Floral oil production in a family dominated by pollen flowers: The case of Macairea radula (Melastomataceae)

Abstract

The Melastomataceae family, the largest radiation of pollen flowers, has been reported to offer floral oils exclusively in the Olisbeoideae subfamily. However, species from other clades such as Macairea radula (Marcetieae, Melastomatoideae) exhibit staminal glands that secrete oil-like viscous substances whose chemical composition and function are still unknown. We used anatomical sections and histochemical tests to characterize these staminal glands and their exudate. We also used GC–MS to characterize the chemical composition of the oil from the flowers and the scopae of visiting oil bees (Centris aenea). The staminal glands consist of glandular emergences with a multiseriate stalk and a conspicuous multicellular secretory head. Histochemical tests revealed the presence of lipids and phenolic compounds inside the glandular head cells. Although histologically different from trichomes, these glands are morphologically similar to trichomatic elaiophores. GC–MS confirmed the non-volatile lipidic nature of the staminal gland secretion, which consists of a mix of medium to long chain alkanes and nutritious fatty acids. Therefore, M. radula staminal glands produce oils similar in composition to the oils produced as bee reward by other angiosperm flowers. Some of these compounds were also found in the oils extracted from visiting bee scopae, suggesting that the oils produced by the staminal glands can be collected by bees. In addition, or alternatively, these oils could promote better adhesion of pollen to the bee's body. Oil production by staminal glands of M. radula may attract oil-collecting bees more consistently, ultimately contributing to the plants’ reproductive success.