Drought-Induced Ultra-Fast Fluorescence Quenching in Photosystem II in Lichens Revealed by Picosecond Time-Resolved Fluorescence Spectrophotometry

Lichens survive under the extreme drought environments. It has been suggested that dried lichens convert excess light energy into heat by unknown mechanism to prevent the accumulation of harmful photoproducts. We studied 18 lichen species by their steady-state fluorescence spectra, PAM and picosecond time-resolved fluorescence decay profiles at 4–300 K. Quantitative analyses of the decay profiles were applied. We obtained the following results: (1) All dried lichens showed a low intensity of PS II fluorescence; (2) the picosecond decays of PS II fluorescence were fast (<10 ps) in most dry lichens; (3) the excitation energy transfer from LHC II to CP43/CP47 was still active; (4) the lifetime of the PS I fluorescence was little affected; (5) the changes were fully reversed within 1 min after the re-hydration; and (6) some lichens showed no fast decay of PS II fluorescence. We noticed two different types of drought-induced energy dissipation mechanisms: Most of lichens dissipated almost all the excitation energy in a few picoseconds by an unknown quencher; some lichens decreased the antenna size of PS II by the state transition mechanism. The new type of the quencher found in this study seems to be situated in the core antenna, and is different from the well-known non-photochemical quenching mechanism.