Detection of Monodehydroascorbic Acid Radical in Sulfite-Treated Leaves and Mechanism of its Formation

The aim of the present study is to detect the monodehydroascorbicacid (MDA) radical in broad bean (Vicia faba L.) leaves whichwere treated by vacuum-infiltration in Na₂SO₃ solution and subsequentcentrifugation (sulfite-treated leaves). When sulfite-treatedleaves were illuminated with white light, the electron spinresonance (ESR) signal of MDA radical was observed. The levelof the MDA radical depended on the concentration of sulfitethat was used for vacuum-infiltration and on the light intensityof illumination. The formation of the MDA radical in sulfite-treatedleaves was inhibited by DCMU or by replacement of air with N₂.Glycolaldehyde also inhibited the formation of MDA radical insulfite-treated leaves. Catalase activity was decreased by thesulfite treatment without affecting significantly the activitiesof ascorbate peroxidase (AA-POX) and of peroxidase which preferentiallyoxidizes phenolics (PhOH-POX). From these results, we concludethat the formation of the MDA radical in sulfite-treated leavesis catalyzed by peroxidases using the H₂O₂ which is generatedby photorespiration and the Mehler reaction. ¹On leave from the Center for Multidisciplinary Studies, Universityof Belgrade, Yugoslavia.