Postharvest
water
relations
in
cut
rose
cultivars
with
contrasting
sensitivity
to
high
relative
air
humidity
during
growth
Dimitrios
Fanourakis
a,
Susana
M.P.
Carvalho
a,b,∗,
Domingos
P.F.
Almeida
b,c,
Olaf
van
Kooten
a,
Wouter
G.
van
Doorn
d,
Ep
Heuvelink
aaWageningenUniversity,DepartmentofPlantSciences,HorticulturalSupplyChainsGroup,Droevendaalsesteeg1,6708PBWageningen,TheNetherlands bPortugueseCatholicUniversity,CollegeofBiotechnology,RuaDr.AntónioBernardinodeAlmeida,4200-072Porto,Portugal
cUniversityofPorto,FacultyofSciences,RuadoCampoAlegre,823,4150-180Porto,Portugal dMannLaboratory,DepartmentofPlantSciences,UniversityofCalifornia,Davis,CA95616,USA
Keywords: Airemboli Floweropening
Stemhydraulicconductivity Rosahybrida
Vaselife Vascularocclusion
a
b
s
t
r
a
c
t
Aconstanthighrelativeairhumidity(RH)duringcultivationcanstronglyreducethevaselifeinsomecut rosecultivars.WestudiedthreecontrastingcultivarsintheirtolerancetohighRHinordertoanalysein detailthewaterrelationsduringpostharvestandbetterunderstandthisgenotypicvariation.Plantswere grownatmoderate(60%)andhigh(95%)RH,andcutflowerswereplacedinwaterimmediatelyafter cutting.Flowersofcv.PinkProphytagrownathighRHdidnotopenthroughoutvaselife,whileflower openingofcvs.FriscoandDreamwasnotaffectedbypreharvestRH.CultivationathighRHresulted inabout80%shortervaselifeinPinkProphyta,whereasinDreamandFriscothenegativeeffectwas considerablysmaller(15and9%shortervaselife,respectively).Theshortervaselifeandreducedflower openingofcutrosesgrownathighRHwasduetoahigherrateoftranspirationbothinthelightanddark periods.ItwasfoundthattheleavesofPinkProphytagrownathighRHcouldpartlyclosetheirstomata uponloweringofthewaterpotentialorwhenflowerstalkswerefedwithabscisicacid,butstomata remainedfarmoreopenthaninleavesgrownatmoderateRH.TheRHduringcultivationdidnotaffect stemhydraulicconductivityanditsrecoveryafterairemboliinduction.Preventingvascularocclusion largelyalleviatedthehigh-cultivation-RHeffectonvaselifeandfloweropening,showingthattheeffect ofhigh-cultivation-RHbecomesonlyimportantifwateruptakeislimited.
1. Introduction
Thewaterrelationsofcutflowersaredependentonanumber ofphysiologicalandanatomicaltraitsthatregulatewaterlossand wateruptakerates(reviewedbyvanDoorn,1997).Thesetraitsare establishedduringthepreharvestperiod,beingtheresultofthe complexinteractionbetweengenotypeandenvironmentduring cultivation,andwillsubsequentlydeterminepotentialvaselife(i.e. maximumvaselife)ofagivencutflower.Forinstance,although therelativeairhumidity(RH)levelduringcultivationhasno sig-nificanteffectoncropgrowthandvisualquality(TorreandFjeld, 2001),cutrosesgrownathighRH(≥85%)oftenhaveaveryshort vaselife(MortensenandFjeld,1998).Nevertheless,thedecreasein thelifespanaftercultivationathighRHisstronglydependenton thegenotype,varyingbetweenaslittleas15%(cvs.Dream,Frisco,
∗ Correspondingauthorat:WageningenUniversity,DepartmentofPlantSciences, HorticulturalSupplyChainsGroup,Droevendaalsesteeg1,6708PBWageningen,The Netherlands.
E-mailaddress:[email protected](S.M.P.Carvalho).
andKardinal)toasmuchas75%(cv.Amadeus)(Mortensenand Gislerød,1999).SensitivecultivarsgrownunderelevatedRHshow precociouspostharvestsenescencesymptoms,whicharetypically relatedtowaterstress,includingprematureflowerandleaf wilt-ingaswellaspedicelbending(Torreetal.,2001;Mortensenand Gislerød,2005).Thedescribedphenotypicvariationinthe sensitiv-itytohighRHduringcultivationisnotyetfullyunderstood.
Alimitedcapacitytoreducewaterloss,duetostomatal mal-functioning, is thoughttobe themain reason for thevase life reductioninplantsgrownunderlong-termhighRH(RezaeiNejad andvanMeeteren,2007;Fanourakisetal.,2011).Theregulation ofwaterlossismostlyunderphysiologicalcontrol(i.e.stomata), whereaswateruptakeisbasicallyaphysicalprocessincutflowers. Anadequatestomatalresponsivenesstodifferentclosingstimuli (e.g.darknessandlowwaterpotential)willlimitthenetlossof waterfromthecutflower,andwillconsequentlydelayearly wilt-ingsymptoms(vanDoornetal.,1989;BleeksmaandvanDoorn, 2003).Theflow rate(wateruptake)isproportional tothe driv-ing force(water potential), and tothe conductance (inverse of resistance)ofthetransportpath(vanDoorn,1997).It hasbeen shownthatdroughtstressresultsinareductionofxylemvessel
diameter(LovisoloandSchubert,1998),whichinturnleadsnot onlytoalowerstemhydraulicconductivitybutalsotoahigher resistancetocavitation(Nijsseetal.,2001;McElroneetal.,2004). Sincelowwaterpotentialsduringdroughtstresscanchangethe xylemanatomy,someoppositechangesmightbeexpectedwhen plantsaresubjectedtolong-termhighwaterpotentials,asaresult of elevatedRHlevels. Theeffect ofRH duringgrowth onstem hydraulicconductivityhasnotbeenpreviouslyinvestigated.
Themainobjectivesofthepresentworkwere:(i)toanalysethe postharvestwaterrelationsofcultivarswithcontrastingsensitivity tohighRHduringpreharvest,(ii)toassessthecultivationRHeffect onthestemhydraulicconductivityanditsrecoveryafterartificial inductionofairemboli,and(iii)totestifimprovementofwater uptake,bypreventingvascularblockage,cancompensateforthe higherwaterlossfoundaftercultivationathighRH.We hypothe-sizedthatashortervaselife,aftergrowthathighRH,resultsfrom thecombinedeffectofahigherwaterlossandachangedxylem anatomy(leadingtoahighersensitivitytoairemboli).Moreover, weexpectthatfactorswhichimprovewateruptakewillalleviate thisnegativeeffectofhighRHoncutflowerlongevity.
