Cell wall swelling, fracture mode, and the mechanical properties of cherry fruit skins are closely related

Abstract

Cell wall swelling, fracture mode (along the middle lamellae vs. across cell walls), stiffness, and pressure at fracture of the sweet cherry fruit skin are closely related. Skin cracking is a common phenomenon in many crops bearing fleshy fruit. The objectives were to investigate relationships between the mode of fracture, the extent of cell wall swelling, and the mechanical properties of the fruit skin using sweet cherry (Prunus avium) as a model. Cracking was induced by incubating whole fruit in deionised water or by fracturing exocarp segments (ESs) in biaxial tensile tests. The fracture mode of epidermal cells was investigated by light microscopy. In biaxial tensile tests, the anticlinal cell walls of the ES fractured predominantly across the cell walls (rather than along) and showed no cell wall swelling. In contrast, fruit incubated in water fractured predominantly along the anticlinal epidermal cell walls and the cell walls were swollen. Swelling of cell walls also occurred when ESs were incubated in malic acid, in hypertonic solutions of sucrose, or in water. Compared to the untreated controls, these treatments resulted in more frequent fractures along the cell walls, lower pressures at fracture (p ), and lower moduli of elasticity (E, i.e., less stiff). Conversely, compared to the untreated controls, incubating the ES in CaCl and in high concentrations of ethanol resulted in thinner cell walls, in less frequent fractures along the cell walls, higher E and p . Our study demonstrates that fracture mode, stiffness, and pressure at fracture are closely related to cell wall swelling. A number of other factors, including cultivar, ripening stage, turgor, CaCl, and malic acid, exert their effects only indirectly, i.e., by affecting cell wall swelling.

Swelling of cell walls in mature sweet cherry fruit: factors and mechanisms

“…In an earlier study, we observed marked decreases in cell wall swelling in the presence of free Ca ions (Brüggenwirth and Knoche 2017). In contrast, swelling increased in the Cheek 2.6 ± 0.1 a a 4.3 ± 0.1 a 1.7 ± 0.1 Suture 2.6 ± 0.1 a 4.5 ± 0.1 a 1.9 ± 0.2 Stylar scar 2.7 ± 0.1 a 5.7 ± 0.1 b 3.0 ± 0.2 Shoulder 2.8 ± 0.1 a 4.5 ± 0.1 a 1.7 ± 0.1 presence of malic acid, probably due to extraction of Ca from the cell wall.…”

“…This was probably due to complexing with Ca ions, which altered the amount of bound Ca in the cell wall matrix. Removal of Ca from the cell wall will result in reduced cross-linking, thus a loosening of cell wall constituents, and thus an increase in swelling (Brüggenwirth and Knoche 2017).…”

“…Extraction and complexing of Ca would also account for differential swelling among buffers of different chemistry but of the same pH. Additional indirect evidence for the involvement of pectins in swelling comes from (i) the exposure of pectins on the fracture surface of the sweet cherry fruit skin (Schumann et al 2019), (ii) the mode of fracture of the skin along and not across cell walls (Brüggenwirth and Knoche 2017) and (iii) data on other fruit crops demonstrating that swelling is associated with the pectin fraction (Redgwell et al 1997;Basanta et al 2013).…”

“…Cracking is now considered to be a multistep process comprising: (i) the formation of microcracks due to cuticular strain caused by downregulation of cutin and wax synthesis and deposition during early growth (Knoche et al 2004;Peschel and Knoche 2005;Alkio et al 2012) and the later exposure of the strained cuticle to wetness (Knoche and Peschel 2006), (ii) localized water penetration through microcracks (Winkler et al 2016), (iii) the bursting of individual mesocarp cells and the consequent leakage of malic acid into the cell-wall-free space that further damages adjacent cells including those of the epidermis (Winkler et al 2015;Grimm et al 2019), and (iv) the complete loss of the (already low) turgor that results in cell wall swelling (Grimm and Knoche 2015). Lastly, cell wall swelling decreases cell wall stiffness, fracture tension and cell:cell adhesion resulting in the separation of neighboring cells (Brüggenwirth and Knoche 2017). The tension generated by the skin strain is sufficient to cause cells to separate along their swollen walls and to rupture the skin.…”

Spatial heterogeneity of flesh-cell osmotic potential in sweet cherry affects partitioning of absorbed water

“…As determined from the juice expressed from sweet cherry mesocarp tissue, the symplast of these cells contains high concentrations of malic acid (70 mM 19 ) and is very acidic (pH 4.2 20 ). Malic acid is known to decrease the modulus of elasticity of the cell wall material, possibly by desorbing and complexing cell-wall bound Ca 20,21 . The effect of symplastic leakage would remain localised if diffusion of malic acid in the apoplast was slow.…”