Rind blemish of citrus. II. Structure and ultrastructure

Abstract

Four types of rind blemish on ‘Valencia’ oranges were studied by light and scanning electron microscopy. They were the smooth translucent blemishes on immature and mature fruit, the fine textured corky blemish on mature and immature fruit, scabby irregular blemishes, and stippling that occurs on the periphery of blemishes. All blemish types were formed by wound periderm as a result of wind scarring, while stippling was caused by copper fungicides. The smooth translucent type, formed as a result of cork abscission from the fine textured corky blemish, had no prominent phelloderm and evidence of surface injury was slight. The other types had prominent phelloderm, the greatest amount of cork production being associated with the scabby blemish and stipples. The surface of the fine textured corky blemish was made up of cork plates which may abscise and leave remnants of cork cell walls on the surface. These cell walls are subject to weathering as the fruit matures. The angular cork cells on immature fruits were prominent but these were coated with a waxy substance on mature fruit. The phelloderm of blemishes of immature fruits did not stain with toluidine blue O, but the reaction on mature fruit indicated the presence of polyphenols. Staining was most intense with stipples. The influence of stomatal apertures on the blemish was discussed in relation to stippling.