As a result of continuing volume and hence surface-area growth, the skins of most fruit species suffer ongoing strain throughout development. Maintenance of surface integrity is essential to protect the underlying tissues from desiccation and pathogen attack. Fruit skins are commonly “primary” in structure. They comprise a polymeric cuticle overlying an epidermis and a hypodermis. The cuticle is responsible for the skin's barrier function and the cellular layers for the skin's load-bearing functions. Skin failure can be just of the cuticle layer (microcracking) resulting in barrier impairment or it can involve cuticle and cellular layers (macrocracking) resulting in both barrier and structural impairment. Fruit skin failure is associated with a number of disorders including shriveling, cracking, russeting, and skin spots. All result in reduced market value. Our objective is to review the literature on the strategies adopted by fruit to cope with the challenge of continuing skin expansion. We uncover a multistep strategy to prevent or minimize the risk of fruit skin failure. This comprises: (1) area expansion of the load-bearing skin-cell layer(s) by ongoing cell division and (2) the avoidance of skin stress or strain concentrations by matching patterns of skin-cell division to those of area expansion. Also involved, (3) are the partitioning of cuticle strain into plastic and viscoelastic components at the expense of the elastic one. For this, wax and cutin are deposited in the cuticle during growth. Wax and cutin deposition “fix” the strain in the cuticle. Cutin is preferentially deposited on the inner surface of the cuticle, which fixes the strain, but it leaves the outer cuticle surface more strained. Last, (4) if the primary skin is damaged, the barrier functions are restored by the formation of a “secondary” fruit surface (periderm). Lignin can also be used to strengthen the underlying cells following structural failure.

Cuticles envelope primary surfaces of the above-ground portion of plants. They function as barriers to water movement and to gas exchange, and in pathogen defense. To serve as a barrier on growing organs, cuticles must remain intact but at the same time must accommodate ongoing growth. Minimizing cuticle failure has stimulated significant research on the cuticle’s mechanical properties. The objective here is to review the literature on the mechanical properties of isolated fruit and leaf cuticles. Cuticles are viscoelastic polymers. Viscoelasticity results mainly from the cutin matrix. Impregnation by waxes, flavonoids, and cutan increases stiffness and strength but decreases extensibility. On the inner side, the cutin matrix is impregnated by cell wall polysaccharides, which are responsible for its elastic behavior. Across species, the maximum forces sustainable by hydrated cuticles in uniaxial tensile tests averaged 0.82 N (range 0.15–1.63 N), the maximum stresses averaged 13.2 MPa (range 2.0–29.0 MPa), the maximum strains averaged 8.8% (range 1.6–28.0%), and the moduli of elasticity averaged 224 MPa (range 60–730 MPa). Among the environmental factors, high temperature and hydration both decreased stiffness. Therefore, the mechanical properties of cuticles in vivo depend largely on the relative proportions of their constituents. These proportions change during development and are also affected by environmental factors such as temperature.

Russeting in apples (Malus × domestica Borkh.) and pears (Pyrus communis L.) is a disorder of the fruit skin that results from microscopic cracks in the cuticle and the subsequent formation of a periderm. To better understand russeting, rheological properties of cuticular membranes (CM) and periderm membranes (PM) were studied from the russet-sensitive apple ‘Karmijn de Sonnaville’ and from ‘Conference’ pear. The CM and PM were isolated enzymatically, investigated by microscopy and subjected to tensile tests, creep/relaxation tests and to stepwise creep tests using a material testing machine. The isolated CM formed a continuous polymer, whereas the PM represented a cellular structure of stacked cork cells. Tensile tests revealed higher plasticity of the hydrated PM compared with the CM, as indicated by a higher strain at the maximum force (ɛ_max) and a lower modulus of elasticity (E). In apple, the maximum force (F_max) was higher in the CM than in the PM but in pear the higher F_max value was found for the PM. In specimens obtained from the CM : PM transition zone, the weak point in apple was found to be at the CM : PM borderline but in pear it was within the CM. In both apple and pear, creep/relaxation tests revealed elastic strain, creep strain, viscoelastic strain and viscous strain components in both the PM and CM. For any particular force, strains were always greater in the PM than in the CM and were also greater in pear than in apple. The ɛ_max and F_max values of the CM and PM were lower than those of non-russeted and russeted whole-fruit skin segments, which included adhering tissue. In russeting, stiff CM are replaced by more plastic PM. Further, the cell layers underlying the CM and PM represent the load-bearing structure in the fruit skin in apple and pear.

The effect of four applications of gibberellin A ₄₊₇ [GA ₄₊₇ (10 mg·L ⁻¹ at 10-day intervals beginning with petal fall)] on water-induced russeting, formation of microcracks. and on fruit growth and deposition of the cuticular membrane (CM) was studied in developing ‘Golden Delicious’ fruit ( Malus × domestica Borkh.). Submerging developing apple fruit in deionized water for 48 h induced russeting in untreated control but not in GA ₄₊₇ -treated fruit. Immersing in water during early fruit development, 19 days after full bloom (19 DAFB), resulted in more russeting than immersions occurring later (139 DAFB). Water on the outer surface of epidermal segments increased the frequency of microscopic cracks in untreated controls but to a lesser degree in GA ₄₊₇ -treated fruit. The effect of GA ₄₊₇ on water-induced russeting and formation of microcracks was larger during early as compared with later stages of fruit development. Fruit treated with GA ₄₊₇ consistently had fewer microcracks as compared with non-treated control fruit. GA ₄₊₇ had no effect on amounts or rates of cutin or wax deposition, strain, or mechanical properties of the CM as compared with the non-treated control. Thus, the decrease in russeting and formation of microcracks in the cuticle of GA ₄₊₇ -treated fruit must be accounted for effects on underlying epi- and hypodermal tissues.