the increased osmotic potential of the plants [65]. However, with the increase in salt concentration and
the decrease in soil moisture, plants try to adjust themselves to drought by accumulation of salts. Thus,
the accumulation of salts in plants decreases their osmotic potentials to the level of highest stress, so
that plants are able to take up maximum water during the hot summer, resulting in a gradual increase
in their osmotic potentials. Some plants exhibit lower values of osmotic potentials during winter peri-
ods associated with low temperature and/or high salinity. Thus, it is clear from the study of Mohammed
[23] that the water relations of plants are directly related to the amount of moisture present in the soil
[135,136]. The study [23] showed higher stomatal density and opening during the rainy season when
soil water status remained higher (Table 6) [137]. More water was lost when water was abundant in the
soil, that is, during the rainy season.X. SUMMARY AND CONCLUSIONSSalinity, water, and temperature are most critical factors that determine the vegetation pattern of a region.
The excess of salts in the soil inhibits plant growth. Halophytes survive under conditions of high salinity
and exhibit succulence, which might resort to other physiological adaptations to overcome the adverse
saline environment in the soil. The ionic and toxic effects of various salts, especially of NaCl, play a ma-
jor role in halophytism. Salinity is known to affect many aspects of plants and induce numerous changes
in their morphology. Most of the halophytes avoid salinity, some evade it, and a few others tolerate it. Xe-
rosucculents are characteristics of halophytes showing a thick cuticle and a cover of waxy layers, such as
Suaeda fruticosa,Salsola baryosma, and Haloxylon recurvum. The accumulation of proline in plants is
correlated with the extent of the water stress in the plant. In halophytes, a positive correlation is seen be-
tween the proline content and the amount of Naand Clin the cell sap. Salt stress induces accumula-
tion of more proline in halophytes and perhaps plays an essential role in their survival. Ion uptake by
plants is largely dependent upon the availability of the ions in the soil. Higher level of minerals in halo-
phytes has been observed during dry periods. Although NaCl is the major salt present in most salt-affected
soils, other salts such as MgCl 2 , MgSO 4 , and Na 2 SO 4 are also present and play a combined role in the salt
tolerance of a species at the time of seed germination. Seed germination is controlled by both osmotic and
ionic factors. Maximum germination of seeds in halophytes has been reported during the rainy season due
to leaching of salts in deeper soil layers through rainfall. Because of the leaching action, there is a de-
crease in soil salinity. Salinity also affects almost all the aspects of plant metabolism. Maximum values
of carbohydrate and crude protein in halophytes have been observed during the rainy season, when plant
water status is higher than in winter or summer. The maximum stomatal opening and water loss are noted
during the rainy season.576 SEN ET AL.TABLE 6 Seasonal Variations in Stomatal Density (mm^2 ) in Leaves of Different Plant Species from Two Sitesa
Pachpadra (site I) Jodhpur (site III)
RWS R WSSpecies U L U L U L U L U L U L
Cressa cretica 129 125 53 39 44 35 —b —————
(23) (24) (17) (11) (9) (7)
Salsola baryosma — 205 — 53 — 35 — 188 — 99 — 103
(13) (17) (12) (17) (14) (17)
Sesuvium sesuvioides 56 52 — — — — 43 20 — — — —
(35) (33) (30) (22)
Suaeda fruticosa 46 43 30 29 25 35 100 82 59 69 52 39
(33) (28) (24) (24) (16) (27) (23) (16) (27) (23) (23) (25)
Trianthema triquetra 59 — 40 — — — 102 — 56 — 63 —
(31) (25) (28) (24) (26)
Zygophyllum simplex 59 63 26 29 — — — — — — — —
(red strain) (33) (33) (15) (16)
aValues in parentheses are stomatal index (SI). R, rainy; W, winter; S, summer; U, upper; L, lower.
bPlant absent.
Source: Ref. 137.