Ultrastructure and Secretion of Extrafloral Nectaries of Plumeria rubra L.

Abstract

Extrafloral nectaries situated on the adaxial side of the petiole base are differentiated into a long head, comprising subepithelial ground tissue surrounded by a layer of elongated palisade-like epithelial cells and a short stalk from the nectary meristem. Many ultrastructural changes occur in epithelial and subepithelial cells of the nectary, from the young to secretory stages, such as an increase in the amount of cytoplasm rich in mitochondria with well developed cristae, rough endoplasmic reticulum (rER), smooth endoplasmic reticulum (sER) tubules and Golgi bodies. Plasmalemma invaginations with secretory vesicles occur long the radial walls. Substantial amounts of secretory material accumulate in the gap between the radial walls and subcuticular space, probably carried by the secretory vesicles from the cytoplasm at the secretory stage. Before cessation of secretion the cytoplasm becomes vesiculated and the volume of the vacuome increases. At the post secretory stage, cytolytic processes and death of cells occur. The subepithelial cells attain their maturity prior to epithelial cells. Histochemical localization reveals the presence of lipids, proteins and insoluble polysaccharides within the epithelial cells and in the secretory material deposited in the subcuticular space as well as the gap between the radial walls of the epithelial cells and outside the cuticle.