Surface moisture increases microcracking and water vapour permeance of apple fruit skin

Abstract

Surface moisture induces microcracking in the cuticle of fruit skins. Our objective was to study the effects of surface moisture on cuticular microcracking, the permeance to water vapour and russeting in developing 'Pinova' apple fruit. • Surface moisture was applied by fixing to the fruit a plastic tube containing deionized water. Microcracking was quantified by fluorescence microscopy and image analysis following infiltration with acridine orange. Water vapour permeance was determined gravimetrically using skin segments (ES) mounted in diffusion cells. • Cumulative water loss through the ES increased linearly with time. Throughout development, surface moisture significantly increased skin permeance. The effect was largest during early development and decreased towards maturity. Recovery time courses revealed that following moisture treatment of young fruit for 12 days, skin permeance continued to increase until about 14 days after terminating the moisture treatment. Thereafter, skin permeance decreased over the next 28 days, then approaching the control level. This behaviour indicates gradual healing of the impaired cuticular barrier. Nevertheless, permeance still remained significantly higher compared with the untreated control. Similar patterns of permeance change were observed following moisture treatments at later stages of development. The early moisture treatment beginning at 23 DAFB resulted in russeting of the exposed surfaces. There was no russet in control fruit without a tube or in control fruit with a tube mounted for 12 days without water. • The data demonstrate that surface moisture increases microcracking and water vapour permeance. This may lead to the formation of a periderm and, hence, a russeted fruit surface.

Russeting in Apple is Initiated after Exposure to Moisture Ends: Molecular and Biochemical Evidence

“…Moisture exposure occurring later in fruit development (for example between 66 and 78 DAFB or 93 and 105 DAFB) resulted in only slight decreases in expression of cutin- and wax-related genes and no increases in expression of suberin-related genes. This is consistent with the observed lack of periderm formation [ 16 ] and the lack of visual symptoms of russeting [ 15 ]. The higher susceptibility to russet during early fruit development results from the high relative area growth rates at this stage [ 49 ].…”

“…These included genes involved in the synthesis of monomers and constituents ( GPAT6 , KCS10 , SHN3 , WSD1 , CER6 ) and their transport across the plasma membrane ( ABCG11 ). The down-regulation occurred at the same time as microcracks formed [ 15 , 16 ], as cuticle thickness around microcracks decreased [ 16 ] and as the amounts of the amounts of the major constituents of cutin and wax decreased. These observations suggest a causal relation between moisture exposure, a reduction in the expressions of genes involved in cuticle synthesis, a decrease in cuticle mass and the subsequent formation of a periderm and the onset of suberin synthesis and deposition.…”

“…Deposition of wax in microcracks is an effective repair mechanism that re-establishes the cuticle’s barrier function [ 15 , 45 , 46 ]. Furthermore, wax deposition in the cuticle of an expanding fruit surface converts elastic strain into plastic strain, thereby fixing both strain and stress [ 47 ].…”

“…Microscopic cracks (‘microcracks’) in the cuticle are the first visible symptom of russeting [ 10 , 15 , 16 ]. Microcracks form when the fruit skin is strained during periods of rapid surface expansion [ 17 ].…”

“…Microcracking is exacerbated by surface moisture [ 18 , 19 , 20 ]. Recently, a system was developed that allows microcracks, and hence also russet, to be induced in the skins of developing apples by localized exposure to moisture [ 15 ]. The remaining unexposed skin of the same fruit may serve as the control.…”