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Climate variability is intensifying the mismatch between crop water demand and irrigation supply, making deficit irrigation (DI) an increasingly important strategy for sustaining agricultural production with reduced water availability. Yet, the success of DI depends not simply on the amount of water withheld, but on the capacity of crops to perceive, transmit and integrate water-deficit signals while protecting reproductive development and harvestable yield. This review synthesizes crop acclimation to DI across four interacting levels: hydraulic signalling, hormonal crosstalk, molecular reprogramming and yield resilience. Early declines in soil and root-zone water potential alter root hydraulic conductance, xylem tension, aquaporin activity and root-to-shoot communication, rapidly constraining stomatal conductance and canopy water loss. These hydraulic responses interact strongly with abscisic acid (ABA), auxin, cytokinins, ethylene, jasmonates, salicylic acid and strigolactones to coordinate stomatal behaviour, root-system plasticity, source-sink relations and reproductive survival. At the molecular level, ABA-dependent PYR/PYL–PP2C– SnRK2 signalling, ABA-independent DREB pathways, NAC/MYB/WRKY transcription factors, Ca2+ and reactive oxygen species signalling, osmoprotectant metabolism, antioxidant defence, aquaporins and stress-associated proteins collectively reprogram plant growth and metabolism. An additional emerging dimension is the contribution of rhizosphere and endophytic microbiomes to hormonal regulation, root architecture, nutrient acquisition, osmotic adjustment and redox homeostasis under water limitation. Evidence from meta-analyses indicates that DI commonly improves water productivity while imposing context-dependent yield penalties, with responses governed by crop, genotype, soil texture, climate, irrigation method, deficit intensity and phenological timing. Moderate and stage- specific deficits can therefore function as controlled acclimation signals, whereas severe or poorly timed deficits, particularly when combined with heat stress, disrupt carbon assimilation, reproductive processes and grain or fruit filling. Integrating plant signalling biology, microbiome functions, high-throughput sensing and precision irrigation can shift DI from a fixed water-saving practice toward a biologically informed strategy for stabilizing yield and water productivity under climate uncertainty.
Keywords: deficit irrigation; drought acclimation; hydraulic signalling; abscisic acid; hormonal crosstalk; deficitirrigation; droughtacclimation; hydraulicsignalling; abscisicacid; hormonalcrosstalk
Highlights deficitconsideredmoderateunderonecombination oftemperature,relativehumidityandvapour-pressure • Deficitirrigationactsbothasawater-savingtreat- deficitmaybecomephysiologicallysevereunderan- mentandasacontrolledphysiologicalacclima- other[35,42,47].
• Root hydraulics, xylem tension, aquaporins Evidenceaccumulatedacrossdifferentcropsdemon- and ABA jointly regulate rapid root-to-shoot stratesthattheresponsetoDIisstronglycontextde- responsestosoildrying[13,14,25,26].
pendent [3–5]. Global syntheses generally indicate • Hormonalcrosstalkdetermineswhetherwater higherwater-useefficiencyorwaterproductivityun- conservationcanbeachievedwithoutunaccept- derDI,althoughtheseimprovementsarefrequentlyac- ablepenaltiestogrowthandreproductivedevel- companiedbysomereductioninyield[3–5].Inwheat, opment[15–18].
aglobalmeta-analysisbasedon381pairedobserva- • Beneficialrhizosphereandendophyticmicroor- tionsreportedanaverageimprovementinWUEofap- ganisms can reinforce drought acclimation proximately6%togetherwithayieldreductionofap- throughphytohormoneregulation,rootmodi- proximately16%,althoughparticularcombinationsof fication, osmotic adjustment and antioxidant irrigationintensityandenvironmentalconditionsal- defence[46,49,50,52,53].
lowedimprovementsinbothyieldandWUE[3].These • Heat can intensify drought-induced oxidative findingsdemonstratewhynosingleDIthresholdcan damage, reproductive failure and yield loss, beuniversallyrecommended.
making the physiological severity of a given Plantsdonotpassivelydeteriorateassoildries.They irrigationdeficitclimatedependent[47]. activelyperceivechangesinwateravailabilitythrough • Decision-supportsystems,multispectralsensing alterationsinwaterpotential,hydraulicconductance, andmachinelearningprovideopportunitiesfor cellturgor,membranestatusandchemicalsignalling adaptive,plant-responsiveirrigationundercli- [6,7,9,14]. Roots and shoots communicate through maticvariability[48,51].
hydraulic signals, hormones, ions, peptides, ROS andcalcium-dependentsignallingnetworks[13–24]. In addition, increasing evidence indicates that root- 1.Introduction associated microbial communities can modify root Waterscarcityisnolongerrestrictedtochronicallyarid architecture,nutrientacquisition,hormonebalance environments.Increasingatmosphericdemand,irreg- and stress-related metabolism, thereby adding a ularrainfall,heatwavesandshiftsinprecipitationtim- biological rhizosphere component to whole-plant ingareexposingirrigatedandrainfedcropstomore droughtacclimation[46,49,50,52,53].
