Identification of North American soybean lines that form nitrogen-fixing nodules with Rhizobium fredii USDA257

Rhizobium fredii produces nitrogen-fixing (Fix+) nodules on primitive soybean lines, but most strains do not form such structures with the small number of agronomically advanced lines that have been tested. We systematically evaluated the ability of R. fredii USDA257 to produce Fix+ nodules on 197 soybean lines available in the midwestern United States. Thirty-four of 197 such lines were Fix+. The frequency of this response was positively correlated with increasing maturity group. The acetylene-reduction rate of one advanced cultivar, Davis, was greater than that of the primitive cultivar, Peking. Our data indicate that the capacity to nodulate effectively with USDA257 is widespread in contemporary North American soybean lines.Key words: Acetylene-reduction, nodulation, Rhizobium, soybean