全 文 ：应用与环境生物学报　 2005, 11(1):115～ 122 　 　
ChinJApplEnvironBiol=ISSN1006-687X 　 2005-02-25
　RESPONSESOFCONIFERSTODROUGHTSTRESS＊
DUANBaoli, YINChunying＆LIChunyang＊＊
(ChengduInstituteofBiology, ChineseAcademyofSciences, Chengdu　610041, China)
Abstract　Droughtstressisoneofthemajorenvironmentalstressesthatafectplantsurvivalandgrowth.Theefectsofdrought
canbemitigatedbyanumberofstrategies, includingmorphological, physiologicalandmolecularacclimationtodrought.Many
speciesexhibitchangesinpartitioninginfavorofthestructuresinvolvedinwateruptakeandtransport, andincreasewateruse
efficiency(WUE)inresponsetowaterdeficits.Greaterallocationtorootsystemmayincreasetheamountofsoilwateraccessi-
bleforaplant.Changesinfoliagearea/sapwoodarearatioalsoinvolveinthewaterpotentialgradientfromroottoshoot.Syn-
thesisandaccumulationofosmoprotectantscanincreasethedroughttoleranceofplant.Osmoticadjustmentinresponseto
droughtstressallowsthenormalfunctioningofphysiologicalprocessestotakeplace.Xanthophylscycle-dependentthermal
dissipationandtheintegratedsystemofenzymaticandnon-enzymaticantioxidantsareinvolvedinthephotoprotectivemecha-
nismsduringtheperiodsofdrought.Water-stressrelatedgenesareinducedbyabscisicacid(ABA), whichapparentlyisin-
volvedinthesignaltransductionduringdroughtstress.Inthisreview, weexaminetherolesofmorphologicalchanges, osmotic
adjustment, xylemcavitation, photosynthesis, WUE, ABAandmolecularmechanismforacclimationandadaptationofconifers
todroughtstress.Moreover, theuseofphysiologicalcharacteristicsasselectioncriteriafordroughttoleranceisalsodiscussed.
Duringwaterstress, drought-tolerantmechanismsarenotindependentandtheyareneededtointegratediferentaspectsabout
wholeplantresponses.Ref99
CLC　Q949.665 ∶ Q945.78
Keywords　conifers;droughtstress;morphologicalstructure;physiologicalresponse;molecularmechanism
松科植物对干旱胁迫的反应＊
段宝利 　尹春英 　李春阳＊＊
(中国科学院成都生物研究所　成都　 610041)
摘　要　水分亏缺是制约树木生长的重要环境因子 , 植物通过形态 、生理以及分子水平来适应水分亏缺.渗透调节使
植物在低水势下维持正常生理活动 ,是植物忍耐水分亏缺的重要生理机制.干旱胁迫下 , 植物形态结构变化有利于水
分吸收和传导 ,从而提高水分利用效率;同时 ,生物量向根部的分配增加 ,叶面积 /边材面积比发生变化 , 这种生物量分
配转移提高了根和茎向叶片输水能力 ,从而防止气穴现象;干旱胁迫容易引起光能过剩 , 过剩的光能会对光合器官产
生潜在的危害.依赖于叶黄素循环的热耗散是光保护的主要途径;同时酶促及非酶促系统也是防止光合器官破坏的重
要途径;脱落酸作为一种激素逆境信号 , 活化了与抗旱诱导有关的基因.本文从形态变化 、渗透调节 、气穴现象 、光合作
用 、水分利用效率 、脱落酸以及分子机理等方面阐述了松科植物对干旱胁迫的响应 , 并对耐旱指标的筛选进行了讨论.
干旱胁迫下 , 各耐旱机理相互制约 , 需要联合各个方面的因素来考虑整个植物对干旱的反应.参 99
关键词　松科;干旱胁迫;形态结构;生理响应;分子机理
CLC　Q949.665 ∶ Q945.78
　　Plantsareoftensubjectedtotheperiodsofsoilandatmos-
phericwaterdeficitduringtheirlifecycle.Theincreasedfrequency
ofsuchphenomenaislikelytooccurinthefutureevenoutsideto-
daysarid/semi-aridregions[ 1].Covering11% oftheearthster-
restrialsurface[ 2] andcontainingabout800Pgcarbon[ 3] , thebore-
alforestisoneoftheearthslargestterrestrialbiomes, andisalso
ofenormouseconomicimportance.Intheborealforest, evena
Received:2004-03-16　　Accepted:2004-04-22
＊SupportedbytheChinaNationalMajorFundamentalScienceProgram
(No.G2002CB111504), andtheProgramof100DistinguishedYoung
ScientistsandKnowledgeInnovationProjectofCAS(No.KSCX2-
SW-115)
＊＊Correspondingauthor(E-mail:licy@cib.ac.cn)
decreaseinwatercontentofthesurfacehorizonsofsoilcansubject
halow-rootedseedlingstoseveredroughtstress.Evidence
showedthattrees, especialyconifers, inthestageofseedlings
tendtoexperiencedroughtstress[ 4].Becausedroughtstressinhib-
itsthegrowthofconiferseedlings, anyphysiologicalormorphologi-
caladjustmentthatamelioratesthegrowthreductionduetowater
stresswilimprovetheirrateofplantationestablishment[ 4].Suc-
cessfulestablishmentofseedlingsdependspartlyontheirdrought
resistance[ 5].Thus, abetterunderstandingoftheresponsesofco-
niferoustreestodroughtstresscanbeofimportanceforunder-
standingtheresponseofconiferousforeststoglobalclimate
change, andalsoisessentialtoevaluateitsintroductioninthe
semi-aridzones.Despitealotofpublicationsondroughttoler-
anceofconifers, theadaptivemechanismsutilizedbyconifersto
surviveunderdroughtstressconditionsarenotwellunderstood.
