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 acti vities of metals in the soil solution. To examine the effects of Zn2and Cd2+ activities on white clover (Trifolim repens L.), Rhizobium le guminosarum bv. trifolii, the infection process, and nodulation, a gro wth chamber study was conducted. The chelator EGTA [ethylene-bis(oxyet hylenetrinitrilo) tetraacetate] was used to buffer Zn2+ and Cd2+ activ ities in nutrient solution from pZn(2+) = 8.0 to 5.0 and pCd(2+) = 10. 5 to 8.0 mol L-1. Plants gown in pZn(2+) = 5.0 and pCd(2+) = 8.75 and 8.65 were stunted and chlorotic with interveinal necrosis. Shoot conce ntrations of Zn and Cd averaged >200 and 40 mg kg(-1), respectively, i n 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 r eductions, respectively, were observed. In treatments where mineral N was supplied to plants, 20 to 35% greater shoot yield was recorded com pared,vith plants inoculated with rhizobia, presumably due to a period of starvation for N due to the delay in the onset of fixation. Nodula tion was observed at all Zn2+ and Cd2+ activities. At pCd(2+) = 8.75 a nd 8.65, however, there was a delay in nodulation. Most nodules were e ffective in NL fixation irrespective of Zn2+ and Cd2+ activity. Enumer ation of viable cells showed a significant decline of total cell count s from 10(5.5) cells mL(-1) in the control to 10(5.8) and 10(4.5) cell s mL(-1) at the highest Zn2+ and Cd2+ activities, respectively. At the activities studied, Cd was more toxic to rhizobia and nodulation than Zn. At the highest Cd2+ activity, however, plants were killed within 48 h due to metal toxicity. This technique showed for the first time t hat toxicity of Zn and Cd was greater to plants than to rhizobia.