Sweet Cherry Skin Has a Less Negative Osmotic Potential than the Flesh

摘要

The skin is the primary load-bearing structure in a sweet cherry fruit (Prunus avium L.). Failure of the skin in rain cracking is considered to be related to water uptake. Little is known of the skin's water potential, its osmotic potential (Psi(S)(Pi), and turgor. The objective here was to quantify Psi(S)(Pi) relative to the osmotic potential of the flesh (Psi(F)(Pi)). Spatial resolution was achieved by monitoring plasmolysis in epidermal cells in tissue sections, incubated in selected osmotica using a light microscope method. Decreasing the osmotic potential [Psi(Pi) (more negative)] of the incubation medium increased the proportion (percent) of plasmolyzed epidermal cells. The pattern of increasing plasmolysis was sigmoidal with increasing osmolyte concentration. The value of Psi(Pi) for 50% of cells plasmolyzed, depended to some extent on the osmolyte used. The value of Psi(Pi) became slightly less negative for the osmolytes tested in the order: 1) mannitol, 2) sucrose, and 3) artificial cherry juice (a solution comprising the five major osmolytes of sweet cherry juice in the appropriate proportions and concentrations). There was little difference in the value of Psi(Pi) at 50% plasmolysis between the cultivars Hedelfinger, Sam, and Sweetheart. In all three cultivars, the value of Psi(F)(Pi) (measured for expressed juice using an osmometer) was markedly more negative than that of Psi(S)(Pi) (measured for 50% plasmolysis). Incubating skin segments in juice from the same fruit resulted in the plasmolysis of most.(85.7% to 96.4%) of the epidermal cells. As fruit development progressed from stage II [27 day after full bloom (DAFB)] to the fully mature stage III (97 DAFB), plasmolysis occurred for increasingly more negative values of Psi(Pi). Moreover, the difference between the osmotic potential values recorded for the flesh Psi(F)(Pi) and for the skin Psi(S)(Pi) increased. Plasmolysis of epidermal cells was accompanied by a marked swelling of their walls. The results indicate a marked difference in the osmotic potential of flesh (Psi(F)(Pi) trended more negative) and skin cells (Psi(S)(Pi) trended less negative).