Zinc and cadmium toxicity to alfalfa and its microsymbiont

The availability of heavy metals in soil depends on their speciation in solution and chemical activities of free metals. To examine the sensitivity of alfalfa (Glycine max L.), its microsymbiont, and the process of N 2 fixation to Zn 2+ and Cd 2+ activities, a growth chamber study was conducted. Nutrient solution was used with the chelator, EGTA, to buffer Zn 2+ and Cd 2+ activities. The activities of Zn 2+ and Cd 2+ ranged from pZn 2+ = 8.0 to 5.0 and pCd 2+ = 10.5 to 8.0 mol L -1 . Plants grown in pZn 2+ = 5.25 and 5.0 ; and pCd 2+ = 8.75 and 8.65 were stunted and chlorotic with interveinal necrosis. In these treatments, shoot concentrations of Zn and Cd averaged about 300 and 50 mg kg -1 , respectively. This resulted in a 20 and 90% yield reduction for plants grown in pZn 2+ = 5.25 and 5.0, respectively. Yield reduction was 50 and 75% for pCd 2+ = 8.75 and 8.65, respectively. Plants supplied with mineral N had about 40% greater shoot yield than plants inoculated with rhizobia. No nodulation occurred at pZn 2+ = 5.0, but at pZn 2+ = 5.25 and pCd 2+ = 8.75 and 8.65, there was a delay in nodulation. All nodules formed were mostly effective in N 2 fixation. Enumeration of viable cells showed a significant decline of total cell counts from 10 5.5 cell mL -1 in the control to 10 5.1 and 10 4.8 at the highest Zn 2+ and Cd 2+ activities, respectively. At the activities studied, Cd was more toxic to rhizobia than Zn. However, at the highest activities, toxicity of metals were so severe that root development was inhibited and in some cases resulted in plant death. These results suggest that toxicity of Zn and Cd was greater to plants than to rhizobia.