2. Materialsandmethods
2.1. Plantmaterialandgrowthconditions
Rooted cuttings of three cut rose cultivars (Rosa hybrida L. cvs.Pink Prophyta,Frisco,and Dream)knowntohavedifferent decreases in vase life after cultivation at high RH (Mortensen andGislerød,1999),wereobtainedfromacommercialpropagator (Kordes,DeKwakel,TheNetherlands).Thecuttingswereplanted in 3.6L potscontaininga mixtureof cocopeat(Jongkind Grond BV,Aalsmeer,TheNetherlands)andperlite(Agraperlitenr.3,Pull, Rhenen,TheNetherlands),3:1(v/v).CultivarPinkProphyta (regis-teredcultivarnameRUIkuiros)willbecalledProphytaintherest ofthispaper.
Fiveexperimentswereconducted.Ineachexperiment,plants weregrowninfourgrowthchambersasasingleshoot(oneplant perpot)atadensityof30plantsm−2.Intwogrowthchambersthe RHwas60±3%(moderateRH)andintwoothersitwas95±1% (highRH).Thefourchambershadaconstantdayandnight tem-perature(19±1◦C),resultinginvapourpressuredeficits(VPDs)of 0.88±0.12kPa(moderateRH)and0.11±0.03kPa(highRH). Cli-mateparameterswererecordedautomaticallyevery5minbydata loggers(FourierMicrologEC650,MicroDAQ.comLtd.,Contoocook, NH,USA).
Fluorescent tubes (TLD 58W/84, Philips, Eindhoven, The Netherlands) provided an 18h photoperiod and 300±20molm−2s−1 of photosynthetically active radiation (PAR,determinedwithaModelLI-250,LI-COR,Lincoln,NE,USA). Radiationlevelsweremeasuredat70cmfromtheroot-shoot inter-face,whichcorrespondstothetopoffullygrownplants.TheCO2 concentrationduring thelight period was 370±50molmol−1 (determinedusingIndoorAirQualityMeter,Model8760,TSI Incor-porated,Shoreview,MN,USA).Plantswerewateredautomatically withanutrientsolutionasdescribedbyFanourakisetal.(2009).
Experimentsonpostharvestcharacteristicsusedsecond-order shoots,eachoriginatingfromanindividualplant,atthecommercial harveststage[stage2accordingtotheAssociationofDutchFlower Auctions(VBN,2005);describedforexperiment1].Theharvested shootshadalengthofapproximately50–60cm,measuredfromthe primaryshoot/secondaryshootjunctiontothetopoftheshoot. Thenightbeforetheexperiment,theplantswerewellirrigated andplacedindarknessfor12h,toensuremaximalturgidityand minimizethepresenceofnaturalairemboli(vanDoornandSuiro, 1996).
2.2. Vaselifeundernon-optimumwateruptakeconditions (experiment1)
Thefloweringstemsofthethreecultivarswerecutinair,left in air for 2–3min, and their cut ends were then washed with sodiumhypochloritesolution(2%,(v/v);theconcentrationof com-mercialbleachsolution).Allflowerswerecuttothesamelength (49±2cm),andthesamenumberoffive-leafletleavesper culti-varwasleft(cvs.FriscoandProphyta,fourleaves;cv.Dream,five leaves).Subsequently,thecutfloweringstemswereputinflasks (oneflowerperflask)containing300mLofanartificialvase solu-tion(0.7mMCaCl2·2H2O,1.5mMNaHCO3,5MCuSO4·5H2O;van Meeterenetal.,2000).Thepresenceofcoppersulphateinthevase solutionleadstoamoderateinhibitionofbacterialgrowth.Thetop oftheflaskswascoveredwithParafilm,toensurethatwaterloss couldonlyoccurviatheflowerstalks.Theseflaskswereplacedina climate-controlledroomat20◦C,50%RHand10–12molm−2s−1 PAR at a 12h on-off cycle,provided by fluorescenttubes (TLD 58W/84,Philips,Eindhoven,TheNetherlands).Theheightofthe vasesolutioncolumnwasheldconstantovertheevaluationperiod toavoidhydrostaticpressuredifferencesbetweenflowerswith dif-ferenttranspirationrates(MensinkandvanDoorn,2001).
Theterminationofvaselifewasdeterminedbasedonthe occur-renceofatleastoneofthefollowingcriteria:(i)bendingofthe pedicel(bent-neck;i.e.pedicelbendsandfloweranglebecomes largerthan90◦fromtheverticalposition);(ii)abscissionofmore thantwopetals;(iii)visiblewiltingoftheflower,i.e.lossofpetal turgor;and(iv)morethan50%ofthenumberofleaveshadabscised, turnedyellow,orhaddesiccated(VBN,2005).Totalleafareawas determinedattheendofvaselife,usingaleafareameter(model 3100AreaMeter,LI-COR,Lincoln,NE,USA).
