**1. Introduction**

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Relationships

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Processes. *Fiziol. Biokhim. Kul't. Rast.*, 41(4), 279-296.

Inoculation with efficient rhizobia at the ordinary dose does not increase appreciably the seed yield of soybean because the occupation ratio of the inoculated rhizobial strains in the nodules is very low due to competition with less efficient indigenous rhizobia [1, 2]. In order to increase the seed yield by rhizobial inoculation, the occupation ratio of the inoculated strains must be increased. The increase of the occupation ratio has been examined from vari‐ ous viewpoints such as improvement of inoculation method [3]. For the screening of effi‐ cient and competitive strains, a large number of useful strains had been isolated from mutagenized and recombinant rhizobia [4, 5, 6].

Furthermore, Williams and Lynch [7] who identified a non-nodulating line of soybean among the progenies from the cross between cvs. Lincoln and Richard, showed that the ab‐ normal nodulation reaction was controlled by a single recessive gene of the host plant, *rj*1. Thereafter, cv. Hardee was found to nodulate ineffectively with *Bradyrhizobium japonicum* belonging to the strains 3-24-44 and 122 serogroups [8, 9]. It was demonstrated that the inef‐ fective nodulation was controlled by a host dominant gene, *Rj*2. Furthermore, this cultivar was found to nodulate ineffectively with the *B. elkanii* strain 33 due to the presence of anoth‐ er *Rj*-gene, *Rj*<sup>3</sup> [10]. Cvs. Hill and Amsoy 71 harbor a gene (*Rj*4) that was responsible for the ineffective nodulation. These *Rj*4-cultivars were nodulated ineffectively with *B. elkanii* strain 61 [11]. Soybean plants harboring these *Rj*-genes (*Rj*-cultivars) were considered to restrict ef‐ fective nodulation with appropriate serogroups of strains and to prefer certain types of rhi‐ zobia for nodulation. If this assumption holds true, planting of *Rj*-cultivars could increase the populations of rhizobial strains highly compatible with those cultivars in soils. There‐ fore, the relationship between the *Rj*-genotypes of soybean and the preference of the *Rj*-cul‐ tivars for various types of *Bradyrhizobium* strain was examined [12, 13]. These *Bradyrhizobium*

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strains were classified into three nodulation types, type A, B, and C, based on the compati‐ bility with *Rj*-cultivars. Nodulation type A strains nodulated with almost all the cultivars except for the *rj*1-ones (non-nodulating lines) and were preferred by non-*Rj-*ones. Type B or type C strains nodulated soybean cultivars other than the *Rj*2*Rj*3-ones or *Rj*4-ones, respective‐ ly except for *rj*1-ones and were preferred by *Rj*4-ones or *Rj*2*Rj*3-ones, respectively.

This chapter deals with the developmental process and experimental trial of inoculation methods using effective *Bradyrhizobium* strains and various *Rj-*genotypes to increase the yield of soybean.
