Wherever sweet cherries are grown, rain-induced fruit cracking imposes a major 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 (Brüggenwirth 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 (Brüggenwirth 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, Brüggenwirth 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 a skin strip is associated with severe narrowing, and hence a gross overestimation of skin extensibility (Brüggenwirth et al., 2014).
The purpose of this study was to 1) quantify the key mechanical properties of two sweet cherry cultivars of contrasting cracking susceptibility, using a biaxial tensile test and 2) investigate the mechanistic basis of any differences between the two cultivars. Because many fruit may vary diurnally in diameter, and hence surface area (Lang, 1990; Measham et al., 2014; Montanaro et al., 2012; Ohta et al., 1997) and because this may cause the skin to fatigue, we also investigated the effects of repeated loading and unloading cycles on the mechanical properties of the fruit skin of the two cultivars.
Bargel, H., Spatz, H.C., Speck, T. & Neinhuis, C. 2004 Two-dimensional tension tests in plant biomechanics: Sweet cherry fruit skin as a model system Plant Biol. 6 432 439
Basanta, F.M., de Escalada Plaì, M.F., Stortz, C.A. & Rojas, A.M. 2013 Chemical and functional properties of cell wall polymers from two cherry varieties at two developmental stages Carbohydr. Polym. 92 830 841
Batisse, C., Buret, M. & Coulomb, P.J. 1996 Biochemical differences in cell wall of cherry fruit between soft and crisp fruit J. Agr. Food Chem. 44 453 457
Beyer, M., Lau, S. & Knoche, M. 2005 Studies on water transport through the sweet cherry fruit surface: IX. Comparing permeability in water uptake and transpiration Planta 220 474 485
Brüggenwirth, M., Fricke, H. & Knoche, M. 2014 Biaxial tensile tests identify epidermis and hypodermis as the main structural elements of sweet cherry skin. Ann. Bot. Plants 6:plu019
Chanliaud, E., Burrows, K.M., Jeronomidis, G. & Gidley, M.J. 2002 Mechanical properties of primary plant cell wall analogues Planta 215 989 996
Christensen, J.V. 1996 Rain-induced cracking of sweet cherries. Its causes and prevention, p. 297–327. In: A.D. Webster and N.E. Looney (eds.). Cherries. CAB Intl., Wallingford, UK
Christensen, J.V. 1999 An evaluation of sweet cherry cultivars. Danish Institute of Agricultural Sciences, Årslev, Denmark
Hovland, K.L. & Sekse, L. 2004a Water uptake through sweet cherry (Prunus avium L.) fruit pedicels: Influence of fruit surface water status and intact fruit skin Acta Agr. Scand. Sect. B Soil Plant Sci. 54 91 96
Hovland, K.L. & Sekse, L. 2004b Water uptake through sweet cherry (Prunus avium L.) fruit pedicels in relation to fruit development Acta Agr. Scand. Sect. B Soil Plant Sci. 54 264 266
Knoche, M. & Peschel, S. 2006 Water on the surface aggravates microscopic cracking of the sweet cherry fruit cuticle J. Amer. Soc. Hort. Sci. 131 192 200
Knoche, M., Peschel, S., Hinz, M. & Bukovac, M. 2000 Studies on water transport through the sweet cherry fruit surface: Characterizing conductance of the cuticular membrane using pericarp segments Planta 212 127 135
Matas, A.J., Cobb, E.D., Bartsch, J.A., Paolillo, D.J. & Niklas, K.J. 2004 Biommechanics and anatomy of Lycopersicum esculentum fruit peels and enzyme-treated samples Amer. J. Bot. 91 352 360
Measham, P.F., Bound, S.A., Gracie, A.J. & Wilson, S.J. 2009 Incidence and type of cracking in sweet cherry (Prunus avium L.) are affected by genotype and season Crop Pasture Sci. 60 1002 1008
Measham, P.F., Bound, S.A., Gracie, A.J. & Wilson, S.J. 2010 Vascular flow of water induces side cracking in sweet cherry (Prunus avium L.) Adv. Hort. Sci. 24 243 248
Montanaro, G., Dichio, B., Xiloyannis, C. & Lang, A. 2012 Fruit transpiration in kiwifruit: Environmental drivers and predictive model. Ann. Bot. Plants 6:pls036
Ohta, K., Hosoki, T., Matsumoto, K., Ohya, M., Ito, N. & Inaba, K. 1997 Relationships between fruit cracking and changes of fruit diameter associated with solute flow to fruit in cherry tomatoes J. Jpn. Soc. Hort. Sci. 65 753 759
Peschel, S. & Knoche, M. 2012 Studies on water transport through the sweet cherry fruit surface: XII. Variation in cuticle properties among cultivars J. Amer. Soc. Hort. Sci. 137 367 375
Salato, G.S., Ponce, N.M.A., Raffo, M.D., Vicente, A.R. & Stortz, C.A. 2013 Developmental changes in cell wall polysaccharide from sweet cherry (Prunus avium L.) cultivars with contrasting firmness Postharvest Biol. Technol. 84 66 73
Schumann, C., Schlegel, H.J., Grimm, E., Knoche, M. & Lang, A. 2014 Water potential and its components in developing sweet cherry J. Amer. Soc. Hort. Sci. 139 349 355
Weichert, H. & Knoche, M. 2006 Studies on water transport through the sweet cherry fruit surface: 10. Evidence for polar pathways across the exocarp J. Agr. Food Chem. 54 3951 3958