Theaimofthisreviewistosummarizetheresultsofthenumerous
studiesofwaterrelationsofconifersunderdroughtstress.Accord-
ingtopreviousstudies, drought-tolerantmechanismofconifers
canbedividedintoseveralaspectsaspresentedinthefollowing
chapters.
1　MorphologicalResponses
Ithasbeenobservedthat, inacommondroughtstressenvi-
ronment, plantsoriginatingfromxerichabitatshaveslowerratesof
shootgrowththanthosefrommesichabitats.Thistrendhasbeen
observedinconiferousplants[ 6].Presumably, slowershootgrowth
andasmalerleafsurfaceareaexposedtotheatmospherecanre-
ducethedangeroflethaldesiccationbyextremeatmosphericand
soildrought.Therefore, aslowrateofshootgrowthisacommon
evolutionaryresponseofplantstohabitatswhereenvironmental
stressisfrequent.
Changesinbiomassdistributionareusualyinterpretedasac-
climationstodroughtstress.Foranindividualtree, totalleafareais
closelycorrelatedwiththecross-sectionalareaofthewater-con-
ductingportion(sapwood)ofthemainstemandisexpressedasthe
leaf/sapwoodarearatio(Al/As).Originally, theworkwasreferredas“pipemodeltheory” whichcharacterizedthesapwoodconducting
tissueasapipeconductingwatertoleaves[ 7].Itwashypothesized
thatforaspecificenvironmenteveryleafwouldrequireaspecifica-
mountoftranspirationalwatertomaintainopenstomataforphotosyn-
thesis, andthataspecificamountofconductingtissuewasrequired
tosupplythewater.Clearly, therelationbetweensapwoodareaand
leafareademonstratesabasicoptimizationincarbonacquisitionand
allocation.AdeclineinAl/Asinresponsetodrierclimatesmaybe
themostsignificantresponsetoincreasingaridity[ 7, 8].Thisshiftin
biomassallocationincreasesthephysicalcapacityofrootsandstems
tosupplyleaveswithwater(leafspecifichydraulicconductance)
andmaintainsminimumleafwaterpotentialabovethelevelsthat
causexylemcavitation.Whiteheadetal.[ 8]interpretedthefoliageto
sapwoodarearatiointermsofhydraulicresistanceandpredicted
thattheratioshoulddecreaseinresponsetodrierair, andsuggested
thatplantsindryenvironmentshavelowerAl/Asthanplantsinwet
environments.Thisstudyreflectsrelativelygreaterevaporativede-
mandsinmorexericareas.
Rootsystemdevelopmentcanalsoadapttodrought.Theroots
canactasawaterreservoirandtoproduceadeeptaprootisan
advantageforatreetosurviveinadryregion[ 9 , 10].Maximizing
growthoftherootsisimportantinestablishmentofyoungseed-
lings[ 9].Generaly, treesnativetoaridenvironmentsoftenhavea
highroot/shootratio.Themoreistheexposuretodrought, the
higheristheratiobetweenrootandshootmassshiftedfurtherinfa-
voroftheroots[ 11].Cregg[ 12]alsofoundthat, duringlimitedwater
supply, seedlingsfromaridandsemi-aridregionsshowedahigh-
erallocationofdrymatertorootsthanthosefromhumidregions.
Somemodificationsmayalsotakeplaceinthestructureofleavesas
aresponsetodrought[ 13].Underwaterdeficits, plantstendtopro-
duceseedlingswithshorterneedles, lesssurfacearea, andfewer
stomataperneedle[ 12].
2　PhysiologicalResponses
2.1　OsmoticAdjustment
Itiswellestablishedthatmaintenanceofcelturgorisapre-
requisiteforalmostalformsoflifeastheyprovideamechanismfor
theexpansionofcelenvelope.Plantscanregulateturgorbysolute
accumulation, osmoticadjustment(OA)andelasticadjustmentof
theircellwals[ 14].
