English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

Rain cracking of sweet cherry fruit (Prunus avium L.) may be the result of excessive water uptake and/or of mechanically weak skins. The objectives were to compare mechanical properties of the skins of two cultivars of contrasting cracking susceptibility using biaxial tensile tests. We chose ‘Regina’ as the less-susceptible and ‘Burlat’ as the more-susceptible cultivar. Cracking assays confirmed that cracking was less rapid and occurred at higher water uptake in ‘Regina’ than in ‘Burlat’. Biaxial tensile tests revealed that ‘Regina’ skin was stiffer as indexed by a higher modulus of elasticity (E) and had a higher pressure at fracture (pfracture) than ‘Burlat’. There was little difference in their fracture strains. Repeated loading, holding, and unloading cycles of the fruit skin resulted in corresponding changes in strains. Plotting total strains against the pressure applied for ascending, constant, and descending pressures yielded essentially linear relationships between strain and pressure. Again, ‘Regina’ skin was stiffer than ‘Burlat’ skin. Partitioning total strain into elastic strain and creep strain demonstrated that in both cultivars most strain was accounted for by the elastic component and the remaining small portion by creep strain. Differences in E and pfracture between ‘Regina’ and ‘Burlat’ remained even after destroying their plasma membranes by a freeze/thaw cycle. This indicates that differences in skin mechanical properties must be accounted for by differences in the cell walls, not by properties related to cell turgor. Microscopy of skin cross-sections revealed no differences in cell size between ‘Regina’ and ‘Burlat’ skins. However, mass of cell walls per unit fresh weight was higher in ‘Regina’ than in ‘Burlat’. Also, the ratio of tangential/radial diameters of epidermal cells was lower in ‘Regina’ (1.86 ± 0.12) than in ‘Burlat’ (2.59 ± 0.15). The results suggest that cell wall physical (and possibly also chemical) properties account for the cultivar differences in skin mechanical properties, and hence in cracking susceptibility. Wherever sweet cherries are grown, rain-induced fruit cracking imposes amajor limitation to production (Christensen, 1996). Susceptibility to rain cracking differs among cultivars (Christensen, 1995, 1999, 2000; Measham et al., 2009), but the mechanistic basis of differential cracking susceptibility among cultivars is not clear. From the coincidence of rainfall and fruit cracking, it is inferred that cracking is related to water uptake into the fruit. Water uptake, in turn, leads to an increase in volume, causing the fruit surface area to increase. When the limits of extensibility are exceeded, the fruit is expected to crack. Based on the above logic, differential cracking susceptibility among cultivars could result from either (or both) of two, mechanistically unrelated, factors. First, the net import of water into the fruit will affect cracking by causing fruit volume to increase, thereby straining the skin beyond some defined upper limit. Second, mechanical properties of the fruit skin will affect the fracture limits. Recent investigations (Br€uggenwirth et al., 2014) have established that it is the epidermal and hypodermal cell layers (not the cuticle) that represent the structural ‘‘backbone’’ of the skins of sweet cherry fruit. Thus, differences in cracking susceptibility among sweet cherry cultivars could be the result of different water-uptake characteristics and/or of the mechanical properties of the epidermal and hypodermal cell layers. Water uptake across the fruit surface has been studied in detail in the last decade. The pathways and mechanisms of transfer have been identified (Beyer et al., 2005; Knoche et al., 2000; Weichert and Knoche, 2006). However, among 29 cultivars, there was no close relationship between variation in cracking susceptibility and variation in the major characteristics of the fruit surface: water permeance, strain, mass of cuticle per unit area, stomatal density, etc. (Peschel and Knoche, 2012). Some information is available on vascular transport, but to our knowledge differences among cultivars have not been identified (Br€uggenwirth and Knoche, 2015; Hovland and Sekse, 2004a, 2004b; Measham et al., 2009, 2010, 2014). Little information is available on the mechanical properties of the sweet cherry fruit skin (Bargel et al., 2004). Recently, Br€uggenwirth et al. (2014) modified the biaxial bulging test to quantify the mechanical properties of excised fruit skins under standardized laboratory conditions. Modifications included dimensional fixation of a skin segment in a washer before excision, to prevent any release of elastic strain [built up in the skin due to growth (Grimm et al., 2012)]. Silicone oil was used to give a hydrostatic pressure to the skin segment from the physiological inside to prevent uncontrolled water uptake from that side—this could have confounded the test results (Simon, 1977). In contrast to the more common, uniaxial tensile tests of engineering, biaxial tensile tests better mimic the growth stresses and strains occurring in vivo. In a spherical organ of nearly isotropic growth, stresses and strains are of course fairly uniformly biaxial, not uniaxial. Also, sweet cherry skin exhibits high Poisson ratio properties, so a uniaxial tensile test using Received for publication 18 Nov. 2015. Accepted for publication 23 Dec. 2015. This research was funded in part by grants from the Deutsche Forschungsge-

Mechanical Properties of Skins of Sweet Cherry Fruit of Differing Susceptibilities to Cracking