SUMMARYA 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 //.M) 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.

The predominant pathway for human exposure to chromium in non-occupationally exposed individuals is via food with a daily intake of around 30-100 µgd -1 , with vegetables providing a major contribution. Unlike reports of chromium essentiality to man and animals, plants appear not to require chromium in spite of some early reports of a stimulatory growth effect.Most reports on chromium in plants have been concerned with their growth on soils amended with sewage sludge, pF-ash, tannery waste, or on ultra basic soils, which contain extreme concentrations of the element. Experimental studies with plants grown in hydroponic solution have often been undertaken at unrealistically high concentrations to examine the uptake of chromium in various forms, either as CrIII or CrVI at different pHs. In most cases, reports on chromium in plants deal with element concentrations and plant/soil relationships rather than detailed biochemical and physiological processes.In general, chromium is largely retained in the roots of plants, although the oxidation state of chromium, pH, presence of humates and fulvates and plant species, affect plant uptake and transport. Leaves usually contain higher concentrations than grains. The uptake of CrIII is largely a passive process, whereas CrVI uptake is a metabolically mediated process via the sulphate pathway and is thus readily transported around the plant. The presence of a compound similar to trioxalate CrIII has been recorded while little chromium has been reported to be associated with cell organelles or soluble proteins.

No abstract