Abstract
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.
Rain-cracking severely limits sweet cherry production worldwide (Christensen, 1996). By breaching fruit skin integrity, it exposes the underlying flesh to rapid drying and to invasion by insects and pathogens. Cracking is thought to be related to excessive water uptake by the fruit (Christensen, 1996; Sekse, 1998). Water uptake increases fruit volume and turgor, thus subjecting the peripheral dermal tissues to tangential strain and stress. When the limits of extensibility are exceeded, the dermal tissue fails causing the fruit to crack (Considine and Kriedemann, 1972). The following two different and unrelated groups of factors are critical in cracking: 1) the water-transport characteristics of the fruit surface (in wet conditions) and possibly also of the vascular systems of the fruit and pedicel (at all times); and 2) the mechanical properties of the flesh and, especially, of the skin.
Although considerable progress has been made in understanding and analyzing water transport through the fruit surface in sweet cherry (Beyer and Knoche, 2002; Beyer et al., 2002, 2005; Weichert and Knoche, 2006), the mechanical properties of the sweet cherry fruit have received little attention. In grape berries (Vitis vinifera L.), another example of a soft, fleshy fruit susceptible to rain-cracking, the distribution of stresses in the fruit surface were modeled using the physics theory developed for analyzing thin-walled, steel pressure vessels (Considine and Brown, 1981). According to this, the fleshy parenchyma of the fruit is held under pressure relative to the atmosphere by the stressed berry skin as the tensile, load-bearing structure. This model predicts stress distributions and failure patterns that are consistent with field observations in vineyards (Considine, 1982). Although sweet cherries differ somewhat in structure and texture from grape berries, this model may in principle also be applied to sweet cherries (Considine and Brown, 1981). Here, the strained exocarp holds the internal mesocarp and endocarp tissues under compression. Circumstantial evidence supports this hypothesis. First, the orientation of macroscopic cracks on the sweet cherry is similar to that on the grape berry and consistent with predictions based on the model; i.e., circular cracks around the shoulder, longitudinal cracks on the cheek, and circular cracks around the stylar scar (Simon, 2006; S. Lang, personal communication). Second, the epidermal and hypodermal cells are small and have thick cell walls compared with the mesocarp (Glenn and Poovaiah, 1989), which indicates a role in stress containment. Third, the cuticle, as the outermost layer of the exocarp, is markedly strained in the course of development (Knoche et al., 2004; Peschel et al., 2007). Fourth, the epidermal cells are oriented perpendicular to the style/pedicel axis, the microcracks in the cuticle parallel to the style/pedicel axis suggesting that failure of the cuticle is caused by strain in the underlying epidermis (Peschel and Knoche, 2005). Fifth, the water potential of sweet cherries becomes more negative when the exocarp is severed by multiple cuts (2 mm apart, 2 mm deep) using razor blades (Knoche et al., 2004). However, when the cuticle is abraded away by rubbing lightly with fine carborundum powder, there is no change in mesocarp water potential. Both observations are consistent with the hypothesis that straining of the sweet cherry’s elastic exocarp (but not of the cuticle) induces in it a tension (stress), which holds the mesocarp under pressure. If the exocarp tension is released, then the mesocarp turgor pressure falls (Knoche et al., 2004). Unfortunately, direct evidence for stress in the exocarp is lacking.
The objectives of this study were, therefore, to detect and quantify any elastic strain in the exocarp of sweet cherry fruit. Two different assays were used. A “gaping” assay was used to quantify the release of linear elastic strain in vivo after the making of deep incisions into the mesocarp (adapted from Skene, 1980). The release of biaxial elastic strain was also monitored in vitro using ESs excised from the cheek of the fruit.
Materials and Methods
Plant material.
