Colleters in Asclepiadoideae (Apocynaceae): Protection of Meristems against Desiccation and New Functions Assigned

Premise of research.Although colleters are one of the most studied glands of the Apocynaceae, very little is known regarding the composition of their secretion. The aim of this study was to analyze histochemically vegetative and floral colleters of Asclepiadoideae (Apocynaceae) and verify their function during organ development.Methodology.Shoot apex, leaves, and flowers in different developmental stages were processed according to usual techniques in plant anatomy and submitted to histochemical tests.Pivotal results.Foliar and floral colleters of Peplonia secrete mucilage exclusively, while those of Asclepias, Blepharodon, Fischeria, Matelea, and Oxypetalum secrete mucilage, proteins, phenolic compounds, and lipids, including fatty acids. The secretion permeates and involves shoot apex, floral buds, and the developing organs, protecting these meristematic regions.Conclusions.The secretion of colleters of Peplonia is efficient in preventing the desiccation of organs but allows fungal proliferation among colleters. The heterogeneous secretion of Asclepias, Blepharodon, Fischeria, Matelea, and Oxypetalum protects meristematic regions against desiccation due to the presence of mucilage; inhibits the growth of fungi due to the presence of lipids; and can immobilize phytophagous insects due to its high viscosity. Despite the general description of protection against desiccation by colleters in many plant families, colleters of Asclepiadoideae can protect their meristems in different ways. We demonstrate the antifungal property of the lipids present in some colleter secretions for the first time in angiosperms, which indicates the need to reevaluate the functions assigned to colleters in other groups.