Nodulation efficiency of Bradyrhizobium japonicum strains with genotypes of soybean varying in the ability to restrict nodulation

Competition from existing soil rhizobia has limited the benefits from nitrogen fixation for soybean grown in the American Midwest. A strategy being considered to overcome this problem is the use of varieties that are restricted in nodulation with soil strains, but nodulate normally with inoculant bradyrhizobia. In this study we examine the efficiency in nodulation of strains of Bradyrhizobium japonicum that have been reported as restricted in nodulation with specific genotypes of soybean, using a root-tip marking procedure in growth pouches. When B. japonicum USDA110 was applied to the soybean cultivars Hardee and Fiskeby V at the rate of 3.50 × 104 cells/pouch, more than 75% of the plants were nodulated above the root-tip mark, and average uppermost nodule position was above the root-tip mark. By contrast, when this strain was applied in similar concentration to the soybean cultivar Peking, few plants developed nodules above the root-tip mark, and the average position of the uppermost taproot nodule was ...