Duringthepostharvestphase,theflowerandflaskweightswere recordedseparatelytwotimesaday(time0and12haftertheonset ofthelightperiod).Thetranspirationratewascalculatedperunit leafarea.Treatmentswerecomparedbasedontheaverage transpi-rationrateoverthecompletepostharvestperiod.Thefreshweight (FW)ofeachfloweringstemwasexpressedrelativetoitsinitial weight.Theflowerdiameterandopeningstagewererecordeddaily duringvaselife.Theflowerdiameterwasmeasuredbyassessing themaximumdiameterandthediameterperpendicularlytothat one.Thesetwovalueswereaveraged.Flowerstageswere deter-minedusingthescaleofVBN(2005)(i.e.stage2:loosepointedbud withcylindricalshape;stage3:half-openflower; stage4:open flower;stage5:maximallyopenedflowerwithvisibleanthers).In thisexperimenttwelveflowerspertreatmentwereassessed. 2.3. Vaselifeunderoptimumwateruptakeconditions
(experiment2)
Inordertotestifimprovingthewateruptakeconditionsduring vaselifecouldcompensateforthenegativeeffectofhighRHduring cultivationonkeepingquality,vascular blockage(causedbyair emboliatthecutsurfaceandbacterialgrowthinthevasewater) wasprevented.Flowering shootsofthethreecultivarswerecut underwatertopreventairentranceintothexylemconduitsthat wereopenedbycutting.Thus,justpriortocutting,potswereplaced inbucketscontainingdegassedsterilizedwater,wherebythewater level wasabout5cm abovetheprimary shoot/secondaryshoot junction.Eachcutwasmadeusingshearsthathadbeensterilizedin ethanolandthroughaninternodethathadbeensurface-sterilized byrubbingwithaclothdrenchedinthesamesolution(sterile treat-ment;vanDoornetal.,1991).Thefloweringstemswerecuttothe samelength,andthesamenumberoffive-leafletleavespercultivar asdescribedearlier(experiment1).Subsequently,tofurtherreduce theeffectofbacteriaonxylemocclusion,thestemswereplaced insterilizedflaskscontaining300mLofanartificialvasesolution
(detailsinexperiment1),whichwassterilized(autoclavedat121◦C for15min)anditspHwasreducedto3withadditionofcitricacid. Thisapproachwasusedinplaceofavasesolutionbiocide,common invaselifework,toavoidpossibleeffectsofchemicalsotherthan onmicrobes(vanDoornetal.,1990).Theflaskswereplacedunder testroomconditionsandcutflowersweresubmittedtothesame procedures and measurements as described for experiment 1. Additionally,inthecvs.FriscoandProphyta,theflowerwiththe flaskwasweighedatregularintervalsduringthelightperiod(time 2,4,8,and10haftertheonsetofthelightperiod).Inexperiment2, themeasurementswerecarriedoutineightflowerspertreatment. 2.4. Recoveryfromthedecreaseinstemhydraulicconductivity (Kh)duetoairaspiredatthestemcutsurface,incv.Prophyta (experiment3)
Incv.Prophyta(sensitive)weinvestigatedtheeffectofhighRH onstemhydraulicconductivity(Kh).Wealsoartificiallyinduced thepresenceofairemboliatthecutsurface.Allmanipulationswith plantsandstemsegmentsinthelaboratoryweredoneunderwater topreventtheentranceofairintothexylemvesselsatthecut sur-face.Stemsegmentsof35cmlengthwerecutfromtheplantsat 5cmabovetheprimaryshoot/secondaryshootjunctionwithsharp shears.Thestemswererecut,removing5cmfrombothends,with anewrazorblade.Stemlengthwasthenapproximately25cm.The numberofxylem vesselsexceeding20cmlengthisverylow in rose(lessthanorequalto5;vanDoornandReid,1995).Leaves wereremovedfromthestemsegmentwitharazorblade,leaving 0.2cmofthepetiolesonthestem(vanIeperenetal.,2001).Each stemsegmentcontainedthesamenumberofnodes,sinceanodal structureoffershigherresistancetowaterflow(Salleoetal.,1984), andhadsimilardiametersatbothcutendscomparedtotheother replications.Thetimebetweencollectionfromtheplantandthe startofthemeasurementwasapproximately20min.
Asiliconetubewaspushedovertheuppercutendofthestem segment(cutend atlargestdistancefromtheroots),whilethe lowerendofthestemsegmentwasplaced inacontainerfilled withdegassedaqueoussolutionofsodiumbicarbonate(1.5mM), calciumchloride(0.7mM)and coppersulphate (5M)atroom temperature(20±2◦C)(vanMeeterenetal.,2000).Thetubewas thenconnectedtoapump(7550-62,Barnant,Barrington,IL,USA) creatinga pullingpressure differenceof40kPa.Actual pressure wasmeasuredusingapressuretransducer(DVR5,Vacuubrand, Wertheim,Germany).Duringthesemeasurementssolutionflow wasalwaysinthe naturaldirection,from thelower(closestto theroots)totheuppercutend.Flowthroughthestemsegments wascalculatedfromweightchangesofthesolution,correctedfor evaporation.Flowratesstabilizedtypicallyafter5±2min. Subse-quently,theKhwascalculatedaccordingtovanMeeterenetal. (2000)(Eq.(1)),byusingthestemsegmentlength(x),theapplied pressuredifference(P)andtheflowrate(q).
Kh=P/xq (1)
AftertheKhmeasurementwithoutinitialairemboli,andwhile keepingthestemsegmentsundertension(40kPa),thesegments wereliftedoutofthesolutiontoallowairentrancefor approx-imately3min.Thepressureexertedwasfarlowerthantheone neededtomovetheair-waterinterface(1.5MPa)throughthepores ofthepitmembranes(vanDoornandSuiro,1996),buthigherthan theoneneededtofillmostxylemconduitsatthecutsurfacewithair (vanIeperenetal.,2001).Afterthisperiodofexposuretoair,the stemsegmentswereloweredbackintothesolutionandKhwas followedfor2.4h.Inthisexperimentsevenstemsegments(one segmentperplant)pertreatmentwereused.
2.5. Stomatalresponsetoadecreaseinleafwaterpotential (leaf),incv.Prophyta(experiment4)
Theeffectofambienthumidityduringcultivationonthe sto-matalresponsivenesstoadecreaseinleafwaterpotential(leaf) wasinvestigatedincv.Prophyta(sensitive).Thetranspirationrate asafunctionofleafrelativewatercontent(RWC)wasevaluatedin onesetofmeasurements.Terminalleafletsofthefirstfive-leaflet leavescountingfromtheapexweredetached.Theirpetioleswere immediatelyrecutunderdegassedwater(toprevent cavitation-inducedembolism),placedinflasksfilledwithwater,andfurther incubatedfor1hatabout100%RH(21◦C,VPDcloseto0)to estab-lishtheirsaturatedFW.Sincetheleafletsweredetachedduringthe lightperiodinthegrowthchamber,therehydrationprocesswas thereforeconductedinthelight(15molm−2s−1PAR;following darknessthelight-inducedstomatalopeningrequiresseveralmin; Mottetal.,1999).Subsequently,theleafletswereremovedfrom waterandplacedonatable(abaxialsurfacedown)at21◦C,50±3% RH,and50molm−2s−1PAR.Transpirationratewas gravimetri-callyrecordedduring4h.Theleafletswerethendriedat80◦Cfor 24h.TheRWCwascalculatedaccordingtoSlavik(1974).
Inanothersetofmeasurements,theleafasafunctionofleaf RWC was determined. Terminal leaflets of the first five-leaflet leavescountingfromtheapexweredetached,theirpetioleswere recutunderdegassedwaterandplacedinvialsfilledwithwater. Subsequently,theleafletswererehydratedindarkness(topromote stomatalclosure).Thiswasdoneinplaceofovernightrehydration, commoninpressure–volumework,topreventchangesinosmotic potentialwhichcanoccurwithinseveralhours(Augeetal.,1986). Afterwards,theleafletswerecoveredwithapolyethylenesheet ofaknownmassandweighedtodeterminetheirsaturatedFW. Thentheleafletwiththepolyethylenesheetstillaround,wasplaced inaScholander-typepressurechamber(SoilMoistureEquipment Corp.,SantaBarbara,CA,USA)andthebalancepressure(−leaf) wasdetermined.Thepressureinthechamberwasincreasedata ratenothigherthan0.02MPas−1toavoidcellinjury(Kikutaetal., 1985).Leaveswereallowedtodryonatableat21◦C,50±3%RH, and50molm−2s−1PAR.Incrementalweightlosswas gravimet-ricallydetermined.Thedryweightoftheleafletswasobtainedas describedabove.Allmeasurementsincluded14leaves(oneleafper plant)pertreatment.
