Evaluating mechanisms for plant-ion (Ca2+, Cu2+, Cd2+ or Ni2+) interactions and their effectiveness on rhizotoxicity

Electrical properties of plant cell membranes (CMs) provide a new avenue for exploring the mechanisms of plant-ion interactions and the biotic effects of ions. The modulating effect of Ca2+ on rhizotoxicity of metallic ions was studied to evaluate the mechanisms of plant-ion interaction in terms of the electrical potential at the CM exterior surface CMSe (ψ0o). Adding Ca2+ to root bathing media (BM) reduced the negativity of ψ0o at the CMSe. This reduction caused decreases in the activities of toxic metal ions at the CMSe and hence alleviated the toxicity (denoted as Mechanism I). Calcium is an essential element for growth and adding Ca2+ could also restore metal-displaced Ca2+ at the CMSe and alleviated Ca2+ deficiency (Mechanism II). The reduced surface negativity increased the surface-to-surface transmembrane potential difference (Em,surf), thus increasing the electrical driving force for transport of toxic metallic ions across the CM (Mechanism III). The Mechanism III would increase toxicity, but did not offset the alleviation by Mechanisms I and II. Regression analysis of relative root elongation in appropriate nonlinear equations incorporating the effects of above mechanisms provided evidence that under the current experimental conditions, Mechanism I is the most important mechanism for plant-ion interactions. In addition, the intracellular concentration of metals in root was a better predictor of toxicity than the free metal ion activity in the BM and could be predicted with an electrostatic uptake model developed previously. Based on the intracellular concentration and the EUM, the toxicity threshold (EC50, activities producing 50% growth inhibition) could be predicted generally within a factor of 2 of the observed values, indicting its potential utility in risk assessment of toxic metals.