Mutant Strains of Rhizobium japonicum with Increased Ability to Fix Nitrogen for Soybean

A strain of Rhizobium japonicum used in commercial inoculants was mutagenized and screened by a rapid effectiveness assay with soybean plants. Two mutant strains nodulated the roots earlier than the wild type and also expressed greater symbiotic nitrogen-fixing activity than the wild type in the presence and absence of fixed nitrogen. In addition, one of the mutants formed more root nodules than the wild type. Plants inoculated with these strains had increased dry weights ( approximately 60 percent) and nitrogen content ( approximately 100 percent) when grown in growth chambers.