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⁺, 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 ( ${\rm EC}50_{{\rm cu}^{2 + } } $). However, these variations could not be interpreted entirely as a function of the concentrations of these cations alone. In particular, only the relationship between ${\rm EC}50_{{\rm cu}^{2 + } } $ 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 ${\rm EC}50_{{\rm cu}^{2 + } } $ values. This difference was significant, as indicated by non-overlapping standard deviations of the negative logarithm of ${\rm EC}50_{{\rm cu}^{2 + } } $ ( ${\rm pEC}50_{{\rm cu}^{2 + } } $) 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 K_HBL = 6.27, log K_CuBL = 7.40, and $f_{{\rm CuBL}}^{{\rm 50}\%} $ = 0.36 at pH = 7. The lack of significant relationships between ${\rm EC}50_{{\rm cu}^{2 + } } $ 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 [ABSTRACT FROM AUTHOR]