Anatomical and histochemical study of abscission process in Japanese persimmon fruit at young fruit stage

In Japan, physiological fruit drop of young persimmon fruit is observed from June to July. It has been known that the fruit drop occurs at the juncture tissue between the peduncle and fruit (hereinafter referred to as “peduncle-fruit tissue”). However, histochemical studies on the constituents of abscission cells have not been found. In this study, we used persimmon cultivar ‘Fuyu’, and anatomically and histochemically observed abscission process by ethephon treatment. The peduncle-fruit tissue was fixed in FAA solution (Formalin + acetic acid + alcohol), and embedded in paraffin, cut in 15 μm and stained with toluidine blue-O, ruthenium red, zinc chloride and IKI (iodine + potassium iodide). As a result, an abscission zone was observed in the peduncle-fruit tissue. In the abscission process, the cells composed of the epidermis, cortex or vascular bundle were broken down, and cracks was formed to the central parenchyma. The boundary between the cells in the parenchyma tissue region was unclear. Finally, the cell walls were broken down and the peduncle was completely separated from the calyx, and fruit abscission was occurred. The cells compose of the abscission zone were stained dark shade blue with zinc chloride, black with IKI, and pale red with ruthenium red. Pectin, starch grains and cellulose were contained in the cells of the abscission zone. In contrast, cells composed of abscission zone after fruit detachment was not stained. These results indicated that the abscission cells were broken down by the cell wall degrading enzymes such as cellulase and pectinase. Starch grains were consumed for energy needed for the enzyme activities in abscission process. We conclude that fruit drop in June to July is induced by physiological processes in the abscission layer of the peduncle-fruit tissue.