frequentepisodesofsoildrying[6,7,33–35,42]. The simultaneousoccurrenceofdroughtandheatispar- Theimportanceofsuchplant–microbeinteractionsis ticularly important because their interaction can in- illustrated by field evidence showing that microbial tensifydehydration,oxidativeinjury,photosynthetic consortiacanimprovenutrientavailability,biological inhibition,reproductivefailureandyieldlossbeyond activityandcropproductivityunderreducedexternal theeffectsproducedbyeitherstressindividually[47]. inputs[46].Functionalprofilingofbeneficialmicroor- ganismsfurtherindicatesthatPGPR,arbuscularmyc- Undertheseconditions,irrigationmanagementmust orrhizalfungiandendophytescaninfluencephytohor- balancetwopotentiallyconflictingobjectives:conserv- monalsignalling,osmolyteaccumulation,antioxidant inglimitedwaterresourcesandmaintainingeconomi- defenceandplanttolerancetodroughtandotherabi- callyacceptablecropyields[1,2].Deficitirrigation(DI), oticstresses[49,50,53].
inwhichirrigationwaterisdeliberatelysuppliedbelow fullcropevapotranspirationrequirementsduringpart Stomata can adjust transpiration within minutes, orallofthegrowingseason, hasemergedasanim- whereas root growth, aquaporin activity, osmotic portantstrategyforincreasingcropwaterproductivity adjustment, microbiome interactions and transcrip- [1,2,40,41].
tionalreprogrammingdevelopoverlongertimescales [14,19–26,49,50].Theseresponsesmayconservewater ThebasicprincipleofDIistoacceptacontrolledde- for later developmental stages but simultaneously gree of water limitation when the marginal yield re- restrictcarbonassimilation[27,28,31,32]. sponsetoadditionalirrigationisrelativelysmall,while protectingdevelopmentalstagesduringwhichrepro- Therefore,theagronomicvalueofacclimationdepends ductivedevelopmentandyieldformationarehighly onwhetherwaterconservedthroughstressresponses sensitive[1,2,40].Thisapproachbecomesincreasingly ultimatelyprotectsmoreyieldthanislostthroughre- complexunderclimatevariabilitybecauseanirrigation ducedphotosynthesisandgrowth. Thisreviewinte- grateshydraulicsignalling,hormonalcrosstalk,molec- tion or damaging stress depending on crop stage, ularreprogramming,microbiome-assistedacclimation atmospheric demand, genotype and rhizosphere andyieldformationunderDI,withparticularemphasis conditions.
onclimatevariabilityandfield-levelmanagement. 3.HydraulicSignalling:TheFirstLayerofAc- 2. Deficit Irrigation as a Controlled Acclima- climation tionEnvironment 3.1 Soil–Root Hydraulic Continuity and Water- Deficit irrigation encompasses several operational PotentialGradients strategies. SustainedDIappliesarelativelyconstant Hydraulic signalling begins as water availability fractionoffullcropwaterrequirementduringmuchof declinesatthesoil–rootinterface. Watermovement thegrowingseason[1,2].Regulateddeficitirrigation throughthesoil–plant–atmospherecontinuumfollows imposes stronger deficits during relatively drought- gradients in water potential and is constrained by tolerant developmental stages while maintaining hydraulic resistance in the soil, rhizosphere, roots, adequate irrigation during sensitive stages [1,40]. xylemandleaves[9,10,25,26].
Partial root-zone drying alternately wets and dries Astherhizospheredries,hydraulicconductivitycande- different portions of the root system so that roots clinesharply,increasingthewater-potentialgradient exposedtodryingsoilgeneratechemicalsignalswhile requiredtosustaintranspiration[9,10].Thesechanges rootsinwettersoilcontinuesupplyingwater[37–39]. cangeneraterapidalterationsinleafwaterstatusand Thesestrategiesproducedistinctspatialandtemporal stomatalbehaviourbeforeextensivebiochemicalac- patternsofsoilandplantwaterpotentialandconse- climationoccurs.
quentlydifferentsignallingenvironments[37–39].PRD Rootarchitecture,roothairsandrhizospherecharac- is especially relevant physiologically because it par- teristicsinfluencetheabilityofplantstomaintainhy- tiallyseparatesroot-derivedchemicalsignallingfrom drauliccontactwithprogressivelydryingsoil[10–12]. severewhole-plantdehydration[38,39].
Rhizospheremicrobialrestructuringcanfurtherinflu- Thedistinctionbetweenstressavoidance,acclimation encerootarchitecture,soilaggregation,nutrientcy- and injury is essential. Moderate decreases in soil clingandsoilecologicalfunctioning,potentiallyalter- water availability can reduce stomatal conductance ingtheenvironmentthroughwhichrootsacquireboth beforesevereleafdehydrationdevelopsandthereby waterandnutrients[50,52].
improve intrinsic WUE [13,27]. Root proliferation at Regenerativemanagementsystemscharacterizedby depth,osmoticadjustmentandchangesinaquaporin greater microbial diversity and organic matter have activity may improve access to residual soil water alsobeenassociatedwithimprovedsoilaggregation [10–12,25,26].
