Young leaves and stems of Atriplexspp. contained quite high values of sodium and chloride in com-
parison with mature parts because of fleshiness and the presence of more salt bladders in the epidermis of
tender plant parts. Young leaves of A. triangularisalso contained more Naand Clthan mature leaves
with intact bladders [26].
Halophytic cells need to have high osmotic pressure and at the same time prevent the excess ions
from inhibiting the enzymatic processes. If the excess ions are stored in the vacuole, the metabolic activ-
ity can be carried on in the cytoplasm, where the ion content is lower. The lower salt concentration pre-
vents organelles such as chloroplasts from being damaged by excess ions [5]. When a change in
metabolism results in a change in the ability to resist stress conditions, anthocyanin may develop in the
leaves or stems of plants. Some halophytes of the Indian region such as S. fruticosa,S. baryosma,Tri-
anthema triquetra, and Zygophyllum simplexexhibit these characteristics under osmotic stress conditions.
The effect of salinity as a specific and dominant factor in a saline environment determines to a great
extent the ability of halophytes to reproduce and perpetuate. Information regarding the germination be-
havior of Indian halophytes is still scant. Rajpurohit and Sen [42] concluded that under field conditions
the highest germination percentage in C. cretica,S. fruticosa,S. baryosma,Sesuvium sesuvioides, and T.
triquetracan be achieved after rain that is heavy enough to leach out the salt from the closed environment
of the seeds. Several authors found that the increase in salinity leads to dormancy of seeds in halophytes
and glycophytes [42,43].
In spite of the preceding discussion, how halophytes handle salts is still not fully understood. Breckle
[41] and Weber [37] stated that we know many mechanisms, and it has been estimated that over 1000
genes are turned on or off as a response to salinity. This does not count the genes that are “hard wired”
into the enzyme system.
IV. MECHANISMS OF SALT TOLERANCE
Whether it is drought, cold, heat, salt, metal, or any other (pollution stress) stress or a combination of
some or all of these, the end result is a dehydration stress. Levitt [44] speculated that plants may have
a general mechanism of resistance to every stress. At the morphological level, wilting, leaf rolling, and
decrease in stomatal aperture, succulence, leaflessness, etc. constitute a general mechanism for con-
serving water. At the physiological level, reduction in evapotranspiration and decrease in water poten-
tial are major manifestations.
Biochemical studies have revealed similarities in processes induced by various abiological stresses,
leading to accumulation of compounds such as ascorbate, glutathione, -tocopherol, betaine, proline and
other amino acids, quaternary ammonium compounds, polyamines, sucrose, polyols (mannitol, sorbitol,
pinitol), and oligosaccharides in plant tissue. In addition, changes in the activity of certain key enzymes,
gene expression, and biosynthesis of abscisic acid (ABA) have been noted [45].
For many plants there is a correlation between increases of metabolites and osmotic stress toler-
ance, but the mechanisms that cause this protection are not clear. During salt stress, cells retained more
of the six-carbon polyols than glycerol. To understand the role of glycerol in salt tolerance, salt-toler-
ant suppressor mutants were isolated from the glycerol-deficient strains, and results compare with the
“osmotic adjustment” concept typically applied to accumulating metabolites in plants. The accumula-
tion of polyols may have dual functions: facilitating osmotic adjustment and supporting redox control
[46].
High selectivity during nutrient uptake is one mechanism used by halophytes to avoid a deficiency
of essential ions such as K, Mg, and Ca. The results of Koyro et al. [47] suggest that Laguncularia race-
mosadeveloped an intercellular and intracellular K and Ca buffer that enabled it to grow and to exclude
Na. However, higher salinities led to high energy metabolism and decreased growth, starch content, and
nectar production.
The most characteristics associated with salt tolerance are under polygenic control, as observed by
Jefferies and Rudmik [48]. However, halophytes exhibit a high level of physiological plasticity in this re-
spect, and there is evidence in some that morphogenetic changes occur in response to salinity [48].
Wu and Seliskar [49] stated that the response of the plasma membrane Hadenosinetriphosphatase
(ATPase) in Spartina patenssuggests that this species has evolved mechanisms that can regulate this im-
portant enzyme when cells are exposed to NaCl.
566 SEN ET AL.