretentionofwateratlow leafwaterpotentials(Rodríguezetal.,2012).

Despiteitsgoodresistancetodrought,pomegranatefor com-mercial production requires regular irrigation throughout the season,especiallywhenitiscultivatedinaridandsemiaridareas, toreducetheincidenceoffruitphysiopathies(e.g.fruitsplitting)

(Galindoetal.,2014b;Rodríguezetal.,2017)andtoreachoptimal

growth,yieldandfruitquality(Levin,2006;Hollandetal.,2009).In thissense,theperiodcorrespondingtotheendofpomegranatefruit growthandripeningisclearlycriticalfortheincidenceoffruit split-ting.Galindoetal.(2014b)showedthatatveryseverewaterdeﬁcit levels,despite leafturgor being maintained, fruitturgor is lost inducingareductioninfruitexpansion.Then,whenanimportant rainfalleventtakesplace,previouslywaterstressedpomegranate fruitsarerehydratedasymmetricallybecausearilturgorincreases toamuchgreaterextentthanpeelturgor,thepressureofthearils onthepeelfavouringsplitting.

Intriglioloetal.(2013)estimatedpomegranate

evapotranspi-ration to be around 412–514mm, but reports on the effect of irrigationmanagementonpomegranatefruityieldandqualityare relativelyscarce.Theﬁrstresultsindicatedthatit ispossibleto controlthedesiredripeningtimeinpomegranatesbyapplying dif-ferentirrigationregimes(SonawaneandDesai,1989).Recently,

Galindoetal.(2014a)indicatedthatSDIappliedthroughoutthe

pomegranateseasontoachievepronounced waterdeficitlevels reducestotalyieldpertree,thenumberoffruitspertreeandthe sizeofthefruits;however,suchastrategycanbringforwardthe availabilityoffruitsresultingfromlateflowerings,which,despite theirsmallersize,areofgreatinterestforthepomegranate trans-formation industry due to their very highcontent of bioactive compounds.Incontrast,otherstudiesmentionambiguousresults concerningthe effect of SDI onthe chemical characteristics of pomegranatefruit.Inthissense,Mellishoetal.(2012)concluded thatSDI,undermoderatewaterstress,producedsomechangesin colourandchemicalcharacteristics,whichreflectedearlier ripen-ing.However,Menaetal.(2013)indicatedthatpomegranatejuice fromtreessubmittedtoSDI regimesthatproduceseverewater stresslevelswasoflowerqualityandlesshealthythanthejuice fromfullyirrigatedtrees.Thisreductioninqualitywasduetothe

factthatthewaterstresslevelscausedadramaticdecreasein bioac-tivephenoliccompounds,especiallyanthocyaninsandpunicalagin

(Table3);besides,thepomegranateswerelessattractivefor

con-sumersduetotheirpaleredcolour.Ontheotherhand,Laribietal.

(2013)showedthatpomegranatesfromSDItrees, submittedto

mildwaterstressduringﬂoweringandfruitsetandmoresevere waterstressduringthelinearstageoffruitgrowthandripening, hada redderpeelandhigherlevel oftotalsoluble solidsin the juice.

Intrigliolo et al. (2013) and Laribi et al. (2013) studied the

pomegranateresponsetoRDI involvingirrigationwater restric-tionsduringdifferentfruitstagesandconcludedthattheperiod comprisedbyﬂoweringandfruitsetcouldberegardedas non-critical from the yield point of view and that irrigation water restrictionduringpomegranatefruitgrowthandripeningenhances peelredness andTSS inthejuice.However,restricting the irri-gationwaterduringthelinearfruitgrowthperiodincreasedthe concentrationofmanybioactivecompoundsinthejuice,suchas anthocyanins,thatarerelatedtohealthandtaste.Recently,Galindo

etal.(2017)showedthatashortperiodofirrigationrestrictionat

theendofripeningperiodbringstheharvesttimeforward,saves irrigationwater,enhancesthefruitbioactivecompoundscontent (anthocyanins,phenoliccompounds,punicalaginandellagicacid) andincreasesthepriceofthefruitwithoutaffectingmarketable yieldandfruitsize.