2.6. Stomatalresponsetoabscisicacid(ABA)feedingduring postharvest,incv.Prophyta(experiment5)
Theefficiencyofanantitranspirant compound(abscisicacid, ABA)indecreasingthetranspirationrateofcv.Prophyta(sensitive) grownatdifferentmoistureambientconditionswasassessed.Care wastakenthatthevascularblockagewaspreventedinthetested cutflowers(asdescribedforexperiment2).Floweringstalkswere placedinartificialvasesolutionwith0(control)or100M(±)ABA (Sigma,St.Louis,MO,USA),andkeptundertestroomconditions (asdescribedforexperiment1).Thetopoftheflaskwascovered withParafilm,whileitssideswerewrappedinaluminumfoil(to reflectlight),sinceABAislightsensitive(DaviesandJones,1991). Theexperimentwasstoppedwhenleafabscissionwasobserved inABA-fedflowers,andtotal leafareawasthen measured.The transpirationrate wasdetermined asdescribed earlier (experi-ment 1). In this experiment eight flowers per treatmentwere used.
2.7. Statisticaldesignandanalysis
DataweresubjectedtoanalysisofvarianceusingGenstat soft-ware (10th edition, VSN International Ltd., Hemel Hempstead, Herts,UK).Experiments1,2and5hadasplit-plotdesign,withRH
Table1
Thevaselife,flowerstageandflowerdiameterattheendofvaselifeofthreecutrosecultivars,grownatmoderate(60%)orhigh(95%)relativeairhumidity(RH).Inexperiment 1thestemswerecutinairandplacedinvasesolutionwithmoderateinhibitionofbacterialgrowth(vaselifeundernon-optimumwateruptakeconditions).Inexperiment 2xylemocclusionwaslargelyprevented,asthestemsweresurface-sterilized,cutunderdegassedwaterandplacedinsterilizedvasesolution(vaselifeunderoptimum wateruptakeconditions).TheflowerstageswereaccordingtothescaleofVBN,rangingfrom2(loosepointedbudwithcylindricalshape)to5(maximallyopenedflower withvisibleanthers).Theflowerdiameteristheaverageofthemaximumdiameterandthediameterperpendicularlytothis.Valuesarethemeansof12(experiment1)or 8(experiment2)replications.
Cultivar RH(%) Experiment1 Experiment2
Vaselife(days) Flowerstagea Flowerdiameter(mm) Vaselife(days) Flowerstagea Flowerdiameter(mm)
Prophyta 60 19.4e 4.3 94.0bcd 29.9a 4.8 100.0 95 4.0a 3.0 68.4a 19.3c 4.1 91.0 Frisco 60 18.0de 5.0 104.0e 26.6b 5.0 102.0 95 16.4d 5.0 99.0de 20.8c 4.6 99.0 Dream 60 13.6c 4.3 90.7bc 20.3c 4.8 97.2 95 11.5b 4.2 88.1b 18.1c 4.4 93.8 Fprob Cultivar <0.001 – <0.001 <0.001 – 0.019 RH <0.001 – <0.001 <0.001 – 0.001 Cultivar×RH <0.001 – <0.001 <0.001 – 0.250
MeansineachcolumnfollowedbydifferentlettersindicatesignificantdifferencesaccordingtoLSD-test.
aFlowerstagedidnotshowanormaldistribution,thereforenoFprobabilityisgiven.
levelasthemainfactor,andcultivar(experiments1and2)or dura-tionofABAfeeding(experiment5)asthesplitfactors,respectively. Experiments3and4wereanalysedbyone-wayANOVA.Treatment effectsweretestedat5%probabilitylevelandmeanseparationwas carriedoutusingleastsignificantdifferencesbasedonStudent’s t-test(P≤0.05).
3. Results
3.1. Vaselife,floweropeningstageandflowerdiameter
Thelifespanoffloweringstemscutinairandplacedinvase solutionwithmoderateinhibition ofbacterialgrowth(i.e. non-optimumwateruptakeconditions;experiment1)wasreducedin stalksfromplantsgrownathighRH(95%),comparedwiththose fromplantsgrownatmoderateRH(60%).Theeffectwaslargest incv.Prophyta.AtmoderateRHitsvaselifewasabout19days, whereasthevaselifeoffloweringstemsgrownathighRHwasonly 4days(Table1;experiment1).AsmallnegativeeffectofhighRH duringcultivationwasfoundonthevaselifeofcv.Dream,whileno significanteffectwasobservedincv.Frisco(Table1;experiment 1).Moreover,flowerdiameterwasonlysignificantlyinhibitedin cv.ProphytawhengrownunderhighRH,resultinginabout30% reductionattheendofvaselife(Table1;experiment1).Asimilar trendwasobservedforthefloweropeningstage,butsincethese datawerenotnormallydistributed,noanalysisofvariancewas per-formed(Table1;experiment1).Symptomsthatresultedinearly terminationofvaselifeaftercultivationathighRHwerepedicel bendingandleafdesiccationincv.Prophyta,andpedicelbending incv.Dream. Vaselifeterminationwasthusduetoearlywater stresssymptoms(Table2;experiment1).Suchprecocious senes-cencesymptomswerenot observedincv.Friscogrown athigh RH,whichendeditsvaselifeasaconsequenceofanaturalflower
wiltingasobservedinnearlyallflowersofthestudiedcultivars grownatmoderateRH(Table2;experiment1).
Inexperiment2theflowersweresurface-sterilized,cutfrom theplantunderdegassedwater,andplacedinsterilizedvasewater. Uptakeofairintothecutstemswastherebypreventedand micro-bialeffectsweredrasticallyreduced(i.e.optimumwateruptake conditions).Theseconditionslargelyalleviatedthenegativeeffect of highRH duringcultivationonthelengthofvase lifeand on thefloweropeningstageanddiameterincv.Prophyta(Table1; experiment2).Incv.Dreamtheseconditionscompletelyprevented theeffectofhighRHduringcultivation(Table1;experiment2). Contrarytotheexpectationtherewasstillanegativeeffectof cul-tivationRHonthevaselifeofcv.Frisco(Table1;experiment2).In thisexperimentnowaterstresssymptomswerenoted,exceptleaf desiccationincv.DreamgrownathighRH(Table2;experiment2). 3.2. Transpirationrateanditsdiurnalrhythminthelightperiod
A two day vase life trial, in which the leaves were totally removed on the second day, showed that leaves in cv. Frisco accountedforabout80%ofthetotaltranspirationduringthelight periodinflowersgrownbothatmoderateandhighRH(datanot shown).
