Feeding a Cutin Precursor (Oleic Acid) to Developing Apple Fruit Does Not Increase Cuticle Deposition

Cuticular microcracks impair the barrier functions of the fruit cuticles of many fruit-crop species, including of apple ( Malus ×domestica ). Among other factors, microcracking depends on the balance between the rate of cuticle deposition and the rate of fruit growth. The objective of our study was to test the hypothesis that feeding the developing apple fruit with a precursor of cutin monomers increases cuticle deposition. Carbon-13–labeled oleic acid applied to the surface of ‘Gala’ apples was taken up and incorporated in the cutin fraction at all application concentrations. The rate of cuticle deposition decreased during development. Compared with the untreated controls, spray applications of oleic acid, submersion in an emulsion containing oleic acid, or feeding oleic acid using a tube mounted on the surface did not result in significant increases in the mass of the cuticular membrane (CM) or the dewaxed CM (DCM), or the wax per unit area. Similarly, spray applications of oleic acid had little effect on cuticle deposition. CM, DCM, and wax mass did increase slightly as the concentration of oleic acid increased, but this effect was only occasionally significant statistically. CM and wax mass were greater at 151 days than at 79 days after full bloom. Spray applications of the emulsion of oleic acid caused severe russeting at all concentrations. Uniaxial tensile tests showed no treatment effects on cuticle stiffness or maximum strain, but fracture force did decrease as the oleic acid concentration increased. Our results reveal that external applications of a precursor of cutin monomers or wax constituents do not lead to gravimetrically detectable increases in cuticle mass nor (most likely) to physiologically relevant increases in cuticle mass.