Selection of symbiotic efficient and high salt-tolerant rhizobia strains by gamma irradiation

Gamma irradiation was used on Rhizobium tropici CIAT 899 and Mesorhizobium ciceri 835 nodulating, respectively, common bean and chickpea in order to select individual rhizobia mutant highly tolerant to salt and symbiotically effective. Radiosensibility analysis showed that 800 Gy was the highest dose of gamma rays supported by tested strains. The irradiated clones at 700 and 800 Gy were more tolerant to salt stress and phosphorus deficiency compared to control strains, which proved that gamma irradiation was effective in changing physiological and phenotypic characteristics via random mutations. Likewise, biochemical analysis showed that tested clones had a variable superoxide dismutase and catalase activities especially in Mesorhizobium ciceri 835. The increase of antioxidant enzyme activities in some mutants was related to tolerance acquisition that protects effective symbiosis against oxidative damage induced by abiotic stresses. Thus, efficient symbiotic nitrogen fixation rhizobia under salt stress were identified.