StudiesontheresponseofpomegranatetreestoPRDhavebeen performedbyParvizietal.(2014,2016)andParviziandSepaskhah

(2015).Theseauthorscomparedthefollowingstrategies:SDI(50%

and75%ofETc),irrigatingonlyonesideoftrees(north) through-outthegrowingseasonandkeepingtheotherside(south)ofthe treedry,PRD(50%and75%ofETc)andFI,maintainingbothsidesof thetreewetted.ThefirstauthorsshowedthatbothSDIstrategies andPRDat50%ETcinducedadecreaseinpomegranateyield,and recommendedPRD(75%ETc)because,inadditiontosavingwater, yield,intrinsicwateruseefficiency(WUE)andtranspiration effi-ciencyincreased.Asregardpomegranatefruitqualityattributes,

ParviziandSepaskhah(2015)indicatedthatbothPRDstrategies

increasedthepomegranatefruitjuicecontentandmaturityindex anddecreasedthetitratableacidityvaluescomparedwithFIfruits, whiletheresponseoffruitstobothSDIstrategieswastheopposite ofthatobservedinresponsetoPRD.

Deficitirrigationcanbeconsideredasatoolthatsignificantly improvesthepostharvest performance ofpomegranate. Several authorsreportedthatthefruitsresultingfromSDIandRDI treat-mentsshowedbetterpostharvestbehaviourthan thosefromFI becauseofretardedchillinginjuryincidence(Pe ñaetal.,2013), highersensoryandnutritionalqualityandlongershelflife(Laribi

etal.,2013;Pe ˜naetal.,2013;Pe ˜na-Estévez etal.,2016).

More-over, in a study of the effect of different irrigationtreatments andtheefficacyofavapourtreatment(7–10sat95◦C)andusing NaClOassanitizingagentsonthequalityandshelflifeoffresh-cut pomegranatearils,Pe ña-Estévezetal.(2015)observeda synergis-ticeffectofthewaterdeficittreatmentandthepostharvestthermal treatment.Bestresultswereobtainedforarilsfrompomegranates grownontreesfromwhichirrigationwaswithheldfor16or26days priortoharvest,forwhichashelflifeof18daysat5◦Cwas estab-lished.

4. Summary,conclusionsandfutureresearchneeds

Bearing in mind the characteristics of the emerging crops consideredin thisreview, itis clearthattheypresent different mechanismstoconfrontwaterdeﬁcitsituations,andthat differ-entlevelsofresistanceareachieved.Inthissense,pistachiotrees canbeconsideredthemostdroughtresistantbecausetheycan

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Fig.3.Quadraticrelationshipbetweensecondarymetabolitescontentinthefruitsandplantwaterstatus(blueline)(Horner,1990).Undermildwaterdeﬁcit,stomatal

regulationmayleadstoareductioninplantgrowth,increasingconcentrationofnonnitrogenoussecondarymetabolites(centraltree).Whenwaterdeﬁcitincreases(right

tree),CO2assimilationisreducedandcarbonispreferentiallyallocatedtothesynthesisofprimarymetabolites,whichdonotexceededtheamountusedforfruitgrowthto

thedetrimentofthesynthesisofcarbon-basedsecondarymetabolites.(Forinterpretationofthereferencestocolourinthisﬁgurelegend,thereaderisreferredtotheweb

versionofthisarticle.)

