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Formation of microcracks in the cuticular membrane (CM) of epidermal segments (ES) of apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf., ‘Golden Delicious’, ‘Braeburn’, ‘Idared’, ‘Jonagold’, and ‘Topaz’; all grafted on ‘Malling.9’ rootstocks] fruit was studied after exposure of the surface of the ES to water. Potential strain of the CM on the ES was preserved by mounting a stainless steel washer on the fruit surface using an ethyl-cyanacrylate adhesive. Subsequently, ES were excised by tangentially cutting underneath the washer. The number of microcracks in the CM was established by light microscopy before and after a 48-h incubation period in deionized water. Within 48 h, the number of microcracks rapidly increased when the outer surface of ES of ‘Golden Delicious’ apple was exposed to water, but there was essentially no increase in microcracks when exposed to the ambient atmosphere. The occurrence of microcracks depended on the region of the fruit surface and increased from the rim of the pedicel cavity to the calyx. Increasing the relative humidity (greater than 75% relative humidity at 22 °C) above the outer surface of the ES exponentially increased the number of microcracks. Water-induced microcracking was not limited to ‘Golden Delicious’, but also occurred in ‘Braeburn’, ‘Jonagold’, ‘Topaz’, and, to a markedly smaller extent, in ‘Idared’ apple. The mechanism of formation of microcracks in the CM of apple fruit and their role in fruit russeting are discussed.

Neck shrivel is a fruit disorder of european plum (Prunus domestica L.). We investigate whether an asymmetrical distribution of osmolytes might explain the observations of a turgid stylar end and a flaccid stem end, in a selection of 17 plum cultivars sourced from two sites. The osmotic potential (Ψ_Π) of the juices expressed from stem or stylar end fruit samples decreased (became more negative) during development. The cell turgor (Ψ _P ) slightly increased during development up to 352 ± 42 kPa at 78 days after full bloom (DAFB) in the stem end and up to 331 ± 51 kPa at 92 DAFB in the stylar end, and then decreased. At maturity, Ψ _P averaged 22 ± 3 kPa in the stem end and 22 ± 4 kPa in the stylar end. These values are negligibly low compared with the very negative values of Ψ_Π in the stylar end (−3188 ± 73 kPa) and stem end (−3060 ± 74 kPa). There was a transient gradient in Ψ_Π between stylar end and stem end that almost disappeared by maturity. Marked differences in Ψ_Π and its distribution were measured across 17 cultivars. In the majority (14), Ψ_Π was more negative at the stylar end than at the stem end. A more negative Ψ_Π in the stem was only detected in ‘Aprimira’, ‘Topfive’, and ‘Tophit’. Our results demonstrate that cell Ψ _P is very low and is essentially independent of Ψ_Π in developing european plums. In most cultivars, Ψ_Π in the stylar end is more negative than in the stem end. The absence of an axial gradient in Ψ _P and the small differences in Ψ_Π between the stem and stylar end make both factors unlikely candidates for explaining neck shrivel.

Russeting is an important surface disorder in fruit and mechanical growth stresses, among other factors, are considered causal in russet induction. To test this hypothesis, fruit development and russeting were monitored on a whole fruit level and also in the calyx, cheek, and neck region of developing ‘Conference’ and ‘Condo’ pear fruit (Pyrus communis L.). To quantify growth, the pear fruit was geometrically modeled as approximating to half of a prolate spheroid for the calyx region and two truncated cones for the cheek and neck regions, respectively. Mass and surface area of ‘Conference’ and ‘Condo’ fruit increased in a single sigmoidal pattern with time. Fruit volume, determined by buoyancy, using a hydrostatic balance, and the Archimedes’ principle was closely related to that predicted by the model from fruit geometry. Growth rates of surface area in ‘Conference’ and ‘Condo’ peaked at ≈90 and 100 days after full bloom (DAFB), respectively, and were highest in the calyx followed by the cheek and neck regions. Relative growth rates, calculated by dividing growth rates by the absolute surface area present at that time, were at maximum during early development and thereafter continuously declined. In general, relative growth rates were highest for the cheek region, intermediate in the calyx, and lowest for the neck. ‘Conference’ fruit were always more russeted than ‘Condo’ with russeting generally decreasing from calyx to cheek and neck. Furthermore, russeting increased rapidly in ‘Conference’ during early development until ≈70 DAFB, particularly in the calyx and cheek regions and, to a lesser extent, in the neck region. There was little change in russeting after ≈70 DAFB. Plotting rates of russeting vs. relative growth rates in surface area indicate a positive and common relationship across regions where russeting increased when relative growth rates exceeded 0.03/day. Thus, differential growth rates between regions within ‘Conference’ or ‘Condo’, but not across the two cultivars, accounted for topical differences in russeting.

Skin spot is a commercially important disorder of the fruit skin of ‘Elstar’ apples (Malus ×domestica Borkh.). The disorder is characterized by patches of small brownish dots (“skin spots”) that usually appear on the skin after fruit are removed from storage. Water-induced cuticular microcracks are implicated in the etiology of skin spot. The objectives of our study were 1) to establish the effect of surface wetness on the severity of skin spot; and 2) to identify possible relationships between meteorological records of rainfall over a number of seasons and the severity of skin spot in those seasons. Surface wetness treatments were imposed on fruit using overhead sprinklers installed above trees grown under a plastic rain shelter. During early fruit development [14 to 44 days after full bloom (DAFB)], surface wetness did not affect the severity of skin spot. However, during the later part of the growing season (greater than 44 DAFB), increased surface wetness increased the incidence and severity of skin spot and also the severity of cuticular microcracking. Most skin spots and microcracks were already present at harvest before storage, but skin spots and microcracking did increase slightly during subsequent controlled atmosphere (CA) storage. Over a 9-year period, the severity of skin spot in ‘Elstar’ apples grown and stored locally under standard orchard and storage conditions was positively correlated to the number of rainy days. This correlation was greater for the period between 1 Aug. to harvest than for periods 1 June to harvest or 1 July to harvest. Fruit treated with 1-methylcyclopropene (1-MCP) were more susceptible to skin spot than untreated control fruit. Calculating regression equations for the relationship between the severity of skin spot and the number of rainy days (period 1 Aug. and harvest) revealed that the (commercially important) threshold score for skin spot of 2 is predicted to occur after 44 rainy days for control fruit and after 34 rainy days for 1-MCP-treated fruit. Our data demonstrate that skin spot arises from cuticular microcracks, which in turn result from numerous exposures to surface wetness (rain, dew), especially those occurring during later stages of fruit development.