Onexposuretoosmoticstressasaresultofdrought, plants
accumulatearangeofmetabolicalybenignsolutes, colectively
knownascompatiblesolutesorosmolytes.Astheresultofthenet
soluteaccumulation, theosmoticpotentialofthecellislowered,
whichinturnattractswaterintothecelandthusmaintainturgor
pressure.Inhigherplantssoluteaccumulationincludeslowmolec-
ularweightsugars, organicacids, polyolsandnitrogencontaining
compounds, suchasaminoacids, amides, proteinsandquaternary
ammoniumcompounds.Inducedbywaterstress, changesinfree
aminoacidconcentrationsandsimultaneouslyanaccumulationin
pralinewereobservedinsomeconifers[ 15 ～ 17].Theseresultssug-
gestthattheaminoacidandproline(PRO), actingasosmolytes,
haveanimportantroleinplantadaptationtodroughtstress.Gener-
ally, theprimaryfunctionofpolyaminesisturgormaintenancebut
theymayhaveotherprotectiveeffectsonmacromoleculesinde-
hydratingcels.Someauthorsreportedpolyaminesplaytheroleof
notonlypromotingosmoticadjustment, butalsostablingmem-
brane[ 18, 19].Droughtstressdecreasesmembranestabilityandin-
creasesleakageofions, aminoacidsandothermaterialsfrom
cells[ 20 , 21].Thereisanexperiment[ 18]showedtheprotectiveaction
ofpolyaminesonmembraneintegrityandosmoticadjustment.For
example, spermineandspermidinewerefedintothexylemof1.5
-year-oldjackpine(PinusbanksanaL.)for7 daysandplants
weredroughtedbywithholdingwaterfor12 days, theresults
showedthatthespermidineandsperminepromotedtheOAandre-
ducedmembranedamageinjackpineunderdrought.Thisprotec-
tiveabilityofpolyamineswasalsoprovedbysimilarexperi-
ment[ 19].
AwealthofstudiessuggestedthatOAcontributestodrought
resistanceinmanyconifers[ 22 ～ 26].Evidence[ 26]showedthatthea-
bilityofOAandturgormaintenanceunderdroughtstresscouldbe
ausefulcriterionfortheearlyselectionofdroughttolerantgeno-
types.ThekeyroleofOAisturgormaintenanceduringwaterdefi-
cits, whichisessentialformaintenanceofturgor-relatedprocess,
especialystomatalregulation[ 24].Foranygivenleafwaterpoten-
tial, aleafwithamorenegativeosmoticpotentialhasmoreturgor
pressuretoexpendandcanthereforewithstandgreaterdehydration
beforeacriticallossofturgoroccurs.Alowercelularosmoticpo-
tentialalsoconservesthecellularvolumeandmaintainsgradientsof
waterpotentialfavorableforwaterinflux[ 27].Plantgrowingunder
waterdeficitsmaycontributetotheirosmoticadjustmentbymain-
tainingthetissuewatercontentabovethevaluescriticalforcelular
damage.Theconifersmoretoleranttodroughtmayhaveahightis-
116 　　　　　　　　应 用与 环 境生 物学 报　　ChinJApplEnvironBiol　　　　　　　　　　　　　　 　　　11卷
suetolerancetolowrelativewatercontent(RWC)[ 28].
Highelasticityalowscelwalltoshrinkaroundshrinkingcy-
toplasm.Thesqueezingofthecelwallaroundthecytoplasmmain-
tainsgreaterturgorataparticulartissuewatercontent.Markedre-
ductionsinboththesaturatedandturgor-lossvolumesandlarge
increasesintheelasticcoeficientsofthewhitesprucetissuesindi-
catedthatelasticadjustmentwascriticalforturgormainte-
nance[ 28].Ashighercellelasticitypermitsalowercelularosmotic
potentialforturgormaintenanceunderwaterstress[ 29] , increasesin
theelasticmodulusofcelwalsmayprovideaneffectivemecha-
nismofwaterstresstolerance.
2.2　XylemCavitation
Thereareseveralexplanationsforxylem cavitations.The
leadingexplanationistheair-seedinghypothesis[ 30] , which
proposesthatembolismsaretriggeredbyairaspiratedintoavessel
viapitsinthewalwhereitadjoinsanairspace.Onceinsidethe
vessel, theairdisruptsthecohesionofthewatercolumn, thereby
causingasuddenretractionofthewatercolumnandleavingbehind
avesselfilledwithwatervaporandair.Cavitationshavebeen
viewedasaseriousdysfunction[ 31, 32].However, someexperiments
haveshowedthatcavitationwasanadaptivemechanismasthewa-
terreleasedfromthelumensofxylemconduitsbycavitationcould
acttobuferleafwaterstatusovershorttimeperiodsandthereby
conservingsoilwaterandoptimizingstomatalconductance[ 33].
