Stress and Strain in the Sweet Cherry Skin

Abstract

<jats:p>Rain-cracking of sweet cherry (<jats:italic>Prunus avium</jats:italic> 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 (<jats:italic>r</jats:italic><jats:sup>2</jats:sup> = 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.</jats:p>

Evidence for a radial strain gradient in apple fruit cuticles

“…Third, CM often roll up and curl upon isolation with the morphological outer surface being located inside the curl. Finally, a radial gradient in strain has been reported for the skin composite of sweet cherry fruit where the CM as the outer most layer is more strained than the underlying epi-and hypodermis (Knoche and Peschel 2006;Grimm et al 2012).…”

Biomechanical properties of the tomato (Solanum lycopersicum) fruit cuticle during development are modulated by changes in the relative amounts of its components

“…The epidermal tissue, as the interface with the environment, has been shown to act as a barrier that limits and controls organ growth (Savaldi-Goldstein et al, 2007). The epidermis is in tension in planta as excised epidermal peels become shorter (Skene, 1980;Kutschera et al, 1987;Grimm et al, 2012) and cuts in the epidermis tend to remain open (Dumais & Steele, 2000). Epidermal cells form a continuous and uniform layer of strongly attached cells with thicker cell walls which are expected to be less extensible (Javelle et al, 2011;Mirabet et al, 2011).…”

Mottling on Sweet Cherry Fruit Is Caused by Exocarp Strain

“…PREPARATION OF EXOCARP SEGMENTS FOR MORPHOMETRIC ANALYSIS. The exocarp of sweet cherry fruit is subjected to elastic strain and on excision of an exocarp segment (ES), this strain is released (Grimm et al, 2012). To maintain the elastic strain of an ES beyond excision, stainless steel washers (6.4 mm inner diameter, 32.2 mm 2 area) were mounted on the fruit surface using an ethyl-cyanacrylate adhesive (Loctite 406; Henkel Loctite Deutschland, München, Germany; Knoche and Peschel, 2006).…”

“…Recent investigations demonstrate that the exocarp is markedly strained during Stage III. First is because mature sweet cherry fruit gape when the fruit is cut longitudinally radially using a razor blade (Grimm et al, 2012). Second, exocarp segments decrease in area on excision from the fruit surface (Grimm et al, 2012).…”

“…First is because mature sweet cherry fruit gape when the fruit is cut longitudinally radially using a razor blade (Grimm et al, 2012). Second, exocarp segments decrease in area on excision from the fruit surface (Grimm et al, 2012). Third, the cuticular membrane is markedly strained during Stage III development (Knoche et al, 2004).…”