Combination toxicology of copper, zinc, and cadmium in binary mixtures: Concentration-dependent antagonistic, nonadditive, and synergistic effects onroot growth in Silene vulgaris

Using Silene vulgaris (Moench) Garcke as a test plant, the combination toxi cology of Cu, Cell and Zn was studied. Plants were exposed to single metals and binary metal mixtures in a factorial design. Root elongation after 3 d of exposure was used as a toxicity endpoint. The responses to mixtures wer e classified using nonaddition, response addition, and,where possible, conc entration addition as reference models. In all the metal combinations teste d, the nature of the joint effect appeared to be strongly concentration-dep endent. The responses were approximately nonadditive (Cu/Zn, Cu/Cd) or anta gonistic (Zn/Cd) with respect to root growth inhibition as long as the conc entrations of both components of the mixture were kept within the slightly toxic range. As soon as one of the mixture components exceeded a critical l evel of toxicity, synergism was the predominant joint effect, even when the concentration of the other component was nontoxic under single-metal expos ure. Further increases of the metal concentrations eventually decreased the degree of synergism, possibly as a result of increasing antagonisms at the level of metal uptake. The strongly synergistic effects of root-internal m etals in the morel concentrated mixtures seemed to be due to sensitizing se condary effects other than saturation of common cellular detoxification mec hanisms.