Influence of Glycine spp. on Competitiveness of Bradyrhizobium japonicum and Rhizobium fredii

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Abstract
The displacement of indigenous Bradyrhizobium japonicum in soybean nodules with more effective strains offers the possibility of enhanced N 2 fixation in soybean ( Glycine max (L.) Merr.). Our objective was to determine whether the wild soybean ( G. soja Sieb. & Zucc.) genotype PI 468397 would cause reduced competitiveness of important indigenous B. japonicum strains USDA 31, 76, and 123 and thereby permit nodulation by Rhizobium fredii , the fast-growing microsymbiont of soybean. In an initial experiment, PI 468397 nodulated and fixed moderate amounts of N 2 with USDA 31 and 76 but, despite the formation of nodules, fixed essentially no N 2 with USDA 123. In contrast, PI 468397 formed a highly effective symbiosis with R. fredii strain USDA 193. In two subsequent experiments, Williams soybean and PI 468397 were grown in a pasteurized soil mixture or in soybean rhizobium-free soil and inoculated with both USDA 123 and USDA 193. In each experiment, more than 90% of the nodules of Williams contained USDA 123, while only a maximum of 2% were occupied with USDA 193. In contrast, in the two experiments, 16 and 11%, respectively, of the nodules produced on PI 468397 were occupied by USDA 123, while in both experiments 87% contained USDA 193. Thus, in relation to the cultivar Williams, which is commonly grown and used as a parent in soybean breeding programs in the United States, PI 468397 substantially reduced the competitive ability of B. japonicum strain USDA 123 in relation to R. fredii strain USDA 193.