greater than proline at a high salinity level [163]. Accumulation of amides may exceed proline in certain
species under water stress [153]. Mineral deficiencies of K, P, Mg, S, Cl, and Zn cause accumulation of
asparagine and glutamine along with the basic amino acid arginine [156]. Deficiencies of many mi-
cronutrients cause accumulation of asparagine. In certain cases, e.g., Zn deficiency, asparagine accumu-
lation may occur up to 50-fold [153]. The imino acid pipecolic acid accumulated in Mg-, Cl-, K-, and Fe-
deficient plants [153,156]. Saline conditions also favor the accumulation of pipecolic acid.
Hydroxypipecolic acid accumulated in Limoniumplants subjected to salt stress [153].
D. Nonprotein Amino Acids
Among nonprotein amino acids, citrulline, ornithine, and -aminobutyric acid accumulate in plants un-
der certain stresses. Deficiency of mineral nutrient elements such as K in Sesamumand P in Citruscaused
accumulation of citrulline and ornithine [156]. Barley plants subjected to water stress accumulated or-
nithine in the leaves [153]. In conditions of anaerobiosis, the most striking response was the accumula-
tion of -aminobutyric acid [167]. Following anaerobiosis, -aminobutyric acid accumulated rapidly in
leaves due to increased decarboxylation of glutamate and decreased transamination of 4-aminobutyrate
[167]. Accumulation of -aminobutyric acid has also been observed in copper-deficient citrus plants
[163] and in the leaves of tomato plants infected with tobacco mosaic virus [168].
E. Diamines and Polyamines
Accumulation of the diamine putrescine and stimulation of its biosynthetic enzyme arginine decarboxy-
lase have been reported in several forms of environmental stresses, for example, deficiencies of nutrients
such as K and Mg [169] and Ca [86], salinity [86,170], water stress [158], ammonium toxicity [171], SO 2
fumigation [172], and acid stress [173]. It has been suggested that putrescine accumulation under stress
conditions has adaptive significance as it serves as an organic ion and can compensate partly for Kin
K-deficient plants [153]. Many plants accumulate the polyamines spermine, spermidine, and agmatine
under deficiency of K [169], P [174], Mg [86], S, Ca, and Mn [174]; salinity stress [170]; acid stress [165];
and ammonium toxicity [171]. Concentrations of these amines are very low in nonstressed plants, but
stress conditions induce a severalfold increase in their level. The type of polyamines accumulated de-
pends on the type of stress as well as the plant species. In detached oat leaves, osmotic treatment induces
a rise in the level of putrescine and stimulation of arginine decarboxylase activity. Other species show in-
creases in the levels of spermidine as well as spermine and decline of putrescine as well as its biosynthetic
enzymes. It is suggested that changes in the level of putrescine under stress conditions might be impor-
tant in regulating the ionic environment within the cell [173].
It appears from the preceding discussion that various environmental stresses induce accumulation of
soluble nitrogenous compounds, the extent and nature of the compounds accumulated depending on the
type of stress and the plant species. Levels of accumulation of some of these compounds, i.e., amino acids
and betaine, are associated with the stress sensitivity or tolerance of the plant species. For instance, salt-
tolerant plants possess inbuilt higher levels of the amino acid proline [153], glycine betaine [165], and -
alanine betaine [153]. Accumulation of these compounds is greater in tolerant plants than in sensitive
ones. The sensitive species have low levels of these compounds in the nonstressed plants and show less
accumulation under salinization [153]. Functions of these nitrogenous compounds are diverse. Amino
acids and betaine accumulating under salt and water stresses serve as osmoregulators, protect
biomolecules, decrease the water potential of the cytoplasm, and improve moisture uptake. Accumulation
of amides and the amino acid arginine appears to have a role in detoxifying ammonia, which attains ele-
vated levels during mineral deficiencies, water stress, low-temperature stress, etc. Rabe [156] has advo-
cated that most of the nitrogenous compounds that accumulate during environmental stresses serve to
detoxify the cell of ammonia.
V. CONCLUDING REMARKS
Nitrogen is one of the most essential elements in plant nutrition; however, its availability is limited under
harsh environmental conditions of salinity, water deficit, extremes of temperature, metal toxicities, etc.
These stresses considerably reduce NO 3 uptake, metabolism, and protein synthesis and drastically affect
650 DUBEY AND PESSARAKLI