exchange, conductance, transpiration, and internal CO 2 increased in salinities up to 100 mol/m^3 and there-
after decreased. Chandrashekar and Sandhyarani [126] found a decrease in sugar and starch content at dif-
ferent increasing salinity levels in Crotalaria striata. Abd-El Samad and Shaddad [127] reported that the
sensitivity of a soybean cultivar was due to decreased saccharide content under NaCl salinity. Muthuku-
maraswamy and Panneerselvam [60] found that NaCl salinity decreased the accumulation of starch, sugar
content, and activity of -amylase in Raphanus sativusseedlings.
Bankar [64] reported that the carbohydrate content was more than the control value at all levels of
NaCl salinization and maximum content was recorded at ECe 7.5 mS cm^1 inCarthamus tinctoriusstem.
Under Na 2 SO 4 salinization, the content was less than control at all levels. In leaves, the carbohydrate con-
tent increased up to ECe 10.0 mS cm^1 and decreased further under NaCl salinity. With Na 2 SO 4 treat-
ment, the carbohydrate content increased up to ECe 12.5 mS cm^1 and decreased at higher salinity lev-
els. The results of Zidan and Elewa [128] revealed that soluble carbohydrate remained unchanged at low
and moderate levels of NaCl in four plant species of Umbelliferae. Thus, these investigators [122,123]
concluded that the maximum sugar content at site I during the rainy season may be due to low soil salin-
ity because of leaching of the salts during rainfall compared with summer and winter.
From the preceding account it is clear that in some plants soluble sugar increases, whereas in others
it decreases and in some others its content remains unaffected. In general, plants use soluble sugars as an
osmoticum under saline conditions. Hence, the plants that can tolerate low or medium levels of salt stress
synthesise more soluble sugars and tolerate salt stress. The plants that fail to increase soluble sugar
biosynthesis could not tolerate salts.
The nutritive pattern of plants is very important when fodder values and productivity are taken into
consideration. Root zone salinization presents a challenge to plant productivity that is effectively coun-
tered by salt-tolerant halophytic plants but, unfortunately, much less successfully by major crop plants.
The way in which salt affects plant metabolism was reviewed by Volkmar et al. [129]. Protein synthesis
and turnover in growing plants is a basic component of metabolic regulation that provides a way to vary
the enzymatic complement during the response to environmental conditions [130]. Protein is the most im-
portant constituent of cells from both structural and functional points of view. Changes in the ion content
of plant cells induced changes in the activity of certain metabolic systems. Such changes may have seri-
ous consequences for membrane proteins.
Vera-Estrella et al. [131] observed that increasing concentrations of NaCl stimulated the activities of
tonoplast and plasma membrane H-ATPases in Mesembryanthemum crystallinum. Immunodetection of
the ATPases showed that the increased activity was not due to changes in protein amount that could be
attributed to treatment conditions. A specific role for these mechanisms in salt adaptation is supported by
the inability of mannitol-induced water stress to elicit the same responses and the absence of enzyme ac-
tivity and protein expression associated with Crassulacean acid metabolism in the cells. Under conditions
of extreme salinity, proteins are precipitated. The protein content of various plant tissues generally de-
BIOLOGY AND PHYSIOLOGY OF SALINE PLANTS 573
TABLE 5 Seasonal Variations in Total Sugar (mg/g Dry Weight) and Crude Protein (% Dry Weight)
Contents in Halophytes Growing at Sites I and III
Total sugar Crude protein
Site I Site III Site I Site IIISpecies Ra Wa Sa RWS RWS RWS
Aeluropus lagopoides 34 —b ————12—————
Cressa cretica 19 11 17———211917———
Salsola baryosma 33 16 18 51018201615361622
Sesuvium sesuvioides 22 9 —1617—2016—19 9—
Sporobolus helvolus 29 8 14———1812 8———
Suaeda fruticosa 39 35 18 51425292519282517
Trianthema triquetra 10 4 — 515191917—211512
Zygophyllum simplex 18 25 ————1817————
(red strain)
aR, rainy; W, winter; S, summer.
bPlant not available.
Source: Ref. 124.