New insights into the hydraulics of trees Hervé Cochard UMR547 PIAF INRA Clermont-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 Europe The occurrence of extreme droughts is thought to increase in the future (global climate change) How sustainable are our forests ?

Distribution of drought resistant species in France for the next century NOW Frequency distribution Quercus ilex 2050 2100 Badeau and Dupouey 2005

Distribution of drought vulnerable species in France for the next century NOW Frequency distribution Fagus sylvatica 2100 2050 Badeau 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

Time / Drought Intensity Tree drought «resistance » Survival Growth Stomatal Conductance Photosynthesis ↑ Productivity ↑ Resilience 100 Xylem Cavitation Hydraulic Resistance 80 60 Intensity of the processes 40 20 Time / Drought Intensity Hydraulic traits may provide new insights into our understanding of tree drought resistance

The Hydraulic behavior of trees

The ‘Hydraulic’ behavior of trees RH=1/KH T P DY = – RH*Flow Ohm’s law analog for water transport in trees Cochard et al 1997

The hydraulic limit of sap transport: Cavitation Sap is transported in xylem conduits under large negative pressures Water nucleation (cavitation) can occur under negative pressure Air-seeding process

Trees operate close to the point of xylem cavitation Percent Xylem Cavitation Stomatal conductance Xylem Pressure, MPa

‘Stomatal control of xylem cavitation’ Y Provoking 10 % embolism Y Provoking 90 % stomatal closure % Cavitation Stomatal conductance Xylem Pressure, MPa Cruiziat, Cochard, Améglio 2002

Experimental evidence for a stomatal control of cavitation Populus cv ‘ peace ’ ABA+ Percent Cavitation Distance to apex, cm ABA- Cochard, Ridolfi, Dreyer 1996

Hydraulic traits with high functional significance More efficient RH=1/KH Safer CAVITATION

The significance of hydraulic efficiency for trees

How significant is the hydraulic efficiency for trees ? Walnut Bryophytes Ferns Conifers Angiosperms Cochard et al, 2002 Brodribb et al, 2007 Hydraulic efficiency scales with leaf gas exchanges

Where are the located the main hydraulic resistances along the sap pathways ? Root Resistance ≈ Shoot Resistance Cochard et al, 2004 T i g e s F e u i l l e s F . e x e l s i o r J . r e g i a B . v e r r u c o s a S . f r a g i l s P . m a l u s P . p e r s i c a F . s y l v a t i c a Q . r o b u r Q . p e t r a e a Q . i l e x C . l i b a n i C . a t l a n t i c a 2 4 6 8 1 R é s i s t a n c e H y d r a u l i q u e , % Leaves ≈ 80% of shoot Resistance Veins ≈ 10-50% of leaf Resistance

Apoplasmic Symplasmic Gaseous Sap pathways in leaves Mesopyll cell symplasm Apoplasmic Mesopyll cell wall Gaseous Evaporation in stomatal chambers

Leaf conductance is variable and under environmental control Cochard et al 2007 Nardini, unpublished

Leaf conductance can vary rapidly Sack et al (2002) : light decreases leaf hydraulic resistance A variable Symplasmic pathway Cochard et al 2007

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

Functional significance of leaf aquaporins Cochard et al 2007

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

Hydraulic efficiency Key parameter Correlates tightly with gas exchanges Highly variable across species Highly sensible to environmental factors Under biological control : Aquaporins Hydraulic conductances are tightly regulated to optimized leaf gas exchanges

The significance of xylem cavitation for trees

% Xylem cavitation Xylem vulnerability to cavitation across species Juniperus Quercus robur Pinus Populus Prunus % Xylem cavitation Buxus -12 -10 -8 -6 -4 -2 Xylem pressure, MPa

Cavitation resistance correlates with species ecological preferences Maherali et al 2004

Cavitation resistance across Prunus species Cochard et al 2007

Pinus sylvestris Cochard et al unpublished

Cavitation resistance seems an adaptive trait for drought resistance How 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 genotypes Intrinsic, 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 cavitation Loss of hydraulic conductance (Sperry et al 1988) : Reliable but not fast (1genotype/week) Acoustic emissions (Tyree et al 1985) Not reliable and not fast XYL’EM www.bronkhorst.fr

Techniques for measuring cavitation Air injection (eg, Cochard et al 1992) Rather Fast , reliable ? (1genotype/day) Centrifuge technique (Cochard et al 2005) Very fast, reliable ? (5 genotypes/day) r 0.5 1

Reliable for conifers and species with short vessels Evaluation of the ‘cavitron’ technique Betula Max vessel length 3 different sample length ‘true curve’ 15 cm Prunus Reliable for conifers and species with short vessels 30 cm Quercus 140 cm With this technique about 5 genotypes/day. More accessible traits ?

Anatomical traits correlated with cavitation across species Hacke et al 2001 Wood density P50, MPa

Anatomical traits do not seem to correlate with cavitation across genotypes Cochard et al 2007

The molecular basis of xylem cavitation ? A better understanding of the mechanism of cavitation Angiosperms Effect of water surface-tension on cavitation Conifers Cochard unpublished

Identify the structural/textural characteristics of pit membranes determining cavitation

How to identify genes involved xylem cavitation ? Global techniques cDNA-AFLP Manipulate plants Experimentally Screen mutant banks for specific genes coding for the primary cell wall (Arabidopsis) UPD-glucose dehydrogenase(UGHD). UPD-glucuronate 4-epimerase. Pectine methylesterase. Glycosyltransferase. Glucosyltransferase. UPD-glucose pyrophosphorylase. Cellulose synthase(CSL). Boron links in pectins shade Boron + Boron - Full light

Conclusion Two keys aspects of tree hydraulics Cavitation Aquaporins Two keys aspects of tree hydraulics Physiological implications Molecular basis Ecological significance More drought performing forests