Abstract
The skin of a fruit protects the vulnerable, nutrient-rich flesh and seed(s) within from the hostile environment. It is also responsible for the fruit’s appearance. In many fruitcrop species, russeting compromises fruit appearance and thus commercial value. Here, we review the literature on fruit russeting, focusing on the factors and mechanisms that induce it and on the management and breeding strategies that may reduce it. Compared with a primary fruit skin, which is usually distinctively colored and shiny, a secondary fruit skin is reddish-brown, dull and slightly rough to the touch (i.e., russeted). This secondary skin (periderm) comprises phellem cells with suberized cell walls, a phellogen and a phelloderm. Russeted (secondary) fruit skins have similar mechanical properties to non-russeted (primary) ones but are more plastic. However, russeted fruit skins are more permeable to water vapor, so russeted fruits suffer higher postharvest water loss, reduced shine, increased shrivel and reduced packed weight (most fruit is sold per kg). Orchard factors that induce russeting include expansion-growth-induced strain, surface wetness, mechanical damage, freezing temperatures, some pests and diseases and some agrochemicals. All these probably act via an increased incidence of cuticular microcracking as a result of local concentrations of mechanical stress. Microcracking impairs the cuticle’s barrier properties. Potential triggers of russeting (the development of a periderm), consequent on cuticular microcracking, include locally high concentrations of O2, lower concentrations of CO2 and more negative water potentials. Horticulturists sometimes spray gibberellins, cytokinins or boron to reduce russeting. Bagging fruit (to exclude surface moisture) is also reportedly effective. From a breeding perspective, genotypes having small and more uniform-sized epidermal cells are judged less likely to be susceptible to russeting.
Highlights
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The fruit skin is exposed to a broad range of abiotic and biotic challenges, serving as a critical barrier protecting the fruit tissues against (a) uncontrolled water loss/uptake [1], (b) uncontrolled exchanges of respiratory gasses (O2, CO2 ) and the hormone ethylene (C2 H4 ) [2], (c) UVradiation [2,3] and (d) invasion by pathogens [4,5]
The periderm reaches the surface as growth proceeds and as the residues of the primary fruit skin tear and are sloughed off
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. This review deals with the development of a secondary surface (a periderm) on the skins of commercial fruit types that usually retain their shiny, distinctively-colored, primary surfaces through to harvest and consumption. Horticulturae 2022, 8, 231 properties, while the cuticle is responsible for the skin’s barrier properties [7,8] It is the primary fruit skin that determines the appearance and attractiveness of the fruit to end consumers, as well as to seed-dispersing animals. The skins of a significant number of commercial fruit types are partially or wholly covered by areas of a secondary surface. Horticulturists refer to this as russeting [11,12]. The entire fruit skin is russeted in most kiwifruit cultivars/species. For a comprehensive review of the biochemistry and molecular biology of russet formation, the reader is referred to the excellent recent reviews by Macnee et al [23] and Wang et al [24]
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