Effect of calcium and trace metals (Cd, Cu, Mn, Ni, Zn) on root iron uptake in relation to chemical properties of the root-excreted ligands.

Interferences of major cations (Ca, Mg) and trace metals (TM, i.e. Cd, Cu, Mn, Ni and Zn) in root Fe uptake were evaluated. Root Fe uptake was modelled including the reactions of the root exuded ligand with the soil major and trace cations. Fe uptake was simulated with different ligands representing various affinities for the cations, the latter varying in concentration. The stability constant of Fe complexes (K) does not influence Fe uptake, contrarily to the ligand parameters for Fe-hydroxide dissolution. Fe uptake decreases when K or Ca in solution increases. Presence of TM has nearly no influence on Fe uptake when the TM complexes have low stability constants (K), as in the case of oxalate and citrate complexes. When ligands have high K, like EDTA, DFO-B or mugineic acid (MA), TM reduces Fe uptake by 51-55%, and much more in the case of TM contamination. Exudation of Fe ligands with low K has no negative effect on TM uptake, which can increase if the dissociation rate is high, as for Cu complexes. Ligands with high K (EDTA, DFO-B, MA) greatly reduce TM uptake, only if their hydrated cations can be absorbed. Calcium does not significantly reduce Fe uptake when Ca-complexes have K < 10. Consequently, ligands like oxalate or MA should be efficient in most soils. TM should perturbate Fe uptake mediated by ligands with high K such as MA, but not oxalate. Plants exuding phytosiderophores should also absorb TM complexes to avoid micronutrient deficiencies.