Environmental Context. Toxic effects of trace metals are often related to the amount of metal that is internalized by the organism. Uptake of metals by biota is usually predicted with equilibrium models, which assume that transport of the metal from the solution to the biosurface does not limit uptake. In this study, uptake of cadmium by higher plants is shown to be limited by the transport of the free ions to the root surface under a range of conditions.

Abstract. Uptake of cadmium (Cd) by spinach and wheat was higher in the presence of fast-dissociating complexes than in unbuffered solutions with the same free ion concentration. This contribution of metal complexes to metal uptake cannot be explained by equilibrium free-ion-based models, which assume that transport of the free ion to the biosurface is not limiting the uptake. To demonstrate diffusion limitations to metal uptake, we used an agar gel technique in which Cd and Zn concentrations around the roots, after 6 h of uptake, were compared with bulk concentrations. Metal depletion around the roots was clearly observed in agar where the ion activities were not buffered by complexes, whereas the depletion was less pronounced in buffered agar. Metal uptake by the plants in unbuffered media was greater as the degree of agitation increased (stirred solution > unstirred solution > agar), while no such dependence on hydrodynamic conditions was found in buffered media, which is in agreement with theoretical predictions.