Mottling on Sweet Cherry Fruit Is Caused by Exocarp Strain

Abstract

<jats:p>Mottling (pale spots) is clearly visible to the naked eye in all regions of the surface in all except for yellow cultivars of sweet cherry fruit (<jats:italic>Prunus avium</jats:italic> L.). The objective was to characterize these spots and their distribution on the exocarp. Within the spots, anthocyanins were limited to the epidermal cell layer but, in areas immediately adjacent to the spots, anthocyanins were present in the epidermal and in the hypodermal cell layers (making these areas darker). In ‘Sam’ sweet cherries, the median length and width of a spot in the cheek region were 390 and 162 μm, respectively, and the median area was 0.053 mm<jats:sup>2</jats:sup> per spot. The spatial density in the cheek region averaged 1.94 (± 0.13) spots per mm<jats:sup>2</jats:sup> and the percentage of surface area covered by the spots was 12.5% (± 1.07%). Epidermal cells within a spot had slightly larger projected surface areas than those in the adjacent region and thicker cell walls. The margins of the spots did not align with the anticlinal walls of the epidermal cells. The spots’ long axes were oriented parallel with the stem/stylar scar axis, whereas the slightly elongated epidermal cells within and adjacent to the spots were orientated perpendicular to the stem/stylar scar axis. The spatial density of spots and the cumulative spot area were highest in the region of the stylar scar, intermediate in the cheek and stem cavity, and lowest in the suture region. Spot spatial density on small fruit exceeded that on larger fruit, but the areas of individual spots was smaller. When an exocarp segment was excised from the cheek of a fruit, it contracted slightly as elastic strain was released. The projected surface area of the spots and that of the whole segment decreased to a similar extent. Our data suggest that spots result from a tensional failure during Stage III development in which the anthocyanin-containing hypodermal cell layer tears (schizogenously) and separates from the epidermis. This being the case, the pale spots (mottling) can be referred to as “strain spots.”</jats:p>

Mature Sweet Cherries Have Low Turgor

“…A reasonable explanation accounting for water uptake without an associated increase in Y fruit P is that the skin suffers an increase in strain without an associated increase in stress. Recent studies by Grimm et al (2012Grimm et al ( , 2013 demonstrate that the strain in the exocarp of mature sweet cherry fruit is largely reversible and is released in a time-dependent manner after excision. This behavior is referred to as viscoelasticity (Niklas, 1992;Vincent, 1990) and is a common characteristic of cellwall composites.…”