However, similarinformationislittleknownonconifers.
Ithasbeensuggestedthatstomatamaycontroltheriskofxy-
lemembolism[ 34 , 35].Adeclineinstomatalconductanceinresponse
toachemicalorhydraulicsignalfromtheroots[ 36, 37] reducesthe
transportofwaterinthexylemandhenceavoidsthecriticalten-
sionscausingcavitation.InmatureScotspine, thestomatalclosure
inresponsetosoildrynessatathresholdsoilwaterdeficitpreven-
tedthedevelopmentofsubstantialxylemembolisminaboveground
woodytissue[ 38].However, thereisacontraryexperiment.Forex-
ample, withthestudyofScotspineandSitkaspruceafter3weeks
ofdroughttreatment, cavitationratesinScotspinecontinuedtoin-
crease, althoughthelowstomatalconductanceswereobservedin
bothScotspineandSitkaspruce[ 39].Sothemechanismforthe
controlofstomatalaperturealsoremainspoorlyunderstood.
Cochard[ 40]reportedthevulnerabilityofaspeciestoairem-
bolismwasconsistentwithitsecophysiologicalbehaviorunderwa-
terstress, drought-tolerantspeciesbeinglessvulnerablethan
drought-avoidingspecies.Highratesofcavitationhavealsobeen
foundinlargevesselortracheidsize[ 41].Thelargeststemstended
tocavitatebeforethesmalestoneswhenabranchexperienced
drought[ 41].Inaddition, evidence[ 42]showedrootxylemwithless
negativepressuresiscommonlymorevulnerabletocavitationthan
stemxylemaslessxylemnegativepressureisessentialtoembol-
ism.Ithasalsobeenreportedthatthepitmembraneflexibilityac-
countedforvariationinvulnerabilitytocavitations[ 40].Sperryand
Tyree[ 43]concludedlessvulnerabilitytocavitationwouldoccurina
morerigidmembranewithsmallerwaterconductingpores.
Thereisincreasingevidencethatrefilingoftracheidsafter
cavitationoccursdespiteofthepersistenceofnegativepressuresin
adjacentconduits[ 44, 45].Forexample, PeaandGrace[ 31] withheld
waterfrom5 -to6 -year-oldScotspinefor25 days, and14
daysafterre-watering, theyfoundacompleterecoveryofxylem
densitytothepre-droughtvalue, indicatingcompleterefilingof
tracheidsaftercavitationoccured.Buttherefilingwasnotob-
servedinotherexperiments[ 39 ～ 42].SperryandTyree[ 43] suggest
thatconiferslacktheabilitytorefillcavitatedtracheidsbecause
theycannotgenerateapositivexylempressure.Themechanismof
refilingoftracheidsfollowingdrought-inducedcavitationremains
unclear.
2.3　Photosynthesis
Wateravailabilityisanimportantdeterminantofseedlingsur-
vival[ 46, 47].Acommonresponsetowaterstressisadecreasein
stomatalconductancewhichdecreasestranspiration, photosynthesis
andgrowth.Lowgasexchangerateshavebeensuggestedtobean
attributeofdroughtadaptationinPseudotsugaspecies[ 48].Thiscan
notbegivenasageneralrule, sinceinthestudyofconiferousspe-
cies, thedroughtresistantP.macrocapahadthelowestgasex-
changerates, butexhibitedtheleastconservativewaterecono-
my[ 49].Thecapacityforphotosyntheticacclimationtowaterdeficit
isknowntovaryinpopulationsofconifersspecies[ 50 ～ 52].Black
sprucepopulationsthathavegreaterdehydrationtoleranceareable
tomaintainhigherphotosyntheticcapacityunderdrought, andgrow
fasterondrysitesthanpopulationswithmorelimiteddehydration
tolerance[ 53].Treesoccurringnaturallyondrysitesaregenerally
moredehydrationtolerantandmoreabletomaintainhighgasex-
changeratesunderdroughtconditionsthantreesofthesamespe-
ciesoccupyingmoremesicsites[ 54 ～ 56].
