Cadmium and nickel uptake by tomato and spinach seedlings: Plant or transport control?

Abstract

Equilibrium models for metal uptake by biota assume that the uptake is rate limited by the internalisation of the metal across the cell membrane (plant control). However, evidence has been found that Cd uptake by plants is controlled by the diffusive transport of the free ion to the root at low Cd²⁺ activities. In this study, the uptake of Cd and Ni by tomato and spinach in nutrient solution was compared to assess whether Ni uptake is under plant or transport control. The diffusive gradient in thin films (DGT) technique was used to measure the maximal diffusive flux. In unbuffered solutions, the uptake flux of Ni was approximately three-fold smaller than that of Cd at free ion concentrations below 1 µM. Stirring the solution increased the uptake of Cd, but not that of Ni at low free ion concentration. The presence of DGT-labile complexes increased the uptake of Cd, but not that of Ni, except at high pH (pH 7). Increasing the solution pH increased Cd uptake only when solution Cd²⁺ activities were strongly buffered. Overall, the results indicated that the Cd uptake was strongly diffusion limited and that uptake is likely to be under transport control in natural environments. Uptake of nickel, on the other hand, appears to be at the border between plant and transport control. This finding has practical applications, e.g. competition effects at the root have little effect on Cd uptake and chelator-assisted phytoextraction is expected to have less effect for Ni than for Cd.