New insights into the hydraulics of trees

1. New insights into the hydraulics of trees Hervé Cochard UMR547 PIAF INRA Clermont-Ferrand France

2. How sustainable are our forests ? 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)

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

4. Distribution of drought vulnerable species in France for the next century NOW Frequency distribution Fagus sylvatica 2100 2050 Badeau and Dupouey 2005

5. 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

6. ↑ Productivity ↑Resilience Survival Photosynthesis Hydraulic Resistance Xylem Cavitation Stomatal Conductance Growth Time / Drought Intensity Tree drought «resistance » 100 80 60 Intensity of the processes 40 20 0 Hydraulic traits may provide new insights into our understanding of tree drought resistance

7. The Hydraulic behavior of trees

8. 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

9. Air-seeding process 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

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

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

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

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

14. The significance of hydraulic efficiency for trees

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

16. 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 0 2 0 4 0 6 0 8 0 1 0 0 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

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

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 Leaf conductance is variable and under environmental control Cochard et al 2007 Nardini, unpublished

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

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

21. Functional significance of leaf aquaporins +Aquaporins - Aquaporins Cochard et al 2007

22. 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)

23. 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

24. The significance of xylem cavitation for trees

25. Xylem vulnerability to cavitation across species Juniperus Populus Quercus robur Pinus Prunus % Xylem cavitation Buxus 0 -12 -10 -8 -6 -4 -2 Xylem pressure, MPa

26. Cavitation resistance correlates with species ecological preferences Maherali et al 2004

27. Cavitation resistance across Prunus species Cochard et al 2007

28. Pinus sylvestris Cochard et al unpublished

29. 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 ?

30. 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 ?

31. Techniques for measuring cavitation Acoustic emissions (Tyree et al 1985) Not reliable and not fast Loss of hydraulic conductance (Sperry et al 1988) : Reliable but not fast (1genotype/week) XYL’EM www.bronkhorst.fr

32. r 0 0.5 1 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)

33. 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 ?

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

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

36. 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

37. Identify the structural/textural characteristics of pit membranes determining cavitation

38. 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). shade Boron links in pectins Boron + Boron - Fulllight

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