INHERITANCE OF HOST-CONTROLLED RESTRICTION OF NODULATION BY BRADYRHIZOBIUM-JAPONICUM STRAIN USDA-110

The use of soybean [Glycine mar (L.) Merr.] genotypes which specifical ly restrict nodulation by competitive indigenous Bradyrhizobium popula tions has been proposed as an approach to favor increased nodulation b y more effective, inoculant-quality strains. Soybean plant introductio n (PI) PI 417566 restricts nodulation by Bradyrhizobium japonicum (Kir chner, Jordan) strains in serogroup 110. In this study, the interactio n of PI 417566 with strains in serogroup 110 is being used as a model system to determine whether host-controlled nodulation restriction can be used to alter the competitiveness of indigenous bradyrhizobia. The objectives of this study were to determine the type of inheritance an d the number of genes in PI 417565 which condition restriction of nodu lation by strain USDA 110 and whether this gene(s) could function to p revent nodulation of serogroup 110 strains under held conditions. Inhe ritance of host-controlled nodulation restriction was determined by cr ossing PI 417566 with the nonrestricting cultivars Agassiz and Evans. The nodulation phenotype of 126 F-2 plants from Agassiz x PI 417566 an d 44 F-2 plants from Evens x PI 417566 was assessed in growth chamber studies. In the former cross, 98 nodulating and 28 non-nodulating plan ts, and in the latter, 32 nodulating and 12 non-nodulating plants were observed. In both cases, a Chi-squared test indicated a good fit to a 3:1 segregation ratio, indicating that a single recessive gene condit ions restricted nodulation in PI 417566. A 2-yr field study utilizing F-2:4 and F-2:5 progeny from Agassiz x PI 417566 and F-2:4 progeny fro m Evans x PI 417566 demonstrated that the lines had the ability to res trict nodulation by serogroup 110 strains under held conditions and th at restriction occurred even in the presence of large numbers (5 x 10( 7) cells/g soil) of added USDA 110.