Could root-excreted iron ligands contribute to cadmium and zinc uptake by the hyperaccumulator Noccaea caerulescens?
Résumé
Purpose Evaluation of various mechanisms of Cd2+ and Zn2+ root uptake through modelling, including interactions with Fe uptake. Methods Four increasingly complex models were developed and tested against measured uptake by the hyperaccumulator Noccaea caerulescens grown on three contaminated soils. Model 1 formalises root uptake of free hydrated ions only, to which the dissociation of soil-borne metal complexes was added in Model 2, together with the root uptake of these complexes through the Fe pathway in Model 3. Model 4 represents Strategy I for Fe uptake, with Cd2+ and Zn2+ complexed by a root organic exudate hijacking the Fe adsorption/reduction pathway. Results Model 1 enables a good prediction of Cd and Zn uptake in acidic or neutral soils. In contrast, the high Cd uptake in the alkaline soil cannot be reached either by simulating additionnally the dissociation of Cd complexes with soil-borne ligands (Model 2), nor by replicating their uptake through the Fe pathway (Model 3). These results suggest that the plant increases the Cd availability in alkaline soil during the uptake process. Supporting this idea, the uptake of Cd desorbed from iron hydroxide and calcium carbonate by root exuded ligands for Fe acquisition (simulated in Model 4) can explain the high Cd uptake from alkaline soil. Conclusions In acid soils, Cd and Zn uptake can be predicted from their free hydrated concentrations, while in calcareous soils with very low concentrations of free hydrated ions, root exudation for Fe acquisition could make other soil Cd species phytoavailable.