CutrosesgrownathighRHhadhighertranspirationratesinthe lightperiod,comparedwithrosesgrownatmoderateRH(Fig.1A, CandE).Inthecvs.DreamandFriscotheeffectwasonlyfound duringthefirstdaysofvaselife(Fig.1CandE).CultivarProphyta hadanaveragetranspirationrateinthelightperiodthatwasthree timeshigherwhengrownathighRH,comparedtostalksgrownat moderateRH(Fig.1A).Theeffectwassmallerinthecvs.Frisco and Dream (32and 22% higher transpiration in flowersgrown at high RHcompared toroses grown at moderate RH, respec-tively;Fig.1C and E).These datarefer tonormalcuttingand a Table2
Percentageoftheincidenceofsymptomsterminatingthevaselifeofthreecutrosecultivars,grownatmoderate(60%)orhigh(95%)relativeairhumidity(RH).Detailsof theexperimentsaregiveninTable1.Datareferto12(experiment1)or8(experiment2)replications.
Cultivar RH(%) Experiment1 Experiment2
Pedicelbending Desiccatedleaves Desiccatedpetals Wiltedflower Pedicelbending Desiccatedleaves Desiccatedpetals Wiltedflower
Prophyta 60 – – – 100 – – 50 50 95 25 17 – 58 – – 25 75 Frisco 60 – – – 100 – – 75 25 95 – – – 100 – – 37 63 Dream 60 8 – – 92 – – 88 12 95 8 – – 92 – 25 75 –
0 0.2 0.4 0.6 0 10 20 30 Time (days) BProphyta 0 0.2 0.4 0.6 0 10 20 30 Time (days) DFrisco 0 0.2 0.4 0.6 0 10 20 30 Time (days) FDream 0 0.2 0.4 0.6 0 10 20 30 L ig h t tra n s p ira tio n r a te (mmo l m -2s -1) Time (days) AProphyta 0 0.2 0.4 0.6 0 10 20 30 L ig h t t ran sp ir at io n r a te (m m o l m -2s -1) Time (days) CFrisco 0 0.2 0.4 0.6 0 10 20 30 L ig h t t ran sp ir at io n r a te (m m o l m -2s -1) Time (days) EDream
Fig.1.Transpirationrateinthelightperiodduringvaselifeundernon-optimum(A,CandE;experiment1)andoptimum(B,DandF;experiment2)wateruptakeconditions ofthreecutrosecultivars,grownatmoderate(60%,opensymbols)orhigh(95%,closedsymbols)relativeairhumidity.DetailsoftheexperimentsaregiveninTable1.Values arethemeansof12(experiment1)or8(experiment2)replications±S.E.
moderatelylargevasewatermicrobial population.Whenflower stems were surface-sterilized, cut under degassed water, and placedinsterilevasesolution,thetranspirationwasalsohigher inflowerscultivatedathighRHcomparedtothosegrownat mod-erateRH(Fig.1B,Dand F;seealsoFig.7A).Therewasnoclear differencebetweenthisexperiment(Fig.1B,DandF)andtheone inwhichthestemshadaspiredairandthevasewatermicrobial populationwasmuchhigher(Fig.1A,CandE).
Thediurnal courseoftranspirationratewasrecordedduring thevaselifeofthecvs.ProphytaandFrisco,underwateruptake conditionsthatwereclosetooptimum(stemsurface-sterilization followedbycuttingstemsfromtheplantunderdegassedwater andplacementinsterilizedvasesolution).Arelativelylarge diur-naloscillationinthetranspirationratewasobservedinflowers grownatmoderateRH,andamuchsmalleroscillationinflowers grownathighRH(Fig.2AandB).Thetranspirationrateduringthe lightperiodshowedapeakduringthefirsthours,whilethelowest valuewasalwaysobservedattheendofthelightperiod(Fig.2Aand B).Theamplitudesbetweenthehighestandthelowest transpira-tionrateswithinthelightperiodareshowninFig.2CandD.Roses grownunderhighRHexhibitedaconsiderablysmalleramplitude oftranspirationduringthelightperiod,comparedtothosegrown atmoderateRH.
3.3. Transpirationrateindarkness
Inallthreecultivarstested,ahighRHduringcultivation signif-icantlyincreasedthetranspirationrateinthedarkperiodduring vaselife(Fig.3A,CandE).CultivarProphytagrownathighRHhad onaverageafivetimeshighertranspirationrateinthedark com-paredtorosesgrownatmoderateRH.Thecvs.FriscoandDream showedanincreaseofapproximatelya factoroftwo.Compared to these data on roses placed under suboptimal water uptake conditions,theeffectofcultivationRHonthetranspirationratein darknesswassimilarin rosesthatweresubjectedtooptimized
wateruptakeconditionsduringvaselife(Fig.3B,DandF;seealso Fig.7B).
Thedifferencebetweenthetranspirationrateduringthelight periodandtherateofnocturnaltranspirationisshowninFig.4.This differencewaslargeatthebeginningofvaselifeinrosesgrown undermoderateRH,butbecamelowerlateron, inrosesplaced undersuboptimalwateruptakeconditions(Fig.4A,CandE).This decreaseinthedifferencebetweentranspirationduringthelight andindarkness,duringthecourseofvaselife,wasnotfound(cv. Prophyta;Fig.4B)orwaslesspronounced(cvs.FriscoandDream; Fig.4DandF)inrosesthatweresubjectedtomoreoptimalwater uptakeconditions.
3.4. Freshweight(FW)
AlargeinitialincreaseinFWwasobservedinrosesthatwere held under optimum water uptake conditions during vase life (Fig.5B,DandF)whileasmallerFWincreasewasfoundin flow-ersthatwereexposedtonon-optimumconditions(Fig.5A,Cand E).TheRHduringcultivationhadlittleeffectontheFWinthecvs. Dream(Fig.5CandD)andFrisco(Fig.5EandF).Onlyincv.Prophyta theFW remainedlower,almostthroughoutvaselife,in flowers grownunderhighRHcomparedtothosegrownundermoderate RH(Fig.5AandB).
