new insights into the hydraulics of trees hervé cochard umr547 piaf inra clermont-ferrand france

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  • New insights into the hydraulics of treesHerv Cochard

    UMR547 PIAFINRAClermont-Ferrand France

  • Drought resistance is a major issue for European forests Extreme drought events during the recent decades (1976, 1990, 2003)Severe forest diebacks across EuropeThe occurrence of extreme droughts is thought to increase in the future (global climate change)

  • NOW20502100Frequency distributionDistribution of drought resistant species in France for the next centuryQuercus ilexBadeau and Dupouey 2005

  • NOW20502100Frequency distributionDistribution of drought vulnerable species in France for the next centuryFagus sylvaticaBadeau and Dupouey 2005

  • Challenging issues from foresters (and researchers)

    Adopt now new cultural practices ?Can current species acclimate/adapt to drier conditions ?Can we identify genotype/ecotype of current species more resistant to drought ?Can we substitute current species with more drought resistant ones ?

    Better understanding of the physiological and molecular basis of tree drought resistance

  • 020406080100Intensity of the processesTree drought resistanceHydraulic traits may provide new insights into our understanding of tree drought resistance

  • The Hydraulic behavior of trees

  • The Hydraulic behavior of treesPTDY = RH*FlowOhms law analog for water transport in treesRH=1/KHCochard et al 1997

  • The hydraulic limit of sap transport: CavitationWater nucleation (cavitation) can occur under negative pressureSap is transported in xylem conduits under large negative pressures

  • Trees operate close to the point of xylem cavitationPercent Xylem CavitationStomatal conductanceXylem Pressure, MPa

  • Stomatal control of xylem cavitationCruiziat, Cochard, Amglio 2002

  • ABA+ABA-Experimental evidence for a stomatal control of cavitationCochard, Ridolfi, Dreyer 1996

  • RH=1/KHCAVITATIONHydraulic traits with high functional significance

  • The significance of hydraulic efficiency for trees

  • Brodribb et al, 2007How significant is the hydraulic efficiency for trees ?Cochard et al, 2002WalnutBryophytesFernsConifersAngiospermsHydraulic efficiency scales with leaf gas exchanges

  • Where are the located the main hydraulic resistances along the sap pathways ?Veins 10-50% of leaf Resistance Cochard et al, 2004 Root Resistance Shoot ResistanceRsistance Hydraulique, %020406080100C. atlanticaC. libaniQ. ilexQ. petraeaQ. roburF. sylvaticaP. persicaP. malusS. fragilsB. verrucosaJ. regiaF. exelsiorTigesFeuillesLeaves 80% of shoot Resistance

  • Sap pathways in leaves

  • Leaf conductance is variable and under environmental controlCochard et al 2007Nardini, unpublished

  • Leaf conductance can vary rapidlyCochard et al 2007Sack et al (2002) : light decreases leaf hydraulic resistance

  • Tajkhorshid et al 2002Cochard et al 2007Molecular basis of variable leaf conductance : Aquaporins

  • - AquaporinsCochard et al 2007+AquaporinsFunctional significance of leaf aquaporins

  • Aquaporins do not transport only H20 CO2-porins Uehlein et al 2003 Control CO2 diffusion in the leaf mesophyll (photosynthesis)Future issues for aquaporins and leaf hydraulicsGreat diversity of leaf response to light

    Great diversity of aquaporinsunpublished

  • Hydraulic efficiencyKey parameterCorrelates tightly with gas exchangesHighly variable across speciesHighly sensible to environmental factorsUnder biological control : Aquaporins

    Hydraulic conductances are tightly regulated to optimized leaf gas exchanges

  • The significance of xylem cavitation for trees

  • Xylem pressure, MPa0-2-4-6-8-10-12% Xylem cavitationPopulusQuercus roburPinusPrunusJuniperusBuxusXylem vulnerability to cavitation across species

  • Maherali et al 2004Cavitation resistance correlates with species ecological preferences

  • Cochard et al 2007Cavitation resistance across Prunus species

  • Pinus sylvestrisCochard et al unpublished

  • Cavitation resistance seems an adaptive trait for drought resistanceHow cavitation could cause tree dieback ?(Still speculative)

    Direct effects in the short term: run-away cavitation bud and meristem mortality by dehydration

    Indirect effects in the longer term: lower carbohydrate reserves (frost resistance; bud growth)- Impair impairment by loss of hydraulic conductance (less competitive)Can cavitation resistance be used as a criterion for screening more drought-resistent genotypes ?

  • Screening cavitation-resistant genotypesIntrinsic, structural property of the xylem

    Do we have fast and reliable techniques for screening hundred of genotypes ?

    Can we identify more accessible traits correlated with cavitation resistance ?

    Can we identify genes involved in cavitation resistance ?

  • Techniques for measuring cavitationwww.bronkhorst.frXYLEMLoss of hydraulic conductance (Sperry et al 1988) :

    Reliable but not fast (1genotype/week)Acoustic emissions (Tyree et al 1985)

    Not reliable and not fast

  • Techniques for measuring cavitationAir injection(eg, Cochard et al 1992)

    Rather Fast , reliable ?(1genotype/day)

    Centrifuge technique(Cochard et al 2005)

    Very fast, reliable ?(5 genotypes/day)

  • BetulaPrunusQuercusEvaluation of the cavitron techniqueWith this technique about 5 genotypes/day.More accessible traits ?Max vessel length3 different sample lengthtrue curve15 cm30 cm140 cmReliable for conifers and species with short vessels

  • Anatomical traits correlated with cavitation across speciesHacke et al 2001

  • Anatomical traits do not seem to correlate with cavitation across genotypesCochard et al 2007

  • The molecular basis of xylem cavitation ?A better understanding of the mechanism of cavitationAngiospermsConifersCochard unpublishedEffect of water surface-tension on cavitation

  • Identify the structural/textural characteristics of pit membranes determining cavitation

  • How to identify genes involved xylem cavitation ?UPD-glucose dehydrogenase(UGHD).UPD-glucuronate 4-epimerase.Pectine methylesterase.Glycosyltransferase.

    Glucosyltransferase.UPD-glucose pyrophosphorylase.

    Cellulose synthase(CSL).Boron +Boron -Boron links in pectinsManipulate plantsScreen mutant banks for specific genes coding for the primary cell wall (Arabidopsis)ExperimentallyshadeFull lightGlobal techniques cDNA-AFLP

  • ConclusionAquaporinsCavitationTwo keys aspects of tree hydraulicsPhysiological implications Molecular basisEcological significance

    More drought performing forests