Ultrastructural variability and chemical composition of Orchidaceae pollen

The Orchidaceae show an enormous diversity in flower morphology, which is also reflected in the orchid specific pollen dispersal units, the so called pollinia and pollinaria. Packaging of pollen by elastoviscin, an orchid characteristic sticky material, results in a huge variety of this morphological and structural complex pollen units. Despite the fact that orchids are one of the most diverse families among angiosperms, available palynological data are still meagre and contradicting. This study provides new insights into pollen morphology and ultrastructure of 11 selected orchid species belonging to subfamilies Epidendroideae and Orchidoideae, studied with combined light microscopy (LM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).
The main aims were to compare the morphology and ultrastructure of pollen dispersal units, and to clarify the controversial chemical nature of the pollen wall layers and of elastoviscin. The combined LM and SEM study exhibited the presence of all known tetrad types even within a single pollinium, which is unique for the orchids. The application of different TEM staining methods confirmed the assumed lipidic nature of elastoviscin and the differences in contrast and ultrastructure assume a mixture of sticky materials with dissimilar chemical compositions. Chemical detection reactions conducted on extracted elastoviscin also indicate the presence
of aldehydes and neutral polysaccharides. The study affirmed that sporopollenin is limited predominantly to outer pollen grains of peripheral tetrads in compact and sectile pollinia, whereas inner tetrads exhibit highly reduced non-sporopollenin pollen walls.