Micromorphological, histochemical and ultrastructural analysis of flower secretory structures in two species pollinated by flies (Diptera) of Asclepiadoideae Burnett

Abstract This micromorphological, chemical and ultrastructural study is a continuation of research conducted on fly-pollinated species. The aim of proposed project is to examine the features of floral secretory tissue of stapeliads (Eudicots, Apocynaceae, Asclepiadoideae) and compare them with previously published Bulbophyllum species (Monocots, Orchidaceae, Epidendroideae). The Echidnopsis cereiformis and Stapelia scitula flowers comprised features that characterize a fly-pollinated syndrome. Secretory activity was observed on the adaxial (upper) surface of flowers of both species, however, the chemical composition of secretion differs between species. In S. scitula it is lipid-rich material, whereas in E. cereiformis it is protein-rich. The ultrastructural studies revealed dense cytoplasm in the epidermis and subepidermal tissue with large nuclei and numerous mitochondria, the profiles of SER and RER, and dictyosomes. Large amounts of secreted material were present on the cuticle surface, similar to the unusual prominent periplasmic space with flocculent secretory material. The most remarkable feature was the presence of a prominent periplasmic space in the epidermal cells of S. scitula. Interestingly, subcuticular spaces have been noted only in a few other species, including three orchids from the section Lepidorhiza. Since Asclepiadaceae and Orchidaceae share other features which developed independently, such as pollen mass coherent in pollinia, presence of a periplasmic space could have also developed as a consequence of pollinators pressure and could be a feature important taxonomically.