INTERACTION OF MUGINEIC ACID WITH SYNTHETICALLY PRODUCED IRON-OXIDES

Interactions between mugineic acid (MA) purified from root washings of Fe-deficient barley (Hordeum vulgare L., cv. Minorimugi) and syntheti cally produced Fe oxides (goethite, hematite, lepidocrocite, and ferri hydrite) were studied in the equilibrium pH range of 3 to 11. The amou nt of MA adsorbed on Fe oxides was related to their specific surface a rea and followed the order: ferrihydrite > > goethite greater than or equal to lepidocrocite greater than or equal to hematite. The adsorpti on of MA on Fe oxides also decreased with increasing equilibrium pH an d increased with increasing MA concentration. The MAFe complexes were also adsorbed on Fe oxides, especially on ferrihydrite and goethite, a t pH 3 to 7. At pH > 10, however, MAFe complexes were decomposed to MA and Fe(OH)(3) colloids. The amount of Fe dissolved from Fe oxides by MA was in the following order: ferrihydrite > > lepidocrocite greater than or equal to hematite = goethite. The Fe dissolution from Fe oxide s by MA was related to their crystallinity and the maximum amount of F e dissolved by MA was in the pH range of 7 to 8. The amounts of MA ads orbed on Fe oxides and Fe dissolved by MA from Fe oxides depended on M A concentration, pH, and the type and the amount of Fe oxides added in the system. The Fe dissolution processes from Fe oxides by MA could i nvolve two factors, namely (i) the complexation of MA with Fe exposed on the surface of Fe oxides by ligand exchange; and (ii) the release o f MAFe complexes from adsorption sites on Fe oxides by nucleophilic su bstitution. Our data further clarify the chemistry of Fe nutrition of graminaceous plants in Fe-deficiency-causing soils.