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
Frequency distribution
NOW
2050
Fagus sylvatica
2100
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
Tree drought «resistance »
Intensity of the processes
↑ Productivity
↑ Resilience
100
80
60
40
20
0
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
15
600
10
400
5
Sap Flow Density
dm3 dm-3 h-1
0
0
leaf , MPa
800
200
-1
-2
RH=1/KH
1.5
1.0
0.5
-3
0.0
T
P
-1
0.5
1.0
1.5
D = – RH*Flow
-2
-3
0
6
12
Hours
Cochard et al 1997
2.0
Sap flow density, dm3 dm-2 h-1
0.0
0
leaf , MPa
0
20
vpd, hPa
Rg, Wm-2
1000
18
24
Ohm’s law analog for water transport in trees
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
Stomatal conductance
Percent Xylem Cavitation
Trees operate close to the point of
xylem cavitation
Xylem Pressure, MPa
‘Stomatal control of xylem cavitation’
Conductance stomatique / Transpiration relative
Stomatal conductance
stomatal closure
 Provoking 90%%Cavitation
 induisant 90% dePLCfePLCrmeture stomatique
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Xylem Pressure, MPa
Cruiziat, Cochard, Améglio 2002
Experimental evidence for a stomatal control of cavitation
ABA-
Populus cv ‘ peace ’
Percent Cavitation
ABA+
Distance to apex, cm
Cochard, Ridolfi, Dreyer 1996
Hydraulic traits with high functional significance
leaf , MPa
0
-1
More efficient
RH=1/KH
-2
CAVITATION
-3
0.0
0.5
Safer
1.0
1.5
Sap flow density, dm3 dm-2 h-1
2.0
The significance of
hydraulic efficiency for
trees
How significant is the hydraulic efficiency for trees ?
Relative Transpiration
1.2
1.0
Bryophytes
Ferns
Conifers
Angiosperms
Walnut
0.8
0.6
0.4
Drought
Pressurization
Chilling
0.2
0.0
0.0
0.5 1.0 1.5 2.0 2.5 3.0
Plant Hydraulic Conductance
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
Tiges
Cochard et al, 2004
Feuilles
F. exelsior
J. regia
B. verrucosa
S. fragils
P. malus
P. persica
F. sylvatica
Q. robur
Q. petraea
Q. ilex
C. libani
C. atlantica
0
20
40
60
80
100
Résistance Hydraulique, %
Leaves ≈ 80% of shoot Resistance
Veins ≈ 10-50% of leaf Resistance
Sap pathways in leaves
Apoplasmic
Mesopyll cell wall
Symplasmic
Mesopyll cell symplasm
Gaseous
Evaporation in
stomatal chambers
Leaf conductance is variable and
under environmental control
20
25
15
Kleaf,
Kleaf,
20
10
15
5
0
0
10
20
Temperature, °C
30
40
10
0
500
1000
1500
2000
PAR
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
+Aquaporins
- Aquaporins
Cochard et al 2007
Future issues for
aquaporins and leaf
hydraulics
Dark
Light
20
Kleaf
Great diversity of leaf response to light
unpublished
25
15
10
Great diversity of aquaporins
5
Q
ue
rc
us
r
ob
ur
Be
tu
la
Fa
gu
Ro s
bi
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Fr ia
ax
in
us
Ac
er
Po
pu
Al
lu
s d nus
Po
e
pu ltoi
d
lu
s t es
re
m
Po ula
pu
lu
Sa s X
lix
al
ba
0
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
-12
-10
-8
-6
Xylem pressure, MPa
-4
-2
0
Cavitation resistance correlates with species ecological preferences
Populus euphratica
Salix fragilis
Populus trichocarpa
Populus alba
Alnus glutinosa
Salix caprea
Juglans regia
Salix caprea
Betula pendula
Quercus rubra
Populus tremula
