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.