Callose wall and sporoderm characteristics during the anther development stage of Dendrobium officinale

The pollen developmental characteristics in plants with pollinia remain unknown. In this study, Dendrobium officinale (Orchidaceae) pollinium development was investigated on the basis of a histochemical examination of polysaccharides and lipids. The D. officinale anther wall is composed of one layer of epidermal cells, two layers of endothecial cells, one layer of middle layer cells, and one layer of tapetal cells. The microspore mother cells, which have no obvious callose wall structure before meiosis, undergo simultaneous microsporogenesis. After meiosis, four microspores in a tetrad do not separate and subsequently develop as pollen tetrads, which ulti-mately form two pollinia. During microspore development, sporopollenin covers the pollinium surface and forms the exine of the pollinium, but the pollen tetrads inside the pollinium lack the exine structure. A pollen germinal pore was not detected in the pollinium exine. An in vitro analysis of pollen germination revealed that pollen grains at the surface of pollinia covered with an exine layer do not germinate. Furthermore, pollen tubes are mainly produced by the pollen grains inside the pollinium.