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Published on September 7, 2007; 10.1104/pp.107.106872

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Received August 3, 2007
Accepted August 30, 2007

Photoprotection in the Lichen, Parmelia sulcata. The Origins of Desiccation Induced Fluorescence Quenching

John Veerman , Sergej Vasil'ev , Gavin D. Paton , Justin Ramanauskas , and Doug Bruce *

Department of Biological Sciences, Brock University, 500 Glenridge Ave., St. Catharines, Ontario, L2S 3A1 Canada

* Corresponding author; email: dbruce{at}brocku.ca.

Lichens, a symbiotic relationship between a fungus (mycobiont) and a photosynthetic green algae or cyanobacteria (photobiont), belong to an elite group of survivalist organisms termed resurrection species. When lichens are desiccated they are photosynthetically inactive, but upon re-hydration they can perform photosynthesis within seconds. Desiccation is correlated with both a loss of variable chlorophyll a (Chl a) fluorescence and a decrease in overall fluorescence yield. The fluorescence quenching likely reflects photoprotection mechanisms which may be based on desiccation induced changes in lichen structure that limit light exposure to the photobiont (sunshade effect) and/or active quenching of excitation energy absorbed by the photosynthetic apparatus. To separate and quantify these possible mechanisms we have investigated the origins of fluorescence quenching in desiccated lichens with steady state, low temperature and time-resolved Chl fluorescence spectroscopy. We found the most dramatic target of quenching to be photosystem II (PSII), which produces negligible levels of fluorescence in desiccated lichens. We show that fluorescence decay in desiccated lichens was dominated by a short lifetime, long wavelength component energetically coupled to PSII. Remaining fluorescence was primarily from PSI and although diminished in amplitude, PSI decay kinetics were unaffected by desiccation. The long wavelength quenching species was responsible for most (about 80%) of the fluorescence quenching observed in desiccated lichens, the rest of the quenching was attributed to the sunshade effect induced by structural changes in the lichen thallus.




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