Ontogenesis, Ultrastructure and Morphometry of the Petiole Oil Ducts of Celery (Apium graveolens L.)

Abstract

Oil ducts in celery petioles originate from a group of four initial cells which undergo an asynchronous periclinal (tangential) division to give four internal mother epithelial cells and four peripheral cells. In the centre of the mother epithelial cells a cavity is formed by schizogenous separation of the cell walls. Epithelial cells increase in number to 8–12 through a series of anticlinal (radial) divisions. When duct cell divisions have finished and the central cavity is well-opened, epithelial cells switch into secretory cells. The organelle which appears directly involved in the process of essential oil secretion is considered to be the leucoplast. During the stage of secretion, leucoplasts increase in number by about 170% and in volume by 237%. Within their stroma, a network of smooth tubular elements locally develops, in which osmiophilic secretory droplets are formed. The volume of the plastidal secretory product was assessed to be 91.32 μm3 per epithelial cell. Endoplasmic reticulum-elements (ER) are observed to arrange parallel to the plastid surface and also to accumulate in the parietal cytoplasm facing the cavity of the duct. Ultrastructural evidence suggests that the secretory product is transported from the leucoplasts to the central cavity of the duct with the assistance of the ER-elements.