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Heat and drought stresses are among the most dominant environmental constraints limiting crop productivity worldwide, particularly under the current scenario of climate variability. While individual stress responses have been widely documented, their simultaneous occurrence presents a more severe and complex challenge to crop systems. Heat stress accelerates plant metabolism and phenological development, whereas drought stress restricts water availability, leading to reduced turgor pressure and impaired physiological functioning. When these stresses occur together, they intensify cellular damage, disrupt metabolic pathways, and significantly reduce crop yield. The combined stress adversely affects photosynthesis, nutrient uptake, reproductive development, and grain formation. It also enhances oxidative stress and alters hormonal balance, ultimately leading to premature senescence and yield loss. This review provides a comprehensive understanding of the physiological, biochemical, and molecular mechanisms underlying heat–drought interactions and discusses adaptive strategies for improving crop resilience.
Keywords: Heat stress; Drought stress; Stress interaction; Crop physiology; Yield loss; Climate change; Heatstress; Droughtstress; Stressinteraction; Cropphysiology; Yieldloss; Climatechange
stress increases photorespiration, while drought re- Highlights ducesinternalCO concentration,furtheraggravating • Combinedheatanddroughtstressexertssyner- theinefficiencyofcarbonfixation. Thesecombined gisticnegativeeffectsoncropgrowth,physiology, effectssignificantlyreducebiomassproductionand andyieldformation.
• Photosynthesis,waterrelations,andreproduc- tiveprocessesarethemostseverelyaffectedun- derstressinteraction.
2.2 WaterRelationsandTranspiration • Oxidativestressandhormonalimbalanceplay Water balance in plants is severely disrupted under akeyroleinmediatingplantresponsestocom- droughtstressduetoreducedsoilmoistureavailabil- binedstressconditions.
ity. Thisleadstodecreasedleafwaterpotentialand • Integratedapproachesinvolvingbreeding,agro- lossofcellturgidity(16,17).Heatstress,ontheother nomicmanagement,andbiotechnologyarees- hand,increasesevapotranspirationdemand,acceler- sentialforimprovingcropresilience.
atingwaterlossfromplanttissues.Undercombined stress,plantsexperienceseveredehydration,forcing stomatalclosuretoconservewater(18). Whilestom- 1.Introduction atalclosurehelpsreducewaterloss,italsolimitsCO Climatechangehasresultedinamarkedincreaseinthe entryintoleaves,reducingphotosynthesis.Thistrade- frequencyofextremeweatherevents,includingpro- offsignificantlylowerswater-useefficiencyandgrowth longeddroughtsandheatwaves.Thesestressesrarely (19–21).
occurinisolationunderfieldconditions,makingtheir combinedimpacthighlyrelevantformodernagricul- 2.3 OxidativeStressandCellularDamage ture(1).Heatstress,definedastemperaturesexceeding Oneofthemajorconsequencesofcombinedheatand optimalthresholdsforplantgrowth,directlyaffects droughtstressistheexcessiveaccumulationofreactive enzymeactivity,membranestability,andmetabolic oxygenspecies(ROS).Theseincludesuperoxideradi- processes.Ontheotherhand,droughtstresslimitssoil cals,hydrogenperoxide,andhydroxylradicals,which wateravailability,affectingplantwaterrelationsand damagecellularcomponents(22). Undercombined nutrienttransport(2,3).Whenthesestressesoccursi- stress,ROSproductionincreasesbeyondthedetoxi- multaneously,theireffectsarenotmerelyadditivebut fication capacity of antioxidant systems. This leads oftensynergistic.Forexample,drought-inducedstom- tolipidperoxidation,proteindegradation,anddam- atalclosurereducestranspirationcooling,whichfur- agetonucleicacids(23–25). Plantsactivateantioxi- therelevatesleaftemperatureunderheatstresscondi- dantenzymessuchassuperoxidedismutase,catalase, tions.Thisinteractionamplifiesphysiologicaldamage andperoxidasetomitigateoxidativestress.However, andreducescropproductivitymoreseverelythanindi- under severe conditions, these defenses are insuffi- vidualstresses(–6).Severalstudieshavereportedthat cient,resultinginirreversiblecellularinjury(26). combinedheatanddroughtstresscancauseyieldre- ductionsexceeding50%inmajorcropssuchaswheat andmaize(7,8,9). Understandingtheseinteractions 3.EffectsonCropGrowthandDevelopment isthereforeessentialfordevelopingclimate-resilient 3.1 GerminationandSeedlingEstablishment agriculturalsystems.
