| Summary: | Russeting is an economically important surface disorder in apple (<i>Malus × domestica</i> Borkh). Indirect evidence suggests an irregular skin structure may be the cause of the phenomenon. The objective of this study was to characterize epidermal and hypodermal cell morphology and the mechanical properties of the skins of apple cultivars of differing russet susceptibility. Dimensions of epidermal and hypodermal cells were determined using microscopy. Stiffness (<i>S</i>), maximum force (<inline-formula><math display="inline"><semantics><mrow><msub><mi>F</mi><mrow><mi>m</mi><mi>a</mi><mi>x</mi></mrow></msub></mrow></semantics></math></inline-formula>), and maximum strain (<inline-formula><math display="inline"><semantics><mrow><msub><mi mathvariant="sans-serif">ε</mi><mrow><mi>m</mi><mi>a</mi><mi>x</mi></mrow></msub></mrow></semantics></math></inline-formula>) at failure were quantified using uniaxial tensile tests of skin strips. Particularly during early fruit development, epidermal cells (EC) and hypodermal cells (HC) in russet non-susceptible cultivars occurred in greater numbers per unit area than in russet-susceptible ones. The EC and HC were lower in height, shorter in length, and of reduced tangential surface area. There were little differences in <i>S</i> or <inline-formula><math display="inline"><semantics><mrow><msub><mi>F</mi><mrow><mi>m</mi><mi>a</mi><mi>x</mi></mrow></msub></mrow></semantics></math></inline-formula> between non-susceptible and susceptible cultivars. However, the <inline-formula><math display="inline"><semantics><mrow><msub><mi mathvariant="sans-serif">ε</mi><mrow><mi>m</mi><mi>a</mi><mi>x</mi></mrow></msub></mrow></semantics></math></inline-formula> were higher for the skins of non-susceptible cultivars, than for those of susceptible ones. This difference was larger for the young than for the later growth stages. It is concluded that russet-susceptible cultivars generally have larger cells and a wider distribution of cell sizes for both EC and HC. These result in decreased <inline-formula><math display="inline"><semantics><mrow><msub><mi mathvariant="sans-serif">ε</mi><mrow><mi>m</mi><mi>a</mi><mi>x</mi></mrow></msub></mrow></semantics></math></inline-formula> for the skin during early fruit development when russet susceptibility is high. This increases the chances of skin failures which is known to trigger russeting.
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