New insights into the hydraulics of trees

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)

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
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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
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The Hydraulic behavior of trees
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The Hydraulic behavior of trees
RH1/KH
T
P
DY RHFlow
Ohms 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

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Trees operate close to the point of xylem
cavitation
Percent Xylem Cavitation
Stomatal conductance
Xylem Pressure, MPa
11
Stomatal control of xylem cavitation
Cruiziat, Cochard, Améglio 2002
12
Experimental evidence for a stomatal control of
cavitation
ABA
ABA-
Cochard, Ridolfi, Dreyer 1996
13
Hydraulic traits with high functional significance
RH1/KH
CAVITATION
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The significance of hydraulic efficiency for trees
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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 ?
Cochard et al, 2004
Root Resistance Shoot Resistance
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Veins 10-50 of leaf Resistance
Leaves 80 of shoot Resistance
17
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
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
-2
-4
-6
-8
-10
-12
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
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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
XYLEM
www.bronkhorst.fr
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)
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

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. Glycosyltrans
ferase. Glucosyltransferase. UPD-glucose
pyrophosphorylase. Cellulose synthase(CSL).
shade
Boron links in pectins
Boron
Boron -
Full light
39
Conclusion
Cavitation
Aquaporins

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