both low and high NP fertility conditions, water stress reduced NR activity at different growth stages of
plants. However, activity was always greater in highly fertilized plants than those with low fertilizer treat-
ments. A slow decline in NR activity with water stress may be attributed to a partially maintained NO 3
flux inside the plant despite increased stomatal resistance and decreased rate of transpiration under stress
conditions.
In maize plants, desiccation leads to a steady decrease in NR activity with a concomitant decrease in
leaf water potential, leaf NO 3 content, and NO 3 flux [120,121]. Water-stressed maize plants when re-
watered recovered partially, showed increased NR activity and increased NO 3 flux [115]. In maize cal-
lus tissue, a decrease in relative humidity caused a gradual decrease in NR activity [119]. While examin-
ing NR activity in different organs of two chickpea (Cicer arietinumL.) varieties in relation to soil
moisture stress, Wasnik et al. [120] observed a significant reduction in leaf NR activity due to moisture
stress. In cacao (Theobroma cacaoL.) plants, which experience a periodic drought during January to May
in the coastal regions of India, water stress induced by withholding irrigation for 7 days caused a sub-
stantial decrease in NR activity in the seedlings [122].
Several explanations have been put forth for decreased NR activity in plant parts subjected to water
stress [63,115,121]. The most plausible explanation, suggested by Morilla et al. [121], is that reduction in
NR activity in Zea maysplants subjected to water stress is due to a decline in the rate of synthesis of NR
protein rather than its increased rate of degradation or a direct effect of water potential on enzyme activ-
ity. According to these investigators, desiccation of plants leads to a decrease in leaf water potential. This,
in turn, decreases NO 3 flux and causes slow delivery of NO 3 to the transpiration stream. Thus, move-
ment of NO 3 to the induction site is prevented, resulting in decreased NR activity. These investigators
believe that decreased NR activity in water-stressed plants is primarily due to a decrease in NO 3 flux and
not a decrease in water potential or the NO 3 content of leaves [115]. Similarly, Singh and Sawhney (63)
suggested that the decline in NR activity during water stress is due to a lowered capacity of tissues to syn-
thesize NR protein because of degradation of polyribosomes to monoribosomes. More conclusive evi-
dence is still required to ascertain whether decreased NR activity in water-stressed plants is due to a de-
creased rate of enzyme synthesis or an increased rate of enzyme degradation.
Certain measures have been suggested by different groups of investigators to overcome partially the
effects of drought stress. By increasing soil fertility, especially with nitrogenous fertilizers, the adverse
effects of drought can be substantially alleviated [118]. More N fertilizer application to water-stressed
plants improved NO 3 uptake and increased NR activity. Such plants showed better performances and
grain yield compared with low-fertilized plants [113]. Similarly, in plants such as sesame (Sesamum in-
dicumL.), moderate water stress, when imparted at an early vegetative stage, partly helped to overcome
the adverse effects of subsequent severe stress [123]. Such prestressed plants maintain high plant water
status, show higher activities of NR, and have better plant performance. Foliar application of chemicals
such as chlormequat, cycocel, or ABA also increased the relative water content in wheat [124] and cacao
[122] plants, and such plants showed increased NR activity.
In water-stressed plants, activities of enzymes of ammonium assimilation remain high as evident
from little or no accumulation of NH 4 in the leaves of such plants [125]. However, the pathway of am-
monium assimilation under stress conditions depends on the plant species, growth stage, and the plant or-
gans studied. It has been shown that water stress lowers the activity of GOGAT in the root nodules of al-
falfa (Medicago sativaL.) and cicer plants [126,127]. In these plants, GOGAT is more sensitive than GS
to water stress. Koundal and Chopra [127] reported a decline in NADH-GOGAT and GS activities in nod-
ules of chickpea plants with water stress with a greater percentage decline in the activity of GOGAT than
GS compared with the nodules of unstressed plants. Rewatering of such plants caused increases in
GOGAT and GS activities, whereas the NR activity remained comparable to that in controls [127]. These
observations indicate that in alfalfa and chickpea nodules, ammonium may be assimilated by the GDH
pathway under water stress. At the flowering stage in Brassicaand in the shoots of Poterium, increased
GDH activity has been observed with water stress [105].
C. Light
Light remarkably influences N uptake and its assimilation. Decreased light intensity reduces the uptake
of NO 3 , causes NO 3 accumulation in the tissues, and decreases the rate of its reduction by lowering the
activities of NO 3 reducing enzymes [63,97]. Plants grown under low light intensity show decreased NO 3
644 DUBEY AND PESSARAKLI