Sweet cherry fruit were obtained from orchards located within a 30-km radius of Halle [‘Burlat’, ‘Hedelfinger’, ‘Kordia’, ‘Sam’ (lat. 51°49′ N, long. 12°00′ E)], from Hannover [‘Regina’ (lat. 52°27′ N, long. 09°84′ E)], and from the Federal Plant Cultivar Office at Marquardt [‘Katalin’, ‘Summit’ (lat. 52°28′ N, long. 12°58′ E)]. Off-season fruit (‘Bing’ and ‘Lapins’) from the southern hemisphere was obtained directly from the importer. The site of production and conditions of storage were unknown but there were no signs of shrivelling on either fruit or pedicels with the latter appearing green, fresh, and fully turgid. Except for the ‘Hedelfinger’ fruit that was sampled at weekly intervals between 45 and 94 DAFB, all fruit was harvested when fully mature based on color. Domestic fruit was processed within 3 d of harvest and that obtained from the importer within 7 d of purchase. Fruit mass was determined gravimetrically on a representative sample of 20 fruit per cultivar.
Gaping assay for quantifying release of linear elastic strain.
Sketch of experimental procedures for determining the linear elastic strain on a whole fruit basis using the gaping assay (A–B) and the biaxial elastic strain using a strain release assay using exocarp segments (ESs) excised from sweet cherry fruit. (A) Principle of gaping assay. Two longitudinal cuts are made through the exo- and mesocarp down to the pit. The subsequent release of elastic strain by gaping is quantified. (B) Sketches of fruit having two, four, eight, and 12 longitudinal cuts. (C) Sweet cherry with dot pattern applied to the cheek surface. (D) Enlarged view of dot pattern on the fruit before (left) and after (right) excision and subsequent relaxation of the ES submerged in oil. For details see text.
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 137, 6; 10.21273/JASHS.137.6.383
Because the amount of water transpired during shipment of the imported, off-season fruit was unknown and transpiration may have decreased turgor and hence reduced the elastic strain in the skin, two preliminary experiments were conducted to quantify the potential effects of turgor loss on gaping. First, 50 fruit were immersed in deionized water at 22 °C and the numbers of cracked fruit monitored over time. After 4.5 h, ≈50% of the submerged fruit had cracked. At this time, 10 of the remaining, submerged and non-cracked fruit plus 10 control fruit (not submerged) were photographed and their equatorial perimeters were determined. These were then cut twice longitudinally (i.e., parallel to the pedicel/stylar axis), one cut on each opposing face (see Fig. 1A). Fruit was then incubated as described previously and gaping was quantified after 24 h. There was no significant difference in gaping between the “rehydrated” and the control (“non-rehydrated”) fruits (8.6% ± 0.2% vs. 8.1% ± 0.2% release of elastic strain for rehydrated vs. control fruit, respectively; P < 0.275). Nor was there a significant relationship between the amounts of water taken up by the submerged fruit and the release of strain (r2 = 0.07; P < 0.452). In the second preliminary experiment, fruit was incubated at 22 °C above dry silica and the amount of water transpired quantified gravimetrically at 0, 6, 24, and 48 h. Subsequently, fruit was cut twice longitudinally, incubated, and gaping quantified after 24 h as described previously. The amount of water transpired increased linearly with time, but the release of strain slightly decreased. The regression equations were: mass loss (milligram per fruit) = 32.8 (± 28.4) – 17.2 (± 1.1) × time (hours), r2 = 0.993, P < 0.004 and strain (percent) = 8.64 (± 0.09) – 1.32 (± 0.19) × mass (grams per fruit), r2 = 0.961, P < 0.020; respectively. Thus, a hypothetical decrease in turgor resulting from transpiration has a significant but only a small effect (less than 1%) on the release of strain in the gaping assay. These experiments demonstrate that any transpiration that might have occurred in the off-season fruit during transit would have had only a marginal and negligible effect on the strain released in the gaping assay.
The time course of elastic strain release was assessed by making opposing, longitudinal cuts [two cuts per fruit (see Fig. 1A)] with measurements of equatorial perimeter being made at 0, 1, 3, 6, and 24 h.
To compare elastic strain in the skin in latitudinal and longitudinal directions, two opposing cuts per fruit were made either in longitudinal or in equatorial latitudinal directions. Again, in each case, the cut depth was until the razor blade touched the pit.
