Variability among Soybean Genotypes in Response to Nodulation by a Rhizobitoxine-Producing Strain of Bradyrhizobia

Nodulation by rhizobitoxine-producing (RT⁺) strains of Bradyrhizobium elkanii (syn. Bradyrhizpbium japonicum; DNA homology group II) has been shown to reduce shoot growth, N₂ fixation, and seed yield of ‘Forrest’ soybean (Glycine max [L.] Merr.). We conducted greenhouse and field experiments to assess the frequency of these negative responses among selected soybean cultivars. In the greenhouse, 38 cuitivars representing a range of yield potentials, maturity groups, and growth habits were inoculated with B. elkanil strain USDA 94 (RT⁺) or B. japonicum strain USDA 110 (RT⁻). Large differences in tolerance to nodulation by RT⁺ bradyrhizobia were noted, as estimated visually and by comparing shoot weights (40 d after planting) of plants inoculated with the two strains. For the field study, two tolerant (Stafford and SS FFR 565), two moderately sensitive (Pharaoh and Williams 82), and two highly sensitive cultivars (Forrest and Hutcheson) were chosen. Seeds sown to vermiculite under greenhouse conditions were inoculated with either USDA 94 or a suspension of soil taken from the study site containing the indigenous bradyrhizobia. After 23 d, the nodulated seedlings were transplanted to 1.8-m² field microplots. Shoots were harvested at the R7 growth stage and analyzed for various response parameters. Only Forrest, Hutcheson, and Pharaoh were significantly affected by inoculation with USDA 94, which decreased seed yield, pod number, vegetative growth, and the amount of N₂ fixed. These results support the hypothesis that significant genotypic variability exists in the response of soybeans to nodulation by RT⁺ bradyrhizobia. The greenhouse screening procedure can provide useful predictions of the field response of tolerant and highly sensitive cultivars, but may not be accurate for moderately sensitive cultivars. Contribution of the Delaware Agric. Exp. Stn. Misc. Paper no. 1502. Supported by a grant from the Delaware Soybean Board, Dover.