Patterns of microcracking in apple fruit skin reflect those of the cuticular ridges and of the epidermal cell walls

Abstract

Microcracks in the cuticle of developing apples are aligned with ridges on the inner cuticle surface and are indicative of stress-strain concentrations above the anticlinal cell walls. Microcracks occur in cuticles of most fruits. Growth strains are considered causal. In apples (Malus × domestica), microcracks usually form a mesh pattern similar to that formed by cuticular ridges. Ridge patterns are similar to those of the epidermal cells' anticlinal walls. Our aim was to identify the mechanistic bases for these pattern similarities. By quantifying ridge depth, ridge width, and the areas enclosed by ridges, we reveal the presence of major and minor ridges. Major ridges enclose two-to-four epidermal cells, minor ridges only one cell. There are similar and overlying patterns of microcracking on the cuticle's outer surface and of ridges on its inner surface-microcracks generally follow the outlines of the major ridges. In biaxial tensile tests at 20 kPa, strains were low and microcracks shallow, but at > 40 kPa, strains were higher and microcracks deeper. Microcracks traversing the cuticle are usually aligned with the anticlinal walls of the underlying epidermal cells. In general, increased skin strain is associated with increased skin transpiration. Transpiration increases are reversible for low strains but irreversible for high strains. The alignment of cuticular microcracks with the major ridges, and these with the anticlinal cell walls, indicates associated stress/strain concentrations.

Russet Susceptibility in Apple Is Associated with Skin Cells that Are Larger, More Variable in Size, and of Reduced Fracture Strain

“…The mechanical properties of the apple fruit skin are determined by the epidermal and hypodermal cell layers, but the cuticle’s contribution is minimal [ 2 , 22 , 36 ]. Hence, mechanical properties must be related to the characteristics of cells of the skin and to those of their cell walls.…”

“…Russet begins with the formation of microcracks in the cuticle. These usually occur above, and are aligned with, the anticlinal cell walls of the epidermis [ 1 , 2 , 3 , 4 ]. Microcracks impair the barrier properties of the cuticle [ 2 , 5 ].…”

“…These usually occur above, and are aligned with, the anticlinal cell walls of the epidermis [ 1 , 2 , 3 , 4 ]. Microcracks impair the barrier properties of the cuticle [ 2 , 5 ]. Initially, microcracks are shallow and do not fully traverse the thickness of the cuticle, possibly due to a gradient in strain across the cuticle [ 6 ].…”

“…Initially, microcracks are shallow and do not fully traverse the thickness of the cuticle, possibly due to a gradient in strain across the cuticle [ 6 ]. Shallow microcracks may be repaired by subsequent depositions of wax [ 2 , 7 ].…”

“…These findings are consistent with the observation that the rheological properties of the apple fruit skin are principally determined by the epidermal and hypodermal cell layers, and not by the cuticle [ 22 ]. Furthermore, even the pattern formed by the microcracks in the cuticle is accounted for by the disposition of the anticlinal cell walls of the underlying epidermis [ 2 ]. All these findings indicate the factor(s) determining russet susceptibility (1) more likely reside with the cellular components of the fruit skin, rather than with the cuticle, and (2) that they are especially relevant during the early stages of fruit development, when susceptibility to russet is highest.…”