Cadmium uptake by durum wheat in presence of citrate

The aim of this study was to determine the mechanisms underlying the uptake of Cd by durum wheat (Triticum turgidum L. ssp. durum cv. "acalou") in the presence of citrate under hydroponic conditions. Wheat seedlings were exposed for 3 h to simplified nutrient solutions initially containing 35 nM of free Cd with or without citrate. Uptake experiments with citrate alone were also performed. Solutions were radio labelled with (109)Cd or citrate-(14)C. The depletion of Cd and citrate in the exposure solution was followed, and at the end of the exposure Cd and citrate were quantified in all compartments of the experimental system (root surface, inner roots and aerial parts). The apparent rates of internalization of Cd (r (Cd)) and citrate (r (cit)) were estimated from the ratio of the uptake flux to the contents adsorbed at the root surface. About two times more Cd was taken up in the presence of citrate. Assuming that citrate and Cd-citrate complexes were taken up at the same rate, a direct uptake of intact complexes could not account for the higher uptake of Cd in presence of citrate. In addition, Cd was internalized at the same rate in the absence or presence of citrate (r (Cd) = 3.62 h(-1)), whereas the internalization rate constants of Cd and citrate were different (r (Cd) a parts per thousand aEuro parts per thousand 8 r (cit)). This strengthens the idea that a direct uptake of non-dissociated Cd-citrate complexes probably did not account for the higher Cd uptake in presence of citrate. A dissociation of Cd-citrate complexes within the diffusion layer or at the root surface and the subsequent additional supply of free Cd were more likely to explain the higher Cd uptake in presence of citrate.