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

2. 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 ?

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. 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

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. 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

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

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

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

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

14. The significance of hydraulic efficiency for trees

15. 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

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 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

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

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Nardini, unpublished

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

22. 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)

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 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

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
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

32. 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

33. 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 ?

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

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).
Boron links in pectins
shade
Boron +
Boron -
Full light

39. 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