New aspects of the photodegradation of iron(III) citrate: spectroscopic studies and plant-related factors.

Iron (Fe) is an essential cofactor for all livings. Although Fe membrane transport mechanisms often utilize Fe, uncoordinated or deliberated ferrous ions can initiate Fenton reactions. Fe citrate complexes are among the most important complexed forms of Fe especially in plants that, indeed, can undergo photoreduction. Since leaves as photosynthetic organs of higher plants are generally exposed to illumination in daytime, photoreaction of ferric species may have biological relevance in iron metabolism, the relevance of which is poorly understood. In present work Fe citrate transformation during the photodegradation in solution and after foliar application on leaves was studied by Mössbauer analysis directly. To obtain irradiation time dependence of the speciation of iron in solutions, four model solutions of different pH values (1.5, 3.3, 5.5, and 7.0) with Fe to citrate molar ratio 1:1.1 were exposed to light. Highly acidic conditions led to a complete reduction of Fe together with the formation of Fe citrate and hexaaqua complexes in equal concentration. At higher pH, the only product of the photodegradation was Fe citrate, which was later reoxidized and polymerized, resulting in the formation of polynuclear stable ferric compound. To test biological relevance, leaves of cabbage were treated with Fe citrate solution. X-ray fluorescence imaging indicated the accumulation of Fe in the treated leaf parts. Mössbauer analysis revealed the presence of several ferric species incorporated into the biological structure. The Fe speciation observed should be considered in biological systems where Fe citrate has a ubiquitous role in Fe acquisition and homeostasis.