Nectar resorption in the spur of Platanthera chlorantha Custer (Rchb.) Orchidaceae - structural and microautoradiographic study

In the flowers of Platanthera chlorantha nectar is secreted and accumulated in a spur. Previous studies of this species revealed that after the period of secretion and cessation, rapid nectar resorption occurs. The aim of this study was the observation of nectar resorption on the structural level using tritium labelled sucrose. For this purpose, during the peak of nectar secretion, 10 mul of nectar accumulated in the spur was replaced with the same volume of labelled sucrose (10 muCi). Small fragments of spurs were sampled between 12-36 h of incubation, at the resorption phase. Afterwards, they were fixed and embedded in epoxy resin. Material for the microautoradiographic study was prepared with the dipping technique. The cells of secretory epidermis and unicellular secretory papillae had dense, strong stained cytoplasm, a large nucleus and small vacuoles. A characteristic feature of these cells was the presence of numerous starchless plastids, mitochondria and ER profiles. Many vesicles occurred in the close vicinity of the cell wall. In the cuticle covering cell walls no pores or cracks were observed. Presence of [H-3] sucrose was detected mainly in the walls of the nectary cells, which would indicate an apoplastic route of resorbed nectar.