Technical advance: Reduction of Fe(III)-chelates by mesophyll leaf disks of sugar beet. Multi-component origin and effects of Fe deficiency

The characteristics of the Fe(III)-chelate reductase activity have been investigated in mesophyll disks of Fe-sufficient and Fe-deficient sugar beet leaves. The Fe(III)-chelate reductase activity of mesophyll disks was light dependent and increased markedly when the epidermis was removed. Iron(III)-citrate was photo-reduced directly by light in the absence of plant tissue. Total reductase activity was the sum of enzymatic mesophyll reduction, enzymatic reduction carried out by organelles exposed at the disk edge and reduction caused by the release of substances both by exposed mesophyll cells and at the disk edge. Compounds excreted were shown by HPLC to include organic anions, mainly oxalate, citrate and malate. When expressed on a leaf surface basis, Fe deficiency decreased the total mesophyll Fe(III)-chelate reductase activity. However, Fe-sufficient disks reduced less Fe than the Fe-deficient ones when expressed on a chlorophyll basis. The optimal pH values for Fe(III) reduction were always in the range 6.0-6.7. In control leaves Fe(III)-citrate and Fe(III)-malate were the substrates that led to the highest Fe reduction rates. In Fe-deficient leaves Fe(III)-malate led to the highest Fe reduction rates, followed by Fe(III)-EDTA and then Fe(III)-citrate. Km values for the total reductase activity, enzymatic mesophyll reduction and enzymatic reduction carried out by organelles at the disk edge were obtained.Supported by grants AGR97–1177 from the Comisión Interministerial de Ciencia y Tecnología to A.A., and PB94–0086 from the Dirección General de Investigación Científica y Técnica and AIR3-CT94–1973 from the Commission of European Communities to J.A. A.L. was supported by fellowships from the International Center for Advanced Mediterranean Studies-Agronomic Mediterranean Institute of Zaragoza and the Spanish Institute of International Cooperation. A.-F.L.-M. was supported by a fellowship from the Spanish Ministry of Science and Education. F.M. and Y.G. were scientists on contracts from the Spanish Ministry of Education and Culture and the Spanish Council for Scientific Research, respectively.Peer Reviewe