Pinus nigra
Quercus robur
Fraxinus excelsior
Populus nigra
Fagus sylvatica
Pinus sylvestris
Pinus cembra
Quercus petraea
Pseudotsuga
Cytisus scoparius
Picea abies
Pinus pinaster
Abies alba
Pinus mugho
Carpinus betulus
Euonymus europaeus
Cedrus atlantica
Pinus corsicana
Quercus suber
Lonicera etrusca
Quercus ilex
Pinus Halepensis
Amelanchier ovalis
Prunus spinosa
Crataegus monogyna
Taxus baccata
Buxus sempervirens
Hygrophilous
Mesophilous
Xerophilous
Maherali et al 2004
-8
-7
-6
-5
-4
-3
-2
-1
0
Xylem pressure inducing 50% cavitation, MPa
Cavitation resistance across Prunus species
Cochard et al 2007
P50, MPa
-3.0
-3.5
-4.0
Cochard et al unpublished
Sa
lg
es
V ch,
al
lc Sw
e
Pr bre
ad , S
es
p
Fo , S
nt p
fr
G eyd
ra
na e, F
Je da, r
iz
in Sp
Po en,
te Sw
nz
Le a, I
uv t
e
Sc num
ot
la se,
K nd Nl
oo
tw
H ijk
yy , N
tia
la l
,F
i
Pinus sylvestris
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
Acoustic emissions
(Sperry et al 1988) :
(Tyree et al 1985)
Reliable but not fast (1genotype/week)
Not reliable and not fast
www.bronkhorst.fr
Techniques for measuring
cavitation
Air injection
Centrifuge technique
(eg, Cochard et al 1992)
(Cochard et al 2005)
Rather Fast , reliable ?
(1genotype/day)
Very fast, reliable ?
(5 genotypes/day)
r
0 0
.
5
1
Evaluation of the ‘cavitron’ technique
100
Betula
17.5 cm water
27.5 cm water
37.5 cm water
80
Max vessel length
PLC
60
3 different sample length
40
‘true curve’
20
15 cm
100
Prunus
80
PLC
60
40
30 cm
20
Reliable for
conifers and
species with
short vessels
100
Quercus
80
PLC
60
140 cm
40
With this technique about 5 genotypes/day.
More accessible traits ?
20
0
-6
-5
-4
-3
-2
Xylem Pressure, MPa
-1
0
Anatomical traits correlated
with cavitation across species
Hacke et al 2001
6.0
-2
Inter-vessel wall thickness
of larger vessels, µm
Tension de sève, MPa
MPa
P50,
induisant
50%
d'embolie
-1
-3
-4
-5
-6
-7
-8
0.3
0.4
0.5
0.6
0.7
Densitédensity
du bois, g cm-3
Wood
0.8
P. padus
P. cerasus
P. avium
P. persica
P. spinosa
P. mahaleb
P. domestica
P. armeniaca
P. amygdalus
P. cerasifera
5.5
5.0
4.5
4.0
3.5
3.0
-7
-6
-5
P50, MPa
-4
-3
Anatomical traits do not seem to correlate with cavitation
across genotypes
Cochard et al 2007
0.50
Wood density, kg dm
-3
Inter-vessel wall thickness, µm
6
0.45
0.40
0.35
0.30
-2.4
Poplar clones
Salix clones
-2.2
-2.0
-1.8
P50, MPa
-1.6
-1.4
5
4
3
Poplar clones
Salix clones
2
-2.4
-2.2
-2.0
-1.8
P50, MPa
-1.6
-1.4
The molecular basis of xylem cavitation ?
A better understanding of the mechanism of cavitation
Angiosperms
Effect of water surface-tension
on cavitation
Fagus
Cedrus
Picea
Pinus
0
-1
Conifers
P50, MPa
-2
-3
-4
-5
Control
Tween 40
Triton X100
Merpol
-6
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.
Boron +
Boron -
Glucosyltransferase.
UPD-glucose
pyrophosphorylase.
100
Ombre
shade
80
PLC
Boron links in pectins
Fagus sylvatica
60
40
Lumière
Full
light
20
Cellulose synthase(CSL).
0
-5
-4
-3
-2
Potentiel Hydrique, MPa
-1
Conclusion
Aquaporins
-
Cavitation
Two keys aspects of tree hydraulics
Physiological implications
Molecular basis
Ecological significance
- More drought performing forests