Diffusion limitation of zinc fluxes into wheat roots, PLM and DGT devices in the presence of organic ligands

Abstract

Environmental context Zinc is an essential micronutrient for plants and many arid areas of the world have zinc-deficient soils. The bioavailability of Zn to plants is influenced by diffusion limitations and complex lability in the soil solution. To identify the relative importance of these two factors, we investigated the influence of diffusion layer thickness on Zn uptake by wheat and by two bio-mimetic devices in the presence of ethylenediaminetetraacetic acid and two natural ligands found in soil. Abstract Organic ligands can increase metal mobility in soils. The extent to which this can contribute to plant metal uptake depends among others, on complex lability and diffusion limitations in solute transfer from the soil solution to root uptake sites. We investigated the influence of diffusion layer thickness on zinc uptake by wheat seedlings in the presence of ethylenediaminetetraacetic acid (EDTA), citrate and histidine with similar free Zn by measuring 65Zn uptake from stirred, non-stirred and agar-containing solutions. Analogous experiments were performed using permeation liquid membranes (PLM) and ‘diffusive gradients in thin films’ (DGT) probes as bio-mimetic devices. In treatments with low EDTA concentrations (~2µM) or ligand-free Zn solution, increasing diffusion layer thickness reduced Zn fluxes into roots to a similar extent as into PLM and DGT probes, indicating reduced uptake attributable to diffusion limitation. In the citrate treatments root Zn influx was similar to EDTA treatments under stirred conditions, but increasing diffusion layer thickness did not affect Zn uptake. This suggests complex dissociation compensated for reduced Zn2+ diffusion and that the entire complexes were not taken up. The Zn root influxes in the histidine treatments were found to be on average by a factor of 2.5 higher than in the citrate treatments and they also showed no decrease in non-stirred and agar treatments. Dissociation kinetics inferred from PLM measurements explained a large part, although not all, of the increased Zn uptake by the plants in the presence of histidine. The difference may be a result of the uptake of neutral or positive Zn–histidine complexes. The results of this study confirm that labile complexes can contribute to Zn uptake by wheat either through diffusion limitation and complex dissociation or through uptake of entire complexes, depending on the nature of the ligands.