Factors affecting cuticle synthesis in apple fruit identified under field conditions

Microscopic cracks are causal in a number of commercially-important fruit skin disorders of apple, including russeting. Microcracks result from ongoing skin strain during fruit growth. Continuous cuticle deposition in developing apple fruit reduces strain and thus, microcracking. The objective was to identify factors that affect cuticle deposition under field conditions in developing apples. We fed C-13-labelled oleic acid as a precursor for cutin monomers to the surface of developing apple fruit. Cutin accounted for most of the incorporation of the oleic acid. Incorporation was limited to the surface in direct contact with the feeding solution, there was very little lateral diffusion. Increasing the concentration of the feeding solution generally increased the incorporation of C-13-labelled oleic acid. Incorporation was initially rapid, but then slowed down and approached an asymptote within three days. The duration of the incorporation period after feeding had no effect on the amount of precursor incorporated. All incorporation was essentially instantaneous. During fruit development, incorporation rate was maximal between 64 and 78 days after full bloom (DAFB), then decreased towards maturity. There was no effect of natural (foliar) shading on cutin deposition. Across five apple cultivars, incorporation of the precursor was lowest in 'Gala' and highest in 'Pinova'. Those cultivars whose cuticles had higher mass per unit area also had higher incorporations of the C-13-labelled oleic acid in the cutin, but not in the wax. The potential commercial benefits of applying cutin monomer precursors in the field are discussed.