Theearlystagesofcropgrowthareparticularlyvulner- abletoenvironmentalstresses.Droughtstressreduces 2. Physiological Mechanisms Under Com- wateravailabilityforseedimbibition,whichisessential binedStress forinitiatinggermination(27). Heatstresscandena- tureenzymesinvolvedinmetabolicactivationduring 2.1 ImpactonPhotosynthesis germination, further inhibiting seedling emergence. Photosynthesisisoneofthemostsensitiveprocesses Whenbothstressesoccurtogether,germinationrates affectedbyheatanddroughtstress.Underheatstress, declinesharply,andseedlingvigorissignificantlyre- the stability of photosystem II is compromised, and duced(28,29). Poorestablishmentatthisstagehas enzymaticreactionsinvolvedincarbonfixationaredis- long-termconsequencesoncropgrowth,asitreduces rupted. DroughtstressfurtherlimitsCO availability plantpopulationandlimitsyieldpotential. duetostomatalclosure(10). Whenbothstressesoc- curtogether,thereisadrasticreductioninchlorophyll content,electrontransportefficiency,andRubiscoac- 3.2 VegetativeGrowth tivity.Thisleadstoasharpdeclineinphotosynthetic Duringthevegetativestage,plantgrowthdependson rateandcarbonassimilation(11,12).Inaddition,heat celldivision,elongation,andphotosyntheticactivity. Droughtstressreducesturgorpressure,limitingcellex- proportionofeconomicyieldtototalbiomass,alsode- pansion,whileheatstressacceleratesdevelopmental clinesbecauseplantsallocatemoreresourcestoward processes,shorteningthevegetativephase(30).Com- survivalratherthanreproduction(47,48). bined stress results in reduced leaf area, decreased plantheight,andlowerbiomassaccumulation. The 5.MolecularandBiochemicalResponses reductionincanopysizelimitslightinterceptionand Plantsrespondtocombinedheatanddroughtstress carbon assimilation, further affecting plant growth through complex molecular mechanisms. Stress- (31–33). Additionally,leafsenescenceisaccelerated responsivegenessuchasheatshockproteins(HSPs) undercombinedstress,reducingthedurationofactive areactivatedtoprotectcellularproteinsfromdenatu- photosynthesis.
ration(49).Similarly,drought-responsivegenessuch asDREBregulateosmoticbalanceandstresssignal- 3.3 RootDevelopmentandFunction ingpathways.Theinteractionofthesepathwayscre- Rootsystemsplayacriticalroleinstressadaptation atesuniquegeneexpressionpatternsundercombined byexploringdeepersoillayersforwater.Undermod- stress(50–52). Hormonalregulationalsoplaysakey eratedroughtconditions,rootgrowthmayincrease role, withabscisicacid(ABA)levelsincreasingtoin- asanadaptiveresponse(34).However,highsoiltem- ducestomatalclosureandstresstolerance.Osmolytes peraturesunderheatstresscanimpairrootmembrane suchasprolineaccumulatetomaintaincellularstabil- stabilityandenzymeactivity.Undercombinedstress, ity(53,54).
rootgrowthisoftenrestricted,reducingtheplant’sabil- itytoabsorbwaterandnutrients(35,36).Thislimita- tionfurtheraggravatesstresseffectsonabove-ground 6.AdaptiveandMitigationStrategies plantparts.
