Fast‐Growing Rhizobium fredii are Poor Nitrogen‐Fixing Symbionts of Soybean1

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Abstract
Soybean‐Rhizobium genotype combinations with high rates of N2 fixation are useful in soybean (Glycine max L. Merr.) breeding programs. Nitrogen fixation was examined in 13 soybean cultivars and wild soybean (G. soja Sieb. & Zucc.) plant introductions inoculated with Bradyrhizobium japonicum strain 61A76 and four fast‐growing strains of Rhizobium fredii. Five‐week‐old plants were assessed for nodule morphology, acetylene reduction rate, nodule number, nodule fresh weight, and plant‐top dry weight. All fast‐growing strains formed effective nodules on cultivars ‘Peking’, ‘Virginia’, ‘Hardee’, and the G. soja plant introductions. In contrast, three of the fast‐growing strains formed ineffective nodules on the other cultivars. One strain, USDA191, formed effective nodules on all cultivars. No nodules were formed by the fast‐growing R. fredii on ‘Harosoy rj1 rj1’. Rhizobium fredii strain USDA191 fixed N, as well as the slow‐growing control with cultivars Peking, Virginia, ‘Harosoy 63’, and ‘Rampage’, and G. soja plant introductions PI 342622A and PI 101404B, but fixed less NJ than the slow‐growing control with cultivars ‘Evans’, ‘Williams’, Hardee, ‘Hill’, and G. soja PI 407217 and PI 81762.