Secretory Cavity Development and Epidermal Exudation Pathways in Fruits of Ruta graveolens L. (Rutaceae)

Ruta graveolens L. fruits are densely covered with translucent dots corresponding to secretory cavities that accumulate bioactive metabolites, primarily essential oils. Immature fruits present an aromatic surface exudate, indicating the active release of secretory products despite the internal location of the cavities. This study investigated the origin, development, ultrastructure, and secretion-release mechanisms of fruit secretory cavities using light, scanning, and transmission electron microscopy. Secretory cavities originated from clusters of ground meristem cells associated with phloem strands. Lumen formation began with the collapse of central cells, while surrounding cells differentiated into a metabolically active secretory epithelium rich in polymorphic plastids, smooth endoplasmic reticulum, mitochondria, vesicles, and lipid bodies. Mature cavities consisted of a multilayered epithelium surrounding a large lumen and enclosed by a parenchymatous sheath. Progressive lysis of inner epithelial cells contributed to lumen expansion and secretion accumulation. As cavities enlarged, they became positioned immediately beneath the epidermis, whose cells became compressed and flattened. Secretion was released through the rupture of glandular and epidermal cells and through stomata located in epicarp depressions. Ultrastructural evidence indicates the combined operation of eccrine, granulocrine, and holocrine secretion mechanisms. Pectin-cellulosic wall thickenings likely function as apoplastic barriers, directing secretion toward the lumen and protecting adjacent tissues.