3.5. Recoveryfromthedecreaseinstemhydraulicconductivity (Kh)duetoairaspiredatthestemcutsurface,incv.Prophyta
Stemsegmentsofcv.Prophytawerecutunderwater.Asuction forceof40kPawasappliedtotheupperendwhilethelowerend ofthestemsegmentwasmaintainedunderwater.TheabsoluteKh valueswerenotaffectedbythepreharvestRHlevel(P=0.458;data notshown).TheinitialKhwassetto100%(Fig.6A).Aftersome initialmeasurements,airwasallowedtobeaspiredatthebasal cutsurfacefor3min,byliftingthebasalendofthesegmentabove
0 0.2 0.4 0.6 0 1 2 3 4 5 6 7 8 9 10 Tr anspi ra ti o n r a te (m m o l m -2s -1) A Prophyta 0 0.2 0.4 0.6 0 1 2 3 4 5 6 7 8 9 10 Tr anspi ra ti o n r a te (mmo l m -2s -1) Time (days) B Frisco 0 20 40 60 80 0 10 20 30 R e la ti v e tr a n s p ir a ti o n r a te (% re d u c ti o n ) Time (days) CProphyta 0 20 40 60 80 0 10 20 30 Time (days) D Frisco
Fig.2. Transpirationrate(2,4,8,10,12haftertheonsetofthelightperiod,and12haftertheonsetofthedarkperiod)duringthefirst10daysofvaselifeunderoptimum wateruptakeconditions(experiment2)oftwocutrosecultivarsgrownatmoderate(60%,opensymbols)orhigh(95%,closedsymbols)relativeairhumidity.Thelightand darkperiodswere12heach.DetailsoftheexperimentaregiveninTable1.(C)and(D)depicttherelativedecreasebetweenthemaximum(TLmax)andminimum(TLmin)
valuesoftranspirationrateduringthelightperiod[i.e.((TLmax−TLmin)/TLmax)×100].Valuesarethemeansof8replications±S.E.
0 0.2 0.4 0.6 0 10 20 30 D a rk t ran s p ir at io n r a te (mm o l m -2s -1) Time (days) AProphyta 0 0.2 0.4 0.6 0 10 20 30 Time (days) BProphyta 0 0.2 0.4 0.6 0 10 20 30 D a rk tra n s p ir a ti o n r a te (m mo l m -2s -1) Time (days) CFrisco 0 0.2 0.4 0.6 0 10 20 30 Time (days) D Frisco 0 0.2 0.4 0.6 0 10 20 30 D a rk tr a n s p ir a ti o n r a te (m mo l m -2s -1) Time (days) EDream 0 0.2 0.4 0.6 0 10 20 30 Time (days) FDream
Fig.3.Transpirationrateinthedarkperiodduringthevaselifeundernon-optimum(A,CandE;experiment1)andoptimum(B,DandF;experiment2)wateruptake conditionsofthreecutrosecultivars,grownatmoderate(60%,opensymbols)orhigh(95%,closedsymbols)relativeairhumidity.Detailsoftheexperimentsaregivenin
0 20 40 60 80 0 10 20 30 BProphyta 0 20 40 60 80 0 10 20 30 R e la ti v e tr a n s p ir a tio n r a te (% re d u c tio n ) AProphyta 0 20 40 60 80 0 10 20 30 R e la ti v e tr a n s p ir a tio n r a te (% re d u c tio n ) CFrisco 0 20 40 60 80 0 10 20 30 DFrisco 0 20 40 60 80 0 10 20 30 R e la ti ve t ransp ir at io n r a te (% r e du ct ion) Time (days) EDream 0 20 40 60 80 0 10 20 30 Time (days) FDream
Fig.4.Therelativedecreasebetweenthetranspirationrateduringthelight(TL)anddark(TD)periods[i.e.(TL−TD/TL)×100],duringvaselifeundernon-optimum(A,Cand
E;experiment1)andoptimum(B,DandF;experiment2)wateruptakeconditionsofthreecutrosecultivars,grownatmoderate(60%,opensymbols)orhigh(95%,closed symbols)relativeairhumidity.DetailsoftheexperimentsaregiveninTable1.ThetranspirationrateduringthelightanddarkperiodsisshowninFigs.1and3,respectively. Valuesarethemeansof12(experiment1)or8(experiment2)replications±S.E.
thesolutionsurface.Thiswasfollowedbyloweringthesegment backintothe solution.Kh then showedan initialfastrecovery (Fig.6A).Previousresearchfoundthatthisrecoveryisduetothe partialrefillingwithsolutionofxylemconduitsinwhichairhad
beentakenup,resultinginareconnectionbetweenthevasewater andthexylemconduitsthathadnotbeenopenedbycutting(van Ieperenet al.,2002).Lateron(from aboutt=30min), a slower increaseinKhwasfound(Fig.6A).Thishasbeenrelatedtothe
90 100 110 120 130 0 10 20 30 R e la ti v e fre s h w e ig h t ( % ) AProphyta 90 100 110 120 130 0 10 20 30 R e la tiv e fr e s h w e ig h t (% ) CFrisco 90 100 110 120 130 0 10 20 30 R e la ti v e f res h w e ig ht ( % ) Time (days) EDream 90 100 110 120 130 0 10 20 30 BProphyta 90 100 110 120 130 0 10 20 30 DFrisco 90 100 110 120 130 0 10 20 30 Time (days) FDream
Fig.5.Relativefreshweightduringthevaselifeundernon-optimum(A,CandE;experiment1)andoptimum(B,DandF;experiment2)wateruptakeconditionsofthreecut rosecultivars,grownatmoderate(60%,opensymbols)orhigh(95%,closedsymbols)relativeairhumidity.Thefreshweightofeachfloweringstemwasexpressedrelative toitsinitialweight.DetailsoftheexperimentsaregiveninTable1.Valuesarethemeansof12(experiment1)or8(experiment2)replications±S.E.
R² = 0.94 R² = 0.91 0.0 0.4 0.8 1.2 1.6 -3.5 -2.5 -1.5 -0.5 Tr a n spi rat io n r a te (mmo l m -2s -1) Ψleaf (MPa) B B 0 20 40 60 80 100 0 40 80 120 160 Kh (% in it ia l) Time (min) A
Fig.6. Stemhydraulicconductivityrecoveryuponairemboliinduction,andtranspirationrateasafunctionofleafwaterpotentialincv.Prophytagrownatmoderate(60%, opensymbols)orhigh(95%,closedsymbols)relativeairhumidity.(A)Hydraulicconductivity(Kh)changesfollowingartificialinductionofairemboliatthecutsurfaceof
25cmstemsegments(experiment3).Thearrowdepictsthetimewherethestemsegmentwasallowedtoaspireairatthebasalcutsurface(airaspirationdurationwas 3min).Thereaftertheendofthestemsegmentwasagainplacedintothesolution.Valuesarethemeansof7replications±S.E.(B)Transpirationrateasafunctionofleaf waterpotential(leaf)duringdesiccationofdetachedleaves(experiment4).VerticalbarsindicateS.E(n=14).
relativelyslowdissolutionoftheremainingtrappedairatthetopof thexylemconduits(vanIeperenetal.,2002).After2.4hof measure-ment,theKhtendedtostabilizeat63%oftheinitialvalue(beforeair entranceatthelowerstemend).NoeffectsofRHduringcultivation werefoundintheresponsetoairaspirationfor3min(Fig.6A).