andwaterretentioncomparedwithbiologicallysim- Beneficialmicroorganismscanreinforcesomeofthese plifiedsystems[52].Thus,thehydraulicbehaviourof responses. PGPR and endophytes can promote lat- cropsunderDIcannotalwaysbeseparatedfromsoil eralrootdevelopment,modifyphytohormoneconcen- biologicalconditions.
trations, stimulate compatible-solute accumulation Fromanagronomicperspective,maximumextraction andenhanceantioxidantactivityunderwaterdeficit ofavailablewaterisnotuniversallydesirable.Under [49,53]. Microbial inoculation has also been associ- seasonaldrought,rapidearly-seasonwaterextraction atedwithmaintenanceofcellularturgorandimproved canleaveinadequatereservesforfloweringandgrain water-useefficiencyunderabioticstress[49]. filling[6–8,28].Hydraulictraitsthatmoderatecanopy Incontrast,prolongedorsevereDIdecreasesleafwater consumption,maintainaccesstodeeperwaterordy- potential,restrictscellexpansionandphotosynthesis, namicallyadjustconductancemaythereforeincrease increasescanopytemperature,acceleratessenescence yieldresilience[6,10–12].
andcanimpairpollenviability,fertilization,seedset andgrainorfruitdevelopment[31–35,47].Combined 3.2RootHydraulicConductanceandAquaporins droughtandheatareespeciallydamagingduringre- Roothydraulicconductanceisdynamicallyregulated productive development because high temperature throughrootanatomy, suberizationandmembrane impairspollenfunctionwhiledroughtlimitsassimilate aquaporins[25,26]. Plasmamembraneintrinsicpro- deliverytodevelopingsinks[47].
teinsandotheraquaporinfamiliesfacilitatetransmem- Consequently, the same irrigation treatment may branewatermovementandrespondrapidlytophos- function as beneficial priming, moderate acclima- phorylation,cellularpH,Ca2+ andROS[25,26]. Under moderate water deficit, selective regulation potassiumlossandstomatalclosure.Inothertissues, of aquaporins can redistribute water flow among ABA activates genes associated with dehydration tissuesandassistrootextractionfromwetterportions protection, osmotic adjustment and antioxidant ofthesoilprofile[25,26]. Underseverestress,aqua- defence[16,19–22].
porindown-regulationmaylimitcellularwaterloss, TheimportanceofABAbecomesevengreaterwhenwa- althoughexcessivehydraulicrestrictioncanintensify terdeficitinteractswithheatbecausecombinedstress shootdehydration.
producesdistinctmolecularresponsesandalteredhor- Recoveryafterirrigationisequallyimportant. Rewa- monalsignallingratherthanasimpleadditionofsingle- teringrequiresrestorationofrootandleafhydraulic stresseffects[47].
conductance,renewedaquaporinactivityandreopen- ABAresponsesarestronglydependentondose,tissue ingofstomata[14,25,26].Agenotypethatconserves anddevelopmentalstage[13,15–17]. ModerateABA waterefficientlybutrecoversphotosynthesisslowly signallingcanconservewaterandfavourrootrelative maydisplayhighinstantaneousWUEyetrelativelypoor toshootgrowth,whereasprolongedhighABAconcen- seasonalproductivity[6,7,27,28].
trationsmaysuppressleafexpansionandcarbonas- similation[13,18].
3.3 Root-to-Shoot Communication and Stomatal Regulation 4.2Auxin,CytokininsandRoot-SystemPlasticity Root-to-shootsignallingunderdryingsoilinvolveshy- Auxinregulateslateral-rootinitiation,rootelongation draulicchangestogetherwithmobilechemicalsignals andvasculardevelopment, whereascytokininsgen- [13,14,38,39]. ABAisthemostextensivelycharacter- erallypromoteshootdevelopmentandinteractwith ized component, although xylem sap pH, ions, pep- auxinindeterminingroot-to-shootallocation[18,22]. tides,ROSandcalcium-relatedsignalsalsoparticipate [14,15,23,24].
Waterdeficitaltersauxintransportandlocalsignalling, enablingrootarchitecturetorespondtospatialhet- UnderPRD,rootsindryingsoilcangeneratesignals erogeneity in soil moisture [10,18]. Beneficial rhizo- thatrestrictstomatalconductancewhilerootsinwetter spheremicroorganismsaddanotherregulatorylayer soilmaintainshoothydration[37–39].Thispartialsep- becausemanyPGPRandendophytesproduceauxins, arationofsignallingfromwhole-plantdehydrationex- cytokininsandgibberellinsormodifyplanthormone plainswhyPRDmayincreasewaterproductivitywith- metabolism[49,53].
outproducingthesamedeclineinleafwaterpotential asuniformdrying.
