Cuticle deposition in apple fruit: A radial gradient in cuticle age

Abstract

The skins of most fruitcrop species are subject to marked strain due to the continuing expansion of the fruit surface throughout the long developmental period. A mechanical failure of the cuticle during fruit expansion leads to fruit skin disorders. Fruit skin disorders compromise fruit appearance and decrease market value. The cuticular membrane (CM) of apple fruit copes with the ongoing skin strain by continuing to deposit cutin and wax. Recent evidence suggests a radial gradient in strain between the outer (more strain) and the inner surfaces (less strain) of the apple fruit CM. The strain gradient is consistent with the observation that microcracks being initially limited on the outer surface of the CM. It is hypothesized that an ongoing deposition of cutin on the inner surface of the CM would account for this gradient in strain. Unfortunately, direct experimental evidence is lacking. This dissertation is dedicated: (1) to establish a system using the radioactive carbon isotope (14C) and the stable carbon isotope (13C) to quantify CM deposition in developing apple fruit; (2) to identify factors affecting CM deposition in the field and (3) to provide conclusive experimental evidence for a radial gradient in cutin deposition such that the inner surface of the CM is ‘younger’ than the outer surface. Labelled oleic acid or palmitic acid was applied as supposed precursors for cutin monomers to the surface of ‘Idared’ fruit using ‘infinite dose’, ‘finite dose’ or ‘injection’ methods. Of these methods, infinite dose feeding resulted in more consistent and higher precursor incorporation. Incorporation of oleic acid was significantly higher than of palmitic acid. Oleic acid was incorporated in the cutin fraction, but not in the wax fraction. There was no difference in the incorporation of 13C or of 14C labelled oleic acid. Potential gradients in cuticle age were investigated following feeding developing apples with 13C labelled oleic acid. The 13C content was highest in non-ablated CM isolated immediately after the feeding and incorporation period. The 13C content decreased as the CM was progressively ablated from the inner surface. When the 13C content of CMs was quantified in fruit harvested at maturity, the 13C-enriched layer was found deep in the CM following the continued deposition of un-labelled cutin occurring after 13C feeding period. The results recorded the presence of an age gradient in the apple fruit CM between the younger inner surface and the older outer surface. This gradient is the direct result of the deposition of cutin being on the CM’s inner surface.