Abstract

Many scientists are working to identify effective strains of rhizobia to increase nitrogen fixation and reduce nitrogen fertilizer application. Symbiotic nitrogen fixation of leguminous crops has become an alternative to nitrogenous fertilizer, due to its higher efficiency for nitrogen fixation. This experiment was conducted to select strains for improved nitrogen fixation of soybean (Glycine max L.). Bradyrhizobium strains isolated from Myanmar were evaluated for symbiotic efficacy, using the cultivar Yezin-6 (non-Rj). Five Bradyrhizobium strains, B. japonicum SAY3-7, B. elkanii AHY3-1, B. liaoningense SMY3-1, Bradyrhizobium spp. AHY3-6 and B. yuanmingense SMY6-10 were shown to have superior nitrogenase activity. These five strains were evaluated for their effectiveness on different Rj-genes in soybean cultivars. The nitrogen fixation of B. japonicum SAY3-7, B. elkanii AHY3-1 and B. liaoningense SMY3-1 were higher than other indigenous strains and the standard strain, B. japonicum USDA110. This was particularly the case for the cultivars Yezin-6 (non-Rj) and Yezin-11 (Rj₄), but not for the cultivarsYezin-9 (Rj₃) and Yezin-10 (Rj₂Rj₃). Bradyrhizobium japonicum SAY3-7, B. elkanii AHY3-1 and B. liaoningense SMY3-1 were also evaluated on Yezin-8 (non-Rj) and Yezin-3 (Rj₄). Bradyrhizobium japonicum SAY3-7 had significantly higher nitrogenase activity on Yezin-8 (non-Rj), although B. japonicum SAY3-7 was not significantly different than B. elkanii AHY3-1 and B. liaoningense SMY3-1 on Yezin-3 (Rj₄). Therefore, B. japonicum SAY3-7, which was the most effective nitrogen fixing strain in all the experiments, was selected for inoculant production. According to this study, it can be concluded that the strains were specific to cultivars and,thus, selection of a strain compatible to a specific cultivar is necessary to increase symbiotic nitrogen fixation.

Key words: Bradyrhizobium strains, Myanmar, nitrogenase activity, Rj-genes, soybean.