Microbial auxin production can promote lateral roots and root hairs, increasing the effective soil Stomatal closure conserves water but restricts CO 2 volume explored for water and nutrients, while diffusion[27,31,32].Consequently,theobjectiveisnot ACC-deaminase-producing microorganisms can maximumstomatalclosurebutappropriatelytimed reduce stress-induced ethylene and help maintain regulation.GenotypicvariationinABAproductionand rootelongationunderdrought[49,53].
stomatal sensitivity can determine how effectively water is conserved while carbon assimilation is TheresultingABA–auxin–cytokinin–ethylenenetwork maintained[13].
thereforereflectsbothendogenousplantmetabolism andmicrobialactivitywithintherootenvironment. 4.HormonalCrosstalkunderDeficitIrrigation 4.3EthyleneandOtherHormonalSignals 4.1ABAastheCentralDrought-ResponseCoordina- EthyleneinteractsstronglywithABAinregulatingroot tor and shoot growth under water stress [18]. ABA can ABAaccumulationisoneofthecentralresponsesto restrictstress-inducedethyleneproductioningrow- declining water availability [13–17]. ABA is synthe- ingtissues,whereasexcessiveethylenecanaccelerate sized through carotenoid-cleavage pathways and senescenceandreproductiveabortion.
transportedamongroots,vasculartissuesandleaves BeneficialmicroorganismspossessingACCdeaminase [15–17].
can lower the concentration of the ethylene precur- ABA perception by PYR/PYL/RCAR receptors inhibits sorACCandalleviatedrought-relatedinhibitionofroot clade-APP2Cphosphatases,enablingSnRK2protein development[49,53].Endophytescanalsoinfluence kinasestoactivatedownstreamionchannelsandtran- jasmonicacid,salicylicacidandothersignallingpath- scriptionfactors[15–17]. Inguardcells,thiscascade ways,linkingabiotic-stressadaptationwithdefence promotes anion efflux, membrane depolarization, primingandredoxregulation[53].
Theseinteractionsdemonstratethathormonalaccli- mationtoDIshouldincreasinglybeinterpretedasa networkprocessinvolvingplanttissues,environmen- talsignalsandassociatedmicroorganismsratherthan asanABA-onlyresponse.
4.4Microbiome-MediatedHormonalandPhysiolog- icalAcclimation Integratedpathwaysofcropacclimationto Figure1.
Therhizosphereandinternalplantmicrobiomerepre- deficitirrigationunderclimatevariability.Thefigure sentanemergingcomponentofdroughtphysiology. integratesclimatevariabilityanddeficitirrigationwith BeneficialmicrobialgroupsincludingPGPR,AMFand hydraulicsignalling,hormonalcrosstalk,molecular endophytescanenhancestresstolerancethroughhor- reprogramming,microbiome-assistedresponses, moneproduction,ACC-deaminaseactivity,improved whole-plantacclimationandfinalyieldresilience. nutrientacquisition,osmoticadjustmentandactiva- tionofantioxidantdefence[46,49,50,53]. which bind ABA-responsive elements and induce protectivegenes[15,16,19,21].
Fieldevidenceinricehasdemonstratedthatmicrobial consortiacanmaintaincropproductivitywhilereduc- ABA-independent pathways involve dehydration- ingfertilizerdependenceandsimultaneouslyimprov- responsiveelementbindingproteinssuchasDREB2 ingsoilbiologicalactivity,organiccarbonandnutrient [16,19,21]. NAC,MYBandWRKYtranscription-factor availability[46].Althoughthisstudywasdesignedpri- families further integrate drought signalling with marilyaroundnutrientmanagementratherthanDI,its growth regulation, antioxidant metabolism and findingsdemonstratethatmicrobialfunctioningcan senescence[19–22].
improvethebiologicalandnutritionalconditionofthe Downstream products include late-embryogenesis- rootenvironmentinwhichdroughtresponsesoccur. abundant proteins, dehydrins, heat-shock proteins, osmolyte-biosynthesis enzymes and antioxidant Functionalprofilingstudiesfurtherindicatethatmicro- systems[19–24,33,34].
bialinoculationcanincreasecompatiblesolutessuch asproline,glycinebetaine,trehaloseandsolublesug- Undersimultaneousheatanddrought,transcriptional arsandmayhelpmaintainturgorandWUEduringwa- responses can differ from those activated by either terdeficit[49].Rhizospheremicrobiomerestructuring stressindependently,emphasizingtheneedtoinvesti- haslikewisebeenassociatedwithalteredrootarchitec- gatecombined-stressmolecularnetworksratherthan ture,phytohormoneproduction,microbialsignalling extrapolatingfromisolateddroughtresponses[47]. andimprovedstressresilience[50].
Endophytic microorganisms provide an internal 5.2CalciumandROSSignalling Calcium and ROS are not simply products of injury; signalling dimension. They can influence auxins, atcontrolledconcentrationsbothfunctionassecond cytokinins,gibberellins,ethylene,ABA,salicylicacid messengers[14,23,24].Water-deficitperceptiongen- and jasmonates while enhancing root depth, root eratestissue-specificCa2+ signaturesandROSsignals branching, antioxidant activity and WUE during thatinteractwithABAsignallingandproteinphospho- drought[53].
