The role of flavonoids on the weathering of iron and manganese minerals in the rhizosphere

Iron (Fe) and manganese (Mn) are essential micronutrients for plants but, in most agricultural soils, they are present in scarcely bioavailable forms. Iron occurs mainly as Fe(III) in poorly soluble oxides (hematite, maghemite), oxyhydroxides (goethite, lepidocrocite), amorphous hydroxides and poorly crystalline minerals like ferrihydrite, or is included as Fe(II) or Fe(III) in the lattice of primary and secondary minerals. Manganese is present mainly as Mn(IV) and Mn(III) in amorphous secondary phases as well as in crystalline oxides (birnessite) and hydroxides (manganite), but it is taken up by plants only in the reduced form Mn(II). To increase the bioavailability of these nutrients, plants have developed different mechanisms, among which the active release of flavonoids into the rhizosphere. Flavonoids are polyphenolic compounds with multifunctional properties, such as the protection of plants against pests and diseases, the regulation of root growth and function, and the induction of allelopathy. Very few studies have focused on the mechanisms of Fe and Mn mobilization operated by flavonoids in the soil and, particularly, in the rhizosphere. It can be hypothesized that flavonoids dissolve Fe and Mn minerals by means of reducing or complexing processes, or by a combination of these two mechanisms. In a recent experiment, we observed that rutin mobilized a high amount of Fe from an alkaline soil by reducing it to Fe(II), and quercetin was very efficient in Mn solubilisation from an acidic soil by reducing it to soluble ions Mn(II). When quercetin was used in combination with citrate, Mn solubilisation further increased due to reduction and complexation processes. On the basis of these experimental evidences, the present study aims at investigating the effects of some flavonoids (rutin, quercetin and genistein), both alone and in combination, on the alteration of some of the most representative Fe and Mn (hydr)oxides of the soil (goethite, hematite and birnessite). For each flavonoid, saturated aqueous solutions are prepared, containing Na3N (10 mM) as bacteriostatic agent. Synthetic Fe and Mn minerals are mixed at 20% (w/w) with an inert glass powder and let to interact for 24 h with 30 mL flavonoid solutions under continuous stirring. After centrifugation, the liquid fraction is filtered and analysed to determine: (i) the amount of Fe and Mn by ICP-AES and voltammetry; (ii) the concentration of flavonoids and other secondary products by HPLC analyses. The solid fraction is dried and analysed by (iii) XRD and (iv) SEM-EDX in order to study any structural modification of the minerals. Results are compared to a control without flavonoids and a control prepared using citrate, a well-known complexing molecule in plants.