The effect of different numbers of cuts on strain release was also quantified [zero, two, four, eight, and 12 longitudinal cuts were made and these assessed after 24 h (Fig. 1B)].
Strain release assay for quantifying biaxial strain of exocarp segments.
(A) Time course of release of linear elastic strain (
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 137, 6; 10.21273/JASHS.137.6.383
Preliminary experiments established the following: 1) the reproducibility of the area determinations by image analysis was high (cv = 0.5%); and 2) there was essentially no error in measurement resulting from refraction of light in the oil (area between dot pattern on glass slide 3.49 ± 0.23 vs. 3.48 ± 0.22 mm2 for ambient atmosphere vs. submerged in oil; S. Peschel, unpublished data).
The time course of strain release of ES excised from mature ‘Katalin’ sweet cherries was established during a 48-h time period. The number of replications was 10.
The effect of sample position was studied by excising ESs from the cheek, suture, and stylar end of mature ‘Lapins’ fruit. The number of replications was 16.
Potential differences between sweet cherry genotypes were assessed by quantifying the strain in excised ESs and in isolated CM in mature ‘Burlat’, ‘Hedelfinger’, ‘Kordia’, ‘Lapins’, ‘Sam’, and ‘Summit’ fruit. Briefly, epidermal discs were punched using a cork borer (8.9 mm diameter) and incubated in enzyme solution containing pectinase (90 mL·L−1 Panzym Extra; Novo Nordisk Ferment, Dittingen, Switzerland), NaN3 (15 mm) and citric acid buffer [50 mm citric acid buffer/NaOH, pH 4 at 25 °C (Orgell, 1955)]. Solutions were refreshed repeatedly until CM separated from the tissue. The elastic biaxial strain in the CM (
The role of cuticle and turgor in strain release of ES was investigated in mature ‘Regina’ fruit. The CM was substantially abraded away by rubbing gently with carborundum powder (Carborundum SC-1200; K. Schriever, Schleif- and Poliermittel, Hamburg, Germany). To ensure complete removal of the CM and minimize damage of the cell walls of the underlying epidermal cells, the procedure was optimized in preliminary experiments by monitoring uptake of the fluorescent dye acridine orange using fluorescence microscopy. The turgor of ES was eliminated by subjecting ES to a freeze/thaw cycle. The dots were applied to that portion of the fruit surface treated with carborundum as described previously, photographed under the microscope, and the ES was then excised and immediately frozen at –20 °C. After thawing next day, and a further 48-h incubation period, the strain was quantified. ESs without the CM (abraded away) and/or turgor manipulated served as controls. The experiment was carried out twice using a total of 22 replications.
To identify preferential linear strain in the ES in longitudinal or transverse directions, the distances between pairs of dots were quantified in the two directions for ‘Regina’ sweet cherry and linear strains were calculated. The number of replications was 45.
The effect of temperature on strain release was studied using off-season ‘Bing’ fruit. Fruit were equilibrated for 1.5 h at 2, 10, 15, 22, and 35 °C. The dot pattern was applied as described previously and photographed. Next, the ES was incubated for up to 96 h and re-photographed. The number of replications was 12.
The release of biaxial strain in the exocarp during Stage III development (45 to 94 DAFB) was studied in ‘Hedelfinger’ sweet cherry. This cultivar was selected because data on the changes with time in fruit mass, surface area, and strain in the isolated CM were available for the same growing season (Knoche and Peschel, 2006).
Terminology, data analysis, and presentation.
The term “exocarp segment” is used throughout our study to refer to a segment of exocarp that comprises the cuticle, the epidermis, and hypodermal cell layers and some adhering parenchyma from the outer mesocarp.
Strain in viscoelastic materials comprises an elastic (reversible) and a plastic (irreversible) component, the relative contributions of the two to total strain depending on time. Unless specifically mentioned otherwise, the strain quantified in our experiments represents the stored elastic strain at that particular time and hence, a residual elastic strain only.