6.1 BreedingandGeneticApproaches Developingcropvarietieswithimprovedtoleranceto combinedstressisasustainablesolution.Traitssuch 4.EffectsonReproductiveDevelopmentand as deep root systems, efficient water use, and heat YieldFormation tolerancearetargetedinbreedingprograms(55–57). 4.1 FloweringandPollination Thereproductivestageisthemostsensitivephasein 6.2 AgronomicManagement cropgrowth. Heatstressduringfloweringcancause Agronomicpracticessuchasmulching,conservation pollensterility,reducepollenviability,andimpairfertil- tillage,andoptimizedirrigationcanhelpreducesoil ization(37,38).Droughtstresslimitsassimilatesupply moisturelossandtemperaturestress(58). Adjusting todevelopingreproductiveorgans,leadingtoflower sowingdatescanalsohelpcropsescapepeakstress abortion.Whenbothstressesoccurtogether,spikelet periods(59,60).
sterilityincreasessignificantlyincereals,resultingin poorgrainset(39–41).Thisstagelargelydetermines finalyield,makingithighlycriticalunderstresscondi- 6.3 BiotechnologicalInterventions Biotechnologicaltools,includinggeneticengineering tions.
andmicrobialinoculants,canenhancestresstolerance byimprovingantioxidantcapacityandnutrientuptake 4.2 GrainFillingandYieldComponents (61–63).
Grain filling is strongly affected by both heat and droughtstress.Heatstressacceleratesgraindevelop- mentbutreducesitsduration,leadingtosmallergrains 7.Discussion (42).Droughtstresslimitsthetranslocationofassimi- Theinteractionofheatanddroughtstressrepresentsa latesfromleavestograins,reducinggrainweight.Un- significantchallengetocropproductionsystemsworld- dercombinedstress,bothgrainnumberandgrainsize wide.Thecombinedstressleadstogreaterphysiolog- aresignificantlyreduced(43–45). Theseeffectsulti- ical disruption compared to individual stresses due matelyresultinsubstantialyieldlosses,oftenexceed- toitssynergisticeffects(64–66). Theinteractionbe- ingthosecausedbyindividualstresses.
tweenheatanddroughtstressrepresentsoneofthe mostcomplexchallengesincropproductionsystems, 4.3 BiomassandHarvestIndex as these stresses rarely occur independently under Totalbiomassproductionisreducedundercombined fieldconditions.Unlikeindividualstresses,theircom- stress due to impaired photosynthesis and nutrient binedoccurrencetriggersacascadeofphysiological, uptake(46).Theharvestindex,whichrepresentsthe biochemical,andmoleculardisruptionsthatoftenlead todisproportionatelyhigheryieldlosses. Thissyner- tocombinedstressthatdifferfromindividualstress gisticeffectarisesbecausedrought-inducedstomatal responses. Studies have shown that specific genes closurereducestranspirationalcooling,therebyinten- aredifferentiallyregulatedundercombinedstresscon- sifyingleaftemperatureunderheatstressconditions ditions,indicatingtheactivationofdistinctsignaling (3,16,40).
pathways(17,40). Thishighlightstheimportanceof studyingstresscombinationsratherthanextrapolating Oneofthecentralfindingsacrossstudiesisthesevere fromsingle-stressresponses.Hormonalinteractions, impairmentofphotosynthesisundercombinedstress. particularlyinvolvingabscisicacid,alsoplayakeyrole Heatstressdestabilizesthylakoidmembranesandre- inregulatingplantresponsesundercombinedstress ducestheefficiencyofphotosystemII,whiledrought conditions. Thevariabilityincropresponsestocom- stress limits CO availability due to stomatal closure. binedheatanddroughtstresssuggeststhattolerance Whenbothstressesactsimultaneously, bothphoto- mechanismsarehighlyspecies-andgenotype-specific. chemicalandbiochemicalprocessesareaffected,lead- Somegenotypesexhibitbetteradaptivetraitssuchas ingtoadrasticreductionincarbonassimilation(10,22). deeperrootsystems,efficientwateruse,andenhanced Thisduallimitationisparticularlycriticalbecausepho- antioxidantcapacity,whichenablethemtowithstand tosynthesisservesastheprimarysourceofassimilates stress conditions more effectively (51,55). This vari- required for growth and reproductive development. abilityprovidesopportunitiesforbreedingprograms Anotherimportantaspectisthedisruptionofplantwa- aimedatdevelopingstress-resilientcultivars.Froman terrelations.Droughtstressreducessoilmoistureavail- agronomicperspective,managementstrategiessuch ability,loweringplantwaterpotentialandcellturgor, asadjustingsowingdates,improvingsoilmoisturecon- whileheatstressincreasesevaporativedemand.Under servation,andadoptingclimate-smartpracticescan combinedstress,plantsexperiencerapiddehydration, helpmitigatetheadverseeffectsofcombinedstress whichacceleratesleafsenescenceandreducescanopy (44).However,theseapproachesalonemaynotbesuf- duration(16,18,23).Thisreductioninactivephotosyn- ficientunderextremeclimatescenarios,highlighting theticareafurthercompoundsthedeclineinbiomass theneedforintegratedsolutionscombininggenetic accumulation. In many crops, this leads to a short- improvement,agronomicmanagement,andbiotech- enedcropcycle,reducingthetimeavailableforyield nologicalinterventions.
