Development of liquid inoculants for strains of Rhizobium tropici group using response surface methodology

Common bean (Phaseolus vulgaris L.) is the most important legume for human consumption in many countries of South and Central America, Asia and Africa. The crop can benefit from the biological nitrogen fixation process, especially when inoculated with elite rhizobial strains. Strains belonging to the “Rhizobium tropici group” are preferred because they show high tolerance to abiotic stresses, but their survival in liquid formulations is poor, limiting their use by farmers. In this study, response surface methodology (RSM) was used to develop liquid formulations for the commercial strains CIAT 899 (R. tropici) and PRF 81 (Rhizobium freirei). A significant interaction between dibasic potassium phosphate (K2HPO4) and yeast extract was observed, and to reach higher cell concentration, one should employ low concentrations of yeast extract and high concentrations of K2HPO4. A basic formulation which may represent the basis for the development of liquid inoculants for the common bean crop was developed. Key words: Biological nitrogen fixation, inoculation, common bean, Phaseolus vulgaris, Rhizobium freirei.