Effect of calcium and pH on copper binding and rhizotoxicity to pea (Pisum sativum L.) root: Empirical relationships and modeling.

The accumulation and toxicity of Cu to pea (Pisum sativum L.) roots were investigated. The root uptake of Cu and Ca varied with Ca and H activities. Calcium, H, and Cu competed for root binding with high pH and low Ca favoring more Cu uptake. Root elongation was highly sensitive to root Ca content and correlated better with root-bound Ca and Cu content than with merely dissolved Cu concentrations. The prediction of root elongation needs to include both the root-bond Cu and Ca as predictor variables whenever environmental conditions (low pH and low Ca) decrease Ca accumulation. A multielement uptake model was developed to describe Cu and Ca accumulation by treating the pea roots as a collection of three biotic ligands with known site densities (Q(L)(j)) and proton-binding constants (K(HL)(j)). A series of binding constants were derived. The log K(CuL)(j) (j = 1, 2, 3) values were estimated at pH 6 and 0.2 mM CaCl(2) as 2.36, 4.36, and 0.32, respectively. The derived formation constants can be incorporated into standard solution speciation models to estimate the bioaccumulation of Cu in plant roots under multielement conditions.