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

Dep. of Agronomy, Univ. of Maryland, College Park, MD 20742

R. L. Chaney

Environmental Chemistry Lab.

P. van Berkum

Soybean and Alfalfa Research Lab., USDA-ARS, Beltsville MD 20705

*Corresponding author (ja35{at}umail.umd.edu).

ABSTRACT

Toxicity of metals to plants and microbes depends on the chemical activities of metals in the soil solution. To examine the effects of Zn²⁺ and Cd²⁺ 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 Zn²⁺ and Cd²⁺ activities in nutrient solution from pZn²⁺ = 8.0 to 5.0 and pCd²⁺ = 10.5 to 8.0 mol L^–1. Plants grown in pZn²⁺ = 5.0 and pCd²⁺ = 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 pZn²⁺ = 5.0. In pCd²⁺ = 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 Zn²⁺ and Cd²⁺ activities. At pCd²⁺ = 8.75 and 8.65, however, there was a delay in nodulation. Most nodules were effective in N² fixation irrespective of Zn²⁺ and Cd²⁺ activity. Enumeration of viable cells showed a significant decline of total cell counts from 10^5.5 cells mL^–1 in the control to 10^5.0 and 10^4.5 cells 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 that toxicity of Zn and Cd was greater to plants than to rhizobia.

Contribution from the Maryland Agric. Exp. Stn.

Received for publication February 22, 1996.

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