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 II , uncoordinated or deliberated ferrous ions can initiate Fenton reactions. Fe III citrate complexes are among the most important complexed forms of Fe III 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 III 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 II citrate and hexaaqua complexes in equal concentration. At higher pH, the only product of the photodegradation was Fe II 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 III 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 III citrate has a ubiquitous role in Fe acquisition and homeostasis. Graphical abstract