Salt tolerance of Rhizobium species in broth cultures

Salt tolerance of five rhizobia strains was examined in broth cultures. Five levels of NaCl concentration were used and the optical density was taken as a measure for the vigour of bacterial growth. Rhizobium leguminosarum and R. meliloti were tolerant to high levels of salinity and growth curves in saline broth showed a similar pattern to the control level. Rhizobium japonicum, cowpea Rhizobium, and R. trifolii were intolerant to salt and showed a strong growth retardation with increasing salt concentration. Growth was inhibited at high levels of salinity. It is suggested that rhizobia sensitivity to salts may be partly responsible to the inhibition of nitrogen fixation by legumes growing under salt stress.