Abstract
Osmotic water uptake through the skin is an important factor in rain cracking of sweet cherries. The objective was to establish whether a sweet cherry behaves like an ideal osmometer, where: (1) water uptake rates are negatively related to fruit osmotic potential, (2) a change in osmotic potential of the incubation solution results in a proportional change in water uptake rate, (3) the osmotic potential of the incubation solution yielding zero water uptake is numerically equal to the fruit water potential (in the absence of significant fruit turgor), and (4) the fruits' cuticular membrane is permeable only to water. The fruits' average osmotic potential and the rate of water uptake were related only weakly. Surprisingly, incubating a fruit in (a) the expressed juice from fruit of the same batch or (b) an isotonic artificial juice composed of the five major osmolytes of expressed juice or (c) an isotonic glucose solution—all resulted in significant water uptake. Decreasing the osmotic potential of the incubation solution decreased the rate of water uptake, while decreasing it still further resulted in water loss to the incubation solution. Throughout fruit development, the “apparent” fruit water potential was always more negative than the fruits' measured average osmotic potential. Plasmolysis of epidermal cells indicates the skin's osmotic potential was less negative than that of the flesh. When excised flesh discs were incubated in a concentration series of glucose solutions, the apparent water potential of the discs matched the osmotic potential of the expressed juice. Significant penetration of 14C-glucose and 14C-fructose occurred through excised fruit skins. These results indicate a sweet cherry is not an ideal osmometer. This is due in part to the cuticular membrane having a reflection coefficient for glucose and fructose less than unity. As a consequence, glucose and fructose were taken up by the fruit from the incubation solution. Furthermore, the osmotic potential of the expressed fruit juice is not uniform. The osmotic potential of juice taken from the stylar scar region is more negative than that from the pedicel region and that from the flesh more negative than that from the skin.
Highlights
Rain cracking severely limits sweet cherry production in most regions where this high-value crop is grown, and especially when rainfall occurs during the harvest period (Christensen, 1996)
The relationships between rate of water uptake (FH2O) or J and A or fruit were significant but highly variable, as indicated by coefficients of determination ranging from r2 = 0.12∗∗∗ to r2 = 0.23∗∗∗
The FH2O from artificial juice exceeded that from isotonic polyethylene glycol (PEG) 6000 (Figure 2C)
Summary
Rain cracking severely limits sweet cherry production in most regions where this high-value crop is grown, and especially when rainfall occurs during the harvest period (Christensen, 1996). Osmotic water uptake through the fruit surface into the flesh is considered to be the primary cause of rain cracking (Christensen, 1996; Winkler et al, 2016). There is no published evidence that the pit plays any significant role in either water uptake or in rain cracking. The rate of osmotic water uptake (FH2O; g s−1) by a cherry fruit equals the product of the flux density (J; g m−2 s−1) times the surface area (A; m2).
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