Data are presented in figures and tables as means ± se except for Figure 2B where individual observations are shown. Data were subjected to analysis of variance [ANOVA (Proc GLM)] or linear (Proc REG) or nonlinear regression analysis (Proc NLIN) using SAS (Version 9.1.3; SAS Institute, Cary, NC). Percentage strain data were arcsine transformed before ANOVA. Significance of coefficients of determination (r2) at the 0.05, 0.01, and 0.001 P level is indicated by *, **, and ***, respectively.
Results
After cutting, the fruit gaped within minutes indicating a rapid release of linear elastic strain. However, strain release continued more slowly for up to 24 h after cutting (Fig. 2A). Strain release was log-linearly related to time [ɛ (percent) = 7.88 (± 0.08) + 0.50 (± 0.06) × log time (hours); r2 = 0.939; P < 0.0014 (Fig. 2A, inset)]. There was little difference in the release of skin strain in longitudinal and latitudinal directions (8.0% ± 0.2% vs. 8.4% ± 0.1% for longitudinal strain vs. latitudinal strain, respectively; P < 0.085). The strain released was negatively related to fruit mass [
The time course of release of biaxial strain of excised ES revealed rapid relaxation within minutes of excision of the ES [half-time 2.7 min (Fig. 3A)]. Strain release continued at a decreasing rate until an asymptote was eventually approached. The release of strain was linearly related to the logarithm of time [
(A) Time course of release of biaxial elastic strain (
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 137, 6; 10.21273/JASHS.137.6.383
The strain released from the cheek was higher than that from the suture or the stylar end region (Table 1).
Release of biaxial elastic strain (
The strain release differed among cultivars and ranged from 18.7% ± 1.4% in ‘Lapins’ to 36.0% ± 1.8% in ‘Katalin’ (Table 2). Strain measured in an exocarp segment was always lower than in a corresponding isolated CM.
Fruit mass and biaxial elastic strain (
Relationships between strain in the exocarp and fruit mass were not consistent. There was no significant relationship across (n = 102, r2 = 0.01) or within cultivars at maturity in ‘Hedelfinger’ (r2 = 0.00), ‘Kordia’ (r2 = 0.08), ‘Lapins’ (r2 = 0.10), ‘Sam’ (r2 = 0.03), or ‘Summit’ (r2 = 0.05), but strain increased with fruit mass in ‘Burlat’ (r2 = 0.74***), ‘Katalin’ (r2 = 0.44*), and ‘Regina’ (r2 = 0.25*).
Subjecting ES from mature fruit to a freeze/thaw cycle to remove turgor had no effect on the release of biaxial strain when the CM was intact (Table 3). However, when repeating the procedure using fruit with the CM abraded using carborundum, more strain was released from ES with turgor than from those without turgor.
Release of biaxial strain (
The linear elastic strain of the ES in the latitudinal direction (
Increasing temperature from 2 to 35 °C increased the rate of release of biaxial strain and the total strain released at 96 h (Fig. 4; Table 4).
Effect of temperature on parameter estimates of linear regression equations describing the time course of release of biaxial strain (
(A) Effect of temperature on the release of biaxial elastic strain (
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 137, 6; 10.21273/JASHS.137.6.383
Between 45 and 94 DAFB mass and surface area of ‘Hedelfinger’ cherries increased sigmoidally indicating Stage III development (Fig. 5A). The maximum growth rate in surface area was 0.71 cm2·d−1 at 66 DAFB. The biaxial strain of the exocarp depended on developmental stage and the presence of turgor (Fig. 5B). There was essentially no biaxial elastic strain in the exocarp at 45 DAFB regardless of turgor. Thereafter, strain of ES without turgor increased continuously and reached a maximum at ≈47.2% ± 2.5% at 87 DAFB. In the presence of turgor, however, the increase in strain of the ES began later at 63 DAFB and peaked at 47.6% ± 2.5% at 87 DAFB (Fig. 5B). The effect of turgor on strain in the ES (as indexed by the difference in biaxial strains between ES with and without turgor) increased up to a maximum at 59 DAFB and continuously decreased thereafter (Fig. 5C). Compared with ES, the strain in the isolated CM increased more rapidly and exceeded that of the ES (regardless of turgor) from 66 DAFB onward until maturity (Fig. 5B).