formation.Oxidativestressplaysacrucialroleinmedi- Overall,theevidenceclearlyindicatesthattheinter- atingtheadverseeffectsofcombinedstress.Theexces- actionofheatanddroughtstressposesasignificant siveproductionofreactiveoxygenspeciesunderheat threattocropproductivity.Addressingthischallenge anddroughtconditionsleadstooxidativedamageto requiresacomprehensiveunderstandingofplantre- lipids,proteins,andnucleicacids.Althoughplantspos- sponses at multiple levels, from physiological pro- sessantioxidantdefensesystems,thesemechanisms cesses to molecular mechanisms. Future research areoftenoverwhelmedunderseverestresscombina- should focus on identifying key traits and genes as- tions(34,35,37).ThisimbalancebetweenROSproduc- sociated with stress tolerance and developing inte- tionanddetoxificationisamajorfactorcontributing gratedstrategiesforsustainablecropproductionunder tocellularinjuryandmetabolicdysfunction. There- changingclimaticconditions(65,69).Photosynthesis, productivestagehasbeenconsistentlyidentifiedas water relations, and reproductive processes are the themostsensitivephasetocombinedstress.Hightem- mostaffected,resultinginreducedbiomassandyield. peraturescanimpairpollenviabilityandfertilization, The variability in crop response highlights the need whiledroughtlimitsassimilatesupplytodeveloping forintegratedapproachesthatcombinephysiological reproductiveorgans.Thesimultaneousoccurrenceof understandingwithagronomicandgeneticsolutions thesestressesresultsinhighspikeletsterilityincere- (67–69).
alsandreducedfruitsetinothercrops(7,10,41).This explainswhyevenshortperiodsofstressduringflow- eringcanleadtosubstantialyieldlosses.Grainfilling 8.Conclusion isanothercriticalstageaffectedbystressinteraction. Combinedheatanddroughtstresssignificantlyaffects Heatstressacceleratesgraindevelopmentbutreduces crop growth, physiological processes, and yield for- itsduration,whiledroughtstressrestrictsassimilate mation. Theirinteractionleadstoseverereductions translocation.Together,thesestressesresultinsmaller inphotosynthesis,water-useefficiency,andreproduc- grainsizeandreducedgrainweight(19,42).Thecom- tivesuccess.Tosustainagriculturalproductivityunder binedeffectonbothgrainnumberandgrainweightul- climatechange, itisessentialtodevelopintegrated timatelyleadstosignificantreductionsinoverallyield. strategiesinvolvingstress-tolerantvarieties,improved Atthemolecularlevel,plantsexhibituniqueresponses agronomicpractices,andadvancedbiotechnological tools.Futureresearchshouldfocusonunderstanding 20. Sita,K.,etal.(2017).Foodlegumesandtemper- stressinteractionsatmultiplelevelsanddeveloping ature. FrontiersinPlantScience,8,1658. resilientcroppingsystems.
21. Gray, S. B., & Brady, S. M. (2016). Plant re- sponsestoclimatechange. DevelopmentalBi- ology,419(1),64–77.