surviveunderveryextremedroughtconditions.Theirwaterstress resistanceisbasedonmorphologicalcharacteristicssuchasavery extensiverootsystemandthedevelopmentofstressavoidanceand tolerancemechanisms.Pomegranateandjujubetreesarealsoable towithstandseveredroughtduringthegrowingseasonandthe mechanismsdevelopedbythesecropstoconfrontdroughtarealso predominantlystressavoidanceandstresstolerancemechanisms. Incontrast,anentirelydifferentstrategytoconfrontwaterdeﬁcit isshownbyloquat.Its(loquat)strategycanbeconsideredasa droughtescapemechanismbecauseitisbasedonanatypical phe-nology,completelydifferentfromthatoftraditionalMediterranean temperatefruitcrops,bloominginautumn,developingfruits dur-ingwinterandripeninginearlyspring.So,fruitgrowthaccounts whenMediterraneanclimateiswetterandevaporativedemandof theatmospherereachesminimumvalues,thusavoidingtheeffects ofhotanddrysummers.

Theirrigationwaterrequirementsoftheseemergingcropswere notrelatedwiththeresistancetowaterstress.So,loquattrees pre-sentedthehighestseasonalETc,whichwasslightlyhigherthanthat observedinpistachioandpomegranatetrees,andclearlyhigher thanthatobservedinjujubetrees.Itisclearthatloquatcanbe consideredanoutstandingcropforitsresponsetothecontinuous applicationofRDI,buttherearegoodreasonstoconcludethatthe otheremergingcropsstudiedareabletocopewithwaterscarcity duetotheirpositiveresponsetoDIstrategies,includingminimal impactonyieldsandimprovedWP.

TakingintoconsiderationtheeffectofDIstrategiesonfruit qual-ityandthehealth-relatedcompoundstheycontain,itisimportant tounderlinethatresearchneedstobedirectedatsomevery impor-tantaspectsincluding:(i)theeffectofdeﬁcitirrigationonloquat

fruitquality,forwhich,tothebestofourknowledgenoinformation exists,and(ii)identifyingtheoptimalwaterdeficitlevel,itstiming anddurationforeachcropinordertooptimizefruitqualityand theirhealth-relatedcompoundscontent.Thislastconsiderationis basedonthefactthattheliteratureinmostcasessuggeststhatfruit qualityandthehealth-relatedcompoundscontentcanbeimproved byspecificDIstrategies,butfruitresponsetomoderateandsevere waterdeficitisnotproportionalinmanycases.Itisnotpossibleto establishalinearcorrelationbetweenwaterstressandsomefruit characteristics,especiallyinthecaseofsomesecondary metabo-lites(MattsonandHaack,1987;Gobbo-NetoandLopes,2007).In anattempttopredicttheconcentrationofphenoliccompounds asa function ofwater status, Horner(1990)proposeda model basedonaquadraticrelationshipbetweenbothvariables(Fig.3). Whenplantsareundermildosmoticstressthereisareductionin plantgrowthandtheconcentrationofnon-nitrogenoussecondary metabolitesincrease.Whenplantsareunderseverewaterstress, strongstomatalregulationtakesplaceandCO2assimilationismuch

reduced;carbonispreferentiallyallocatedtothesynthesisof pri-marymetabolites,whichdonotexceedtheamountusedforfruit growthandtothedetrimentofthesynthesisofcarbon-based sec-ondarymetabolites(Fig.3).

Bearinginmindallthepreviousconsiderations,itisevidentthat farmerswhoadoptspeciﬁcDIstrategies andcultivate underuti-lizedplantspeciesshouldberewardedfor(i)makingsustainable useofirrigationwater,(ii)improvingcropbiodiversity,(iii) hav-ingtoacceptaslightreductionintheirfruitandvegetableyields, and(iv)producingfruitswithhighercontentsofbioactive com-pounds.Fortunately,consumersarewillingtopayforspecialfoods, particularlythoseassociatedwithenvironmentalfriendlyfarming

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Fig.4.MainagronomicandfruitqualityaspectsneededtoobtainahydroSOSfruitcertiﬁcation.

