Agriculture Reference
In-Depth Information
the morning to -1.2 MPa in the afternoon under drought, and from -0.1 MPa to -0.4 MPa under
full watering. Net CO
2
assimilation (Amax) 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
m
-2
s
-1
in the
morning to 10 μmol CO
2
mm
-2
s
-1
in the afternoon under full watering. At these times, stomatal
conductance (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
,
respectively. Drought reduced the internal CO
2
concentration (Ci) by about 55 μmol mol
-1
on
day
-1
, and by about 100 μmol mol
-1
on day
-2
and increased leaf temperature (Tl) by about 2-
5°C. The reductions in g
s
and Ci with drought suggest that lower photosynthesis was associ‐
ated with stomatal closure. However, in each treatment, Amax decreased during the day, while
Ci was stable, suggesting that photosynthesis was also reduced by a direct effect of heat on
leaf biochemistry. Both Amax and gs correlated with Tl and with the leaf-to-air vapor pressure
deficit (VPDl), 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 Ci) a single relationship
between Amax and Tl described all the data (R
2
= 0.81). Thus, photosynthesis was limited by
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 soil water deficit.
Under normal physiological conditions, electron transport is directed toward sequential and
fully coordinated reduction of intermediate electron acceptors PS2 and PS1. However, drought
and high temperature can provoke a state of hyper-reduction in the electron transport chain,
enhancing generation of superoxide radicals as has been shown in cotton [183-184] and rice
[186]. Theoretically, high photosynthetic efficiency can increase water-use efficiency as more
carbon is assimilated per unit water transpired. In walnuts, a positive correlation was reported
between photosynthesis and stomatal conductance—an important determinant of water use
efficiency [121; 223]. The effect of salinity stress on the photosynthetic enzyme activities is
postulated to be a secondary effect mediated by the reduced CO
2
partial pressure in the leaves
caused by the stomatal closure [224]. The present review also reveals that in all the walnuts
grown in non-saline and desiccated soils, an increased rate of assimilation is coupled with
increased stomatal conductance [180].
4.7. Total phenols and PPO activity under abiotic stress condition
Walnut nuts have high amount of phenolic compounds. Walnut kernels are rich in oils
composed of unsaturated fatty acids, such as linoleic and oleic acid, and are susceptible to
oxidation. However the content of a-tocopherol, an antioxidant, is lower in walnut than in
other nuts such as almonds, hazelnuts, peanuts [212]. This implies that the nut contains
antioxidants inhibiting lipid auto-oxidation. Recently, a walnut extract containing ellagic
acid, gallic acid, and flavonoids was reported to inhibit the oxidation of human plasma and
low density lipoproteins (LDL) in-vitro [158]. Although the presence of ellagic acid suggests
the occurrence of its bound forms, ellagitannins, there are some reports on the tannin
constituents of walnut [233-235]. Muir et al. [235] demonstrate that a shikimate pathway
enzyme, SDH (shikimate dehydrogenase), is directly responsible for GA [gallic acid]
Search WWH ::
Custom Search