In russeting of apple and pear fruit, a stiff cuticle is replaced by a more plastic periderm. Furthermore, the cell layers underlying the cuticle and the periderm represent the load-bearing structure in the fruit skin in both apple and pear.

Rain-cracking of sweet cherry (Prunus avium L.) fruit involves failure of the exocarp caused by excessive stress and strain. The objective of our study was to quantify exocarp strain in developing cherries. The release of linear elastic strain was followed in vivo using a gaping assay, whereas the release of biaxial elastic strain was followed in vitro after excision of small exocarp segments (ESs) that were submerged in silicone oil and strain release quantified by image analysis. When mature sweet cherry fruit were cut (by making two or more deep, longitudinal incisions parallel to the stylar/pedicel axis and on opposing sides of the fruit down to the pit), the incisions rapidly “gaped.” The gaping wounds continued to widen as they progressively released the linear elastic strain in the skin. By 24 hours the combined widths of two gapes represented 8.8% ± 0.1% of the fruit circumference. Increasing the number of cuts from two to 12 increased the cumulative gape widths to 14.9% ± 0.2%. In ES, monitoring the time course of relaxation after excision revealed a rapid release of biaxial strain, having a half-time of ≈2.7 minutes. Relaxation continued, but at a decreasing rate, for up to 48 hours. Across eight cherry cultivars, the biaxial strain in the exocarp at maturity ranged from 18.7% ± 1.9% in ‘Lapins’ to 36.0% ± 1.8% in ‘Katalin’. Elastic strain in the ES was always lower than that measured in an isolated cuticular membrane (CM). Increasing the temperature from 2 to 35 °C increased the rate of strain release and also the total percent strain released at 96 hours. In developing ‘Hedelfinger’ sweet cherry fruit, there was essentially no elastic strain in the exocarp at 45 days after full bloom (DAFB). Thereafter, significant elastic strain developed, reaching a maximum of 47.6% ± 2.5% at 87 DAFB. The effect of exocarp cell turgor on strain in the ES (evidenced by the difference in the reversible strain between ES with and without turgor) was closely and positively related to the relative area growth rate of the skin (r2 = 0.957). Strain release peaked at ≈59 DAFB, and there was no effect of turgor on strain release in mature fruit. Our data demonstrated the following: 1) the exocarp is a viscoelastic material composite; 2) at maturity, plastic and elastic strain components make up 66% and 34% of the total percent strain, respectively; 3) elastic strain in the exocarp increases during Stage III development; and 4) the strain in the exocarp is unaffected by strain in the CM. Thus, the epidermis and hypodermis layers must represent the main, load-bearing structure in sweet cherry fruit with the cuticle making a mechanically insignificant contribution.

SummaryThis paper investigates the effects of cuticular wax on the release of strain and on the tensile properties of enzymatically isolated cuticular membranes (CMs) taken from leaves of agave (Agave americana), bush lily (Clivia miniata), holly (Ilex aquifolium), and ivy (Hedera helix) and from fruit of apple (Malus 9 domestica), pear (Pyrus communis), and tomato (Lycopersicon esculentum).Biaxial strain release was quantified as the decrease in CM disc area following wax extraction. Stiffness, maximum strain and maximum force were determined in uniaxial tensile tests using strips of CM and dewaxed CMs (DCMs).Biaxial strain release, stiffness, and maximum strain, but not maximum force, were linearly related to the amount of wax extracted. Apple CM has the most wax and here the effect of wax extraction was substantially accounted for by the embedded cuticular wax. Heating apple CM to 80°C melted some wax constituents and produced an effect similar to, but smaller than, that resulting from wax extraction.Our results indicate that wax 'fixes' strain, effectively converting reversible elastic into irreversible plastic strain. A consequence of 'fixation' is increased cuticular stiffness.

The pressure inside a mature sweet cherry (Prunus avium L.) fruit is thought to be an important factor in rain cracking. However, to our knowledge, this pressure has never been quantified directly. The objectives of this study are to quantify: 1) the cell turgor () in fruit using a cell pressure probe (CPP) and a vapor pressure osmometer (VPO); and 2) the tissue pressure in a fruit () using both a fruit pressure probe (FPP) and a compression-plate technique (CP). The value of in mesocarp cells of mature sweet cherry fruit averaged 28.1 kPa in ‘Samba’ and 17.5 kPa in ‘Sam’ at depths below the fruit surface between 200 and 400 μm. A (range 38 to 64 kPa for different cultivars) calculated from the tissue water potential ( = –2968 to –4035 kPa) and the osmotic potential () ( = –3020 to –4116 kPa) of excised mesocarp discs as determined by VPO was of the same order of magnitude as that by CPP. Similar low values were obtained by FPP (range 8.0 to 11.8 kPa across cultivars). The were consistently lower than the values measured by CPP or by VPO. The value in the mesocarp increased slightly with increasing depth below the surface. However, was always negligible (e.g., ‘Samba’ = 10 kPa) compared with either (‘Samba’ = –2395 kPa) or calculated water potential () (‘Samba’ = –2385 kPa). When subjecting intact fruit to CP, linear relationships were obtained between the forces applied and the resulting aplanation areas. The values obtained by CP (range in sweet cherry 18.4 to 36.1 kPa) were somewhat larger than the values obtained by FPP (range in sweet cherry 8.0 to 11.8 kPa). Incubating fruit for up to 7.5 h in deionized water or for up to 96 h in air enclosed above dry silica gel had no measurable effects on . The low and the low values are not unique to sweet cherry. Values of the same order of magnitude were obtained also in mature sour cherry (Prunus cerasus L.), european plum (Prunus domestica L.), grape (Vitis vinifera L.), gooseberry (Ribes uva-crispa L.), red currant (Ribes rubrum L.), black currant (Ribes nigrum L.), blueberry (Vaccinium corymbosum L.), and tomato (Solanum lycopersicum L.). Possible explanations for the very low values of and are discussed.

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.