Nitrogen fixation in mutant soybean lines inoculated with two Bradyrhizobium japonicum strains

A pot experiment was conducted to study the nitrogen fixation in two soybean varieties and four of their mutants with two Bradyrhizobium japonicum strains (RG and FA3) using N-15 isotopic dilution method. Random amplified polymorphic DNA (RAPD) determined that the two rhizobial strains were different as well as the soybean mutants and their mother lines. Both strains of B. japonicum formed effective nodules on all soybean plants. The number of nodules, dry matter yield N uptake and N, fixation were affected by both plant genotype and bradyrhizobial strains. Percentages of fixed N-2 (Ndfa) ranged between 35 and 49%; and the actual amounts of fixed N-2 were between 105 and 2 10 mg N/pot. Amounts of N, fixed by B. japonicum FA3 were higher in both soybean varieties, whereas; B. japonicum RG fixed more N-2 on the mutant soybean lines than the parent. Total plant N estimation may not be a sufficient indicator for N-2 fixation, and therefore, the determination of both the amount of nitrogen derived from N-2 fixation and N derived from soil are required for evaluating nitrogen fixation ability. Moreover, the performance of symbiotic N-2 fixation in soybean was shown to depend on both plant genotype and rhizobial strain. The amount of N-2 fixation can be enhanced by mutation plant breeding technique in parallel with selecting the best compatible bradyrhizobial strain.