The theoretical prediction of interactions between soluble silicon, iron (III) and carboxylate anions in plant fluids

Abstract

Recently, we proposed the presence of iron (Fe)-polysilicate chelate-like complexes as one of the possible mechanism involved in silicon (Si)-mediated alleviation of Fe deficiency in plants (see Pavlovic et al., this volume). The presence of metals and organic ligands (e.g. oxalate and citrate) in aqueous solution can result in the complexation with dissolved silica. In aqueous solutions with low dissolved silica concentration (<10 mM) the principal species are monomeric (e.g. H4Si04(aq), H3Si04·, H2Si0/ ), and polymeric forms to various extents. The anionic species Sil/ which is formed according to the reaction: Si(OH)4 + 3H2L =SiL32-+ 2H+ + 4H20, is a hexacoordinated charge-transfer complex in which the ligands (L) are bidentately bound. A likely assumption is that the main interactions between Si and polycarboxylates do not take place in the aqueous phase, I nit rather on the surface of the dissolved particles (such as the surface of plant cell walls). As a strong Fe chelator, citrate (Cit4-) has been considered as the most likely major candidate for Fe transport in plants via xylem and apoplastic fluids. Ferric-citrate chemistry is complicated and a definitive description of Its aqueous speciation at the neutral or weakly acidic pH va lues (typical for plant fluids) remains elusive. The predominant Fe(lll)-citrate species at the physiological pH values are the mononuclear biologically relevant dicitrate [Fe(Cit)2)5- complex and multinuclear species of low nuclearity, in particular trinuclear complexes The [Fe(Cit)i]5· complex predominates in the Fe: citrate molar ratio range 1:100 to 1:10 (the normal range found in the apoplastic fluids of Fe-adequate plants). Aqueous silica forms stable complexes with polymeric ferric oxy-hydroxide species, which may be formed in the root apoplast. The structure of the Fe-Si complexes appears to be similar at both acid and alkaline pH. The presence of single-corner Fe-0-Si bonds is expected in the monomer complex FeOSi(OH)3 2+ in the acidic solutions. Silica substitutes for double-corner FeO6-octahedra in Fe oxy-hydroxide polymeric complexes existing in the early stages of Fe(III) hydrolysis, likely by forming 2C-type (double corner) complexes with small Fe oxy-hydroxide polymers whose structure consists of Fe06-octahedra linked together by common edges. In low activities aqueous silica solutions, Fe3+ remains essentially hexa-coordinated.