In order to investigate the correlation between the preference of soybeans (Glycine max L.) for Bradyrhizobium japonicum for nodulation and their Rj-genotypes, rhizobia were isolated from eight cultivars (non-Rj-cultivars: T202, Tol-1, and CNS-J; Rj2Rj3-ones: Hardee, IAC-2, and Bonminori; Rj4ones: Hill and Amsoy 71) grown in the field and their cornpatibilities with Hill and IAC-2 were examined. Sixty-three percent of the nodules of the Rj2Rj3-cultivars were occupied by rhizobia of nodulation-type C, which were compatible with the Rj2Rj3-cultivars but incompatible with the Rj4-one. Sixty-one percent of the nodules of the Rj4-cultivars were occupied by isolates of nodulation-type B, which were compatible with the Rj4-cultivars but incompatible with the Rj2Rj3-one. Non-Rj-cultivars preferred rhizobia of nodulation-type A, which were compatible with both Rj-cultivars. To estimate the effect of planting of Rj-cultivars on the rhizobial population in soils, Rj2Rj3-(IAC-2) and Rj4-(Hill) cultivars were inoculated with dilutions of supernatants of suspended soils in which the Rj4-cultivar (Fukuyutaka) and non-Rj-one (Akisirome) were planted in the preceding year, and the nodule number was counted. Based on the results, it was shown that the planting of the Rj4-cultivar increased the population of compatible rhizobia with the Rj4-cultivar and decreased that of the compatible rhizibia with the Rj2Rj3cultivar. Planting of the non-Rj-cultivar did not influence remarkably the rhizobial population in soil. The cultivars IAC-2 (Rj~Rj3), Hill (R j4), and Orihime (non-R j) were planted in the soils used in the above experiment, and the nodulation-types of the isolates from the nodules were identified. Marked preference of IAC-2 for type C rhizobia was observed in the soil planted with the non-Rj-cultivar (87%), while for the type A ones in the soil planted with the Rj4-eultivar (87%). IAC-2 significantly preferred the type C rhizobia in the soil containing type C rhizobia, but the type A rhizobia when the number of type C rhizobia was very small as in the Fukuyutaka (Rj4)-cultivated soil. On the basis of these results, it was suggested that the R j-genes were involved in the inhibition of effective nodulation by certain serogroups of rhizobia as well as in preference for appropriate rhizobia in relation to nodulation and increase of those populations in soil.

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The compatibility between rhizobia and host plants for nodulation was determined based on the genetic and physiological properties of both symbionts. It has been observed that soybean varieties carrying the Rj-gene were not nodulated effectively by certain strains or groups of rhizobia. Soybeans carrying the Rj2-gene, Rj2-varieties, were found to nodule ineffectively by the rhizobial strains belonging to the 3-24-44 and 122 serogroups (Caldwell 1966). In the same way, Rj3-and Rj4-varieties were found to nodule ineffectively by strains USDA 33 (Vest 1970) and USDA 61 (Vest and Caldwell 1972), respectively. Keyser et al. (1982) reported that Rhizobium fredii (R. fredii) also effectively nodulated an unimproved Chinese soybean cultivar, Peking and wild soybean, Glycine soja, but that it was largely incompatible with common commercial cultivars of soybeans for nodulation. Thereafter, Devine (1985) demonstrated that the majority of soybean plants introduced from Southeast Asia nodulated effectively with R. fredii USDA 205. Therefore, it was assumed that the nodulation-conditioning genes which enable to nodulate effectively with R. fredii were involved in the nodulation of these soybeans. Although most of these plants carries the Rj4-gene (Devine and Breithaupt 1980), the relation between effective nodulation with R. fredii and the presence of Rj-genes in host plants had not been elucidated.In legumes, micro nutrients playa significant role in nodulation and nitrogen fixation. Iron deficiency depressed nodule initiation before host plant growth, resulting in a decrease in the nodule number and mass (Tang et al. 1990). Nodulation of soybeans and peas was considerably increased by the application of molybdenum (Hagstrom and Berger 1963). The high concentrations of iron, cobalt, and calcium in soybean root hairs which were target cells for infection were discussed in relation to the high requirement of B. japonicum for these elements (Werner and Kuhlmann 1985). Physiological role of micro nutrients in nodulation still remains to be elucidated.In this paper nodulation with R. fredii USDA 193 was examined in terms of the compatibility with various Rj-genotypes of soybeans and in relation to the effects of nutrient levels in culture media on the nodulation. Materials and methodsPlants and rhizobium. The soybean (Glycine max L. Merr.) varieties, Bragg (non-Rj), Akisirome (non-Rj), CNS (Rj2RjJ, IAC-2 (Rj2 Rj3) , Akisengoku (Rj4)' Hill (Rj4)' and B340 (Rj2Rj3Rj4) were used. The Rj-genotypes are indicated in parentheses (Ishizuka et al.