Agriculture Reference
In-Depth Information
monsoon period, but use of this precipitation declines sharply with decreasing summer rain
input [103]. No other water source was available for trees in this system. However, the roots
might penetrate through the dense gravel layers and may be in contact with groundwater.
Therefore, the influence of groundwater on xylem isotopic signature cannot be completely
excluded, although Williams and Ehleringer [103] found that plants did not use groundwater
in the pinyon-juniper ecosystem of the southwestern USA, a site similar to this study region.
Nevertheless, it should be noted that the seasonal change of water resource partitioning was
based on the two measured depths.
Rosati et al. [121] studied Kaolin applications to mitigate the negative effects of water and heat
stress on walnut physiology and productivity. Kaolin applications were found to improve
A
max
in apple but only under high temperature and vapor pressure difference [122]. Other
authors found no effect or even a reduction in yield, A
max
or both [97-98; 122]. Little data are
available for other tree species: kaolin improved A
max
and stomatal conductance (g
s
) in citrus
at mid-day but not in the morning [99] and no effect was found on pecan [100].
A
max
for walnut was highest in the early morning and decreased throughout the day, for both
the water-stressed (S) and the well-irrigated (W) treatments [121]. A
max
was always lower for
the S treatments, especially in the afternoon. Kaolin application reduced A
max
(by up to 4 mmol
CO
2
m
-2
s
-1
) within each irrigation treatment, especially in the morning when A
max
was high,
whereas in the afternoon this effect tended to disappear in the W treatment and disappeared
completely in the S treatment [123; 125]. The average reduction in A
max
during the day was
minor compared with the reduction due to water stress and was 1-4 mmol CO
2
m
-2
s
-1
in the
S treatments and 2-4 mmol CO
2
m
-2
s
-1
in the W treatment [121].
Also in this study, intercellular CO
2
concentration (Ci) was greatly reduced with water stress
in walnut while the irrigated walnut and the almond trees had similar Ci values [121]. Kaolin
application increased Ci in all cases except for two out of five measurements in the S treatment
in walnut. The average daily increase in Ci with kaolin was 28 mmol mol
-1
in the S and 19
mmol mol
-1
in the W treatments for walnut and 10 mmol mol
-1
for almond [121]. As a result
they concluded that Kaolin application reduced leaf temperature (T
l
) and leaf to- air vapor
pressure difference (VPD
l
), but not sufficiently to compensate for the increase in Tl and VPDl
with water stress in walnut. The kaolin-induced reduction in Tl and VPD
l
did not mitigate the
adverse effects of heat and water stress on Amax. Kaolin application did not affect g
s
and Ys.
The prevailing effect of kaolin application appeared to be the shading of the leaves and the
consequent, albeit minor, reduction of Amax, except at very low A
max
[121].
3.11. Delayed consequences of drought
Irreversible drought-induced damage leads to organ dysfunction, but it seldom results in
direct and immediate tree decline and mortality. Drought induces short term physiological
disorders like decreased carbon and nutrient assimilation, and sometimes even a breakdown
of the photosynthetic machinery itself. These tissues have to be repaired before normal
processes can resume. The tree must allocate existing stored reserves among the demands for
repair, maintenance, growth and defense. As a consequence, tree ring width or leaf area is
frequently smaller during several years following a severe drought [166-167]. Moreover,
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