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Ongoing Growth Challenges Fruit Skin Integrity

Knoche, Moritz, Lang, Alexander
Critical reviews in plant sciences 2017 v.36 no.3 pp. 190-215
cell division, cracking, cutin, fruit quality, fruiting, lignin, market value, pathogens, periderm, risk, surface area, viscoelasticity
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.