Abstract
Flavonoids are polyphenolic compounds that can be released by plant roots into the rhizosphere as a result of root turnover, root injury, root decomposition or through root exudation. In the rhizosphere, the released flavonoids can have multifunctional roles, such as protecting plants against pests and diseases, regulating root growth and functions, influencing nutrient cycles (eg N cycle), interacting with proteins and make protein N more resistant to microbial degradation, and inducing allelopathic growth effects. Much less is known about their role in interacting with soil constituents and promoting the mobilization of mineral nutrients. In this research we studied the interactions of three flavonoids which can be actively released by plant roots (quercetin, rutin, and genistein) with two different agricultural soils, one acidic and one alkaline. In addition, synergistic and competitive effects with three organic acids usually found in root exudates (citric, oxalic, and malic acid) were assessed for the mobilization of Al, Si, Fe, Cu, Zn, Mn, Cd, Cr, Ni and Pb by using different combinations of the above-cited compounds. Mobilization processes strongly depended on the soil type and the nature of the element which is being mobilized. More specifically, citrate out of the three organic acids was the most effective in acid soil while oxalate was able to extract more Cu, Zn and Ni in alkaline soil. Furthermore, in acid soil a synergistic effect by organic acids was observed only for Mn. In alkaline soil this was noticed for Si, while for Al, Cu, Zn and Ni a competitive effect occurred. While organic acids may be mainly involved in complexation and ligand exchange reactions promoting the desorption of elements from soil minerals, flavonoids appear to promote redox reactions which enhance significantly the solubilization of Fe and Mn. In particular, rutin was 25 times more effective than citrate for Fe mobilization in alkaline soil while quercetin was 50 times more efficient than citrate in extracting Mn from the acid soil. In addition, in combination with citrate, quercetin (at µM concentration) can increase as much as five times the amount of Mn which can be extracted by citrate alone. The mechanisms by which Fe and Mn are mobilized by flavonoids were also investigated. Differently from citrate, rutin mobilized Fe from the alkaline soil by reducing it to Fe (II)(reductive dissolution). As for Mn in the acid soil, two different mechanisms seem to be responsible of the marked solubilization of this element. When quercetin acted alone, only a reductive dissolution was observed, with the reduction of Mn to Mn (II). When quercetin was combined with citrate, besides the reductive dissolution to Mn (II), Mn was also partly reduced by quercetin to Mn (III) which, in turn, was immediately complexed and stabilized by citrate. The formation of this complex could have contributed to the large Mn mobilization observed. At last, the effects of flavonoids and organic acids on soil mineralogy are also being evaluated by XRD (X-Ray Diffraction) with Rietveld method and SEM-EDX (Scanning Electron Microscopy coupled to Energy Dispersive X-ray spectroscopy). Our results confirm the importance of studying the combined effect of root exudates and their influence on the biogeochemical cycle of trace elements in rhizosphere soil.