practicesthatusenochemicals(Martínez-RuizandGómez-Cantó, 2016)–whichisthecaseofthefruitsandvegetablesgrownunder DI.However, consumersneed toidentify suchproducts, which shouldbeclearlylabeledanddisplayedseparatefromother prod-uctsofthesametype,otherwisetheirpotentialwillbelostinasea ofproducts.Veryfewgroupshavestudiedconsumeropinion con-cerningDIfruits(Lopezetal.,2016;Fernandes-Silvaetal.,2013),

butNoguera-Artiagaetal.(2016)evenproposedanidentitybrand

toprotectthistypeofproduct,whichmightbecalled hydroSOStain-ableor,in abbreviatedandeasiertoremember form,hydroSOS. Accordingtotheseauthors,hydroSOSproductswillhavea solid identitybasedontwomainfactors:(i)waterdeﬁcitcanincrease theplantsecondarymetabolitecontentand,thus,thefunctionality oftheedibleproducts(Ripolletal.,2014),and(ii)theproductsare environmentallyfriendlybecauseofthesustainableuseofavery scarceresource,water(Fig.4).

Noguera-Artiagaetal. (2016)alsofoundthat consumersare

willingtopayareasonablyhigherpriceforhydroSOSpistachios,if theyareproperlylabeledandidentified.However,furtherresearch isneededtocheckwhetherthisgreaterwillingnesstopayisthe similarforallfruits.Finally,itisessentialtoestablishahydroSOS indextocertifythat theproductsusingthehydroSOSlogohave beenevaluatedfortheirsustainableuseofirrigationwaterand/or theircontentsofbioactivecompounds.Thisindexisunder con-structionandwillbebased,amongotherfactors,onfarmersand tradersbeingabletodemonstrate:(i)knowledgeofthecultural practicesinvolved,includingwatermanagementduringthe non-criticalperiods,(ii)thetiming,levelanddurationoftheapplied waterdeficit,(iii)thatsuitablemonitoringandcontrolofthestress applied hastaken place by measuring,for example, the water potential,(v)theprecisecompositionand contents ofbioactive compounds,e.g.increasedlevelsofproline(anaminoacidused asindicatorofplantwaterstress),and(vi)thegoodsensory qual-ityoftheproductinquestion.Iftheserulesarefollowed,itshould bepossibletoensureconsumersatisfaction,strengthentheir will-ingnesstopayareasonablyhigherprice,andguaranteetheirfuture fidelitytotheseproducts(Fig.4).Iftheindexcanguaranteeallthe

above,consumerdemandwillincrease,aswillthepriceofhydroSOS productsandthepossibleproﬁtforfarmers.Hopefully,farmerswill becomeincreasinglyconvincedabouttheeconomicbeneﬁtsofDI anddedicatelargerareastothecultivationofevenmorecrops.

Acknowledgments

On the occasionof the retirementof Prof. Dr. Félix Moreno (IRNAS-CSIC),the authorsofthis paper shouldliketotakethis opportunitytothankhimforhisscientificknowledge,histireless activity and his willingness to help others during his profes-sionalcareer.WearealsogratefultotheMinisteriodeEconomía y Competitividad deEspa ña(MINECO) (CICYT/FEDER AGL2013-45922-C2-1-R, AGL2013-45922-C2-2-R, AGL2016-75794-C4-1-R andAGL2016-75794-C4-4-R)forgrantstotheauthors.AGandJC-G acknowledgethepostdoctoralfinancialsupportreceivedfromthe RamónArecesFondationandtheJuandelaCiervaprogram, respec-tively.Also,thisworkisaresultofthePRinternship(19925/IV/15) fundedbytheFundaciónSéneca–AgenciadeCienciayTecnología delaRegióndeMurcia(SenecaFoundation–AgencyforScience andTechnologyintheRegionofMurcia)undertheJiménezdela EspadaProgramforMobility,CooperationandInternationalization.

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