Muchworkonacclimationtowaterstressintreeshasfocused
ontheroleofstomatainlimitinggasexchangeinresponseto
stress[ 50 ～ 52].Becausemesophylprocessesinvolvedinthetransfer
ofCO2 intothechloroplastsandCO2 fixation, therehavebeensev-
eralstudiesofphotosyntheticaswellasstomatalacclimationtowa-
terstressinconifers[ 56 , 57].However, ahandfulofstudieshavefo-
cusedontheroleofstomatalandnon-stomatalacclimationtonet
photosyntheticunderwaterstressinconiferstrees[ 57].Eastman
andCamm[ 47]proposedthatlimitationstophotosynthesisinneedles
ofsprucetreesexposedtodroughtstresswerecausedfirstbystom-
atalandthenbynon-stomatallimitations.Inthestudyofblack
spruce, Stewartproposedthatstomatallimitationwaslessthanme-
sophylllimitationinblackspruce, thesimilarresultswereob-
servedinotherconifers[ 49～ 52, 57 ～ 58].Astheresult, ithavebeen
concludedthathighphotosyntheticcapacityratherthanreduced
stomatalwouldbeausefulindicatorwhenbreedingforincreased
droughtresistanceinblackspruce.
Underdrought, stomataclosureandtheinactivationofthe
Calvin-Bensoncycleenzymesoccurduetowaterstress, whichre-
sultsinadecreaseinphotonutilizationandanincreaseinphotoin-
hibition.Consumptionoflightenergyforphotorespiration, theflow
ofelectronsfromacceptorsinphotosystemI(PSI)tomolecular
oxygenandthepH-dependentdissipationofenergyoperateto
protectthephotosyntheticmachineryfromphotoinhibition.Previous
studyshowedthatacceptorsiderestrictionofelectronflowwasa
117　 1期 DUANBaolietal:ResponsesofConiferstoDroughtStress 　　
primaryresponsetowaterstressinwhitespruceseedlings[ 59].Dur-
ingwaterstress, changesinchlorophylfluorescenceinyoung
sprucetreesdemonstratethedown-regulationofprimaryphoto-
chemistryandaneedforenergydissipationtoavoidphotoinhibitory
damage[ 47].Thereisalsoevidencethatdroughtstressleadstoan
increaseintheproductionoffreeradicalinneedles, whichmay
contributetophotosyntheticapparatusinjury.Inordertocounteract
thetoxicityofactiveoxygenspecies, plantsareequippedwithboth
enzymaticandnon-enzymaticmechanismsforreactiveoxygen
species(ROS)scavenging.Superoxidedismutase(SOD)catalyses
thedismutationofsuperoxidetohydrogenperoxideandoxygen.
Andhydrogenperoxidewillbefurtherdetoxifiedbycatalase(CAT)
and/orperoxidases(POX)towaterandoxygen.Non-enzymatic
antioxidantsincludelipophiliccompounds, i.e., carotenoidsand
tocopherols, whichmaintainROSinlowamounts, therebyprotec-
tingphotosyntheticapparatusfromoxidativedamage[ 60].Inaddi-
tiontoantioxidants, thephotosyntheticapparatusisdirectlyprotec-
tedbyxanthophylscyclepigmentsviolaxanthin, antheraxanthin
andzeaxanthin.Studies[ 60, 61] haveshownthatdroughttolerance
specieshavealargerxanthophyllcyclepoolsizesuggestingthat
xanthophylcycledissipatesexcessexcitationenergyasheatinthe
antennacomplexes, therebyprotectingthereactioncentersfrom
photooxidation.Altheseresultsproposedthatxanthophyllcycle-
dependentthermaldissipation, andtheintegratedsystemofenzy-
maticandnon-enzymaticantioxidantsarethetwobiochemical
processesamongthephotoprotectivemechanisms.
2.4　Water-useEficiency
Water-useefficiency(WUE)isameasuretoassessdrought
adaptation.Itcanbedefinedeitherastheratioofdrymatteraccu-
mulationandwaterconsumptionoveraseason, orastheratioof
photosynthesisandtranspirationoveraperiodofsecondsormi-
nutes.WUEseemsgenerallytohaveahighcorrelationwithplant
growth.However, therelationshipmaybepositiveornegative.In-
creasingplantWUEhasbeenshowntoeitherincreaseordecrease
biomassproductivity[ 6, 62 ～ 64].PlantscanachieveahighWUE
througheitherhighnetphotosynthesis, orlowtranspiration, or
both.Bothprocessesareatleastpartialyregulatedbystomatal
conductance[ 65].Thedecreasesofphotosynthesiswhilestomata
fullyopenwouldresultinanegativecorrelationbetweenWUEand
productivity[ 66].Alternatively, whenthewatersupplyislimited,
plantsthatuseafinitewatersupplymoreefficientlybyproducing
greaterbiomassforagivenquantityofwatertranspiredwouldgrow
morerapidly, resultinginapositivecorrelationbetweenWUEand
productivity[ 53].Therefore, underwaterdeficits, plantswitha
highWUEshouldhavehigherbiomassproductivityor/andagrea-
terabilitytosurvivethanplantswithalowWUE[ 66].
