Development, structure and secretion compounds of stipule colleters in Pentas lanceolata (Rubiaceae)

Four types of colleters distributed on the stipules in Pentas lanceolata were studied by light and electron microscopy, and the metabolites they contain were identified. The terminal colleters of one type are formed at the top of the stipule lobes and three types of the basal colleters occur at the base of the lobes. The structure of all colleters matches to the standard type; however, some specific variations also arise. The development of all colleters happens as a result of anticlinal divisions of initial and daughter protodermal cells followed by periclinal and anticlinal divisions of subprotodermal cells. Maturation of the basal colleters occurs after the terminal ones. The secretory structures produce a complex secretion. Histochemistry and fluorescence microscopy demonstrate the presence of proteins, pectins, lipids, terpenoids, phenylpropanoids and tannins in the secretory cells. For the first time gas chromatography-mass spectrometry was used to determine the content of metabolites in extracts from isolated colleters. Seventy-four compounds participating in major biochemical processes were identified. Some iridoids (loganin, tudoside and asperuloside), triterpenes (oleanolic and ursolic acids), sterols (campesterol), and phenolics (4-hydroxycinnamic acid) were found in the colleters in larger quantity than in the stipules without colleters. Several substances including loganin, campesterol, thymol, caryophyllene, and mollugin have been identified in P. lanceolata colleter extracts for the first time.