Sixty strains/isolates of the genera Bradyrhizobium, Rhizobium, Sinorhizobium, Mesorhizobium and Azorhizobium, isolated from different hosts (legume subfamilies: Papilionoideae, Mimosoideae and Caesalpinoideae) and location (Atlantic Forest, Amazon region, crop plantings and heavy metal experiments), were evaluated for Zn, Cu and Cd tolerance in YMA medium modified by the addition of biological buffers (HEPES and MES) and supplemented with Cu (0 to 60 mg L-1), Cd (0 to 60 mg L-1), and Zn (0 to 1,000 mg L-1)sulphates. Growth standards were applied to evaluate rhizobia cultures growth at different metal concentrations, allowing evaluation of highest tolerated concentrations of Zn, Cu, and Cd and the toxic doses (concentrations) of these metals, which reduce the growth standard in 25% (DT25) and 50% (DT50). It was verified that there was no influence of the origin (host and location) on the highest tolerated concentration of each metal; the order of sensitivity to heavy metals, considering the highest tolerable concentrations, was Azorhizobium > Rhizobium = Mesorhizobium = Sinorhizobium > Bradyrhizobium. The DT25 and the DT50 were useful to differentiate strain/isolates of the same genus, which reached the same highest tolerated concentration to Zn, Cu, and Cu; and the order of toxicity was Cu > Cd > Zn.
symbionts; toxicity; heavy metals; growth
