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

MAIN
CONCLUSION: 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 fracture), and lower moduli of elasticity (E, i.e., less stiff). Conversely, compared to the untreated controls, incubating the ES in CaCl2 and in high concentrations of ethanol resulted in thinner cell walls, in less frequent fractures along the cell walls, higher E and p fracture. 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, CaCl2, and malic acid, exert their effects only indirectly, i.e., by affecting cell wall swelling.