Dark Stained Tissues of the Epicarp of Encore Mandarin: Interactions with the Production of Hydroxyl Radicals

Abstract

The interactions between hydroxyl radical production and the composition of the epicarp cells associated with the dark stained tissues of mature fruits of Encore mandarin were investigated. Phosphatidylinositol content of the cells associated with dark stains was lower in unpitted tissues, whereas the concentration of phosphatidylglycerol, phosphatidylcholine and phosphatidylethanolamine did not vary significantly. In these pitted cells, protein content also showed a 1.59-fold increase. Additionally, the relative proportions of gln, thr, asp, glu and gly increased sharply, while ala and tyr decreased. The polypeptide patterns showed quantitative changes when samples of stained and unstained tissues were compared by SDS-PAGE electrophoresis. Qualitatively, the cells associated with pitted tissues revealed a new polypeptide band with an apparent mol. wt. of 50.4 kDa as well as the disappearance of another one of 10 kDa. Ethylene production in the dark stained tissues was lower than in unpitted tissues and these cells showed significant increases in membrane permeability, hydroxyl radical production, and lipid peroxidation. Furthermore, in these tissues the levels of the carotenoids increased significantly but levels of chlorophyll decreased. It was concluded that in the pitted tissues the significant changes of the membrane composition are closely associated with an increasing acyl lipid peroxidation mediated by hydroxyl radical production. The modulation of this metabolism further indicates an incomplete degradation of fatty acids. The implications of the increasing accumulation of carotenoids in the synthesis of oxy radicals are also discussed.