Composition and Variability of Epicuticular Waxes in Apple Cultivars

Abstract

Variation in amount and composition of epicuticular wax among several apple (Malus ×domestica Borkh.) cultivars was characterized by gas chromatography, thin-layer chromatography, and gas chromatography-mass spectroscopy. Across cultivars, wax mass ranged from 366 to 1038 μg·cm-2. Wax mass decreased during the 30 days before harvest. Ursolic acid accounted for 32% to 70% of the hydrocarbons that make up the epicuticular wax. Alkanes, predominantly 29-carbon nonacosane, comprised 16.6% to 49%. Primary alcohols of the hydrocarbons ranged from 0% to 14.6% of the epicuticular wax. Secondary alcohols of the hydrocarbons were the most cultivar specific, making up 20.4% of the epicuticular wax in `Delicious' and only 1.9% `Golden Delicious' strains. Aldehydes and ketones of the hydrocarbons represented a small amount of total wax, ranging from 0% and 6.0%. Percentage of primary alcohol in the epicuticular wax increased as fruit developed. Other components showed no distinct trends with fruit development. Examination of the ultrastructure of cuticular wax using scanning electron microscopy revealed structural differences among cultivars.