CONDITIONS FOR SUCCESSFUL RHIZOBIUM-LEGUME SYMBIOSIS IN SALINE ENVIRONMENTS

The Rhizobium-legume symbiosis in arid eco-systems is particularly important for locations where the area of saline soils is increasing and becoming a threat to plant productivity. Legumes, which are usually present in arid ecosystems, may be adapted to fix more N2 under saline conditions than legumes grown in other habitats. Legumes are known to be either sensitive or moderately resistant to salinity. The salt sensitivity can be attributed to toxic ion accumulations in different plant tissues, which disturb some enzyme activities. Among the basic selection criteria for salt-tolerant legumes and rhizobia are genetic variability within species with respect to salt tolerance, correlation between accumulations of organic solutes (e.g., glycine betaine, proline betaine, and proline) and salt tolerance, and good relationships between ion distribution and compartmentation, and structural adapatations in the legumes. Salt stress reduces the nodulation of legumes by inhibiting the very early symbiotic events. Levels of salinity that inhibit the symbiosis between legumes and rhizobia are different from those that inhibit the growth of the individual symbionts. The poor symbiotic performance of some legumes under saline conditions is not due to salt limitations on the growth of rhizobia. Prerequisites for a successful Rhizobium-legume symbiosis in saline environments include rhizobial colonization and invasion of the rhizosphere, root-hair infection, and the formation of effective salt-tolerant nodules. The possibility of exploring the Rhizobium-legume symbiosis to improve the productivity of saline soils is reviewed in this paper.