Anatomical and ultrastructural study of the secretory cavity development of Citrus sinensis and Citrus limon: evaluation of schizolysigenous ontogeny.

The ontogeny and ultrastructure of secretory oil glands in the fruits of Citrus sinensis and Citrus limon were studied with light and electron microscopy (TEM). Oil glands were initiated by cell division in globular/oval clusters of young meristematic cells under or including also the epidermis. Successively, the central oil cells increased greatly in size by growth and coalescence of one or more vacuoles. Starting from these cells the oil cavity formation and enlargment occurred by schizogenous and lysigenous precesses which overlap one another. The separation of the cell walls, in both Citrus species, was accompanied by strong structural disorganization of the cytoplasm, loss of nuclei, plasmolysis, and followed by autolysis processes. Prior these changes a great quantity of oil droplets filled the cells. In Citrus sinensis this material was formed in the plastids; on the contrary, in Citrus limon oil droplets appeared only in the cytoplasm, and no connection with the plastids was observed. The processes of schizogeny and lysigeny occurred in a centrifugal direction. The peripheral oil chamber cells were so pushed outward and flattened with the progressive expansion of the gland cavity. Also these cells became vacuolated, showed schizogenous processes together with cytoplasm and nuclei breakdown. These cell modifications were preceded by an abundant essential oil synthesis. Electron dense material appeared in the cavity near the boundary cells. Among the cell layers surrounding the cavity, the outer peripheral cells showed very thick walls. These cells, probably, had a protective function, while the inner cells continued, at least in part, to produce and secrete oil compounds. In the two Citrus species the pattern of oil gland development was practically the same, except the different plastid behaviour.