Predicting effects of cations on copper toxicity to lettuce (Lactuca sativa) by the biotic ligand model.

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(+), Ca(2+), and Mg(2+) on Cu toxicity were examined. The addition of these cations resulted in a 50-fold difference in the copper median effective activity (EC50 cu2+). However, these variations could not be interpreted entirely as a function of the concentrations of these cations alone. In particular, only the relationship between EC50 cu2+ 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(+), Ca(2+), and Mg(2+) at concentrations up to 20 mM resulted in a 16-fold difference in EC50 cu2+ values. This difference was significant, as indicated by non-overlapping standard deviations of the negative logarithm of EC50 cu2+ pEC50 cu2+) obtained with (7.37 ± 0.22) and without (6.76 ± 0.22) additions of K(+), Na(+), Ca(2+), and Mg(2+). 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 K(HBL) =6.27, log K(CuBL) =7.40, and [formula in text]. The lack of significant relationships between EC50 cu2+ and concentrations of the cations was not in line with the main assumption of the BLM about the competition between cations for binding sites.