These observations support an expanded model of Guard-cellROScontributestoABA-inducedstomatal DI acclimation in which hydraulic and hormonal re- closure,whereassystemicROS–Ca2+ communication sponsesoftheplantareinfluencedbythebiological helps transmit stress information between tissues stateofboththerhizosphereandinternaltissues. [14,23,24].Productiveacclimationrequiresabalance betweensignallingROSandantioxidantdetoxification. 5. Molecular Reprogramming during Water This balance becomes particularly important under DeficitandRecovery combined heat and drought because simultaneous stress can strongly increase ROS accumulation and 5.1ABA-DependentandABA-IndependentTranscrip- overwhelmantioxidantdefence,resultingindamage tion tolipids,proteinsandnucleicacids[47]. Waterdeficitrapidlymodifiesgeneexpressioninroots and shoots [19–22]. In ABA-dependent pathways, Beneficialmicroorganismscanmodifythisbalanceby SnRK2kinasesactivateAREB/ABFtranscriptionfactors, stimulatingantioxidantenzymesandredox-regulatory Acclimationcom- Major sig- Principalcropresponse Relevance to yield re- Key refer- ponent nals/processes silience ences Hydraulic regula- Soil–root water poten- Rapidadjustmentofplant Prevents excessive dehy- [9,10,25,26] tion tial,xylemtension,root watertransportandstom- dration while regulating hydraulic conductance, atalbehaviour seasonalwateruse aquaporins ABA-mediatedsig- PYR/PYL–PP2C–SnRK2 Stomatal closure, water Conserveswaterbutexces- [13–17] nalling pathway,guard-cellion conservation and activa- siveactivationmayrestrict regulation tionofdrought-responsive carbonassimilation genes Hormonal Auxin, cytokinin, ethy- Root-system plasticity, Balances root develop- [18,22,53] crosstalk lene, jasmonates, sali- senescenceregulationand ment, shoot growth and cylicacid stressadaptation reproductivesurvival ROS–Ca2+ sig- Reactiveoxygenspecies, Signaltransmission,stom- Distinguishes productive [14,23,24,47] nalling calcium signatures, an- atal control and cellular acclimationfromoxidative tioxidantdefence protection damage Molecular repro- AREB/ABF, DREB, NAC, Transcriptional,metabolic Improvestolerance,recov- [19–24] gramming MYB, WRKY, osmolytes andcellularadjustment ery and maintenance of andstressproteins physiologicalfunction Rhizosphere mi- PGPR, AMF, microbial Improvedrootarchitecture, Supportscropfunctionun- [46,49,50,52] crobiome hormones, nutrient nutrientavailabilityandos- derwaterandnutrientlim- mobilization moticadjustment itation Endophyticmicro- ACC deaminase, auxin Improved rooting, photo- Strengthens acclimation [53] biome production,redoxregu- synthesis, antioxidant ac- under drought and inter- lation,defencepriming tivityandWUE actingstresses processes[49,53]. Thus,theoxidativestatusofaDI- metagenomics and metabolomics could therefore treatedcropreflectsinteractionsamongstressinten- reveal how DI simultaneously reprograms the plant sity, temperature, endogenousantioxidantcapacity anditsassociatedmicrobiome.
5.4StressMemory,MicrobiomeMemoryandRecov- 5.3OmicsandEpigeneticRegulation ery Transcriptomics,proteomicsandmetabolomicshave Repeatedmildwaterdeficitsmayalterthespeedand expanded droughtbiologyfromindividualgenesto magnitudeofsubsequentresponsesthroughphysio- regulatorynetworks[14,19–24]. Drought-responsive logicalandmolecularstressmemory. Suchmemory networkscommonlyinvolvephotosynthesis,carbohy- caninvolvetranscriptionalpriming,chromatinmodifi- drate metabolism, amino-acid metabolism, lipid re- cations,persistentmetabolites,antioxidantstatusand modelling,cell-wallprocessesanddefencepathways. alteredhydraulicbehaviour[19–24].
SmallRNAs,alternativesplicing,DNAmethylationand Anadditionalpossibilityisthatrepeatedirrigationcy- histonemodificationsprovideadditionallevelsofcon- clesalterrhizospherecommunitystructure,rootexuda- trol.However,translationfromseverelaboratoryde- tionandendophyticcolonization.Becausemicrobial hydrationtomoderateandfluctuatingfieldDIremains communitiesrespondtosoilmoisture,cropgenotype amajorchallenge.
andmanagementintensity[50,52],partoftheappar- Multi-omicsapproachesarealsotransformingunder- entacclimationofrepeatedlystressedplantsmayin- standingofplant–microbeinteractions.Functionalpro- volvepersistentchangesinplant–microbiomeinterac- filingandmicrobiomerestructuringstudieshaveem- tions.
phasizedthevalueofmetagenomics,transcriptomics This hypothesis remains insufficiently tested under andmetabolomicsforidentifyingmicrobialfunctions fieldDIandrepresentsanimportantresearchfrontier. associatedwithplantstresstolerance[49,50]. Metagenomic analysis using 16S rRNA and shotgun sequencingcancharacterizemicrobialdiversityand 6.FromAcclimationtoYieldResilience functionalpotential,whileregenerativemanagement Yieldresilienceisthecapacitytomaintaineconomi- hasbeenassociatedwithhighermicrobialdiversity, callyusefulproductiondespitetemporalvariationin nutrient-cycling capacity and ecological resilience watersupply[6–8].Itemergesfromintegrationofwater [52].Integratinghosttranscriptomicswithrhizosphere capture,transpirationregulation,carbonassimilation, nutrientstatus,phenology,reproductivestabilityand Field evidence from tomato demonstrates the post-stressrecovery[6–8,27,28].
