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Competition between Bacterial Strains Effecting Nodulation in Soybeans¹

ABSTRACT

Competition between genotypes of Rhizobium japonicum was studied by using chlorosis-inducing strains 76 and 94 to facilitate identification of strains recovered from the nodules of soybean plants. Chlorosis-inducing strains were individually mixed in varying proportions with each of nine normal strains and the mixtures were used as inoculants on the Hawkeye and Lee varieties. The plants were cultured in the greenhouse under conditions previously described.

Technique studies indicated that with rare exceptions a single nodule contained only one bacterial strain and that chlorosis of sorghum seedlings produced by a water extract of a nodule was a reliable indication that the nodule contained a chlorosis-inducing strain.

Strain 76 had a pronounced competitive advantage over all normal strains regardless of the proportions of the strain in the mixtures. As little as 1.1% of strain 76 in the mixture with strain 38 caused 85% of the nodules. Strains 31 and 71 were more competitive against strain 76 than were the other normal strains. Chlorosis-inducing strain 94 was much less competitive against 8 of the 9 normal strains than was strain 76.

The extent of chlorosis of the soybean plants gave a rough estimate of the competitive relationships between strains. The varieties Hawkeye and Lee were essentially identical in response to the various mixtures.

¹ Contribution from the Soil and Water Conservation and Crops Research Divisions, Agricultural Research Service, USDA, Beltsville, Md.

² Bacteriologist, Research Agronomist, and Bacteriologist, respectively.

Received for publication June 27, 1960. Accepted for publication July 20, 1960.