Cadmium is one of the most important heavy metals that enters soil from various sources particularly from phosphorous fertilizers and causes soil pollution. This metal adversely affects nodulation and symbiotic nitrogen fixation, due to its deleterious effects on rhizobia and on their host plants. This research was undertaken to investigate the tolerance degree of Iranian strains of Sinorhizobium meliloti (S.m.) to different levels of Cadmium (Cd). In order to prepare native strains of S.m. root nodules were obtained from provinces important for alfalfa cropping. Following isolation of the root nodules bacteria, pure culture of each isolate was prepared. Plant infection test with each isolate was performed, and after omission of the isolates with no symbiotic potential, 57 effective strains were selected. Tolerance potential of strains to different levels of Cd were assessed by using H.M. media of different amounts of Cd (0, 5, 10…70 mgl-1). The results showed that on the basis of Cd tolerance, there is a significant difference (p<0.05) among various strains of S.m. to the extent that they were classified into sensitive, partially sensitive and resistant groups. Ahigh percentage of studied strains (approximately 70%) were sensitive to Cd that had visible growth in Cd concentration range only of 5-10 (mg-1) Only 3.8% could tolerate high concentration levels of Cd (60-70 mgl-1). Therefore, these were introduced as resistant strains.
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