Effect of salt stress on soybean growth and nodulation under inoculation with soybean rhizobia

Salinity is one of the major problems when it comes to soil fertility and agricultural productivity. In this regard, this study aimed to elucidate the influence of four salinity levels (0, 20, 50, 100 mM NaCl) on the soybean growth and the nodulation by rhizobia inoculated on the soybean variety CNS. Additionally, the compatibility of soybean varieties with the seven representative strains of rhizobia was also investigated. Through the compatibility test, it was revealed that the Sinorhizobium (Ensifer) fredii USDA 191 and the three bradyrhizobia (Bradyrhizobium japonicum USDA 6(T), B. elkanii USDA 31, and B. diazoefficiens USDA 110(T)) showed compatible nodulation with the Rfg1 genotype soybean, whereas, the S. fredii USDA 192, 193, and 205(T) showed incompatible nodulation. The inoculation tests with bradyrhizobia (USDA 31, 110(T)) and sinorhizobia (USDA 191, 192, 193) indicated a better growth of soybean for bradyrhizobia than the sinorhizobia until 50 mM NaCl. Note that even though the symbiotic efficiency at each strain was not different among 0, 20, and 50 mM NaCl treatments, the symbiotic efficiency of bradyrhizobia was higher than the sinorhizobia at each NaCl treatment. The result of the mix inoculation test indicated that the nodule occupancy rate of strain USDA 191 increased significantly along with the NaCl concentration but strain USDA 110(T) showed an opposite response. These results indicate a positive relationship between the soybean growth and the nitrogen fixation ability of the inoculant under salt-stress conditions. Furthermore, the results obtained from the mix inoculation test partly describe the dominance of sinorhizobia under saline conditions.