Nitrogen symbiotic fixation in legumes has proved to be limited, especially in semiarid conditions where salinity is often a temporary problem because of the poor quality of water irrigation during the dry season. Legumes are classified as salt-sensitive crop species (L ̈auchli, 1984) and their limitation in productivity is associated with lower growth of the host plant, poor symbiotic development of root-nodule bacteria (Georgiev and Atkins, 1993) and, consequently, a reduction in the nitrogen-fixation capacity (Delgado et al., 1993). Under stress conditions, biochemical and physiological mechanisms such as accumulation of compatible osmolytes proline, glycine betaine, sucrose and mannitol (Zhu, 2002) are switched on to protect major processes such as cell respiration, photosynthetic activity, nutrient transport, and nitrogen and carbon metabolism.