These compounds also contribute to osmotic balance in the cytoplasm when electrolytes are lower in cy-
toplasm than vacuoles and play a protective role for enzymes in the cytoplasm in the presence of high
level of electrolytes [155].
A. Amino Acids
Plants subjected to most stressful environments show increased levels of total free amino acids
[1,3,6,12,15,35,77,153,156,157]. Proline seems to accumulate in larger amounts than the other amino
acids in response to salinity [153,157], water stress [158], temperature stress [159], mineral deficiency
[154], pathogenesis, and anoxia [160]. In most of the plants studied, salinity and water stresses caused
substantial increases in proline levels of the plant tissues. Proline along with the other soluble nitrogenous
compounds serves as an osmoregulator in plants. A proline level up to 600 mM did not inhibit enzyme
activities [155]. Higher plants differ markedly in their capacity to accumulate proline. Proline-accumu-
lating species when grown in NaCl-free environments contain low levels of proline, but the level in-
creases in the presence of salinity [153]. In salt-stressed plants, proline accumulation results from its in-
creased synthesis and decreased utilization [153]. As a result of water stress, free proline accumulated
appreciably in leaves and other tissues. The functional role of proline accumulation appears to be as a cy-
toplasmic osmoticum to lower cell water potential, provide hydration to biopolymers, and serve as an en-
ergy and N source under adverse environmental conditions [161].
In addition to proline, other amino acids that accumulate under salt and water stress are arginine,
glycine, serine, alanine, leucine, and valine. Salt-stressed rice plants accumulated arginine, alanine,
leucine, and valine in addition to proline [157]. Under salinity stress conditions, the level of these
amino acids is higher in salt-tolerant plants than in salt-sensitive species [157]. Water-stressed plants
accumulate proline, alanine, arginine, and phenylalanine, which have a distinct correlation with the
stress tolerance mechanism [162]. Crop plants such as barley and radish, when grown under low-tem-
perature conditions, accumulated a substantial amount of proline [153,159]. Other amino acids, such as
serine, glycine, and alanine, also accumulated appreciably in several plant species grown under low-
temperature conditions [153]. Barley and radish plants exposed to high temperature accumulated pro-
line [159]. In lemon and orange leaves, infection by Phytophthoraspp. or anaerobiotic conditions
caused increased levels of proline, arginine, and total free amino acids [160]. Most of the mineral de-
ficiencies cause increases in the level of free amino acids. The types of amino acids accumulated de-
pend on the nature of the mineral deficiency [156]. Copper deficiency caused substantial accumulation
of proline and serine in citrus plants [163], whereas iron deficiency resulted in accumulation of argi-
nine, lysine, histidine, and serine in citrus and macadamia plants [164]. The basic amino acid arginine
has been shown to accumulate under a variety of stress conditions such as Mg, K, S, Ca, Fe, Mn, and
Zn deficiencies, osmotic stress, acid stress, excess ammonium in the growth medium, and infection by
pathogens [156].
B. Quaternary Ammonium Compounds
Among quaternary ammonium compounds, glycine betaine (trimethylammonio-2-acetic acid) accumu-
lates most widely in stressed plants. It is the predominant nitrogenous compound accumulating under
salinity stress [165]. Together with proline, glycine betaine serves as a compatible cytoplasmic solute and
has an important role as an osmoregulator in salinity stress. Betaine levels up to 1.0 M do not inhibit en-
zyme activity in vitro. Another compound, -alanine betaine (trimethylammonio-3-propanoic acid), also
accumulates in laboratory-grown salinized plants and in plants growing in saline habitats. Glycine betaine
accumulated in many species, whereas -alanine betaine was restricted to halophytes of Plumbaginaceae
[163]. Plants differ in their capacity to accumulate betaine. Certain other quaternary ammonium com-
pounds such as stachydrine, homostachydrine, and trigonelline accumulated in alfalfa plants in response
to water stress, salinity, and abscisic acid treatments, respectively [166].
C. Amides and Imino Acids
Glutamine and asparagine together with amino acids and certain imino acids such as pipecolic acid and
5-hydroxypipecolic acid accumulate in plants subjected to saline stress; however, their level is much
lower than that of proline or betaine. In plants such as Agrostis stolonifera, asparagine accumulation was
NITROGEN ABSORPTION UNDER STRESS 649