A biotic ligand model (BLM) was developed to estimate Cu toxicity to lettuce (Lactuca sativa) in terms of root elongation after 4 d of exposure. Effects of Na+, K+, Ca2+, and Mg2+ on Cu toxicity were examined. The addition of these cations resulted in a 50-fold difference in the copper median effective activity (). However, these variations could not be interpreted entirely as a function of the concentrations of these cations alone. In particular, only the relationship between and the activity of protons was found to be significant in the whole range of pH examined from 5.0 to 7.0. The addition of K+, Na+, Ca2+, and Mg2+ at concentrations up to 20 mM resulted in a 16-fold difference in values. This difference was significant, as indicated by non-overlapping standard deviations of the negative logarithm of () obtained with (7.37 ± 0.22) and without (6.76 ± 0.22) additions of K+, Na+, Ca2+, and Mg2+. The variations were not statistically significantly related to concentrations of these cations; therefore, only protons can be integrated in the BLM predicting Cu toxicity to lettuce L. sativa with the important parameters: log KHBL = 6.27, log KCuBL = 7.40, and = 0.36 at pH = 7. The lack of significant relationships between and concentrations of the cations was not in line with the main assumption of the BLM about the competition between cations for binding sites. Environ. Toxicol. Chem. 2012;31:355–359. © 2011 SETAC
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