Winkler Andreas, Grimm Eckhard, Knoche Moritz
Fruit Science Section, Institute of Horticultural Production Systems, Leibniz-University Hannover, Hannover, Germany.
Front Plant Sci. 2019 Mar 5;10:164. doi: 10.3389/fpls.2019.00164. eCollection 2019.
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 C-glucose and C-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.
通过皮肤的渗透吸水是甜樱桃出现裂果的一个重要因素。目的是确定甜樱桃是否表现得像一个理想的渗透计，即：（1）吸水速率与果实渗透势呈负相关；（2）孵育溶液渗透势的变化会导致吸水速率成比例变化；（3）导致零吸水的孵育溶液的渗透势在数值上等于果实水势（在果实无明显膨压的情况下）；（4）果实的角质膜仅对水具有通透性。果实的平均渗透势与吸水速率之间的相关性较弱。令人惊讶的是，将果实置于（a）同一批次果实的榨汁、（b）由榨汁的五种主要渗透溶质组成的等渗人工果汁或（c）等渗葡萄糖溶液中孵育，均导致显著的吸水现象。降低孵育溶液的渗透势会降低吸水速率，而进一步降低则会导致果实向孵育溶液失水。在果实发育过程中，“表观”果实水势始终比果实测量的平均渗透势更负。表皮细胞的质壁分离表明果皮的渗透势比果肉的渗透势负性更小。当将切下的果肉薄片在一系列不同浓度的葡萄糖溶液中孵育时，薄片的表观水势与榨汁的渗透势相匹配。¹⁴C - 葡萄糖和¹⁴C - 果糖通过切下的果实表皮发生了显著的渗透。这些结果表明甜樱桃不是一个理想的渗透计。部分原因是角质膜对葡萄糖和果糖的反射系数小于1。因此，葡萄糖和果糖会从孵育溶液中被果实吸收。此外，榨出的果汁的渗透势并不均匀。从花柱痕区域取出的果汁的渗透势比从果柄区域取出的更负，而果肉的渗透势比果皮的更负。
