and plants respond to these stresses by different types of physiological and biochemical adjustments. Like
various physiological processes, N uptake, translocation, and assimilation are severely affected by dif-
ferent types of stresses [1,2,4–6,12–15,20–62,78–80]. Because the availability of nitrogenous nutrients in
the soil, their uptake, and their assimilation are directly related to each other as well as to the growth and
yield of the crops, considerable efforts have been made by various groups of investigators to study the
possible implications of various stress conditions for N nutrition in plants [1,2,4–6,12–15,20–62,72]. In
the following sections, the influence of diverse environmental stresses on the overall process of N uptake
is summarized.
A. Salinity
Soil salinity is one of the major environmental stresses affecting crop productivity. Effect of salinity on
plants may vary depending on the developmental stage of the plant [11,73] as well as the types and con-
centration of salts [1–6,12,74–76]. The responses to salinity on N uptake differ in different plant species
and also depend on the type and extent of salinity. In the majority of the plant species studied, saliniza-
tion in the soil affects N uptake, whereas in the halophytes and in many salt-tolerant crop species no sig-
nificant effect of NaCl on NO 3 uptake is observed [81]. Barley (Hordeum vulgareL.) plants growing un-
der saline conditions show reduced growth [81,82] as well as decreased N uptake [81]. In young barley
seedlings, salinity severely inhibited NO 3 uptake, whereas little effect on NO 3 reduction was observed
[78]. In wheat (Triticum aestivumL.) plants, reduction in growth was even more than that in barley at
higher NaCl salinity levels [82], and uptake of N decreased with increasing salinity [81]. However, by in-
creasing the N supply to the soil, the effect of salinity was alleviated [81]. Khalil et al. [83] found similar
results for cotton (Gossypium hirsutumL.) and corn (Zea maysL.) plants. Soltani et al. [79] observed that
when barley seedlings were grown in the presence of 200 mM NaCl, growth of the seedlings decreased
with a concomitant reduction in the uptake and translocation of N compared with nonsalinized seedlings.
When seedlings of maize genotype differing in drought resistance were grown at 0.84 MPa NaCl
salinity, the supply of reduced N for the synthesis of amino acids and proteins in the tissues was reduced
[80]. The effect was more pronounced in drought-resistant genotypes, in which salinity reduced the ac-
tivity of the metabolic pathway supplying reduced N accompanied with a corresponding reduction in the
relative growth rate of the seedlings. Reduced growth in terms of dry-matter production and decreased
absorption of N have been reported by several investigators for various plant species with different de-
grees of salt tolerance [2–6,12–14,22–26,29–32,35,36,41,42,44–47,51,73–75].
Under salinization, reduced uptake of N by crops appears to be due to more intake of Naand Cl
by the roots. Increased levels of Nain the plant tissues cause nutrient imbalance and displace Ca^2
from the exchange sites on the membranes and cell walls [81]. Chloride present at more than 100 mM
in the saline medium inhibits NO 3 uptake, possibly because of increased accumulation of Cl in the
roots [84]. Smith [85] observed that NO 3 uptake in barley was dependent on the internal rather than
the external concentrations of Cl. Reduced uptake of N could lead to N deficiency in plants and thus
could become a limiting factor for growth of plants under saline conditions [84]. Because salinity leads
to N deficiency, fertilization of plants growing in a saline environment with increasing doses of ni-
trogenous fertilizers has proved beneficial. It minimizes salt-induced damage and apparently provides
salt tolerance [81]. However, in certain crop species such as corn, rice (Oriza sativaL.), wheat, and
spinach (Spinacia oleraceaL.) with excess application of nitrogenous fertilizers a decrease in salt tol-
erance is observed [81].
The presence of Ca^2 in the medium increases NO 3 uptake under saline conditions. Ward et al. [84]
observed that NaCl decreased NO 3 uptake in barley seedlings, whereas the uptake rate increased with in-
creasing level of Ca^2 between 1.0 and 3.0 mM in the saline medium. These investigators observed 31 to
35% more uptake of NO 3 by increasing Ca^2 in the medium compared with a salt-free (control) medium.
Manganese and Mg^2 also enhanced NO 3 uptake under saline conditions, but Ca^2 was more effective
than these two ions [84]. The presence of Ca^2 in the saline medium possibly decreased Naas well as
Cluptake and also reduced membrane disruption in saline solutions, leading to increased NO 3 uptake
[84]. Calcium plays a significant role in maintaining the integrity of the root membranes; thus its depri-
vation, under salinization, decreases ion transport and NO 3 uptake by disrupting the NO 3 transporter that
is located in the plasmalemma of roots [84]. Under saline conditions, Ca^2 has been shown to increase
the activity of the NO 3 transporter [84].
640 DUBEY AND PESSARAKLI