3.6. Stomatalresponsetoadecreaseinleafwaterpotential (leaf),incv.Prophyta
Transpirationratesandleafweremeasuredindetachedleaves ofcv.Prophyta.Thetranspirationratewastakenasameasureof stomatalopening.Inleavesfromplantsthathadbeencultivatedat moderateRH,thestomatashowedarapidclosurereaction,starting whentheleafhaddroppedto−2.0MPa.Thestomatawerealmost fullyclosedwhentheleafwas−2.5MPa(Fig.6B).Thereaction wasquitedifferentinleavestakenfromplantsthathadbeen culti-vatedathighRH.Thestomatashowedasmallclosingreactionalso startingatabout−2.0MPa,buttheyclosedonlyslightlyfurtherat alowerleaf.Whentheleafhadreached−3.0MPathestomata werestillabouthalfopen(Fig.6B).
3.7. Stomatalresponsetoabscisicacid(ABA)feedingduring postharvest,incv.Prophyta
TheefficacyofaddingABAintothevasesolution (antitranspi-rantcompound)ondecreasingpostharvestwaterlosswas evalu-atedincv.ProphytarosesgrownathighRH.Long-termABAfeeding (100M)throughthestembaseresultedinlowertranspiration ratesduringboththelightanddarkpostharvestperiods(Fig.7). However,thelong-termABAfeedingviathevasesolutionwasonly partlyabletocounteracttheeffectofhighRHduringcultivation ontheincreasedwaterloss.Thisisparticularlyevidentduringthe nocturnalperiod,wherethetranspirationrateinABA-fedstalks fromplantsgrownathighRHisfivetimeshigher,ascomparedto unfedstalksfromplantsgrownatmoderateRH(Fig.7B).
4. Discussion
Cutroseflowersoften showpostharvestwater stress symp-toms.Inpreviousstudiesithasbeenfoundthatinsomecultivars these water stresssymptoms become aggravated bylong-term highRHduringcultivation(MortensenandGislerød,1999,2005). Thepresentresultsconfirmthosefindings(Table1).Moreover,we havestudiedindetailthewaterrelationsofcutflowersgrownat moderateandhighRHandweaddressedthequestionofhow sto-matalopeningchangesintime andreacts todarknessandtoa decreaseinwaterpotential.Inthisstudywealsoinitiatedwork toexplainthecultivardifferencesintheirsensitivitytolong-term highRH.
Weobservedthatinthecultivarthatshowedamuchshorter vaselifewhengrownathighRH(cv.Prophyta,i.e.sensitive cul-tivar), theratesof transpirationduringboth thelight and dark periodswere,respectively,three-andfive-foldhigherincutflowers grownatelevatedRH,comparedwiththosegrownatmoderateRH (Table1;Figs.1and3).Theseroseswere,therefore,largelyunable torecoverduringdarkness(stomatalclosingstimulus)fromthe waterstressthatnaturallyoccursinthelightperiod(deStigter andBroekhuysen,1989).Thus,therateofwateruptakeduringthe postharvestperiodrapidlybecameinsufficienttocompensatefor theenhancedleaftranspirationrate.Thisresultedinearlywater stresssymptomsintheseflowers(Table2,experiment1),thusin adecreaseofleaf.Furthermore,itwasfoundthatcv.Prophyta rosesgrownathighRHweremuchlessabletoclosetheirstomata inresponsetothisdecreaseinleaf(Fig.6B).Thecombinedeffects ofahigherinitialtranspirationrateinthelight,amuchhigher tran-spirationrateindarkness,andtheinhibitedresponsetoadecrease inleafseemtoexplaincompletelywhytheflowersthathadbeen grownathighRHshowedearlywaterstresssymptomsandthus hadashortvaselife.CultivationathighRHalsoresultedin inhi-bitionoffloweropeningincv.Prophyta(Table1,experiment1), whichwaslikelyduetothelowwaterpotential.Inotherrose cul-tivarsadecreaseinflowerwaterpotentialalsohasbeenshownto reducefloweropening(vanDoornetal.,1991).
0 0.2 0.4 0.6 12 10 8 6 4 2 0
Dark transpiration rate
(mmol m -2s -1) Time (days) B 0 0.2 0.4 0.6 12 10 8 6 4 2 0
Light transpiration rate
(mmol m
-2s -1)
Time (days)
A
Fig.7.Transpirationratesinthelight(A)anddarkness(B)ofcv.Prophytarosesplacedinavasesolutioncontaining0(solidlines)or100MABA(dashedlines)(experiment 5).Flowerswerecultivatedatmoderate(60%,opensymbols)orhigh(95%,closedsymbols)relativeairhumidity.Xylemocclusionwaslargelypreventedasdescribedfor experiment2inTable1.TheexperimentwasterminatedwhenleafabscissionwasobservedinABA-fedflowers.Valuesarethemeansof8replications±S.E.
Whentheproblemswithwateruptakewerelargelyprevented (stem surface-sterilization, cutting under degassed water, and placementinsterilizedvasesolution;experiment 2)theshorter vase life and the inhibition of flower opening in cv. Prophyta werestronglyalleviatedinhighRH-grownplants(Tables1and2). Similarly,it hasbeenshown that treatmentwithsilvernitrate, anantibacterial compound, increasedthe vase life incut roses cultivatedat highRH(Torreand Fjeld,2001).These datashow thatthewateruptakeproblems,whichoftenoccurinmostrose cultivars,aretheinitialcauseofthewaterstresssymptoms. How-ever,thepoorcontrolofwaterlossinflowerscultivatedathigh RHaggravates this problem, whereas flowersgrown at moder-ateRH react tothe low leaf by rapidly closing their stomata (Fig.6B),whichenablesapositivewaterbalanceduringalonger period.Nonetheless,inflowersgrownatmoderateRHthewater uptake still eventually becomes so low that a net water loss occurs,eventhoughthestomataarelargelyclosed.Thisnetwater lossisdue totheongoing residualstomataland cuticular tran-spiration(Kerstiens,1995).Sowiltingsymptomswilleventually ensue.
