Sodium is the major cation that accumulated in roots and stems as salinity increased (Meneguzzo et al., 2000). It is evident that salt tolerance is associated with low uptake of Na+(Santa-Maria and Epstein, 2001), partial exclusion (Colmer et al., 1995) and compartmentalization of salt in the cell and within the plant (Ashraf, 1994). The preferential accumulation in roots over shoots may be interpreted as a mechanism of tolerance in at least two ways.

First, maintenance of a substantial potential for osmotic water uptake into the roots and second, restricting the spread of Na+ to shoots (Renault et al., 2001). High Na+ levels in the external medium greatly reduce the physicochemical activity of dissolved calcium and may thus displace Ca2+ from the plasma membrane of root cells. In turn, displacement of Ca2+ from root membranes by Na+ affects Na/K uptake selectivity in favor of sodium. A low Ca2+ concentration under saline conditions may severely affect the functions of membranes as barriers to ion loss from cells (Boursier and Lauchli, 1990). Various organic and inorganic solutes such as K+, Na+, Cl-, proline, and glycinebetaine have been reported to contribute to such osmotic adjustment (Saneoka et al., 2001). Salinity inhibits the accumulation of K+ and Ca2+ in roots and stems. The negative effect of NaCl on the allocation of K+, Ca2+, and Mg2+ to the leaf tissues may contribute to their deficiency and the accompanying metabolic perturbations. The altered ion and water relations have a severe impact on the photosynthetic performance of the plant (Netondo et al., 2004). Many plants accumulate high levels of free proline in response to osmotic stress. This amino acid is widely believed to function as a protector or stabilizer of enzymes or membrane structures that are sensitive to dehydration or ionically induced damage. The salt stress caused increases in proline levels. Several investigations have shown that, besides other solutes, the level of free amino acids, especially proline, increases during adaptation to various environmental stresses. Plant salt tolerance has been generally studied in relation to regulatory mechanisms of ionic and osmotic homeostasis (Ashraf and Harris, 2004). In addition to ionic and osmotic components, salt stress, like other abiotic stress, also leads to oxidative stress through an increase in Reactive Oxygen Species (ROS), such as superoxide (02-), hydrogen peroxide (H2O2) and hydroxyl radicals (OH) (Mittler, 2002). It has been reported that most abiotic stress including NaCI salt stress impose injury in plants by osmotic stress, ionic stress and generating reactive oxygen species (Shalata and Tal, 1998). During oxidative stress, the excess production of Reactive Oxygen Species (ROS) causes membrane damage that eventually leads to cell death. For protection against ROS, plants contain antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (GPX) and Glutathione Reductase (GR) or as well as a wide array of non­enzymatic antioxidants (Blokhina et al., 2003). SOD is the major 02- scavenger and its enzymatic action results in H202 and O2 formation. The H202 produced is then scavenged by CAT and several classes of peroxidases. CAT, which is found in peroxisomes, cytosol and mitochondria, dismutates H202 to H20 and O2 (McKersie and Leshem, 1994). Sorghum is a salt tolerant plant therefore it seems that it uses some of the above mechanisms for its adaptation to salt and drought stress.