Studies on practical ways to solve the problem for the symbiosis of Bradyrhizobium japonicum and Soybean plant in warm regions of Iran.

This research started with the collection of soil samples from the soybean areas of Khouzestan and other parts of Iran during 1999. Thirty nine soybean rhizobial strains (Bradyrhizobium japonicum) collected from these locations, along with 19 other strains obtained from collections of Iran or from other countries, were grown on solid or in liquid culture media at 38oC or 41oC. Then, nitrogen fixing potentials of these strains were evaluated in greenhouse experiments at 28 oC and 37oC. Some of the typical bacteria were tested for their growth characteristics. The survivability of the mentioned strains in culture solutions containing six antibiotics at 5, 10, 20 ى g/ml and 0.1 or 0.3 moles of sodium chloride per liter were also evaluated. Four carriers having different formulations were tested at 4oC and 30oC for maintaining rhizobial populations compared with Australian peat used as the most common carrier. Three strains were selected and used in two carriers for inoculant production which were then field tested in Dezful region on soybean variety Semes. The results showed that out of the 58 tested rhizobial strains, 27 were capable of surviving at 38oC and 10 strains at 41oC. Nitrogen fixing potentials of those strains were similar at 28oC but different at 38oC because of the heat sensitivity of most of the bacteria tested in the greenhouse experiments. The tested carriers performed better than the Australian peat at both temperatures, each capable of maintaining population densities of 108 bacteria/gr. Field tests showed superior performance by the heat tolerant strain when maintained by oil less carrier as compared with the other two strains that performed well under normal conditions. However, there was no increases in dry matter production. When the strains were used with oil-containing carrier or when pelleted with rock phosphate after inoculation, their efficiencies increased and they produced from 22 to 27% more dry matter compared with the control. The final crop yields showed significant differences (at 5% level) in seed production due to the different treatments. The best inoculant treatment caused a 74% yield increase compared with the control.