(A) Time course of changes in surface area and biaxial elastic strain (
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 137, 6; 10.21273/JASHS.137.6.383
Discussion
Our study established several important findings: 1) the exocarp of mature sweet cherry fruit is a viscoelastic composite material; 2) elastic strain in the exocarp increases during Stage III development; and 3) there is no relationship between the strain in the exocarp and that in its outermost layer, the CM.
Exocarp as a viscoelastic composite material.
The linear (gaping) and biaxial strain release assays demonstrate that in vivo the exocarp is subjected to elastic (i.e., reversible) strain that is released on in- or excision. This observation is consistent with Considine’s shell model of grape berries where the skin (i.e., the exocarp) of the berry is strained by the pressure developed in the meso- and endocarp (Considine and Brown, 1981). Thus, in sweet cherry also, the exocarp represents the tensile load-bearing structure. From a mechanical point of view, the exocarp is a heterogeneous material (a layered composite), which comprises at least two layers of dissimilar cells (epidermis and hypodermis) overlaid by a polymeric component (the cuticle). The rheological properties of this composite can be classified as viscoelastic based on the following observations. First, on excision this composite releases elastic strain in a time-dependent manner with the release of uniaxial or of biaxial strain being linearly related to the logarithm of time. Second, in vitro, each of the materials that comprise the exocarp composite [cellulose, pectins, xyloglucanes (Chanliaud et al., 2002) and also the cuticle (Petracek and Bukovac, 1995)] exhibits viscoelastic properties when isolated. Third, in addition to the structural cell wall materials, turgid cells also resist deformation in a viscoelastic manner (Niklas, 1992). Fourth, Vincent (1990) suggested that the viscosity of pectins may be inversely related to temperature. This would account for the temperature response of elastic strain release. Finally, the skin of grape berries has been shown to exhibit viscoelastic properties (Hankinson et al., 1977). The skin of a grape has a similar morphology to that of a sweet cherry.
Strain during Stage III of fruit development.
Fruit growth subjects the exocarp to strain and this strain may be partitioned into plastic (irreversible) and elastic (reversible) components (Knoche et al., 2004). We can reasonably infer that until ≈45 DAFB, the increase in surface area of a developing sweet cherry fruit involves almost entirely plastic strain (≈100% plastic strain, ≈0% elastic strain), indicated by there being no detectable release of strain after excision (Fig. 5). However, between 45 and 94 DAFB, the increase in surface area involves both elastic and plastic strain. The relative contributions of elastic and plastic strains to total strain may be estimated from the following calculations. Between 45 and 94 DAFB (Stage III), fruit surface area increased by 14.4 cm2 [from 7.1 to 21.5 cm2 (Fig. 5A)]. Because total strain is the sum of the plastic and elastic strain components, and we measured elastic strain in the mature exocarp at 34% (equivalent to 4.9 cm2), the remaining 66% (equivalent to 9.4 cm2) must be accounted for by plastic strain.
