Ultrastructure of epicuticular wax aggregates during fruit development in apple (<i>Malus domestica</i>Borkh.)

SummaryDevelopment of wax platelets on the surface of ‘Delicious’ (Malus domestica Borkh.) apple fruit was investigated throughout the growing season using field emission scanning electron microscopy. At 5,000 and greater, wax crystalline structures appeared to be composed of microtubules (MT), aggregates of individual MT to form single platelets, and of one or more platelets. The thickness of a single wax platelet ranged from 116–128 nm, approximately the diameter of a single MT; whereas multiple-platelet aggregates ranged in thickness from 307–428 nm. Individual platelets, multiple platelet aggregates and MT were visible throughout fruit development, as well as on different apple cultivars. Examination of the cuticle from young fruitlets (receptacle diameter = 3 mm) of ‘Chinese Crabapple’ (M. hupehensis Rehd.), which develops without trichomes, best demonstrated early platelet formation. A model for wax platelet and cuticle development on apple is proposed based on these data.