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 nearly 30 mm below this mark. Nodulation was improved at higher rates of inoculation, but even when 3.50 × 106 cells were applied to each pouch, less than 50% of the plants were nodulated above the root-tip mark. Bradyrhizobium japonicum strain CB1809 (=USDA136) was also efficient in nodulation with cv. Fiskeby V, but with cv. Hardee, less than 65% of plants were nodulated above the root-tip mark, irrespective of inoculation rate. Because restriction of nodulation with the strains initially tested was not absolute, we examined the patterns of nodulation obtained following the inoculation of two restriction hosts, Peking and PI371607. In pure culture, serogroup USDA110 strains failed to induce significant taproot nodulation of cv. Peking in Leonard jars, but did induce lateral root nodulation. However, in a glasshouse experiment contrasting soil- and seed-applied inoculant, lateral-root nodulation of the restriction host PI371607 by USDA123 was not significant.Key words: Glycine max, competition, restriction, nodulation.
Organization and characterization of genes essential for symbiotic nitrogen fixation from Bradyrhizobium japonicum I110.