Influence of heavy metals on nodulation of red clover

Abstract

The effect of cadmium, cobalt, copper, and zinc on nodulation red clover (Trifolium pratense Pennscott) was investigated. Germinated seedlings at the primary leaf stage were transplanted into soils of three different types, each of which had been amended with the chloride salt of one of the metals. The soil amendments provided additions ranging from 15 to 195 ppm metal expressed on an oven-dry soil weight basis. Plants were harvested after 6 wk. Direct nodule counts made on individual root systems revealed that nodulation was slightly stimulated by low concentrations of metal in some soils, but at higher concentrations, the number of nodules per plant and the number of nodules per g fresh root tissue was reduced. In some soils, the highest metal concentrations were sufficient to kill the plants and no nodulation occurred. The apparent toxicity of the metals to nodule formation was ranked in the order: Cd > Co > Cu > Zn. Lowering of the soil pH or cation exchange capacity of the soil increased the toxicity of all four metals to the nodulation process and to the plants themselves. Toxicity of the metals to the plants was evident as a reduction in root weight, number of leaves,more » and shoot weight with or without marginal chlorosis, anthocyanosis, or necrosis of the foliage.« less