Three mechanisms for the calcium alleviation of mineral toxicities

Ca(2+) in rooting medium is essential for root elongation, even in the absence of added toxicants. In the presence of rhizotoxic levels of Al(3+), H(+), or Na(+) (or other cationic toxicants), supplementation of the medium with higher levels of Ca(2+) alleviates growth inhibition. Experiments to determine the mechanisms of alleviation entailed measurements of root elongation in wheat (Triticum aestivum L. cv Scout 66) seedlings in controlled medium. A Gouy-Chapman-Stern model was used to compute the electrical potentials and the activities of ions at the root-cell plasma membrane surfaces. Analysis of root elongation relative to the computed surface activities of ions revealed three separate mechanisms of Ca(2+) alleviation. Mechanism I is the displacement of cell-surface toxicant by the Ca(2+) induced reduction in cell-surface negativity. Mechanism II is the restoration of Ca(2+) at the cell surface if the surface Ca(2+) has been reduced by the toxicant to growth-limiting levels. Mechanism III is the collective ameliorative effect of Ca(2+) beyond mechanisms I and II, and may involve Ca(2+)-toxicant interactions at the cell surface other than the displacement interactions of mechanisms I and II. Mechanism I operated in the alleviation of all of the tested toxicities; mechanism II was generally a minor component of alleviation; and mechanism III was toxicant specific and operated strongly in the alleviation of Na(+) toxicity, moderately in the alleviation of H(+) toxicity, and not at all in the alleviation of Al(3+) toxicity.