The mechanistic basis for the different contributions of plastic and elastic strain to total strain may be related to the mechanisms of growth of the skin. During Stages I and II, growth occurs primarily by cell division indicated by little change in the average size of the epidermal cells (rapid cell number increases). Stage III growth is predominantly by cell enlargement, although there is some continuing cell division (Knoche et al., 2004). Cell division involves the formation and deposition of new cell walls. It may be visualized that the deposition of newly formed cell walls prevented strain relaxation (elastic) of the exocarp during Stages I and II, but less so during Stage III. That some elastic strain remains throughout Stage III may be attributed 1) to cell elongation becoming the predominant mechanism of area growth; 2) to an increase in turgor that is associated with the elongation of epidermal cells during Stage III (Niklas, 1992; Tukey and Young, 1939); and 3) to the very high rates of area growth (“strain rates”) at this stage; these would favor elastic over a (time-dependent) plastic deformation (Niklas, 1992). This hypothesis is consistent with the transient effect of turgor in developing ‘Hedelfinger’ sweet cherry (Fig. 5B–C). According to current models, cell turgor strains the cell walls and, thus, represents the driving force for cell elongation (Cosgrove, 1986; Hamant and Traas, 2010). Turgor decreases when the strained cell walls “yield” by “creep,” resulting from a reorganization of cellulose microfibrils in the cell-wall matrix (Hamant and Traas, 2010). Thus, the strain caused by the increase in turgor is initially elastic and, hence, reversible. The yielding response of the cell walls essentially “converts” elastic strain (reversible) into plastic strain (irreversible) thereby decreasing turgor and stress (Hamant and Traas, 2010). This mechanism is consistent with the transient effect of turgor on strain release that we observe in this study (Fig. 5C). There was essentially no effect of turgor on strain release in the exocarp at 45 DAFB or at maturity at 94 DAFB. However, strain release did depend on turgor during the middle of Stage III from 59 to 66 DAFB. Apparently, turgor prevented the release of strain during mid-Stage III when growth rates were high but had no significant effect when growth rates were low. Indeed, the effect of turgor as indexed by the difference in strain release between ES with and without turgor (Δɛ in percent) was closely related to the relative growth rate in surface area [RGR in 1/days; Δɛ (percent) = 0.0024 (± 0.0002) × RGR (1/days), r2 = 0.957***].
From a horticultural point of view, it would be advantageous if somehow the elastic strain component could be decreased by increasing the plastic strain component. In this way, if elastic strain and, hence, stress in the exocarp were decreased, then susceptibility to rain-cracking would also presumably be decreased.
Role of the cuticle in straining of the exocarp.
Although at maturity elastic strain in the CM exceeded that in the exocarp by up to twofold, this had little consequence for the release of elastic strain in the exocarp. There was no relationship between strain in the exocarp and that in the CM either in the course of development (Fig. 5; for strain of the CM see Fig. 2 in Knoche and Peschel, 2006) or between selected cultivars at maturity (Table 2). These findings imply 1) that the thin and fragile sweet cherry cuticle has essentially no role in the release of elastic strain in the exocarp nor in its mechanical properties; and 2) that the effective structural member of the skin of a cherry resides in the epidermal and hypodermal cell layers and not significantly in the cuticle.
The two-factorial analysis of variance of our experiment on the effects of CM abrasion on strain release revealed that abrading the CM increased the release of elastic strain in the presence of turgor but not in its absence (Table 3). At present we do not have a good explanation for this observation. If abrading the CM while still attached to the fruit causes an increase in elastic strain in the exocarp, then the potential for strain release after excision should increase. Because in our assay of strain release the dot pattern was necessarily applied after abrading the CM, this effect could have been present but would have passed undetected.
Conclusions
Our study demonstrates that the exocarp represents the primary load-bearing structure in the mature sweet cherry. When strained during Stage III, it responds in a viscoelastic manner. At maturity, irreversible plastic and reversible elastic strains comprise 66% and 34% of the strain in the exocarp, respectively. Turgor had a transient effect on elastic strain during Stage III. This effect essentially mirrored the strain rates during Stage III and, thus, was not detectable at the onset of Stage III or at maturity. Finally, the elastic strain in the cuticle at the outermost layer of the exocarp always exceeded that within the exocarp. There was no relationship between strain in the CM and that in the exocarp suggesting the following: 1) the CM simply extends along with the expanding exocarp cell layers; 2) the contribution of the CM to the strain in the exocarp is negligible; and thus, 3) together, the epidermis and hypodermis most likely represent the main structural member of the exocarp. It should be pointed out that these data do not question the role of the CM in fruit cracking as the primary barrier in water uptake through the fruit surface. The findings of the present investigation provide a basis for further mechanistic studies that will explore the relationship between the failure mechanics of the exocarp and the susceptibility of sweet cherry fruit to rain-cracking. Empirical correlations between morphological characteristics of epidermal cells and cracking susceptibility demonstrate that such relationships may exist (Belmans et al., 1990; Yamaguchi et al., 2002).
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