with Na 2 SO 4 caused a decrease in N and protein content compared with controls [109]. It is suggested
that the two salts NaCl and Na 2 SO 4 show specific ion effects on the N metabolism of Cajanus cajan[109].
Phaseolus vulgarisplants, when grown in greenhouse conditions and irrigated with water containing 44,
88, and 132 mM NaCl, showed increases in the total N content of leaves with increasing salinity [110].
Addition of 4 or 8 mM CaCl 2 or CaSO 4 in the NaCl treatment medium further increased the leaf N con-
tent in such plants, indicating that Ca^2 addition helps in maintaining the selective permeability of the
membranes [110].
B. Water Stress
Water availability is one of the most limiting environmental factors affecting crop productivity. In semi-
arid tropics, the occurrence of drought or water deficit in the soil is common, whereas crop plants of tem-
perate and tropical regions undergo seasonal periods of water stress, especially during the summer. The
plant responses to water stress depend on the severity and the duration of stress and the growth stage of
the plant [111]. Low water potential in the soil as well as inside the plant inhibits plant growth, reduces
developmental activities of cells and tissues, decreases the uptake of essential nutrient elements, and
causes a variety of morphological and biochemical modifications. Plants growing in water-stressed envi-
ronments show reduced N uptake [3,4,7–11,13,20,30,31,35,40,48,51,76] from the culture medium or soil
and decreased activities of N assimilatory enzymes [112–114].
When the water potential inside the plant declines below a threshold value, stomata closure takes
place, which causes reductions in transpiration and water transport through the plant. This, in turn, affects
the roots directly so that the roots are unable to accumulate or absorb NO 3 as effectively as when tran-
spiration is normal [115]. At low water potential, the ability of roots to supply NO 3 to the transpiration
stream decreases, leading to a decrease in NO 3 concentration of the xylem sap [115]. Under nonstressed
conditions, in a freely transpiring plant, a continuous movement of NO 3 from the roots to the leaves
(NO 3 flux) is maintained. This NO 3 flux decreases during water stress.
It was suggested by Viets [116] that, under water stress conditions, roots are unable to take up much
nutrient from the soil because of lack of root activity and slow rates of ion diffusion and water movement.
When examining the uptake of various nutrients by wheat varieties, Rao and Ramamoorthy [117] ob-
served a 39% drop in N uptake of six improved varieties of wheat when moisture stress was imposed at
different stages of plant growth. According to these investigators, the uptake of N was affected by applied
stress mainly through restricted movement of water under such conditions.
Water stress causes a decrease in leaf NO 3 content as well as NO 3 flux from the roots to the leaves
[115]. When the water-stressed plants were rewatered, NO 3 flux increased but not the leaf NO 3 content.
When water-stressed plants were fertilized with more NO 3 , the NO 3 flux increased and plant perfor-
mance as well as grain yield improved [113]. Kathju et al. [113] observed that when wheat plants were
grown under low (N 0 P 0 ) and high (N 80 P 80 ) fertility conditions and water stress was imposed at various
stages of the plant’s life cycle, increasing intensities of stress adversely affected leaf metabolism and plant
performance. However, the performance of plants was better under high-fertility conditions at all stages
with different intensities of water stress. Similar observations by other investigators also indicate that
NO 3 application can partly alleviate water stress–associated damage in plants [118,119]. Lahiri [118]
demonstrated that N application to the soil reduced the adverse effect of drought on dry matter and grain
yield of pearl millet. Sorghum (Sorghum halepenseL.) plants, when fertilized with N, recover faster af-
ter relief from water stress [119]. Although fertilized plants experienced water stress severely, they re-
covered from stress more quickly than unfertilized ones. Such observations have far-reaching conse-
quences in the sense that in dry-land agriculture, where water is a limiting factor, fertilizer application can
be considered for drought mitigation management [119].
Considerable studies have been performed by various groups of investigators to examine the behav-
iors of NO 3 assimilatory enzymes in plants under water stress conditions. In these studies, NR has re-
ceived the most attention. The activity of NR is sensitive to the water potential of the plant and decreases
with decreasing water potential [115]. Even under mild water stress conditions, NR activity declines
rapidly compared with other N assimilatory enzymes [63].
In various crop species examined, NR activity has often been shown to decline with water stress
[89,117,120,121]. In field-grown wheat plants, imposition of water stress caused a gradual decline in NR
activity in leaves [113]. Kathju et al. [113] observed that in wheat plants, increasing the intensity of wa-
ter stress progressively for 3 to 9 days reduced NR activity. These investigators also reported that under
NITROGEN ABSORPTION UNDER STRESS 643