Development of an electrostatic model predicting copper toxicity to plants

The focus of the present study was to investigate the mechanisms for the alleviation of Cu toxicity in plants by coexistent cations (e.g. Al3+, Mn2+, Ca2+, Mg2+, H+, Na+, and K+) and the development of an electrostatic model to predict 50% effect activities (EA50s) accurately. The alleviation of Cu2+ toxicity was evaluated in several plants in terms of (i) the electrical potential at the outer surface of the plasma membrane (PM) psi 0 degrees) and (ii) competition between cations for sites at the PM involved in the uptake or toxicity of Cu2+, the latter of which is invoked by the Biotic Ligand Model (BLM) as the sole explanation for the alleviation of toxicity. The addition of coexistent cations into the bulk-phase medium reduces the negativity of psi 0 degrees and hence decreases the activity of Cu2+ at the PM surface. Our analyses suggest that the alleviation of toxicity results primarily from electrostatic effects (i.e. changes in both the Cu2+ activity at the PM surface and the electrical driving force across the PM), and that BLM-type competitive effects may be of lesser importance in plants. Although this does not exclude the possibility of competition, the data highlight the importance of electrostatic effects. An electrostatic model was developed to predict Cu2+ toxicity thresholds (EA50s), and the quality of its predictive capacity suggests its potential utility in risk assessment of copper in natural waters and soils.