Zinc and Cadmium Effects on Rhizobia and White Clover using Chelator-Buffered Nutrient Solution

Toxicity of metals to plants and microbes depends on the chemical activities of metals in the soil solution. To examine the effects of Zn 2+ and Cd 2+ activities on white clover (Trifolim repens L.), Rhizobium leguminosarum bv. trifolii, the infection process, and nodulation, a growth chamber study was conducted. The chelator EGTA [ethylenebis(oxyethylenetrinitrilo) tetraacetate] was used to buffer Zn 2+ and Cd 2+ activities in nutrient solution from pZn 2+ = 8.0 to 5.0 and pCd 2- = 10.5 to 8.0 mol L -1 . Plants grown in pZn 2+ = 5.0 and pCd 2+ = 8.75 and 8.65 were stunted and chlorotic with interveinal necrosis. Shoot concentrations of Zn and Cd averaged > 200 and 40 mg kg -1 , respectively, in these treatments. This resulted in a 60% yield reduction for plants grown in pZn 2+ = 5.0. In pCd 2+ = 8.75 and 8.65, 40 and 60% yield reductions, respectively, were observed. In treatments where mineral N was supplied to plants, 20 to 35% greater shoot yield was recorded compared with plants inoculated with rhizobia, presumably due to a period of starvation for N due to the delay in the onset of fixation. Nodulation was observed at all Zn 2+ and Cd 2+ activities. At pCd 2+ = 8.75 and 8.65, however, there was a delay in nodulation. Most nodules were effective in N 2 fixation irrespective of Zn 2+ and Cd 2+ activity. Enumeration of viable cells showed a significant decline of total cell counts from 10 55 cells mL -1 in the control to 10 5.0 and 10 4.5 cells mL -1 at the highest Zn 2+ and Cd 2+ activities, respectively. At the activities studied, Cd was more toxic to rhizobia and nodulation than Zn. At the highest Cd 2+ activity. however, plants were killed within 48 h due to metal toxicity. This technique showed for the first time that toxicity of Zn and Cd was greater to plants than to rhizobia.