Colleters in Mabea fistulifera Mart. (Euphorbiaceae): Anatomy and biology of the secretory process

Mabea is an important neotropical genus of the Euphorbiaceae, whose secretory structure diversity is still little known. Mabea fistulifera Mart., one of the most representative species in the genus, is a pioneer plant that is widely distributed across Brazil and Bolivia. We investigated the distribution, morphology, histochemistry and ultrastructure of colleters, reported for the first time to M. fistulifera, aiming to elucidate the gland functions, exudate nature and secretory activity dynamics. Samples of vegetative and reproductive apices and of leaves at different developmental stages were processed following usual procedures for analysis in light, scanning and transmission electron microscopies. Colleters are present exclusively on leaves, occurring on the adaxial surface and apical region of stipules and on the leaf blade margin. Colleters have protodermal origin and are functional since early leaf differentiation. In the colleter secretory portion, central-axis cells and surface cells have similar origin and structure, both showing numerous mitochondria, dictyosomes, rough endoplasmic reticulum and ribosomes. The translucid secretion is of mucilaginous nature, being formed by polysaccharides, proteins and alkaloids, which is compatible with the observed cellular machinery. The presence of a hygroscopic exudate in contact with developing vegetative and reproductive organs is important for the protection of young tissues against desiccation, especially in view of the fact that the species occurs in open environments, which are subjected to water stress, elevated temperature and high light incidence.