Symbiotic Interactions between Strains of Rhizobium phaseoli and Cultivars of Phaseolus vulgaris L.1

Several cultivar‐strain combinations of the Phaseolus vulgaris L.‐Rhizobium phaseoli association were evaluated for N2 fixation, N assimilation, and primary (biomass) production. Plants were grown in a greenhouse. Nitrogen fixation rates were estimated from H2‐ and C2H4‐production data. The relative efficiency of N2 fixation was less than 0.60 indicating a loss of reducing power to H2 evolution. Copious production of H2 by all effective strains used in this study suggests that energy losses through H2 evolution may limit the productivity of the Phaseolus‐Rhizobium symbiosis. Assimilation of N by the symbiotic association in response to N2 fixation was influenced by both strain and cultivar effects. Analysis of variance showed significant differences due to both symbionts in most growth parameters measured. However, there was no significant correlation (P < 0.05) between N assimilation and the estimated rates of N2 fixation in the various effective host‐endophyte combinations. Assimilated N is a better, more direct measure of symbiotic efficiency than the estimate of N2‐fixation rate by C2H4 and H2 production, and the merits of different growth parameters as measures of symbiotic effectiveness are compared and discussed.