Effects of toxic concentrations of Cadmium on nodulation and nitrogen fixation of different strains of Sinorhizobium meliloti in Medicago sativa

Cadmium is a heavy metal with high biological toxicity which accumulates in soil usually by using chemical fertilizers or industrial pollutants. In order to study the effects of cadmium on biological nitrogen fixation and finding more resistant strains, the effects of cadmium on microbial growth, nodulation and symbiotic function of four different strains of Sinorhizobium meliloti were considered. They were cultured in TY liquid medium with different concentrations of CdCl2 and their growth was measured by OD (Optical Density) test. More over the effect of cadmium on legume-rhizobium symbiosis was studied by culturing the germinated alfalfa seeds in perlit medium. Plants at three leaves stage were infected by Sm strains mentioned above and the pots were placed in a growth chamber with 16 h light, 18-22ْْ C temperature and 50% humidity for eight weeks. After this time the number and weight of nodules, root length, root and whole plants dry weight; root and shoot total proteins, photosynthetic pigments content, and nitrogenase activity were measured. The results showed decrease of growth, nodulation and N2 fixation ability of all Sm strains, by increasing Cd concentration. But there were significant differences between strains; so that Sm nifK- (pSRK9) has the most sensitivity to Cd. Sm WT, Sm WT (pSRK9) and Sm nifH- (pSRK9) growth egregiously in presence of cadmium compare to Sm nifK- (pSRK9). Sm WT (pSRK9) has more nodulation and N2 fixation ability in presence of cadmium. Plants with the more nitrogen fixation ability have the more ability for protective component biosynthesis. Protection of the photosynthetic system against oxidative stress may be one of the reasons that cause better growth of the plants with active nitrogen fixing system, compare to others.