, Volume 286, Issue 3-4, pp 141-151
Date: 15 May 2010

Morphological and histological characterization of the osmophores and nectaries of four species of Acianthera (Orchidaceae: Pleurothallidinae)

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Nectar and floral odor are frequently associated with the presence and maintenance of specialized pollination systems in Orchidaceae. We studied flowers of four Acianthera species, a genus of myophilous orchids belonging to the largest fly-pollinated orchid group Pleurothallidinae, in order to characterize the secretory structures related to their pollination mechanism. Flowers at anthesis were sampled to detect volatile compounds and nectar; samples were fixed for light microscopy and scanning and transmission electron microscopy. The labellum presents epidermal cells and the first mesophyll layer involved with secretory processes. Cellular characteristics of these regions associated with the occurrence of sugars allowed us to recognize them as nectaries. Some portions of the sepals also shown to be involved with secretory processes and the presence of nitrogenated volatile compounds characterize them as osmophores. The production of nectar in these species makes the occurrence of sapromyophily questionable, even though these flowers present characteristics of this floral syndrome. The presence of osmophores on the sepals reinforces that this localization is common among the Pleurothallidinae, whilst they occur in a different region (labelum) in the other major fly-pollinated orchid group (Bulbophyllum).