with those growing in nonpolluted areas. It is suggested that the elevated level of heavy metals in the soil
has a direct deteriorative effect on the growth of finer roots and root hairs of the plants contributing to re-
duced N uptake from the soil [147]. Nitrogen deficiency occurs in plants growing in soil with a high level
of heavy metals, which also results in disturbed carbohydrate metabolism. Levels of total carbohydrate
and especially starch and sucrose increase in the leaves of such plants [147].
Indiscriminate use of acid-forming nitrogenous fertilizers causes acidity in the soil. In acid soils be-
low pH 5.0, Al^3 toxicity is a major problem. In such soils, the NH 4 form of N predominates and NO 3
availability is limited. Uptake of many essential nutrients including NO 3 is reduced by NH 4 and Al^3
toxicity in the soil. It has been suggested that plants differ in their sensitivity to Al^3 and that Al^3 -toler-
ant plants are characterized by efficient use of NO 3 in the presence of NH 4 . Such plants have the capac-
ity to increase the pH of their growth medium [148]. When genotypes of sorghum plants differing in Al
tolerance were grown with different NO 3 /NH 4 ratios (39:1, 9:1, and 3:1) with 0 or 300 M Al in the
medium, Al-sensitive cultivar ICA-Natiama showed a greater reduction in NO 3 and NH 4 uptake than the
Al-tolerant cultivar SC-283 when the plants were grown with Al^3 . When the plants were grown without
Al^3 , the sensitive cultivar showed greater NH 4 uptake than the tolerant one [149]. This shows that up-
take of NO 3 and NH 4 is reduced because of Al^3 toxicity. It is suggested that differences in NO 3 and
NH 4 uptake by plants are associated with changes in solution pH. As long as NH 4 is in solution, pH de-
creases, and it increases when NH 4 is depleted from the solution [149].
Among the enzymes of N assimilation, NR is the most sensitive to heavy metal toxicity. Cadmium
ion (Cd^2 ), Cu^2 , and Pb^2 drastically inhibit NR activity. Elemental cadmium has a strong affinity for
MSH groups and thus it inhibits the activity of many enzymes including NR. Muthuchlian et al. [150] re-
ported that when etiolated leaf segments of Vigna sinensiswere treated with Cd^2 up to 10 M, a stimu-
lation of NR activity was observed. Beyond this level Cd^2 strongly suppressed NR activity, and there
was complete inhibition of activity with 1 mM Cd^2 . In similar experiments, Cu^2 caused 91% inhibi-
tion of NR activity. A purified NR preparation from barley seedlings was inhibited up to 80 to 100% with
1 mM Cu^2 , Zn^2 , and Co^2 [151]. In general, the inhibitory effects of Cd^2 and Cu^2 appear to be due
to interference with the sulfhydryl sites of the enzymes [150].
In germinating pea (Pisum sativum) seeds Pb^2 retarded the utilization of N reserves from cotyle-
dons and decreased the activities of N assimilatory enzymes NR, GS, and GDH, whereas NIR remained
relatively insensitive [152]. Mittal and Sawhney [152] reported about a 50% depression in NR activity of
pea seeds 5 days after germination with a medium containing 1.0 mM Pb^2 . The activities of GS and
GDH were suppressed by 70, 43, 45, and 30%, respectively, compared with controls. Decreased activi-
ties of NR, GDH, and aminotransferase in germinating pea seeds disturb the respiratory activity because
of restricted generation of organic acids from amino acids. This would otherwise facilitate the operation
of the tricarboxylic acid (TCA) cycle even under the partially anaerobic conditions existing during ger-
mination of seeds.
IV. ACCUMULATION OF NITROGENOUS COMPOUNDS IN STRESSED
PLANTSPlants subjected to environmental stresses accumulate a number of soluble nitrogenous compounds.
These compounds accumulate in high concentration and have specific roles in plants under stress condi-
tions. Several investigators have observed accumulations of these compounds in a variety of plant species
[1,3,6,12,15,22–33,41–47,49–51,53–59,61,62,77–80,83,84,91,106,109,11 0,113]. Several review arti-
cles have been published on the effects of various stresses on the accumulation and metabolism of these
compounds [1,3,6,12,15,35,40,48,77,153–156].
Soluble nitrogenous compounds that accumulate most widely in stressed plants are the amino acids
proline, arginine, glycine, serine, alanine, and leucine; the quaternary ammonium compounds glycine be-
taine,β-alanine betaine, stachydrine, trigonelline, and homostachydrine; the amides glutamine and as-
paragine; the imino acids pipecolic acid and 5-hydroxypipecolic acid; the diamines putrescine, N-car-
bamyl putrescine, and agmatine; and the polyamines spermine and spermidine. For the complete list of
these compounds, readers are referred to the review article by Rabe [48]. When subjected to stress, plant
species show accumulation of these compounds depending on the type of stress, extent of stress, and the
types of plant species. With most stresses the amino acid proline and the quaternary ammonium com-
pound glycine betaine accumulate and are regarded as components of the stress tolerance mechanism.
648 DUBEY AND PESSARAKLI