Symbiotic Potential of Bradyrhizobium japonicum Strains with Different Growth Rates

To study genetic properties of soybean nodule bacteria with different growth rates, to research the ability of rhizobia to produce phytohormonal substances and estimate their symbiotic potential in the conditions of combined inoculation and mono-inoculation of soybean plants.

Sequencing of the 16S-23S rDNA intergenic spacer of soybean rhizobia was performed on ABI 3130xl Genetic Analyzer. Determination of the qualitative and quantitative composition of phytohormones in culture liquid of Bradyrhizobium japonicum strains was performed by SDTLC-chromatography of high resolution. Symbiotic potential of the strains was estimated in the field experiment in Polissia area of Ukraine.

It has been established that strains of soybean nodule bacteria with different growth rates by the structure of ITS region belong to different genetic groups: USDA 6, USDA 110 and USDA 123. The studied rhizobia differ in the levels of extracellular phytohormones synthesis: the slow-growing B. japonicum strain 46 produces more amounts of auxins, but the intensive-growing B. japonicum strain KB11 significantly exceeds in the amounts of cytokinins. It has been shown that an important factor of increasing soybean productivity is the formation of balanced symbiotic systems of the host plant with several complementary strains but serologically and genetically different strains of rhizobia of the same species.

Combined inoculation of soybean seeds with two active strains B. japonicum 46 and B. japonicum KB11 allows more fully realizing the potential of symbiotic partners and receiving the higher yield.