Water-Nitrogen Use efficiency
of Cork Oak seedlings
H. Khiari, *Z. Nasr, A. Khaldi, ML. Khouja, S-Y Woo
and MN. Rejeb
•Corresponding
author*
•INRGREF, Tunisia
•[email protected]
PLAN

Introduction



Materials and Methods



-Experimental design and Water-Nitrogen treatments
-Methods and measurements
Results



-Problem and Hypothesis in a changing
environment…..
-Objectives
-Chlorophyll content
-Photosynthesis
Conclusions and perspectives
Problem and Hypothesis


Almost zero natural regeneration
rate of Cork Oak Seedling in
natural forest ?
Water stress and Nitrogen
deficiency during long summer
period could be one of major
factors ?
SWC (m3/m3)
Water availability for seedlings in a Cork
Oak forest north Tunisia
Dry period
More than 90 days with
SWC # 0.02 – 0.1
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
0
1000
2000
3000
4000
5000
6000
28/02/08-29/12/09
Climate change scenarios predict
Longer, more frequent and intense Dry spells
In a changing environment …

With increasing atmospheric CO2
during next decades…Nitrogen
availability could be the driving force
for Photosynthesis and growth…

Water and Nitrogen use efficiency for
Cork seedling could be key elements in
the regeneration and growth of the
species …
Objectives

Scientifically : What is the role of
Nitrogen in a context of CC with
increasing of drought and atmospheric
CO2 ?

Technically : Seedling receiving
nitrogen in nursery could have better
survey and growth when planted in
natural forest ?
Materials and Methods
1. Experimental Design
4 Treatments
6 trees / treatment
5 replications
WN :watered + Nitrogen (1+1 gr/plant)
WNo :watered – Nitrogen
SN :stressed + Nitrogen (1+1 gr/plant
SNo :stressed - Nitrogen
RESULTS
1. Chlorophyll Content under
differential water and
nitrogen availabilities
Chlorophyll Content index
WN1
WNo
SN1
SNo
17.72a
12.45b
17.63a
15.81b
- Nitrogen application effects are clearly increasing
chlorophyll content of plants
- No statistical effect of water treatments
RESULTS
2. Photosynthesis under
differential water and
nitrogen availabilities
Photosynthesis with increasing leaf
CO2

Net increase of
photosynthesis for
all treatments when
CO2 increases
Photosynthesis is
significantly
decreasing with
water stress and
Nitrogen deficiency
25
20
SN1
15
10
SN0
5
0
-5

WN1
30
Pn (μmol.m-2.s-1)

0
500
Small increase of Pn
in stressed+nitrogen
plants is noted
Bigger decrease of Pn with water stress
Than the increase with CO2 fertilization
1000
1500
2000
Ci (mol.mol-1)
(SWC=80% to 50%)
(Ci=380 to 760 ppm)
Transpiration decreases with
increasing CO2
120
100
80
60
40
20
0
0
500
1000
Ci (mol.mol-1)



Stress - Nitrogen
Relative % TR or WUE
Relative % TR or WUE
Water + Nitrogen
1500
2000
120
100
80
WUE
60
40
Tr
20
0
0
500
1000
1500
2000
Ci (mol.mol-1)
Net decrease of leaf transpiration (probably attributed to a
decrease of gs)
Increase of WUE : as consequence of Pn increase and Tr
decrease
Faster decrease of Tr in stressed plants than watered plants

In a changing environment with CO2 increase,
cork oak seedlings are subjected to :
- reduction of transpiration :compensation of
deficit rainfall announced by CC scenarios ?
- best WUE…?
- but net carbon assimilation is more affected
by water stress which could cancel the
atmospheric CO2 fertilization…?
Perspectives


-Provenances from “marginal” zones could
have a better tolerance to drought and
better WUE in conditions of drought than
provenances from “central” zones ???
-Provenances from sites where cork oak is
associated to plants fixing atmospheric
nitrogen could have better carbon
assimilation in stressed conditions…???