adapt to saline conditions by conserving water. Plants also adapt to saline conditions by decreasing the
number of stomata, which leads to conservation of water.
According to Perera et al. [147], x-ray microanalysis revealed that the sodium content of the stom-
atal guard cells of Aster tripoliumremained much lower than that of other leaf cells when the plants were
grown at high salinity levels. In contrast, large amounts of sodium accumulated in epidermal and sub-
sidiary cells and particularly in the mesophyll tissue, suggesting that a mechanism exists to limit the ex-
tent of its entry into guard cells. Even in plants grown at high salinity, the content of potassium was much
higher than that of sodium in the guard cells, consistent with the view that this is a major ion involved in
determining stomatal movements in this halophyte. It is suggested that the acquisition by the guard cells
of some ability to restrict the intake of sodium ions may be an important component of sodium-driven reg-
ulation of transpiration and hence of salinity tolerance in A. tripolium[147]. A detailed study of stomatal
behavior to determine monthly and seasonal variations in the state of water balance in some saline plants
of the Indian desert and their relationship with soil moisture conditions was carried out by Mohammed
[23,137] and Sen and Mohammed [138].
Rainwater is the only source of available moisture in the desert of northwestern Rajasthan, India. Al-
though the monsoon season starts here by mid-June and extends to October, the rains are very erratic and
scant. The occurrence of a rather long intermission between successive rain showers, sometimes ranging
from a few days to weeks, is not uncommon here. Whatever rainwater is retained by the soil is exploited
by the roots of annual and perennial species from the months of June and July to November and Decem-
ber. After this, annual species start to disappear because of moisture scarcity in the upper soil layers and
their inability to exploit moisture from the deeper soil layers because of their shallow root system. By this
time, perennial species also start showing various symptoms of water shortage, which are reflected by a
remarkable reduction in the transpiring surface. The maximum soil moisture was recorded in the rainy
season. All halophytic annual species, such as S. sesuvioides,T. triquetra, and Z. simplex, completely dis-
appear with the depletion of soil moisture. Winter showers and premonsoon rains, although scant in quan-
tity, play a significant role in improving the water status of soil and plants [137,138].
In order to maintain lower water potentials within the cell, dicotyledon halophytes normally make
the necessary osmotic adjustment by accumulating Naand Clions. The cellular basis of salt tolerance
in halophytes depends upon the compartmentation of ions necessary for osmoregulation in vacuoles and
upon osmotic adjustment of the cytoplasm by compatible solutes. The central role played by Naand Cl
in osmotic adjustment suggests that the transport of these ions and its regulation must be of primary im-
portance in the physiology of the plant as a whole. The decreases in transpiration rate per unit area of leaf
help to lower the ion input into leaves. Any linked reductions in photosynthesis appear to be due to de-
creases in stomatal frequency [148]. In the grasses, potassium and sugars are used to make the osmotic
adjustment. However, in succulent halophytes, the major use of organic compounds such as vacuolar so-
lutes seems to be precluded on the basis of energetic grounds [37]. Kurban et al. [149] reported that with
increasing salinity levels, the membrane permeability decreased in Alhagi pseudoalhagi, whereas in Vi-
gna radiatait slightly increased at 9.1 dS m^1. The leaf water potential and the osmotic potential de-
creased in both plants along with the seawater salinity levels. The contributions of organic and inorganic
solutes to the osmotic adjustment differed in different species.
The physiological traits involved in leaf water relations were evaluated in Avicennia germinans
seedlings by Suarez et al. [150]. They concluded that the leaves of seedlings adapt to hypersaline soils by
increasing solute concentration and cell elasticity. It is suggested that both processes allow leaf water up-
take and turgor maintenance over a large range of soil water potential.
All the halophytic species studied [23,24] could adjust themselves by changing their osmotic po-
tentials rapidly with a greater range in osmotic potentials of surrounding soil. This is in agreement with
Waisel (12), who stated that it is probably true that the great majority of the halophytic plants belong
to the adjustable group and that their osmotic adjustment occurs rapidly. Recovery from osmotic stress
occurs faster in salt-accumulating halophytes than in salt-enduring halophytes [151]. Osmotic adjust-
ment or osmoregulation enables plants to maintain growth as the plant water potential decreases. Ad-
justment occurs through decreases in osmotic potential by solute accumulation in the cells as the leaf
water potential decreases. In this condition the net result is that the cell turgor potential is kept rela-
tively high, thus maintaining turgor-dependent processes, such as leaf growth and stomatal opening
[152]. Further, it emerged from our study [23,24] that during the rainy season, the higher moisture in
the soil and the leaching of salts resulted in an increase in the osmotic potential of the soil that led to
BIOLOGY AND PHYSIOLOGY OF SALINE PLANTS 575