Ethylene-induced Rough Endoplasmic Reticula in Abscission Cells

An investigation of the effect of ethylene on the fine structure of abscission cells has been prompted by the increasing amount of physiological and biochemical data on ethylene-induced abscission (cf. 1-3, 6, 7, 10-13, 18, 22). Recent investigations (14, 15, 19) have shown that certain changes occur in the fine structure of the cytoplasm and the cell wall as cell separation develops during natural abscission in flower pedicels. It was therefore of interest to study the effects of ethylene on abscission at the ultrastructural level in pedicels. A description of some significant changes induced by ethylene is presented here. The experimental plants (Nicotiana tabacum L. cv. Little Turk.) were grown in the greenhouse to the flowering stage. Terminal 25to 30-cm portions of the plants were then removed and placed in water (basal ends submerged) within vented glass cylinders (capacity 10 liters) under a light intensity of 400 ft-c at 22 C. A continuous flow of moist air was supplied with or without ethylene (25 ppm) to the cylinders. Under these conditions, flowers at the stage of anthesis abscised under slight pressure after approximately 48 hr of exposure to moist air, whereas those receiving air plus ethylene abscised after 9 hr of exposure to the gas mixture. Pedicel tissue containing the abscission zone was harvested at 9 hr and was fixed immediately for electron microscopy studies (14). An examination of the ethylene-treated tissue showed rough endoplasmic reticulum in cells (Fig. 1) distal and proximal (toward the flower and toward the stem, respectively) to the line of separation. The rough ER' appeared at a frequency never observed in cells comprising the control tissue. In earlier studies (15), rough ER was also observed in the abscission cells of naturally abscising pedicels but not at the frequency observed in ethylene-treated cells. These differences may be related to slower rates of abscission under natural conditions. The line of separation in ethylene-treated tissue occurred primarily between two tiers of cells only, the separation proceeding across the pedicel through the central region of the abscission layer. Rough ER was observed throughout at least 10 tiers of cortical cells lying distal and proximal to the line of separation. Cell separation following ethylene treatment is extremely localized. In naturally abscising pedicels, separation occurs between several tiers of cells which comprise the separation layer (21). The separation layer comprises the extreme distal portion of the abscission layer of tobacco flower pedicels (21). Abeles (2) has proposed that ethylene induces abscission by increasing RNA and protein synthesis in the abscission zones of bean petiole explants. Studies by Webster (20) employing histochemical and radioautographic techniques are in agreement with the biochemical data (2). These changes are observed to