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Studies on nitrogen fixation by antibiotic-resistant mutants of Azospirillum brasilense and their interaction with cheena (Panicum miliaceum L.) genotypes in calcareous soil

SUMMARY Azospirillum brasilense was treated with nitrosoguanidine and five antibiotic-resistant mutant strains isolated. Variations in growth, N2-fixation, ultraviolet-dark survival and level of antibiotic resistance were found between the mutant strains. Mutant strains STR 112 and KR 2051 showed maximum nitrogenase activity, glutamine synthetase activity and hydrogenase activity (H2 uptake) at 32 °C and 40 °C respectively. Inoculation of cheena genotypes with A. brasilense and its mutants led to significant increase in associative nitrogen fixation, dry weight of roots and grain yield compared with the uninoculated control, with significant strains × genotypes interactions in calcareous soil. It was also noted that under laboratory conditions Azospirillum inoculation may have produced its response by hormonal means and/or associative N2-fixation.