broader importance of genotype × environment × management interactions. Different combinations Atraitthatimprovesplantsurvivaldoesnotnecessar- of transplanting window and genotype produced ilyimprovecropyield[27,28].Strongstomatalclosure marked differences in disease incidence and yield may increase intrinsic WUE while reducing biomass under changing temperature, rainfall and humidity productionifmaintainedexcessively[27,28,31].Simi- conditions [56]. The highest-yielding combinations larly,extensiverootsystemsareusefulonlywhenwater occurredwithinfavourableplantingwindows,whereas remainsavailableatdepth[10–12].
veryearlyor late establishmentincreasedexposure Phenologicaltimingisoneofthestrongestdetermi- to disease-conducive environments. Although this nantsofDIresponse[1,2,40].Vegetative-stagedeficits experiment addressed disease rather than DI, it canreduceleafareaandtilleringbutmayconserve illustratesanimportantprinciplerelevanttoirrigation waterforreproductivegrowth.Waterdeficitsaround acclimation:genotypeperformanceisconditionalon floweringareespeciallydamagingbecausepollende- theenvironmentalwindowinwhichmanagementis velopment,fertilizationandearlyseedsetaresensitive imposed.
todehydrationandtemperature[33–35,47]. Therefore,breedingforDIresilienceshouldevaluate Combinedheatanddroughtcanintensifyreproductive genotypesacrosscontrastingirrigationtiming,atmo- injury. Heatimpairspollenviabilityandfertilization, sphericdemandandphenologicalstagesratherthan whiledroughtrestrictsassimilatesupply;duringgrain underasinglestandardizeddroughttreatment. filling,heatacceleratesdevelopmentanddroughtre- strictsassimilatetranslocation,resultinginreductions ingrainnumberandgrainweight[47].
8.ClimateVariabilityChangestheMeaningof aModerateDeficit Meta-analysesreinforcethisconditionalresponse.DI generallyimprovesWUEwhilereducingyieldrelativeto Climatevariabilitymodifiescropwaterstressthrough fullirrigation[3–5].Therefore,irrigationrecommenda- simultaneouseffectsonsoil-watersupplyandatmo- tionsshouldbetreatedasenvironmentallydependent sphericdemand[6,7,34,35,42,47].
managementrangesratherthanfixedprescriptions. VPDcanriserapidlyduringhotanddryatmospheric conditions, strengthening the hydraulic gradient 7.Crop-SpecificPatternsandGenotype × En- from leaves to the atmosphere. Consequently, a vironment × ManagementInteractions soil-moisturethresholdthatissafeundermoderate VPDmaybecomedamagingduringaheatwave[6–8]. Cerealsdemonstratetheimportanceofseasonalwater allocation.Inwheat,conservingsoilwaterforanthesis High temperature increases respiratory losses, ac- andgrainfillingcanmaintainproductivityduringter- celerates phenological development, destabilizes minaldrought[3,6,13].Inmaize,reproductivestages reproductiveprocessesandinteractswithdroughtto surroundingtasselling,silkingandearlykernelestab- intensifyoxidativestressandprematuresenescence lishmentareespeciallysensitivetodroughtandheat [35,47].Combinedstresscanshortencanopyduration [34,35,44,47].
andgrain-fillingperiods,producingyieldlossesgreater thanthoseassociatedwitheitherstressinisolation Ricediffersbecauseconventionalfloodedproduction [47].
hasahighirrigationrequirement.Controlledsoildry- ingandwater-savingirrigationcanreducewaterde- Rainfallvariabilitycreatestheoppositemanagement mand,butwaterstressduringpanicleinitiationand problem.Unexpectedrainfallcanremoveanintended floweringrequirescarefulmanagement[1,2]. acclimationsignal,whereasamissedrainfalleventcan Legumescombinedroughtresponseswithsensitivity convertmoderateDIintoseverestress.
ofnodulationandbiologicalnitrogenfixation[36].Wa- The tomato sowing-window study further demon- ter deficit can restrict nodule activity before severe strates how changing environmental exposure can shootsymptomsbecomeapparent.
strongly alter biological outcomes even within Horticulturalcropsprovideopportunitiesforregulated the same crop and geographical region [56]. This DIbecausemoderatewaterlimitationcanmodifyqual- reinforces the need for DI recommendations to be ityaswellasyield[1,37].However,responsesarehighly dynamicallylinkedtoweatherratherthanbasedsolely dependent on crop developmental stage, genotype onfixedseasonalschedules.
andconcurrentenvironmentalconditions. 9. IntegratingPhysiologywithPrecisionIrri- 9.2TowardIntegratedSmartIrrigation gationandDecision-SupportSystems Theintegrationofremotesensing,AI,IoT,bigdataand cropmodelswithinDSScanprovidereal-timemoni- A biologically informed irrigation system should toringandpredictiveirrigationrecommendations[48]. increasingly treat the plant as a sensor rather than Machine-learning systems may help identify nonlin- relyingexclusivelyonsoil-watercontent[29,30].The earrelationshipsamongVPD,soilmoisture, canopy most useful decision variable is likely to combine temperatureandcropresponse,whereasphysiological soil-water status, atmospheric demand and crop modelscanconstrainpredictionswithinbiologically physiologicalresponse.
