Calycinal secretory structures in Calolisianthus pedunculatus (Cham. & Schltdl) Gilg (Gentianaceae): anatomy, histochemistry, and functional aspects

Secretory structures were little studied in Gentianaceae. Glandular areas on the calyx dorsal region are commonly reported for Helieae species, the main tribe of Gentianaceae. So, the elucidation of nature of glandular areas is particularly relevant. Trichomes secreting mucilage, interpreted as colleters, are reported only for the sepals of Gentianinae species. We aimed to anatomically characterize and identify the nature of the calycinal secretory structures in Calolisianthus pedunculatus . Samples from floral buds, flowers, and fruits were collected, fixed, and processed following usual procedures for light and scanning electron microscopies. Histochemical tests were performed to determine the nature of the secretion. Glucose, fructose, and sucrose were measured with an ELISA reader. Colleters occur on the sepal ventral region and are composed of a multicellular secretory head and a stalk. These structures secrete polysaccharides and proteins, and the secretion is probably released through cuticle microchannels. Nectaries, on the other hand, occur on the sepal dorsal region. They are formed by 3–5 cells arranged in rosettes circling a central cell or pore. These structures also secrete polysaccharides (mainly fructose), lipids, and proteins. The identification of the secretory structures in the sepals of Calolisianthus pedunculatus highlights the importance of anatomical studies in this family. The interpretation of the glandular areas on the calyx of the Helieae species as nectaries has been proven, as well as the confirmation of colleters as common structures in the sepals of Gentianaceae. Besides the taxonomic and phylogenetic importance of nectars and colleters, we highlight the importance of the secretion for the protection of floral buds against dehydration.