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 65 Zn 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 (,2 mM) 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 Zn 2þ 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.

Microalgal‐driven pH changes in the boundary layer lead to apparent increases in Pb internalization by a unicellular alga in the presence of citrate

“…As was the case in the present study, citrate assimilation occurred but it could not quantitatively account for the enhancement in metal accumulation if a similar rate of uptake of citrate and metal‐citrate complexes was assumed. Diffusive limitation of Zn and Cd uptake was proposed as the most plausible mechanism to explain these observations, and experimental evidence was presented in the case of Zn (Gramlich et al ).…”

Comparison of different speciation techniques to measure Zn availability in hydroponic media

“…The aim of the present work is to apply different techniques to determine the free (with AGNES and PIM) and labile fractions (with DGT and LASV) of Zn available in hydroponic medium at different concentrations of ligand (EDTA or humic acid). Zinc is an essential metal for plants (and its deficiency has been reported in soils of many arid regions of the world [7]), so its uptake attracts much attention. The relevant medium for plants is the soil solution, which can also be approximated by hydroponic media, much more controlled.…”

“…The uptake of Zn and Cu by spinach and tomato plants, for fixed free metal ion concentrations ([Zn 2+ ] and [Cu 2+ ]), increased when more metal was also bound to ligands [5], indicating that metal complexes might contribute to the uptake by dissociating close to the root surface according to their lability [6]. More recent reports [7][8][9] also indicate the contribution of complexes, either by their dissociation or by their direct internalization on the roots, in some cases, while the key role of the free metal ion is confirmed in most cases.…”

“…Diffusive Gradients in Thin films (DGT) is based on the accumulation of metal in a disc of gel with embedded beads of Chelex resin, while another disc of gel serves to (practically) define the diffusion domain [38]. DGT has been applied to natural waters, sediments and soils [8,39] and to hydroponic media [7,40].…”

The presence and leachability of antimony in different wastes and waste handling facilities in Norway

“…The hydrated gel acts as a lter by preventing larger molecules and colloids from diffusing through. Though concerns have been raised about how accurately DGT can mimic uptake through a cell membrane in certain environments and in the presence of labile organic complexes, 22 DGT can be used as a better indicator of the bioavailability or exposure of a metal or metalloid in environmental samples compared to more traditional methods like ltration or equilibrium solution extractions.…”