Decision-supportsystemsprovideapracticalframe- Suchhybridsystemsareparticularlyattractiveunder workforthisintegration.Climate-responsiveDSScan climatevariabilitybecausetheycanupdateirrigation combineweatherdata,soilproperties,cropmodels decisionswhenheatwaves,rainfallorunexpectedcrop andmanagementinformationtogeneratesite-specific responsesaltertheseverityofanintendeddeficit. recommendationsforirrigation,sowing,nutrientman- agementandriskreduction[48].ModernDSSincreas- inglyincorporateartificialintelligence,remotesensing, IoTsensorsandpredictiveanalytics,allowingrecom- mendationstoresponddynamicallytochangingfield conditions[48].
Soil-moisturesensorscanprovidecontinuousinforma- tiononroot-zonewateravailability,whileautomated systemscanusethesedatatotriggerirrigationwhen
AI/ML,adaptivefeedbackandprecisionirrigation distinguishausefulacclimation-leveldeficitfromdam- decisionsdirectedtowardyieldstabilityandimproved agingstress.
9.1 Multispectral Imaging, Machine Learning and 10.TranslatingDeficit-IrrigationResearchto Plant-StressPhenotyping Farmers’Fields Multispectral and hyperspectral sensing combined Physiologicaloptimizationalonedoesnotguarantee withmachine-learningalgorithmsprovideadditional successfulfieldadoption.Technologiesthatperform opportunities for high-throughput crop monitoring. well experimentally may produce yield gaps under Although these technologies have been extensively farmers’conditionsbecauseofincompleteadoption, developed for disease detection, the physiological input constraints, knowledge gaps and climatic variables detected by spectral imaging include heterogeneity.
changesinchlorophyll,photosynthesis,tissuewater Field studies in banana in northern Tripura provide status,pigmentsandcellularintegrity[51]. a useful example. Integrated nutrient management Multispectral imaging offers rapid, non-destructive combiningorganicinputs,biofertilizersandreduced field assessment, while UAV-based platforms can mineralfertilizersubstantiallyimprovedyieldrelative extendmeasurementsoverlargeareas[51].Machine- to farmers’ practice and demonstrated measurable learningapproachessuchassupport-vectormachines, technology and extension gaps [54]. The study random forests, convolutional neural networks and also showed substantial farmer adoption following vision transformers can identify complex spectral front-linedemonstrations,illustratinghowvisiblefield patternsandclassifyplantstressconditions[51]. performance and technical support can accelerate uptakeofresource-efficientpractices[54]. ForDIresearch,theseapproachescouldbeadapted todistinguishwater-deficitsignaturesfromheat,nu- AlthoughnutrientmanagementandDIaredifferent trientordiseasestressesbyintegratingmultispectral, technologies, the implementation lesson is directly thermal,weatherandsoil-sensorinformation.Theob- relevant: water-savingirrigationwillrequirefarmer- jectiveshouldnotmerelybestressdetectionbutiden- accessiblethresholds,demonstrations,advisorysup- tificationofthetransitionfromreversibleacclimation portandevidenceofyieldandprofitabilityunderlocal toyield-threateningdehydration.
Monitoring Representativeindica- Informationprovided Potentialirrigationdeci- Key refer- domain tor sion ences Soil Soil moisture, matric Availablewateranddeple- Determine onset and in- [29,30,41,48] potential, root-zone tionrate tensityofDI waterstorage Atmosphere Temperature, rainfall, Atmosphericevaporative Modify DI during heat [35,42,47] relativehumidity,VPD demandandclimaticrisk wavesorrainfallevents Planthydraulics Leaf/stemwaterpoten- Actual physiological wa- Detecttransitionfromac- [9,29,30] tial,sapflow terstatus climationtodamagingde- hydration Canopyresponse Canopy temperature, Transpiration restriction Triggerrecoveryirrigation [13,29,30] stomatalconductance andplantstress Spectralsensing Multispectral/hyperspectCrahllorophyll, tissue wa- Spatial identification of [51] reflectance ter and physiological stressedzones changes Remote sens- Thermalandmultispec- Field-scalestressvariabil- Variable-rate irrigation [48,51] ing/UAV tralfieldimagery ity andtargetedmonitoring DSS and crop Soil–weather–crop in- Forecast crop water re- Stage-specific and [48] models tegration quirementsandrisk climate-responsive scheduling AI/ML Patternrecognitionand Integrates complex sen- Adaptivepredictionofirri- [48,51] nonlinearprediction sor and environmental gationneed data Similarevidencefromcommercialmarigoldcultivation severelaboratorydehydrationshouldbetestedunder showedthatrecommendedtechnologywasagronomi- moderate, fluctuating and repeated field deficits callyfeasible,butsignificantextensiongapspersisted [6,7,14,19–24].
becausefarmersadoptedsomepracticesmorereadily Fourth,researchshouldprogressfromsingle-hormone thanothers[55]. Extensioncontact,training,market models toward integrated plant–microbiome hor- accessandinputavailabilityinfluencedtherealized monal networks. ABA remains central, but auxin, performanceofthetechnology[55].
