Cyclic Photophosphorylation in the Mykotrophic Orhid Neottia nidus-avis.

The mykotrophic orchid Neottia nidus-avis (L.) Rich. is not able to evolve oxygen in the light. Plastid preparations from the lip (labellum) of the orchid perform a photosystem I-dependent photoreduction of methylviologen with the artificial electron donor couple 2,6-dichlorophenol indophenol ascorbate. Photosystem II reactions such as the ferricyanide Hill reaction or the photoreduction of 2,6-dichlorophenol indophenol with diphenylcarbazide as the electron donor are not functioning. The plastids exhibit phenazine methosulfate-mediated cyclic photophosphorylation. After infiltration with (32)P-labeled phosphate the labellum forms (32)P-ATP in the light. This rate of ATP formation is enhanced by additional infiltration of phenazine methosulfate prior to illumination. The brown color of the plant is caused by an absorption shift of carotenoids to longer wavelength. By comparison of absorption spectra with the fluorescence excitation spectra of plastid preparations and of the extracted pigments we show that no appreciable energy transfer from carotenoids to chlorophyll occurs.