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Zinc and Cadmium Toxicity to Alfalfa and Its Microsymbiont

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

R. L. Chaney

Environmental Chemistry Lab., USDA, ARS, Beltsville, MD 20705;

P. van Berkum

Soybean and Alfalfa Res. Lab., USDA, ARS, Beltsville, MD 20705.

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

ABSTRACT

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 N₂ 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⁺ ranged from pZn²⁺ = 8.0 to 5.0 and pCd²⁺ = 10.5 to 8.0 mol L^–1. Plants grown in pZn²⁺ = 5.25 and 5.0; and pCd²⁺ = 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 pZn²⁺ = 5.25 and 5.0, respectively. Yield reduction was 50 and 75% for pCd²⁺ = 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 pZn²⁺ = 5.0, but at pZn²⁺ = 5.25 and pCd²⁺ = 8.75 and 8.65, there was a delay in nodulation. All nodules formed were mostly effective in N₂ 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 Zn²⁺ and Cd²⁺ 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.

Scientific Article no. A7893. Contribution no. 9227 from the Maryland Agricultural Experiment Station.

Received for publication September 21, 1995.

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