Plant-soil relationship: Role of humic substances in iron nutrition

Abstract

Soil humic substances have been widely regarded as playing a beneficial role in Fe nutrition of plants. This effect has been mainly attributed to the complexing properties of humic molecules which can modify the solubility of Fe by: a) preventing precipitation and subsequent crystallization as wellordered Fe-(hydro)oxides; b) forming soluble complexes with micronutrients which can then move towards the roots. These Fe sources can be used by both Strategy I and Strategy II plants: while in the first case a role as natural substrate for the inducible Fe3+-chelate reductase can be envisaged, in the latter case a ligand exchange mechanism with plant-borne phytosiderophores is likely to operate. Besides this action, soluble low-molecular weight humic fractions have been shown to positively affect mechanisms involved in plant mineral nutrition, such as the PM H+-ATPase. Furthermore, it has been frequently observed that contact of roots with humic substances induces an enhancement of root growth and the proliferation of root hairs. This kind of biochemical and morphological changes can effectively contribute to the overall process of Fe acquisition by plants. © 2006 Springer.