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full watering. Net CO
2
assimilation (A
max
) ranged from 15 μmol CO
2
m
-2
s
-1
in the morning to
3 μmol CO
2
m
-2
s
-1
in the afternoon under drought, and from 25 μmol CO
2
in the morning to
10 μmol CO
2
mm
-2
s
-1
in the afternoon under full watering. At these times, stomatal conduc‐
tance (g
s
) varied from 0.2 to 0.02 mol H
2
O m
-2
s
-1
and from 0.7 to 0.2 mol H
2
O m
-2
s
-1
, respec‐
tively.
Drought reduced the internal CO
2
concentration (C
i
) by about 55 μmol mol
-1
on day 1, and by
about 100 μmol mol
-1
on day 2 and increased leaf temperature (T
l
) by about 2-5 °C. The
reductions in g
s
and C
i
with drought suggest that lower photosynthesis was associated with
stomatal closure [121]. However, in each treatment, A
max
decreased during the day, while C
i
was stable, suggesting that photosynthesis was also reduced by a direct effect of heat on leaf
biochemistry. Both A
max
and g
s
correlated with T
l
and with the leaf-to-air vapor pressure deficit
(VPD
l
), but with different relationships for droughted and control trees. However, when
stomatal limitations to photosynthesis were accounted for (i.e., based on the assumption that
under stomatal limitation photosynthesis is proportional to C
i
), a single relationship between
A
max
and T
l
described all the data (R
2
= 0.81). Thus, photosynthesis was limited by both the
closing of stomata under drought and by a direct effect of heat on leaf biochemistry. These
results suggest that hot and dry weather reduces photosynthesis and potential productivity
in walnut in the absence of a soil water deficit [121].
To test the hypothesis that A
max
was limited by both T
l
and g
s
, we corrected A
max
for the g
s
(i.e.,
C
i
) limitation and plotted the corrected A
max
(A
maxCorr
) against T
l
. A single fit described all the
data, suggesting that CO
2
assimilation responded directly to T
l
, and that the rest of the variation
in A
max
was due to additional g
s
limitations (i.e., low C
i
), especially under drought. Given the
close correlation between T
l
and VPD
l
, A
maxCorr
was also closely correlated with VPD
l
[121].
Stomatal conductance is probably more related to Ψl [72-74; 84] and not Ψs, but these two
parameters are closely related in droughted walnut [29]. If g
s
was limited by water status at
low Ψs, rather than by VPD
l
, then it remains unclear why g
s
was also closely related to VPD
l
(R
2
= 0.85) under drought (i.e., low Ψs), although with a different relationship than for the
controls [121]. This was probably due to the strong link between Ψs and VPD
l
[180]. A strong
relationship between Ψs and VPD
l
or VPD (i.e., vapor pressure deficit in the air) has been
found in several species, and is commonly used to explain variation in Ψs for fully-irrigated
trees [106].
Also stomatal patterns of A, g
s
, C
i
and E were studied for irrigation treatments under salt stress
conditions by Girona et al [215]. All of the traits studied were highly affected by salt stress.
Gas exchange parameter seasonal patterns showed three groups of responses: A) less affected
plants, B) moderately affected plants and C) highly affected plants [215].
3.10. Relationship between variation in water source partitioning and plant water status
Comparison of the δD values in plant stem water and soil water at different depths demon‐
strated that
J. regia
was compelled to take a higher ratio of water from the deep soil layers in
the dry season. However, measurements of water relationships indicated that the larger water
uptake from deep soil was not able to prevent water stress on the plants. Deep soil water
resources may allow plants with deep root systems to survive in dry seasons [104]. Also, deep
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