Fourteen heat resistant mutant strains were isolated from a wild-type strain (PP201, Nod(+) Fix(+)) of Rhizobium sp. (Cajanus) by giving it a heat shock of 43°C. These mutant strains showed a greater increase in optical density (O.D.) and a higher viable cell count in both rhizospheric and non-rhizospheric soil at high temperature. Symbiotic studies showed that pigeon pea plants inoculated with a few mutant strains had ineffective nodules (Nod(+) Fix(-)) under controlled temperature (43°C) conditions, but under natural high temperature (40-45°C) conditions, the host plants infected with all the mutant strains showed higher total shoot nitrogen than the plants inoculated with the parent strain. Four mutant strains (HR-3, HR-6, HR-10 and HR-12) were found to be highly efficient for all the symbiotic parameters, and thus have the potential to be used as bioinoculants in the North-Western regions of India during the summer season.
Keywords: Growth; Heat resistant mutants; Rhizobium sp. (Cajanus); Survival; Symbiotic properties.