Physiological behavior and antibiotic response of soybean (Glycine max L.) nodulating rhizobia isolated from Indian soils

A total of 92 soybean nodulating rhizobia were isolated from different agro regions and screened for sensitivity against 8 rhizobiophages. Only 33 showed susceptibility with phage strains. Nineteen rhizobial isolates selected, including 9 isolates having high lytic behaviour and 10 resistant to all phages, were subjected to study on symbiotic effectiveness. Strains ALSR12, KNSR16, USDA123 and USDA94 showed highly significant interaction with host plant grown under axenic conditions. Strains USDA123 and ANSR12 had distinct lytic reaction with phage strain SR1 and SR2, respectively. Plaques formed by SR1 on USDA123 were quite clear with sharp margin, whereas central halo was surrounded by turbid concentric by SR2 on strain ALSR12. In view of generation time varying from 9.2 h (USDA94) to 12.6 h (ALSR12), they were marked as slow grower. Of the different carbon sources used, arabinose served as a potent carbon source for supporting highest growth of these strains. As compared to the other strain, ALSR12 was able to grow at 45°C and capable to tolerate a higher range of acidity and alkalinity. Strains ALSR12 and USDA123 showed highest resistance level with the antibiotics streptomycin and gentamicin, respectively. Ex-planta N2-ase activity was determined only in USDA94 and ALSR12. This study provides means for conducting a systematic and scientific investigation towards obtaining a reliable understanding of the bradyrhizobial diversity in terms of their physiological behaviour.   Key words: Soybean, rhizobia, rhizobiophage, diversity.