cytokinins,ethyleneandotherhormonesinteractwith ThesefindingssuggestthatDIresearchshouldmove microbial hormone production and ACC-deaminase beyondphysiologicalwaterproductivityandexplicitly activity[49,50,53].
quantifyadoptionfeasibility,irrigationinfrastructure Fifth,rhizosphereandendophyticmicrobiomesshould requirements, labour demand, risk perception, eco- beincorporatedexplicitlyintoDIstudies.Microbiome- nomicreturnandextensiongaps.Atechnicallyoptimal assistedproduction,functionalmicrobialprofiling,rhi- DIschedulethatfarmerscannotreliablyimplement zosphererestructuringandendophyticinoculationcol- willcontributelittletoclimateresilience. lectivelysuggestpossiblecontributionstonutrientac- quisition,rootarchitecture,osmoticregulation,antiox- 11.ResearchGapsandFuturePriorities idantcapacityandWUE[46,49,50,53].Metagenomics First,DIexperimentsrequiregreaterphysiologicalstan- providesaroutefordetermininghowrepeateddrying dardization.Reportingonlythepercentageoffullirriga- andrewettingrestructurethesecommunities[52]. tionisinsufficient[1–5].Studiesshoulddocumentsoil Sixth, crop multi-omics should be integrated with texture,rootingdepth,soilandplantwaterpotential, microbiomemulti-omics.Combiningplanttranscrip- VPD,cropstage,irrigationfrequency,stressduration tomics, metabolomics and hormone profiling with andrecovery.
metagenomicormetatranscriptomiccharacterization Second, future experiments should incorporate ofrootsandrhizospherecouldrevealthebiological combined heat–drought scenarios. Crop responses networks that distinguish useful acclimation from tosimultaneousstressescannotalwaysbepredicted irreversibledamage[49,50,52,53].
fromindividual-stressexperimentsbecausecombined Seventh, high-throughput phenotyping and digital stressproducesdistinctphysiological,hormonaland monitoringshouldbeintegratedwithDI.DSSplatforms molecularresponses[47].
can combine soil, weather, crop and management Third,thefieldrelevanceofmolecularmarkersneeds data[48],whilemultispectralandmachine-learning improvement. Candidate genes identified under approaches can provide non-destructive spatial informationaboutcropphysiologicalcondition[51]. teruse,improvewaterproductivityandconservesoil moistureforcriticalreproductivestages[1–5].Severe, Eighth, breeding programmes should emphasize prolongedorpoorlytimeddeficitsrestrictcarbonas- dynamic rather than constitutive traits. Responsive similationandreproductivedevelopment,andthese stomata,effectiveroothydraulics,appropriaterooting effectscanbecomesubstantiallymoredamagingwhen depth,reproductivestability,osmoticadjustmentand droughtcoincideswithheat[47].
rapidrecoveryaremorepromisingthanpermanently conservativegrowth[6–13,25,26].
Climate variability therefore makes fixed irrigation thresholds increasingly unreliable. DSS, real-time Ninth, genotype × environment × management in- sensors, remote sensing and machine-learning teractionsrequiremoreexplicitevaluation.Evidence approachescanhelpintegrateatmospheric,soiland fromtomatoshowsthatgenotypeperformanceand crop information into adaptive irrigation decisions yield can change strongly with sowing window and [48,51].
environmentalexposure[56].Comparablefactorialde- signsshouldbeusedforgenotype × irrigationtiming Atthesametime,farmer-levelevidencedemonstrates × climaticenvironment.
thattechnologicaleffectivenessalonedoesnotensure adoption. Demonstration, extension support, local Finally,adoptionandeconomicanalysisshouldbein- adaptability and economic feasibility determine tegratedintoirrigationresearch.Fieldtechnologystud- whether improved management is translated into iesinbananaandmarigolddemonstratethatdemon- stableproductivity[54,55].
stration,extensioncontact,localadaptabilityandpar- tialadoptionstronglyinfluencethegapbetweenexper- ThenextgenerationofDIshouldconsequentlyinte- imentalandfarmer-levelproductivity[54,55].FutureDI grateplanthydraulics,hormonalbiology,molecular researchshouldthereforeevaluatenotonlyphysiolog- reprogramming,microbiomeecology,climatemoni- icalefficiencybutalsopracticalfeasibilityandfarmer toring,artificialintelligenceandfarmer-centredman- adoption.
agement. Suchintegrationcanidentifywhenacrop isproductivelyacclimating,whenstressisapproach- ingirreversibleyieldloss,andwhenirrigationwillpro- 12.Conclusion videthegreatestphysiologicalandeconomicbenefit. Deficitirrigationismosteffectivewhenunderstoodas Thistransitionfromfixedvolume-basedirrigationto- amanagedacclimationprocessratherthansimplya warddynamicresponse-basedmanagementoffersa reductioninirrigationvolume[1–5]. Theearliestre- promisingpathwayforstabilizingcropyieldandwater sponse to soil drying is hydraulic: altered soil–root productivityunderincreasinglyvariableclimates. conductance,xylemtensionandaquaporinregulation changewatertransportandinitiateroot-to-shootcom- munication[9,10,25,26].