Sweet cherry fruit growth follows a double sigmoid growth pattern, which is characterized by rapid increases of fruit mass and surface area during Stages I and III of development with much reduced expansion during the intermediate Stage II (Lilleland and Newsome, 1934; Tukey, 1934). In non-yellow cultivars, the color changes from green to yellow during Stage II and finally to red or black during early Stage III (Serrano et al., 2005). During Stage I, pericarp fruit growth occurs mostly by cell division. During Stage II the endocarp and the embryo develop, whereas cell expansion in the mesocarp accounts for most of the increase in fruit weight during Stage III development (Lilleland and Newsome, 1934; Tukey, 1934; Tukey and Young, 1939). The change in color is related to the degradation of chlorophyll, the presence of carotenoids in chromoplasts, and the accumulation of anthocyanins in the vacuoles (Serrano et al., 2005).
Recent investigations demonstrate that the exocarp is markedly strained during Stage III. First is because mature sweet cherry fruit gape when the fruit is cut longitudinally radially using a razor blade (Grimm et al., 2012). Second, exocarp segments decrease in area on excision from the fruit surface (Grimm et al., 2012). Third, the cuticular membrane is markedly strained during Stage III development (Knoche et al., 2004). Fourth, epidermal cells and microscopic cracks in the cuticle are highly oriented (Peschel and Knoche, 2005). Furthermore, the major axes of the somewhat elongated epidermal cells are oriented perpendicular to the microcracks, which is consistent with a cause-and-effect relationship. Fifth, the exocarp represents the outermost layer of approximately spherical fruit and is therefore subjected to the largest tension (Considine and Brown, 1981). Sixth, Measham et al. (2009) observed a positive relationship between tangential stress determined from the critical turgor pressure and the incidence of cracking.
In recent research we observed that the surface of red and black sweet cherries has a mottled appearance to the naked eye where the dark background color alternates with light, irregular pale spots up to 0.5 mm in length and/or width (S. Peschel and M. Knoche, unpublished data). These spots apparently decreased in number per unit fruit surface area and increased in length/width ratio from the stylar scar to the stem cavity region. Such a change would be consistent with an increasing rate of surface expansion from the stylar scar to the stem cavity region during Stage III development (Knoche et al., 2004). To our knowledge, there are no reports in the literature on the causes and significance of these pale spots. The purpose of this study was to characterize the pale spots and their distribution on the sweet cherry surface.
Considine, J. & Brown, K. 1981 Physical aspects of fruit growth—Theoretical analysis of distribution of surface growth forces in fruit in relation to cracking and splitting Plant Physiol. 68 371 376
Considine, J. & Knox, R.B. 1979 Development and histochemistry of the cells, cell walls, and cuticle of the dermal system of fruit of the grape, Vitis vinifera L Protoplasma 99 347 365
Elsaie, M.L., Baumann, L.S. & Elsaaiee, L.T. 2009 Striae distensae (stretch marks) and different modalities of therapy: An update Dermatol. Surg. 35 563 573
Knoche, M., Beyer, M., Peschel, S., Oparlakov, B. & Bukovac, M.J. 2004 Changes in strain and deposition of cuticle in developing sweet cherry fruit Physiol. Plant. 120 667 677
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
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
Peschel, S. & Knoche, M. 2005 Characterization of microcracks in the cuticle of developing sweet cherry fruit J. Amer. Soc. Hort. Sci. 130 487 495
Ranft, J., Basan, M., Joanny, J., Elgeti, J.F., Prost, J. & Jülicher, F. 2010 Fluidization of tissues by cell division and apoptosis Proc. Natl. Acad. Sci. USA 107 20863 20868
Serrano, M., Guillen, F., Martinez-Romero, D., Castello, S. & Valero, D. 2005 Chemical constituents and antioxidant activity of sweet cherry at different ripening stages J. Agr. Food Chem. 53 2741 2745
Tukey, H.B. 1934 Growth of the embryo, seed, and pericarp of the sour cherry (Prunus cerasus) in relation to season of fruit ripening Proc. Amer. Soc. Hort. Sci. 31 125 144
Wegner, A. 1912 Die Herausbildung der Großformen der Erdrinde (Kontinente und Ozeane), auf geophysikalischer Grundlage Petermanns Geogr. Mitt. 63 185 195, 253–256, 305–309