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.
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