ZINC AND CADMIUM TOXICITY TO ALFALFA AND ITS MICROSYMBIONT

The availability of heavy metals in soil depends on their speciation i n solution and chemical activities of free metals. To examine the sens itivity of alfalfa (Glycine max L.), its microsymbiont, and the proces s of N-2 fixation to Zn2+ and Cd2+ activities, a growth chamber study was conducted. Nutrient solution was used with the chelator, EGTA, to buffer Zn2+ and Cd2+ activities. The activities of Zn2+ and Cd2+ range d from pZn(2+) = 8.0 to 5.0 and pCd(2+) = 10.5 to 8.0 mol L(-1). Plant s grown in pZn(2+) = 5.25 and 5.0; and pCd(2+) = 8.75 and 8.65 were st unted and chlorotic with interveinal necrosis. In these treatments, sh oot concentrations of Zn and Cd averaged about 300 and 50 mg kg(-1), r espectively. 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 wit h mineral N had about 40% greater shoot yield than plants inoculated w ith 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. A ll nodules formed were mostly effective in N-2 fixation. Enumeration o f 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 highe st Zn2+ and Cd2+ activities, respectively. At the activities studied, Cd was more toxic to rhizobia than Zn. However, at the highest activit ies, toxicity of metals were so severe that root development was inhib ited and in some cases resulted in plant death. These results suggest that toxicity of Zn and Cd was greater to plants than to rhizobia.