Thewaterlossofcv.Prophytarosesthatweregrownathigh RH,andplaced inavase solution,wasconsiderablylargerthan thatintheothertwocultivarsstudied(Figs.1and3).Theseresults areconsistentwithanearlierstudy,wherestomatal responsive-nesstoleafdesiccationwassignificantlylowerincv.Prophytaas comparedtocv.FriscoinhighRH-grownplants(Fanourakisetal., 2009).Additionally,it wasfoundthat elevatedRHduring culti-vationresulted in a weakeneddiurnal rhythm duringthe light period,asexpressedbytheamplitudebetweenthehighest and thelowest transpiration rate (Fig.2).Prophyta roses grown at high RH and placed in a vase solution containing ABA had a lower water loss, compared to unfed high RH-grown flower stalks,butstilltheirtranspirationrateespeciallyduringthe dark-ness was higher than in moderate RH-grown plants (Fig. 7). RecentlyithasbeendemonstratedthattheroleofABA,in alle-viatingthenegativeeffectsof highRHonstomatalfunctioning, is restricted to the period of leaf expansion (Fanourakis et al., 2011). Thus, the limited capacity of a long-term ABA feeding toinduce stomatal closure when applied viathe vase solution (Fig. 7)can beexplained by the fact that flowerstalks at har-vestaretotallycomposedoffullydevelopedleaves.Moreover,in thecurrentstudythe100MABAfeedingsolutionwas contin-uouslyreachingtheleafviathetranspirationstream,whereasin Fanourakisetal. (2011)ABA wasbrusheddaily onfully devel-opedleavesatalowerconcentration(30M).Thesedifferences can explain the total absence of stomatal response to ABA in theirstudyandapartial(thoughlimited)responseinthecurrent one.
Althoughstomataldensityin cv.Prophytaisnot affectedby ambienthumidityduringleafdevelopment,stomataformedunder elevatedRHlevelshavelongerporelength(Fanourakisetal.,2011), whichresultsinahigherwaterlossrateatthesamepore aper-turevalues(ParlangeandWaggoner,1970).Highercuticularwater loss,asaresult ofpoorcuticulardevelopment,mightalso con-tributetothehighrates ofwaterlossin highRH-grownleaves (Karbulkovaetal.,2008),thoughthisconclusionhasbeen ques-tionedbyotherauthors(Torreetal.,2001).Thus,partofthehigher water loss observedin cv.Prophyta can bepossibly related to anatomicalfeatures(e.g.biggerstomataandhighercuticular per-meability),whichmightcontributetotheincreasedsensitivityof this cultivar tolong-term high RH during growth.Even in the cvs.Friscoand Dreamtherewasahigher waterlossasaresult ofhighRHduringcultivation,butthisdidnotleadtoearly visi-blewaterstresssymptoms(Table1).Apparently,theincreasein waterlossinthesecultivarswasnothighenoughtoreducethe waterpotentialtoalevelthatinducedearliervisiblesymptoms
ofwater stress.Futureresearchis neededtoevaluatethe rela-tiveimportanceofthephysiologicalandanatomicalcomponents intheenhancedwaterlossamongcontrastinggenotypesgrownat elevatedRH.
Adecreaseofthewaterpotentialincutflowersleadsto cavi-tationevents(Dixonetal.,1988;Spinarovaetal.,2007).Whena highnumberofconduitsbecomesinoperative,asaresultof cavita-tion,wateruptakewillbecomeadditionallyinhibited(vanDoorn, 1997).Bothairemboliatthecutsurface (vanDoornand Jones, 1994)andbacterialocclusion(BleeksmaandvanDoorn,2003)have beenshowntoinducecavitation.Wehavenodataonsensitivityto cavitationinthecvs.ProphytaandDream,butcv.Friscowashighly resistanttocavitation(whichstartedataconsiderablylowerwater potentialthanincv.Soniaroses;vanDoornandSuiro,1996).It isthereforepossiblethatthegenotypicvariationinthevaselife decrease,asaresultofmorehumidairduringcultivation,arises partiallyfromaneffectoncavitation,buttheroleofcavitationin thepresentdifferencesbetweencultivarsisnotyetknown.
Unlikeourinitialhypothesis,noeffectofcultivationathighRH wasfoundontheinitialvaluesofstemhydraulicconductivity.The absenceofsuchaneffectindicatesthatRHduringcultivationhadno effectonstemxylemanatomytoanextentthatitaffectedhydraulic conductivity.Whenthehydraulicconductivityofcv.Prophytastem segmentswasreducedbecauseofaspirationofairintothexylem conduitsopenedbycutting,therewasnoeffectoftheRHlevel duringcultivation(Fig.6A).Thissuggeststhatfactorssuchasthe wettingangleofthexylemconduits(vanIeperenetal.,2002)were alsonotconsiderablyaffected.
Thisstudyclearlydemonstratesthatthemaineffectofambient humidityduringpreharvestonthewaterrelationsduring posthar-vest isclosely relatedtotheregulation ofwater loss,sincethe stemhydraulicconductivityanditsrecoverybyairemboliwere notaffectedbylong-termhighRH.Therefore,itisconcludedthat xylemanatomydoesnotexplainthedifferentialcultivar sensitiv-itytohighRH.Instead,differencesbetweenthecultivarscouldbe largelyexplained bytheircontrastingcapacity tocontrol water loss.Forinstance,theearlywaterstresssymptomsandthe inhi-bitionoffloweropeningincv.Prophyta(sensitive)grownathigh RHwereduetoahigherrateofwaterlosscomparedtothosegrown atmoderateRH,whiletherewasapparentlyasimilarblockagein wateruptake.Thishigherwaterlossseemstobecloselyrelated tostomatalmalfunctioning.Prophytarosesthathad been culti-vatedathighRHwerefoundnottoclosetheirstomatatothesame degreeasthosecultivatedatmoderateRH(1)indarknessand(2) whentheleafdecreased.Theincreaseofcutflowerwaterloss,as aresultofplantgrowthathighRH,wasmuchlesspronounced intwo othercultivars(cvs.Friscoand Dream),compared tocv. Prophyta.Itislikelythatthehigherwaterlossinthesetwo cul-tivarsgrownathighRHresultsinawaterpotential,lowerthan theoneneededtoinducesignificantcavitationevents. Prevent-ingvascularocclusion,causedbyairemboliatthecutsurfaceand bacterialphysicalblockage,considerablyextendedthetimeto wilt-ingandenhancedfloweropeningincv.Prophytarosesgrownat highRH.Thisindicatesthatthehighrateofwaterloss,asaresult ofplant growthatelevatedatmospheric humidity, canonlybe detrimentalforcutflowerlongevityunderlimitingwateruptake conditions.
Acknowledgements
The authorswish tothank theFoundation Alexander Onas-sis (Greece) and the Foundation for Science and Technology (Portugal)forfinancialsupport.ThanksalsotoNikolaosMatkaris and Mereseit Hadush Hailu for their help in conducting the measurements.
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