For the increase of the occupation ratio of inoculum strain in the competition with indigenous rhizobia, the relationship between Rj-genotypes of soybean and the preference of Rj-cultivars for various types of rhizobia for nodulation was investigated by using the Rj2 Rj4-genotype of soybean isolated from the cross between the Rj2 Rj3-cultivar IAC-2 and Rj4-one Hill (Ishizuka et al. 1993: Soil Sci. Plant Nutr., 39, 79-86). Firstly, these Rj2 Rj4-genotypes were found to harbor the Rj3-gene. The Rj2Rj3Rj4-genotypes of soybean were considered to exhibit a more narrow microsymbiont range for nodulation than the Rj2 Rj3-and Rj4-cultivars. Therefore, rhizobia were isolated from the nodules of various Rj-genotypes of soybeans grown in soils, and the preference of the Rj2 Rj3 Rj4-genotype for indigenous rhizobia was examined. The nodule occupancy of serotype 110 was significantly higher in the bacteroids of the nodules from the Rj2 Rj3 Rj4-rgenotypes than in those from the other genotypes, non Rj-, Rj2 Rj3™, and Rj4-cultivars. These results demonstrated that the Rj2 Rj3 Rj4-genotype prefers more actively serogroup USDA110 to the others of rhizobia. Thus, Rj2 Rj3 Rj4-genotype is superior to non- Rj-, Rj2 Rj3-, and Rj4-genotypes for the formation of efficient nodules for nitrogen fixation.
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