New insights into the New insights into the hydraulics of treeshydraulics of trees
New insights into the New insights into the hydraulics of treeshydraulics of trees
Hervé CochardHervé Cochard
UMR547 PIAFUMR547 PIAFINRAINRA
Clermont-Ferrand FranceClermont-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 ?
NOW
2050 2100
Frequency distribution
Distribution of drought resistant species in France for the next century
Quercus ilex
Badeau and Dupouey 2005
NOW
2050 2100
Frequency distribution
Distribution of drought vulnerable species in France for the next century
Fagus sylvatica
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
Su
rvival
Gro
wth
Stom
atal Conductance
Ph
oto
synth
esis0
20
40
60
80
100
Inte
nsit
y of
the
proc
esse
s
Time / Drought Intensity
Tree drought «resistance »
Xyl
em C
avit
atio
n
Hyd
raul
ic R
esist
ance
↑ Productivity ↑ Resilience
Hydraulic traits may provide new insights into our understanding of tree drought resistance
The Hydraulic behavior The Hydraulic behavior of treesof trees
The Hydraulic behavior The Hydraulic behavior of treesof trees
The ‘Hydraulic’ behavior of trees
Hours
0 6 12 18 24
le
af
, MP
a
-3
-2
-1
0
Sa
p F
low
De
ns
ity
dm
3 d
m-3
h-1
0.0
0.5
1.0
1.5vp
d, h
Pa
0
5
10
15
20
Rg
, Wm
-2
0
200
400
600
800
1000
P
TSap flow density, dm3 dm-2 h-1
0.0 0.5 1.0 1.5 2.0
le
af ,
MP
a
-3
-2
-1
0
= – RH*Flow
Ohm’s law analog for water transport in trees
RH=1/KH
Cochard et al 1997
The hydraulic limit of sap transport: Cavitation
Water nucleation (cavitation) can occur under negative pressure
Sap is transported in xylem conduits under large negative pressures
Air-seeding process
Trees operate close to the point of xylem cavitation
Per
cent
Xyl
em C
avit
atio
n
Sto
mat
al c
ondu
ctan
ce
Xylem Pressure, MPa
Epicéa
-5 -4 -3 -2 -1 0
0
20
40
60
80
100
Mais
-5 -4 -3 -2 -1 0
Noyer
-5 -4 -3 -2 -1 0
PL
C
0
20
40
60
80
100
Chêne
Potentiel hydrique-5 -4 -3 -2 -1 0
‘Stomatal control of xylem cavitation’
Epicéa
-5 -4 -3 -2 -1 0
0
20
40
60
80
100
Mais
-5 -4 -3 -2 -1 0
0.0
0.2
0.4
0.6
0.8
1.0
Noyer
-5 -4 -3 -2 -1 0
PL
C
0
20
40
60
80
100
Chêne
Potentiel hydrique-5 -4 -3 -2 -1 0
Con
duct
ance
sto
mat
ique
/ T
rans
pira
tion
rela
tive
0.0
0.2
0.4
0.6
0.8
% C
avit
atio
n
Stom
atal
con
duct
ance
Xylem Pressure, MPa induisant 10% d'embolie, MPa
-6 -5 -4 -3 -2 -1 0
in
duis
ant 9
0% d
e fe
rmet
ure
stom
atiq
ue
-5
-4
-3
-2
-1
0
QR
QP
PA
FS
PTJR
CA
CL
CLCBCS
PH
CS
CA
VMZM
Provoking 10 % embolism
P
rovo
king
90
% s
tom
atal
clo
sure
Cruiziat, Cochard, Améglio 2002
Populus cv ‘ peace ’
Experimental evidence for a stomatal control of cavitation
Cochard, Ridolfi, Dreyer 1996
Perc
ent C
avit
atio
n
Distance to apex, cm
Sap flow density, dm3 dm-2 h-1
0.0 0.5 1.0 1.5 2.0
le
af ,
MPa
-3
-2
-1
0
RH=1/KH
CAVITATION
Hydraulic traits with high functional significance
More efficient
Safer
The significance of The significance of hydraulic efficiency for hydraulic efficiency for
treestrees
The significance of The significance of hydraulic efficiency for hydraulic efficiency for
treestrees
Brodribb et al, 2007
How significant is the hydraulic efficiency for trees ?
Plant Hydraulic Conductance
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Rel
ativ
e T
rans
pira
tion
0.0
0.2
0.4
0.6
0.8
1.0
1.2
ChillingPressurizationDrought
Cochard et al, 2002
Walnut BryophytesFernsConifersAngiosperms
Hydraulic efficiency scales with leaf gas exchanges
Where are the located the main hydraulic resistances along the sap pathways ?