Ontheotherhand, someresults[ 67]challengetheviewthata
greaterWUEisadvantageousinwater-limitedhabitats.Forex-
ample, Warrenetal.[ 67] reportedthatP.pinaster, themore
drought-tolerantspeciesisnotmoreeficientintheuseofwater
thanP.radiata.Inthisinstance, droughttoleranceofP.pinaster
canbeequatedwithavoidanceofwaterstress, probablyviaagrea-
terroot/shootratioanddevelopmentofadeeptaproot.
Theheavier13CisotopeofatmosphericCO2 isdiscriminateda-
gainstduringphotosynthesis, mainlybecauseofdifusionalfrac-
tionationandenzymaticfractionation.Carbondioxidemolecules
containing12Carelighterand, therefore, difuseintotheleafata
fasterratethancarbondioxidemoleculescontaining13C.Theribu-
lose-1, 5-bisphosphatecarboxylasediscriminateagainst13Cand
uses12C.Discriminationisrelatedtotheratioofinternaltoexter-
nalCO2 concentration(Ci/Ca), andCi/Caisafunctionofphoto-
syntheticcapacityandstomatalconductance[ 68].Typicaly, plants
withafavorablewaterstatushaveahighCi/Caandaredepletedin
13CwhereasplantsexposedtodroughtstresshavealowCi/Caand
areenrichedin13C, reflectingthetrade-ofbetweenphotosynthe-
sisandtranspiration.
Thecarbonisotoperatioofplanttissueprovidesanintegrated
measureofinternalplantphysiologicalandexternalenvironmental
propertiesinfluencingphotosyntheticgasexchangeoverthetime
whenthecarbonwasfixed[ 69].Theanalysisofcarbonisotoperatio
hasbeenusedasatoolinthestudyofwater-useprocessesinthe
needlesofconifers[ 70].Advancesintheunderstandingoftherela-
tionshipbetweenstablecarbonisotopecompositionandWUEare
essentialtoevaluatevariationamongpopulationsinimportanttraits
relatedtodroughttolerance[ 68].Geneticvariationincarbonisotope
discriminationmayreflectdiferencesinWUEassociatedwithvari-
ationinphotosynthesisorstomatalconductance.Recentstudies
haveshownsignificantandgeneticvariationincarbonisotopedis-
criminationinanumberofconiferspecies[ 62, 71 ～ 75].Undersimilar
ambientconditions, itwaspredictedthatpopulationfrommesic
habitatswouldhavehighercarbonisotopediscriminationvalues,
whereaspopulationfromxerichabitatsshouldhavelowercarboni-
sotopediscriminationvalues, reflectingthemechanismsforgreater
waterconservation[ 73].Buttheresultshavenotalwaysbeencon-
sistent[ 53].Thismaybebecausecarbonisotopediscriminationval-
uesofC3 plantscanbecorrelatedwithenvironmentalvariationor
withgeneticdiference.
Carbonisotopediscriminationvaluesshouldbenegativelyre-
latedtoproductivitywhenvariationincarbonisotopediscrimination
isaresultofchangesincarboxylationeficiency.Incontrast, if
variationincarbonisotopediscriminationisrelatedprimarilyto
variationinstomatalconductance, thenvaluesshouldbepositively
correlatedwithgrowth, aswasfoundintheworkofFlanaganand
Johnsen[ 73].Negativecorrelationsbetweencarbonisotopediscrimi-
nationandproductivityhavebeenfoundinPseudotsugamenz-
iesi[ 71] , PiceamarianaandLarixoccidentalis[ 64] grownunderlim-
iting-wateravailability[ 73].However, carbonisotopeanalysispro-
videsnoinformationaboutthespecificleaf-levelphysiological
differencesthatmayhavecausedanydifferencesinCi, Forexam-ple, areducedCimayarisefromareducedstomatalconductance,
orfromanincreasedintrinsicleafphotosyntheticcapacity.Thus,
thephysiologicalbasisofdiferencesinWUEcannotbedeter-
minedfrom carbonisotopediscriminationmeasurementsalone.
Therefore, theuseofcarbonisotopediscriminationinbreedingpro-
gramsmustbespeciesspecific[ 76].Whenthegoalistoidentify
sourcesofgeneticvariabilityforWUEtobeutilizedinplantbreed-
118 　　　　　　　　应 用与 环 境生 物学 报　　ChinJApplEnvironBiol　　　　　　　　　　　　　　 　　　11卷
ingprograms, itmaybedesirabletohavemoreinformationabout
thespecificphysiologicaltraitsinvolved.Also, betterunderstand-
ingoftheexacttraitsunderlyingexistingdifferencesinWUEmay
helptoidentifycandidategenestobeusedindirectgenetictrans-
formationapproachestoimprovingdroughttolerance[ 77].However,
therehavebeenfewreportsintheliteraturewheregenotypediffer-
encesinWUEhavebeenfulycharacterized.
