Air pollution is the most important environmental problem of last century that threatens the health of living organisms, especially plants. SO2 is one of the main air pollutants that can cause to imbalance in growth and physiological function of plant in high concentrations. Symbiosis of Rhizobium bacteria with alfalfa can cause increasing plant growth and plant resistance to abiotic stresses. In order to study the effects of rhizobia inoculation on alfalfa antioxidants activity and capacity under air SO2 pollution, 35 days plants (noninoculated and inoculated with native or standard Rhizobium meliloti) exposed to the different concentrations of SO2 (0 as a control, 0.5, 1, 1.5 and 2 ppm) for 6 consecutive days (2 hours per day). Results showed inoculation had no significant effect on antioxidants activity and capacity of alfalfa plant. Different concentrations of SO2 pollution had a significant effect on alfalfa antioxidant system. Increasing SO2 stress increased antioxidant activities (I%) and decreased antioxidant capacities (IC50) of alfalfa leaves significantly in comparison to the control plants (under 0 ppm) as well as increased Superoxide dismutase, catalase and Guaiacol peroxidase activity. Inoculation of alfalfa plant with Rhizobium meliloti reduced significantly the negative effects of high concentration of SO2 on antioxidants activity and capacity. In fact, the rate of this change of antioxidant system under SO2 pollution in inoculated plants was lower than in the noninoculated plants. Therefore inoculation with Rhizobium strains could alleviate the effect of SO2 pollution on antioxidant system.
