Effect of Salinity Stress on Biological Nitrogen Fixation and Growth Indices of Two Soybean Varieties at Different Phenological Stages

In legume crops، the growth of root، its mucilage production، rhizobia penetration into roots، nodulation and finally biological nitrogen fixation decrease in saline condition. The nodule formation stage is very sensitive to salinity compared with plant growth. By recognizing susceptible stages and decreasing soil salinity، it is possible to decrease the adverse effects of salinity on plant. The aims of the present study were to detect the effects of salinity on nitrogen fixation and growth of two soybean cultivars using rhizobium–soybean symbiotic system as an alternative for nitrogen fertilizer. A factorial pot experiment with a randomized complete block design and three replications was carried out and treatments were soybean cultivars (M7 and L90)، salinity levels of irrigation water (0. 53، 2. 5، 4. 5، and 6. 5 dS/m)، levels of nitrogen fertilizer (0 and 150 kg N/ha and inoculation with Bradyrhizobium) and growth stages (one month after sowing، flowering، pod formation، seed formation، and grain filling). The plants were harvested at five different growth stages and in each stage shoot and root dry weights، numbers of nodules، concentrations of N in plant، and effectiveness of symbiosis were measured. The data obtained were statistically analyzed using MSTATC software and comparison of means was made by Dunkan multiple ranges test. In all measured growth indices، except nitrogen concentration and N uptake، the M7 cultivar was superior to L90 cultivar. Salinity decreased all measured growth indices. The highest and the lowest growth indices of soybean including shoot and root dry weights، numbers of nodules، and dry weight of nodule were measured at vegetative (one month after sowing) and grain filling stages، respectively. The symbiotic effectiveness (SE) was highest at flowering stage and lowest at grain filling stage.