RESUMO
Rhizobium are nitrogen-fixing bacteria which possess the nif gene that codes for the nitrogenase enzyme involved in the reduction of atmospheric dinitrogen (N2) to ammonia. Thirty rhizobial strains were identified from ten groundnut plant root nodules collected from semi-arid regions of Rajasthan, India. The isolates were initially identified on the basis of morphological, biochemical, and molecular characteristics. These rhizobium strains were further screened for plant growth promoting activities. Twenty-eight strains were able to produce indole acetic acid, nine strains could solubilize phosphate, and twenty-nine strains exhibited positive results for siderophore and ammonia production. All the bacterial strains were able to efficiently nodulate the groundnut under pot conditions and based on multiple PGP activities six strains were selected for field evaluation. Field experiments confirmed the effectiveness of these selected rhizobium strains resulted in significantly higher nodule number, nodule dry weight, grain yield, and yield components of inoculated plants. Inoculation of the rhizobium strain GN223 followed by GN221 resulted in high yield and field efficiency. Isolation of effective microbial strains is the prerequisite to increase the yield which is evident from the field data of the present study. Hence, these strains might serve as proficient inoculants.