Russeting in apple seems unrelated to the mechanical properties of the cuticle at maturity

Abstract

Russeting is a commercially important disorder of the fruit skin of apples (Malus ×domestica Borkh.). It is thought to result from microscopic cracking of the cuticle on the fruit surface and the subsequent formation of a periderm just below. The study investigates 22 apples cultivars having widely different russeting susceptibilities to determine if susceptibility could be related to the mechanical characteristics of the cuticles at maturity. The mass per unit area of the cuticular membrane (CM), the dewaxed cuticular membrane (DCM), and the cuticle's wax content all varied significantly among the cultivars examined but no simple correlative relationships with russeting susceptibility could be found. Across all cultivars, the mass of wax per unit area was linearly related to CM mass per unit area (R = 0.77, P < 0.0001). The cuticle of all cultivars was markedly strained as indexed by the release of biaxial strain in the CM on extraction of the wax. The release of biaxial strain was linearly and positively related to wax mass per unit area. Maximum force (Fmax) in uniaxial tensile tests, strain at maximum force (εmax), and the stiffness (S) differed widely among the cultivars tested, but, again, there were no relationships between these mechanical properties and russeting susceptibility. Wax extraction from the CM decreased the Fmax in uniaxial tensile tests, increased εmax, and decreased the S. Our results show that none of the cuticle variables measured at maturity nor any of the isolated-cuticle mechanical properties contributes significantly to russeting susceptibility.