2.5　AbscisicAcid
Abscisicacid(ABA)maybeoneofthemostimportantstress
signals, whichplaysvitalrolesinvariousstress-tolerancerespon-
ses.ABAactsasasignalfromroot-to-shootforchangesin
stomatalconductance[ 77] anditwasparticularlyconvincedbythe
splitrootexperiment[ 78] inwhichwateriswithheldfrompartofthe
rootsystem.Thestomatalconductancedeclineseventhoughthe
leavesarestillwellsuppliedwithwaterfromtherestoftherootsys-
tem, butincreasesagainwhenthedryrootisexcised.Increasing
ABAconcentrationleadstomanychangesindevelopment, physiolo-
gyandgrowth.Themaindevelopmentalandmorphologicalefectsof
ABAareinalteringtheplantinsuchawaythatlesswaterislost
throughtranspiration, asreviewedbySeter[ 79].ABAalsoimproves
watertransportamongplantpartsbyincreasingthehydrauliccon-
ductanceforwatermovementfromrootstoleaves[ 79].Atightrela-
tionshipbetweentheconcentrationofABAinthexylemsapandthe
stomatalconductancewasfoundwithvariousstudiesofconi-
fers[ 80 ～ 83]anditindicatesthatABAisinvolvedintheregulationof
stomatalconductanceduringperiodsofdrought.Forexample, inthe
studyofconiferssaplingsexposedtodrought, Jacksonetal.[ 83]re-
portedthatstomatalconductancedisplayedanegativeexponential
relationshipwithABAconcentrationinthexylemandhadnorela-
tionwithABAflux.However, acontrastexperiment[ 84] reportedthe
declineinstomatalconductancecouldnothavebeenmediatedbyin-
creasingABAbecausestomatalclosureappearedtoprecedeanyin-
creaseinABA.Thusitisalmostimpossibletocausestomatalclo-
suretoavoidtissuewaterdeficits, duetothelowsapvelocitiesfor
ABAsignaling.SoariseofABAinconifersiswhetheracontroling
factorornotinstomatalclosureneedsfurtherstudy.
Ithasbeenestablishedthatmanystress-responsivegenesare
up-regulatedbyABA[ 85, 86].ThesemayexplainABAmediates
thedrought-inducedexpressionandthecomplexnatureofABA
responsestodroughttolerance.However, theextentandthemo-
lecularbasisofABAinvolvementinstress-responsivegeneex-
pressionandstresstolerancearenotquietclearinconifers.
3　MolecularMechanism
Droughtstressisabotleneckfactorforplantgrowthandde-
velopment.Up-regulationofgeneexpressionimplicatedinrepair
ofdesiccationinjurycouldincreasethedroughttoleranceofplant.
Muchresearchinthelastdecadewasfocusedonthestress-in-
ducedgeneexpressioninordertofindanswerstothemolecular
mechanismsunderlyingplantcelulardehydrationtolerance[ 87].
Thisworkhasgeneratedinvaluableinformationonmanydiferent
plantgenesinducedbywaterstressandsubsequentlytheproteins
encodedbythesegenes.However, althoughmuchinformationon
droughtstress-inducedgeneshasbeenobtained, fundamental
knowledgeisstillackingregardingthebiologicalfunctionformany
oftheproteinsaccumulatinginresponsetodehydrationintrees,
especialyinconifers.Someofthemostprominentandsubsequent-
lymoststudiedproteinsaccumulatinginresponsetodroughtstress
inhigherplantsarethedehydrins(DHNs)[ 88].Richardetal.[ 89]
alsoreportedthatdehydrinproteinswereinducedinresponseto
wounding[ 89].DHNsaregenerallythoughttoplayanimportantrole
duringplantcellulardehydration, althoughnodirectbiochemical
evidencehasbeenpresented.Inmanysuggestionsontheirbio-
chemicalfunction, DHNshavebeenproposedtoactbyimproving
enzymestability[ 90] andaspossibleosmoregulators[ 91].Indicative
ofdehydrinsisthepresenceofoneorseverallysine-richunits
calledtheKsegmentsconservednearthecarboxyterminusofthe
proteinandrepeatedseveraltimesthroughoutthesequence[ 92].
Somedehydrinsalsopossessastringofserineresidues(S-seg-
ment).Anotherconsensussequence(DEYGNP), theY-seg-
ment, canbefoundneartheaminoterminusofmostofthede-
hydrins[ 92].The15 -amino-acidconsensussequenceoftheLys
-richmotifEKKGIMDKIKEKLPGhasbeenusedforantibodypro-
duction[ 93].Inconifers, dehydrinproteinsandgeneshavebeeni-
dentifiedfromtheseedsofPinus[ 93] andPseudotsuga[ 94] , butstud-
iesongeneexpressionarerestrictedtoneedlesofPiceaglauca
seedlings[ 89].Thus, theidentificationandcharacterizationofwater
-stress-responsivegenesprovidenewinsightsintothenatureof
themachineryinvolvedintheresponsetowaterdeprivationin
plants[ 95] anditmaybeanefectivestrategytoimproveplant
droughtstress-resistancebygenetransfer.
