Some aspects of the kinetics of cadmium and thallium uptake by fronds of Lemna minor L

A study of influx and accumulation of Tl and Cd by Lemna minor has revealed a number of contrasting features which arise from the basic physico-chemical properties of the two elements. Short-term influx of Cd over a range of concentration (0.1-10 μM) followed a linear trend while that of Tl over the same range produced a profile with a plateau at the higher concentrations, indicating saturation of the absorption mechanisms. Application of Ca and K antagonists aiming to indicate the nature of the absorption mechanisms did not reveal any consistent trends. Long-term exposure of the plant to Cd and Tl produced steady-state concentrations after 250 and 140 h respectively, by which stage Lemna had accumulated nearly twice as much Cd than Tl. It appears that a significant amount of Cd is probably in the Donnan free-space and strongly bound to cell constituents from which the element is depurated slowly and continuously in Cd-free medium. It is concluded that Tl is tenaciously held in the plant tissues, perhaps in the cell vacuole, as 80% of the steady-state concentration is retained by the plant even after 140 h in Tl-free medium. The influence of several external physical and chemical factors further emphasizes the contrasting nature of the two elements; Tl uptake is almost entirely an active process and ion activity of Tl is relatively unaffected by changes in pH and ligand concentration. Cd uptake involves both active and passive components, ion activity is markedly reduced at low H+ion concentrations and in the presence of ligands, such as EDTA. Inhibition of uptake was found for both elements in the presence of competing ions.