Selection of thermotolerant bradyrhizobial strains for nodulation of soybean (Glycine max L.) in semi-arid regions of Iran

Nodulation of soybeans grown in semi-arid region of southern parts of Iran is poor due to high air and soil temperatures. Here we identified thermotolerant isolates of soybean bradyrhizobia and evaluated the nitrogen fixation efficiency of the isolates under heat stress conditions in greenhouse and field experiments. The ability of fifty-six bradyrhizobial isolates to grow on solid or in liquid yeast extract mannitol medium at 38 and 41A degrees C was evaluated. We identified 19 isolates, which were able to grow at 38A degrees C and 10 isolates able to grow at 41A degrees C. Greenhouse experiments were carried out at 28 and 38A degrees C to study the nitrogen-fixing capacity of the isolates under optimal and high temperature conditions. Ten isolates had a symbiotic index of effectiveness of 80% or greater compared with nitrogen-fertilized treatments in greenhouse experiments at 28A degrees C. Some thermotolerant isolates demonstrated good nitrogen-fixing performance at 38A degrees C. Eight isolates were selected for use in a field trial in the natural high temperature environment of the Dezful region in Iran. Our results demonstrate that geographical origin can have a great influence on the successful selection of thermotolerant bradyrhizobia. Our thermotolerant isolates were mainly obtained from high-temperature regions, and improved shoot dry matter, nitrogen-uptake and seed yield of the plants.