PREFERENCE IN THE NODULATION OF PHASEOLUS-VULGARIS CULTIVAR RAB39

The low nodule occupancy achieved by superior nitrogen-fixing inoculant strains is a problem in the production of many traditional legume species, including Phaseolus vulgaris. Cultivars that select for inoculant strains, rather than nodulate with ineffective indigenous rhizobia, offer one approach to the resolution of this problem. In this study we identify a bean cultivar, RAB39, that nodulates preferentially with Rhizobium tropici, including the inoculant-quality strain UMR1899. This preference in nodulation was not affected by temperature or pH, though strain preference by a second, alkaline-producing Rhizobium etli strain did vary markedly with temperature. When RAB39 was inoculated with 1:1 mixtures of UMR1899 and seven serologically distinct strains of Rhizobium etli, more than 75% of the nodules formed by each strain pair contained UMR1899. A number of studies have reported close correlations between the percent nodule occupancy and speed of nodulation, measured using the root-tip-marking procedure. Exceptions to this correlation were evident in the present study, suggesting that host preference in nodulation was not related to differences in the parameters normally used to estimate speed of nodulation. The preference of RAB39 for R. tropici, a species of Rhizobium that occurs at low frequency in most bean soils, and especially for the inoculant-quality strain UMR1899, provides a tool to overcome the lock of response to inoculation in common bean.