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The mechanical properties and anatomy of fruit wall peels and their enzyme-isolated cuticular membranes (CM) are reported for three cherry tomato (Lycopersicon esculentum Mill.) cultivars that are crack-resistant, crack-intermediate, and crack-prone (i.e., Inbred 10, Sweet 100, and Sausalito Cocktail, respectively). The resistant and intermediate fruit peels strain-hardened when extended progressively; those of the crack-prone cultivar did so only modestly. The CM of all cultivars strain-hardened when extended with small forces; the CM of the intermediate and crack-prone cultivars strain-softened under tensile forces that did not strain-soften the crack-resistant cultivar. The peels and CM of the resistant cultivar were stiffer, stronger, and required more energy to break than crack-prone peels. The CM of crack-resistant peels developed deeper within the subepidermis than in the crack-prone or crack-intermediate peels. The CM in the outer epidermal periclinal walls of the crack-resistant and crack-intermediate cultivars was thicker than that of crack-prone peels. These data indicate that CM thickness can be used to gauge crack susceptibility among cherry tomato fruit, which can be useful in breeding programs and would facilitate QTL mapping of the underlying genetic factors.

Morphological and physical characteristics of the cuticular membrane (CM) of selected cultivars of sweet cherry (Prunus avium L.) fruit were studied relative to rain-induced cracking. Two characteristics of the CM may be determinants in rain-induced fruit cracking. The surface morphology and chemistry determine surface wettability and water retention, and the morphology and physicochemical characteristics its water permeability. The fruit epidermis as well as the guard cell walls adjacent to the outer vestibule and stomatal pore are covered by a thin lipoidal CM. Stomata were present at a frequency of 0.1 to 2 per mm² depending on cultivar and fruit surface position. However, most appeared nonfunctional with many pores partially or completely occluded with wax-like material. There was no evidence of water (containing fluorescein or AgNO₃) penetration into stomatal pores following surface application or submerging fruit for short periods. There was stomatal pore penetration when submerged fruit were infiltrated by reduced pressure in the presence of 0.1% L-77. Preferential sorption of AgNO₃ and fluorescein by cuticular ledges and guard cells was noted. The epicuticular wax (ECW) had no significant fine-structure. The CM was isolated enzymatically (cellulase/pectinase) and found to be 1 to 2 μm thick with an area weight of 1.2 to 2.3 g·m^–2, of which 25% to 40% was chloroform/methanol (1: 1by vol.) soluble. Fractionation of the chloroform/methanol fraction indicated the presence of four groups of nonpolar constituents. The fruit surface was moderately difficult to wet, forming contact angles of 85% to 105%, and with an estimated critical surface tension in the range of 16-24 mN·m^–1. Fruit water loss (transpiration) and uptake on submersion was followed and found to be complex. Transpiration increased with an increase in temperature, and both rate of transpiration and water uptake increased after removal of the epicuticular and cuticular waxes. Pathways of water uptake and the significance of our findings to rain-induced fruit cracking will be discussed.

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.

Susceptibility of sweet cherry (Prunus avium L.) fruit to rain cracking increases toward maturity and is thought to be related to increases in both tissue pressure () and cell pressure (). Furthermore, at a given water potential (), one might expect the increase in and the to balance the decrease in osmotic potential (). The objectives of our study were to quantify and in developing sweet cherry using vapor pressure osmometry (VPO), compression plate (CP), and the cell pressure probe (CPP). In addition, the tissue water potential was determined by quantifying the bending of strips of fruit skin and the change in projected area of discs excised from the flesh when incubated in a range of sucrose solutions of varying osmotic potentials (). Fruit growth followed a sigmoid pattern with time with the Stage II/Stage III transition occurring at ≈55 days after full bloom (DAFB). The and the were constant up to ≈55 DAFB but decreased to –2.8 MPa at maturity. The calculated by subtracting the from averaged ≈350 kPa up to 48 DAFB and then decreased at a decreasing rate to ≈21 kPa toward maturity. The determined from bending assays using excised skin strips or from water uptake of excised flesh discs was essentially constant up to ≈48 DAFB, then decreased until ≈75 DAFB and remained constant thereafter. These values were in good agreement with those determined by VPO. The as determined by CP passed through a transient peak at ≈41 DAFB, then decreased until ≈63 DAFB and remained constant and low until maturity. Similarly, by CPP increased from 27 to 48 DAFB, remained constant until ≈55 DAFB, and then decreased until maturity. Our data demonstrate a consistent decrease in and that coincides with a decrease in of sweet cherry during Stage III. Because and are low relative to , the change in parallels that in . The reason for the low turgor most likely lies in the accumulation of apoplastic solutes. These prevent a catastrophic increase in pressure that would otherwise lead to the bursting of individual cells and the cracking of entire fruit.