level under salinity was observed in salt-tolerant genotypes [38,39]. Katiyar and Dubey [39] observed a
marked increase in in vivo NR activity in roots as well as shoots of salt-tolerant rice cultivars CSR-1 and
CSR-3 with a salinity level up to 14 dS m^1 NaCl compared with nonsalinized plants. The higher NR
level in seedlings of salt-tolerant cultivars suggests that salinity may promote synthesis or induction of
the enzyme in seedlings of such cultivars. Salt-tolerant crop cultivars thus appear to have better adapt-
ability to saline stress by exhibiting efficient NO 3 reduction under salinization.
The glutamine synthetase (GS)/glutamine-oxoglutarate amido transferase (GOGAT) pathway,
which is the route of ammonia assimilation in plants under normal conditions, is adversely affected by
salinization [100,101]. Miranda-Ham and Loyola-Vargas [100] observed that when Canavalia ensiformis
plants were subjected to NaCl salinity stress, the activity of GS decreased markedly in roots as well as
shoots. In the roots of salt-sensitive pea plants, decreased GS activity was observed under salinization
[81]. Katiyar [101], while studying the behavior of GOGAT in situ in the two sets of rice cultivars dif-
fering in salt tolerance, observed that an NaCl salinity level of 14 dS m^1 was inhibitory to the enzyme.
In tolerant genotypes of crop plants, GOGAT is more tolerant to NaCl than in sensitive genotypes [101].
Decreased activities of GS and GOGAT under salinization suggest possible impairment of N assimilation
and/or amino acid biosynthesis by this pathway as a result of salinity.
The glutamate dehydrogenase (GDH) enzyme plays an important role in ammonia assimilation un-
der stress conditions by detoxifying the ammonia that tends to accumulate under such conditions. In many
crop species examined, under salinization, the activity of GDH increased [81,101–103]. However, in cer-
tain cases, it remained comparable to that in the controls [100] or was decreased by NaCl salinity [81,91].
In the obligate halophyte Suaeda maritima, Boucaud and Billard [102] observed an increase in GDH ac-
tivity with 25 mM NaCl. Similarly, in peanut leaves a salinity-induced increase in GDH activity was ob-
served by Rao et al. [103]. Sharma and Garg [104], while studying the amination and transamination
events in wheat plants, observed that plants grown at 8 and 16 dS m^1 NaCl showed increased activity of
GDH in leaves as well as roots. Seedlings of rice cultivars differing in salt tolerance, when raised under
increasing levels of NaCl salinity, showed a marked increase in GDH activity both in vivo as well as in
vitro compared with controls [101]. The effects were greater in the sensitive than in the tolerant cultivars.
Increased activity of GDH with salinization suggests a possible role of this enzyme in ammonium assim-
ilation under saline conditions [105]. It is suggested that saline conditions favor increased accumulation
of ammonium and related compounds. Thus, the GS/GOGAT pathway of ammonium assimilation is im-
paired, and under such conditions, an increased level of GDH imparts adaptive value to plants by detox-
ifying and assimilating more ammonium [105]. Rice (Oryza sativaL.) crop cultivars appear to have bet-
ter adaptability to saline stress by exhibiting efficient NO 3 reduction under salinization.
Effects of soil salinity on the N content of plant species are varied and depend on the species, the
organs studied, as well as the type of salinity. Lal and Bhardwaj [91] observed decreases in the total-
N and protein-N content of 15-day-old Pisum sativumseedlings salinized with a mixture of NaCl and
CaCl 2 with 4 and 8 dS m^1 salinity. However, an increase in soluble forms of N (NO 3 and NH 4 ) was
observed with salinity. A similar decrease in the content of pea seedlings growing at isosmotic levels
(0.1 to 0.5 M Pa) of NaCl, CaCl 2 , and Na 2 SO 4 salts was observed by Singh et al. [106]. These in-
vestigators observed a decrease in N content with increasing salt stress and found that salt stress was
more harmful to N content than water stress. While investigating the effects of salinization on nodula-
tion and N fixation in pea plants, Siddiqui et al. [107] reported decreases in nodule N and total plant N
and a significant reduction in the N 2 -fixing efficiency of the nodules with increasing level of salinity.
InVigna radiataplants a salinity level of 8 dS m^1 was lethal to plant growth and nodulation. In-
creasing the salinity level in such plants from 0 to 4 and 6 dS m^1 decreased the nodulation and the N
content of roots, stem, and leaves [108].
In certain cases, increased N contents have been observed in various plant species subjected to salin-
ity by several investigators [3,6,22–35,42–47,49,51,83,90–98]. Sharma et al. [6] observed that N con-
centrations in grains and N uptake in grains and straw increased with an exchangeable sodium percent-
age (ESP) up to 25. Pessarakli and Tucker [26] found that the N contents of cotton shoots and roots
increased with NaCl salinity up to 0.8 MPa osmotic potential of the nutrient solution. At the low level
of salinity (0.4 MPa osmotic potential), plants contained significantly higher total-N [26] as well as
crude protein [47] compared with the controls (nonsalinized plants). Khalil et al. [83] reported similar in-
creases in the total-N concentration of cotton and corn under salt stress conditions. In Cajanus cajan, a
protein-rich leguminous crop, a salinity treatment of 10 dS m^1 NaCl caused about a 43% increase in to-
tal-N and protein content in the leaves of 3-month-old plants over controls. A similar salinity treatment
642 DUBEY AND PESSARAKLI