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Significance of Incompatibility Reactions of Rhizobium japonicum Strains with Soybean Host Genotypes¹

A sample of soybean [Glycine max (L.) Merr.] plant introductions from Asia, representing maturity groups 00 through X, was tested for the presence of the Rj₂ and Rj₄ genes. Both of these genes condition ineffective nodulation responses: Rj₂, specifically with the cl and 122 serogroups of Rhizobium japonicum; Rj₄, with R. japonicum strain 61. Rj₂ was present in 19 of the 851 fines tested. The 19 lines were in maturity groups III to VIII and most originated from China and Japan. Rj₄ was present in 261 of the 851 lines tested, and was most prevalent in lines from southeast Asia. This gene occurred in lines from maturity groups 00 to X and was most frequent in Group X with 94% of the lines possessing it. This evidence of geographical differences in gene frequencies suggests that the combinations of host and Rhizobium ecotypes which coevolved in Asia may have been disrupted through separate introduction of the host and microsymbiont to the New World. If this is the case, N₂ fixation may be improved by reassembling the host/strain combinations that evolved in Asia. Use of incompatibility responses may enable soybean breeders to control host specification of symbiosis with genetically improved Rhizobium strains. Host compatibility with indigeneous Rhizobium strains may profoundly affect performance of some plant introductions in evaluation nurseries on soils of low N fertility. Management of N fertility in such nurseries will depend on breeders' objectives.

Key Words: Glycine max (L.) Merr. • Symbiosis • Coevolution • N fixation • Genetics • Nodulation

² Research geneticist and plant physiologist (support scientist), Cell Culture and Nitrogen Fixation Lab., Plant Physiology Inst., AR, SEA, USDA, Beltsville, MD 20705.

Received for publication July 20, 1979.