In halophytes, salinity either induces uptake and accumulation of NO 3 or has no effect on these pro-
cesses [86]. Halophytes can accumulate inorganic ions such as Na, K, Cl, and NO 3 in excess com-
pared with nonhalophytes under saline conditions. Atriplex, Salicornia, and Suaeda maritimaplants show
higher uptake of Na, Cl, SO 42 , and NO 3 in saline environments than under nonsaline conditions [86].
According to Flowers et al. [86], even in conditions of low salinity, the levels of K, NO 3 , and SO 42 are
very high in halophytes compared with other plants. These investigators believe that high uptake of NO 3
by halophytes under salinization is related to the intrinsic properties of these plants as they are adapted to
grow and show normal metabolic functions at high ion concentrations.
The prime enzyme of NO 3 assimilation, NR, which catalyzes the conversion of NO 3 to NO 2 , was
shown to play a major role in the uptake of NO 3 by serving as an NO 3 transporter [87]. This enzyme has
been studied extensively by various groups of investigators for its behavior in different plant species un-
der salinization. Evidence indicates that the uptake and utilization of applied NO 3 are largely dependent
on its assimilation inside the plant tissues [88]. The effects of salinity on NR activity are varied and de-
pend on the type of salinity as well as the plant species. Nitrate reductase is highly sensitive to various
types of environmental stresses including salinity [81]. In many salt-sensitive plant species, NR activity
decreased under NaCl salinity [89–91]. Plaut [89] observed decreased activity of NR in cell-free extracts
as well as intact tissues of wheat (Triticum aestivumL.) seedlings when NaCl was applied to the nutrient
medium and the enzyme was assayed after 24 hr of exposure to salinity stress. Similarly, while studying
the effects of salinity on N metabolism on wheat plants, Abdul-Kadir and Paulsen [90] observed de-
creased NR activity under salinization. In pea (Pisum sativumL.) seedlings, an isosmotic concentration
of NaCl suppressed NR activity and caused accumulation of NO 3 in the plant tissue, and in wheat
seedlings an isosmotic salinity level decreased NR activity without significant accumulation of NO 3 in
the tissues [81]. In young barley plants [78] and rice (Oriza sativaL.) seedlings [92], decreased NR ac-
tivity has been observed under salinization. In pasture plants, NaCl salinity reduced growth with a con-
comitant decrease in NR activity [93–95].
Lal and Bhardwaj [91] observed that after 15 days of salinization of field pea (Pisum sativumL.) with
NaCl and CaCl 2 (1:1), there was significant suppression of NR activity accompanied by decreases in to-
tal-N as well as protein-N and increases in NO 3 -N and NH 4 -N. According to these investigators, NaCl
as well as CaCl 2 salinity impaired NO 3 assimilation in pea plants, leading to accumulation of NO 3 and
NH 4 in the tissues. Tewari and Singh [94], while conducting stress studies in lentil (Lens esculenta
Moench), observed that with increasing exchangeable sodium percentage (ESP) in the cell, there ap-
peared a continuous decrease in NR as well as NIR activities in plants up to 60 days after sowing. Geno-
types of rice plants differing in salt tolerance show varying behaviors of NR as well as NIR under salin-
ity stress [39]. Katiyar and Dubey [39], while studying the mode of N assimilation under salinization in
the seedlings of two sets of rice cultivars differing in salt tolerance, observed decreased NR activity in
seedlings of salt-sensitive cultivars. When desalted enzyme extracts from nonsalinized rice seedlings
were assayed for NR activity in the presence of 1 M NaCl in the assay medium, strong suppression of the
enzyme activity was observed. Other investigators noticed similar suppression of NR activity in salt-sen-
sitive genotypes of rice seedlings when grown in saline medium [38,90].
Several possible explanations have been suggested for the decreased NR activity in salt-sensitive
plants under saline stress [89,96]. A plausible reason appears to be the inhibition of enzyme induction un-
der salinization [94]. As NR is a substrate-inducible enzyme, under saline conditions NO 3 uptake by the
plants is reduced. This causes limited NO 3 availability in the plant tissues so that NR induction is sup-
pressed, which results in decreased NR activity [96].
Several investigators have emphasized that, under salinity stress, enhanced translocation of NO 3 and
assimilates takes place from roots to shoots and from flag leaves to developing grains [78,97].
In certain plant species, an increase in NR activity has been observed due to salinity [38,39,81,98].
Salicornia europeacaplants and corn (Zea maysL.) seedlings showed increased NR activity when grown
in a salinized medium [81]. Joshi [98], while conducting experiments on Cajanus cajanplants, observed
that NaCl salinity stimulated NR activity in the leaves of plants, whereas Na 2 SO 4 salinity inhibited the
enzyme activity. In Cajanusplants a gradual increase in NR activity was observed with increase in NaCl
salinity of the soil in the range 2.5 to 10.0 dS m^1 [98]. In seedlings of Phaseolus aconitifolius, Sankhla
and Huber [99] observed increased in vivo NR activity with salinization.
Rice plants differing in salt tolerance show varying behaviors of NR activity [38,39,95]. Salt-sensi-
tive genotypes of rice plants showed decreased NR activity under salinization, whereas an increased NR
NITROGEN ABSORPTION UNDER STRESS 641