Effects of Cd Pollution on Alfalfa Growth and Nitrogen Fixation of Native Isolates of Sinorhizobium meliloti

The effects of heavy metals, as soil pollutants, on Rhizobia and symbiotic system of Rhizobium-legume are not clearly known. The aims of this study were to determine the effects of Cd on nitrogen fixation of native isolates of Sinorhizobium meliloti in Zanjan province soils. To isolate and obtain pure cultures of sinorhizobium meliloti isolates, root nodules were collected from different alfalfa farms contaminated by heavy metals, especially Cd, in Zanjan province. Plant infection tests were carried out on different strains to identify Rhizobia, then, isolates capable of nodulating were determined. Based on effectiveness of symbiosis and shoots dry weight of alfalfa in greenhouse condition, nitrogen fixation efficiency of different isolates was examined and compared with each other. The results showed that different isolates had different nitrogen fixation efficiencies and the isolates were classified into four groups, namely, very effective, effective, partially effective, and not effective. In the second part of the experiment, resistance of different sinorhizobium meliloti isolates to Cd was examined using selective medium of HEPES-MES containing different amounts of Cd. The highest concentration of heavy metals supporting growth for each isolates was determined as the maximum resistance level (MRL) and, based on it, the isolates were classified into three groups, namely, sensitive, relatively sensitive, and tolerant. In the final experiment, the effective isolates (capable of nodulating) with low and high tolerance to heavy metals were selected and used for a pot experiment. In this experiment, the effects of five sinorhizobium meliloti isolates (S6, S12, S17, S41 and S51) as inoculants, and five concentrations of heavy metals were examined on growth of alfalfa plant in a mixture of perlite and sand as media. A control and a treatment receiving 70 mg/kg N were also included in this experiment. The highest and lowest biomass yields of alfalfa were obtained for control and those treatments inoculated with resistant isolates to Cd. The S51 isolates was tolerant to Cd and had higher nitrogen fixation ability than the other isolates. The S17 and S3 were the most tolerant and the most sensitive isolates to Cd, respectively.