The uptake of trace metals by plants is commonly assumed to depend on the free metal-ion activity, rather than on the total concentration of dissolved metal. Although this free-ion hypothesis has proved to be useful for the interpretation and prediction of metal uptake, several exceptions have been reported where metal complexes also affected metal uptake by plants. In this study, we measured uptake of Zn and Cu by spinach (Spinacia oleracea L.) and tomato (Lycopersicon esculentum L.) in chelator-buffered or resin (Chelex)-buffered solutions, under Zn-deficient and non-deficient conditions. Several ligands, with differing dissociation rates, were used in the chelator-buffered solutions. At the same free-ion activity, Cu and Zn uptake was less in Chelex-buffered than in chelator-buffered solutions. In the chelator-buffered solution, uptake of Cu and Zn at same free-ion activity and same total concentration followed the order: NTA > HEDTA > EDTA > CDTA, i.e., the same order as the dissociation rate. These differences in metal uptake were also reflected in the deficiency symptoms and plant yield in the experiments where Zn deficiency was imposed. The critical Zn2+ activity for Zn deficiency varied by one order of magnitude depending on the buffer, and followed the order HEDTA < CDTA < resin-buffered (no soluble ligand). These results suggest that, when present, aqueous complexes can increase metal uptake by plants because uptake is rate-limited by diffusion of the free ion to the root or cell surface. Thus, the critical free-ion activity in chelator-buffered solutions depends on the type and concentration of the ligand employed.
Keywords: Chelator-buffered solution - Copper - Critical Zn2+ activity - Diffusion limitations - Metal uptake - Zinc deficiency