Veins ≈ 10-50% of leaf Resistance
Cochard et al, 2004 Root Resistance ≈ Shoot Resistance
Résistance Hydraulique, %0 20 40 60 80 100
C. atlanticaC. libani
Q. ilexQ. petraea
Q. roburF. sylvatica
P. persicaP. malusS. fragils
B. verrucosaJ. regia
F. exelsior
Tiges Feuilles
Leaves ≈ 80% of shoot Resistance
Sap pathways in leaves
ApoplasmicMesopyll cell wall
SymplasmicMesopyll cell symplasm
GaseousEvaporation in
stomatal chambers
Temperature, °C
0 10 20 30 40
Kle
af,
0
5
10
15
20
Leaf conductance is variable and under environmental control
PAR
0 500 1000 1500 2000
Kle
af,
10
15
20
25
Cochard et al 2007Nardini, unpublished
Leaf conductance can vary rapidly
Cochard et al 2007
Sack et al (2002) : light decreases leaf hydraulic resistance
A variable Symplasmicpathway
Tajkhorshid et al 2002Cochard et al 2007
Molecular basis of variable leaf conductance :
Aquaporins
- Aquaporins
Cochard et al 2007
+Aquaporins
Functional significance of leaf aquaporins
Querc
us ro
bur
Betula
Fagus
Robini
a
Fraxin
usAce
rAlnu
s
Populu
s delt
oides
Populu
s tre
mula
Populu
s X
Salix
alba
Kle
af
0
5
10
15
20
25
DarkLight
Aquaporins do not transport only H20 “CO2-porins” Uehlein et al 2003 Control CO2 diffusion in the leaf mesophyll (photosynthesis)
Future issues for aquaporins and leaf
hydraulics
Great diversity of leaf response to light
Great diversity of aquaporins
unpublished
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 The significance of xylem cavitation for xylem cavitation for
treestrees
The significance of The significance of xylem cavitation for xylem cavitation for
treestrees
Xylem pressure, MPa0-2-4-6-8-10-12
% X
ylem
cav
itat
ion
Populus
Quercus robur
Pinus
Prunus
JuniperusBuxus
Xylem vulnerability to cavitation across species
Xylem pressure inducing 50% cavitation, MPa-8 -7 -6 -5 -4 -3 -2 -1 0
Buxus sempervirensTaxus baccataCrataegus monogynaPrunus spinosaAmelanchier ovalisPinus HalepensisQuercus ilexLonicera etruscaQuercus suberPinus corsicanaCedrus atlanticaEuonymus europaeusCarpinus betulusPinus mughoAbies albaPinus pinasterPicea abiesCytisus scopariusPseudotsugaQuercus petraeaPinus cembraPinus sylvestrisFagus sylvaticaPopulus nigraFraxinus excelsiorQuercus roburPinus nigraPopulus tremulaQuercus rubraBetula pendulaSalix capreaJuglans regiaSalix capreaAlnus glutinosaPopulus albaPopulus trichocarpaSalix fragilisPopulus euphratica
HygrophilousMesophilousXerophilous
Maherali et al 2004
Cavitation resistance correlates with species ecological preferences
Cochard et al 2007
Cavitation resistance across Prunus species
Salges
ch, S
w
Vallce
bre,
Sp
Prades
, Sp
Fontfr
eyde
, Fr
Granad
a, Sp
Jeizi
nen,
Sw
Potenz
a, It
Leuve
numse
, Nl
Scotla
nd
Kootw
ijk, N
l
Hyytia
la,Fi
P50,
MPa
-4.0
-3.5
-3.0
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 ?
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 ?
Screening cavitation-resistant genotypes
Techniques for measuring cavitation
www.bronkhorst.fr
XYL’EM
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
Techniques for measuring cavitation
Air injection(eg, Cochard et al 1992)
Rather Fast , reliable ?(1genotype/day)
0r0.5
1
Centrifuge technique(Cochard et al 2005)
Very fast, reliable ?(5 genotypes/day)
Betula
Prunus
Quercus
PLC
20
40
60
80
100
PLC
20
40
60
80
100
17.5 cm water27.5 cm water37.5 cm water
Xylem Pressure, MPa-6 -5 -4 -3 -2 -1 0
PLC
0
20
40
60
80
100
Evaluation of the ‘cavitron’ technique
With this technique about 5 genotypes/day.More accessible traits ?
Max vessel length
3 different sample length
‘true curve’ 15 cm
30 cm
140 cm
Reliable for conifers and species with short vessels
P50, MPa
-7 -6 -5 -4 -3
Inte
r-ve
ssel
wal
l thi
ckne
ss
of la
rger
ves
sels
, µm
3.0
3.5
4.0
4.5
5.0
5.5
6.0P. padus
P. cerasus
P. avium
P. persica
P. spinosa
P. mahaleb
P. domestica
P. armeniaca
P. amygdalusP. cerasifera
Densité du bois, g cm-30.3 0.4 0.5 0.6 0.7 0.8
Ten
sion
de
sève
, MP
a in
duis
ant 5
0% d
'em
boli
e
-8
-7
-6
-5
-4
-3
-2
-1
Wood density
P50,
MP
aAnatomical traits correlated with cavitation across species
Hacke et al 2001
P50, MPa
-2.4 -2.2 -2.0 -1.8 -1.6 -1.4
Inte
r-ve
ssel
wal
l thi
ckne
ss, µ
m
2
3
4
5
6
Poplar clonesSalix clones
P50, MPa
-2.4 -2.2 -2.0 -1.8 -1.6 -1.4
Woo
d de
nsity
, kg
dm-3
0.30
0.35
0.40
0.45
0.50
Poplar clonesSalix clones
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
Conifers
P50
, MP
a
-6
-5
-4
-3
-2
-1
0
ControlTween 40Triton X100Merpol
PinusCedrus PiceaFagus
Cochard unpublished
Effect of water surface-tension on cavitation
Identify the structural/textural characteristics of pit membranes determining cavitation
How to identify genes involved xylem cavitation ?
UPD-glucose dehydrogenase(UGHD).UPD-glucuronate 4-epimerase.Pectine methylesterase.Glycosyltransferase.
Glucosyltransferase.UPD-glucose pyrophosphorylase.
Cellulose synthase(CSL).
Boron + Boron -
Boron links in pectins
Manipulate plants
Screen mutant banks for specific genes coding for the primary cell wall (Arabidopsis)
Experimentally
Fagus sylvatica
Potentiel Hydrique, MPa
-5 -4 -3 -2 -1P
LC
0
20
40
60
80
100
Ombre
Lumière
shade
Full light
Global techniques cDNA-AFLP
Conclusion
Aquaporins Cavitation
- Two keys aspects of tree hydraulics- Physiological implications - Molecular basis- Ecological significance
- More drought performing forests