Secretory Tissues and the Morphogenesis and Histochemistry of Pollinarium in Flowers of Asclepiadeae (Apocynaceae)

Abstract

Premise of research. Flowers of some Apocynaceae present an unusual synorganization among parts and organs, which has led to the origin of new organs and a type of pollen dispersal and pollination unique to eudicots. The synorganization of androecium and gynoecium allowed the evolution of the pollinarium, a very complex and species-specific structure composed of pollen grains (pollinium) and secretions produced by the style head (translator). The aim of this study was to investigate the structure and activity of the secretory system related to the morphogenesis of pollinaria and the composition of their secretions in flowers of Asclepiadeae.Methodology. Flowers and buds of Asclepias, Gonioanthela, Matelea, and Oxypetalum were fixed, embedded in Paraplast, and sectioned in a rotary microtome for analysis under light microscopy. SEM was performed for additional structural data, and histochemical tests were carried out to reveal the main chemical composition of the secretions involved in pollinaria morphogenesis.Pivotal results. In the four species of Asclepiadeae studied, the style head possesses a palisade secretory epidermis that is responsible for the secretion of the translator, which is composed of a corpusculum and two caudicles. Mucilage, fatty acids, phenolic compounds, and proteins were detected in the corpusculum, whereas only neutral lipids and mucilage were present in the caudicles. In the anthers, tapetal cells are involved in the secretion of lipids that cover the whole pollinium. Just before anthesis, anthers dehisce, and the pollinia from adjacent anthers contact and adhere to the caudicles of the translator, forming the pollinarium.Conclusions. The translator shape is due to the differential activity of the secretory cells, which are spatially and temporarily coordinated, as well as the amount and composition of the secretion and the undulated outline of the secretory surface. The corpusculum and caudicles are composed of different chemical substances, and these features are related to the processes of removal and insertion of pollinia.