How exogenous ligand enhances the efficiency of cadmium phytoextraction from soils?

Many experiments showed that exogenous ligands could enhance cadmium (Cd) phytoextraction efficiency in soils. Previous studies suggested that the dissociation and the apoplastic uptake of Cd complex could not fully explain the increase of root Cd uptake. Two hypotheses are evaluated to explain enhanced Cd uptake in the presence of ligand: i) enhanced apoplastic uptake of complex due to reduced apoplastic resistance and ii) complex internalization by membrane transporters. RESULTS: show that the ligand affinity for Cd is a key characteristic determining the potential mechanism for enhanced Cd uptake. When low molecular weight organic acids are applied, the complex dissociation could generally be fast (> 10-3.3 s-1) and result in the increased Cd uptake. When hydrophilic aminopolycarboxylic acids (APCAs) are applied in experiments without water or temperature stresses to the plant, the root water uptake flux could very likely be high (> 10-7.8 dm s-1), and the strong apoplastic complex uptake could enhance the root Cd uptake. When lipophilic APCAs are applied, the strong internalization of the complex by membrane transporters could result in the increased Cd uptake if the maximum internalization rate is high (> 10-12 mol dm-2 s-1). However, the complex internalization by membrane transporters must be experimentally confirmed.

Evaluating a mass balance model for soil trace metals using the historical data from the King's Kitchen Garden (Versailles, France).

The mass balance of reconstituted Cd, Cu, Pb and Zn fluxes from 1683 to 2021 was compared to the current levels of the soil used only for vegetable production in the King's Kitchen Garden in Versailles (France). This comparison was made on the basis of 4 scenarios of organic matter application in the 18th and 19th centuries and by an uncertainty analysis over the entire period. The topsoil contamination falls within that of French kitchen gardens. Modelling of past fluxes predicted the correct trend (an increase) and order of magnitude of the soil metal contents. It produced a relatively accurate evaluation of the Cu and Zn contents. The model underestimated the Pb contents by about 80%, revealing a large and unknown source of soil contamination by this metal. The calculation overestimated the current Cd levels by about 100%, probably due to various biases, for example on atmospheric fallout or the composition of organic amendments. This assessment shows that modelling the mass balance of trace metal fluxes can be used to predict the long-term trend in the levels of these elements in cultivated soils, providing the input data are chosen according to realistic scenarios.