Genetransferbetweenecologicallydiferingspeciescanresult
inimprovedenvironmentaladaptation[ 96] asappearstobetruein
thecaseofsitkaandinteriorspruces[ 97].Incorporationofinterior
sprucegenesintothesitkasprucegenomehasproducedhybrids
thathavebothretainedthehighphotosyntheticcapabilityofsitka
spruceundernon-droughtconditions, andacquiredthegreater
droughtandfreezingtoleranceofinteriorspruce[ 97].Intheworkof
Fanetal.[ 97] , seedlingsofalsevensitkasprucepopulationswere
treatedunderidenticalenvironmentalconditions.Differencesin
physiologicalattributes(phenotypicvariation)betweenpopulations
wererelatedtotheircorrespondingSi-rDNAindex(genotypic
variation)values.ThisisconsistentwithCheveruds[ 98] argument
thatphenotypicanalysiscouldsometimessubstituteforgenetica-
nalysis.Theseresultssuggestthatgeneticestimatesprovideamore
directmethodofdefiningpopulationdifferences[ 99].Nevertheless,
cautionisrequiredinusingphysiological(phenotypic)measure-
mentsforscreeningpopulationswithunknowngeneticbackground.
4　SummaryandConclusions
Waterdeficitisafrequentlyenvironmentstressencountered
bymostplantsintheworld.Inordertowithstandperiodsoflow
wateraccessibility, woodyplantshaveevolvedanumberofaccli-
mationandadaptationresponses.Theseresponsescanbeseenat
thewholeplantlevelintheformofchangesinmorphology, growth
anddevelopment.Atthecelularlevelwaterstressinducesstoma-
119　 1期 DUANBaolietal:ResponsesofConiferstoDroughtStress 　　
talclosure, productionofosmolytes, changesinmembranephos-
pholipidcomposition, accumulationofABA, alterationingeneex-
pressionandsynthesisofnewproteins.However, detailsofthe
molecularmechanismsinplantstress-perceptionarestillunclear.
Thestrategiestoopposelossofcelularwatervarylargelyamong
andwithinthespeciesofconifers.Drought-stressedseedlingsof
coniferspopulationfromxerichabitatsshowedhigherresistanceto
droughtthandrought-stressedseedlingsofprovenancesfromme-
sichabitats[ 5 , 6 , 25, 33, 44 ～ 54 , 62, 73].Thus, itisconcludedthatconifers
populationfromxerichabitatswouldbeamoresuitablepopulation
thantheothersforestablishmentonsitespronetosoilwaterdefi-
cits.
Inthenaturalhabitatsofplantstheadverseenvironmentalfac-
torsarealmostneverpresentalone.Underconditionsofsimultane-
ousstresses, thenegativeefectsonplantsandplantadaptivere-
sponsesmaydifferfromthoseunderasingleadverseenvironmental
condition.Theproductivityvariationcausedbydroughtstressmay
dependonspeciessensitivitytodroughtstressandalotherbiologi-
calandenvironmentalfactorssinceplantproductivityrepresentsan
integrationandinteractionoftheenvironmentalfactors.Therefore,
theinteractiveeffectsofdroughtstressandotherenvironmentalfac-
torsonplantgrowthandproductivityunderfieldconditionsshould
beinvestigated.Futurestudiesshouldaddresstheunderstandingof
conifersresponsestotheinteractionsofdroughtandotherclimate
changevariables, particularlyatmospheric[ CO2 ] , temperature,
ozone, UV-Bradiationandmineraldeficiencies.
Duringwaterstress, curtailingthedetrimentaleffectsisofa
basicimportanceformechanismofsurvival.Largenumbersofex-
perimentssuggestthatonespecificmechanismdoesnotconferwa-
tertolerancesolely.Pressinggoalsforfutureresearcharetounder-
standtheroleoftheinterplayofseveralmechanismssimultaneously
onwatertolerance.Withthechangeofglobalclimate, breeders
areparticularlyinterestedinselectingthegenotypesthatcanmain-
tainnormalgrowthunderwaterstress.Thus, selectionfordrought
toleranceisareasonablestrategyondrysites.However, ithasnot
yetprovedpossibletofindanywell-definedcriterionthatcould
beusedbybreederstoselectwater-tolerantgenotypesofconifers.
Folow-upexperimentsshouldfocusongainingdetailedinforma-
tiononwhetherthedetrimentalefectsofdroughtstressarethere-
sultofchangesinphysiological/biochemicalparameters.
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