title: A few molecules of zeaxanthin per reaction centre of photosystem II permit effective thermal dissipation of light energy in photosystem II of a poikilohydric moss.
abstract: The relationship between thermal dissipation of light energy (as indicated by the quenching of chlorophyll fluorescence), zeaxanthin availability and protonation reactions was investigated in the moss Rhytidiadelphus squarrosus (Hedw.) Warnst. In the absence of zeaxanthin and actinic illumination, acidification by 20% CO2 in air was incapable of quenching basal, so-called F0 fluorescence either in the moss or in spinach (Spinacia oleracea L.) leaves. However, 1-s light pulses given either every 40, 60 or 200 s increased thermal dissipation as indicated by F0 and Fm quenching in the presence of 20% CO2 in air in the moss, but not in spinach while reaction centres of photosystem II (PSII) were photochemically open. In the moss, a few short light pulses, which were separated by prolonged dark times, were sufficient to raise zeaxanthin levels in the presence of 20% CO2 in air. Simultaneously, quantum efficiency of charge separation in PSII was decreased. Increasing the CO2 concentration beyond 20% further decreased quantum efficiency even in the absence of short light pulses. Under conditions optimal for fluorescence quenching, one molecule of zeaxanthin per reaction centre of PSII was sufficient to decrease quantum efficiency of charge separation in PSII by 50%. Thus, in combination with a protonation reaction, one molecule of zeaxanthin was as efficient at capturing excitation energy as a photochemically open reaction centre. The data are discussed in relation to the interaction between zeaxanthin and thylakoid protonation, which enables effective thermal dissipation of light energy in the antennae of PSII in the moss but not in higher plants when actinic illumination is absent.
authors.affiliation: Julius von Sachs-Institut für Biowissenschaften, Universität Würzburg, 97082 Würzburg, Germany.
authors.forename: N G
authors.initials: NG
authors.lastname: Bukhov
authors.forename: J
authors.initials: J
authors.lastname: Kopecky
authors.forename: E E
authors.initials: EE
authors.lastname: Pfündel
authors.forename: C
authors.initials: C
authors.lastname: Klughammer
authors.forename: U
authors.initials: U
authors.lastname: Heber
mesh_terms: D019068:Bryopsida / Q000187:drug effects / Q000502:physiology* / Q000528:radiation effects; D002245:Carbon Dioxide / Q000494:pharmacology; D002734:Chlorophyll / Q000737:chemistry* / Q000378:metabolism / Q000528:radiation effects; D004229:Dithiothreitol / Q000494:pharmacology; D004579:Electron Transport; D005453:Fluorescence; D008027:Light; D045342:Light-Harvesting Protein Complexes; D010777:Photochemistry; D010788:Photosynthesis; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000378:metabolism / Q000528:radiation effects; D045332:Photosystem II Protein Complex; D018515:Plant Leaves / Q000187:drug effects / Q000502:physiology / Q000528:radiation effects; D018724:Spinacia oleracea / Q000187:drug effects / Q000502:physiology* / Q000528:radiation effects; D013696:Temperature; D020524:Thylakoids / Q000737:chemistry; D014466:Ultraviolet Rays; D024341:Xanthophylls; D065146:Zeaxanthins; D019207:beta Carotene / Q000031:analogs & derivatives / Q000737:chemistry* / Q000494:pharmacology
chemical_list: D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D024341:Xanthophylls; D065146:Zeaxanthins; D019207:beta Carotene; D002734:Chlorophyll; D002245:Carbon Dioxide; D004229:Dithiothreitol