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 inv estigated in mesophyll disks of Fe-sufficient and Fe-deficient sugar beet l eaves, The Fe(III)chelate reductase activity of mesophyll disks was light d ependent and increased markedly when the epidermis was removed. Iron(III)-c itrate was photo-reduced directly by light in the absence of plant tissue, Total reductase activity was the sum of enzymatic mesophyll reduction, enzy matic reduction carried out by organelles exposed at the disk edge and redu ction caused by the release of substances both by exposed mesophyll cells a nd at the disk edge. Compounds excreted were shown by HPLC to include organ ic anions, mainly oxalate, citrate and malate, When expressed on a leaf sur face basis, Fe deficiency decreased the total mesophyll Fe(III)-chelate red uctase activity. However, Fe-sufficient disks reduced less Fe than the Fe-d eficient 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 F e(III)-citrate and Fe(III)-malate were the substrates that led to the highe st Fe reduction rates, In Fe-deficient leaves Fe(III)-malate led to the hig hest Fe reduction rates, followed by Fe(III)-EDTA and then Fe(III)-citrate, K-m values for the total reductase activity, enzymatic mesophyll reduction and enzymatic reduction carried out by organelles at the disk edge were ob tained.