A few molecules of zeaxanthin per reaction centre of photosystem II permiteffective thermal dissipation of light energy in photosystem II of a poikilohydric moss

The relationship between thermal dissipation energy (as indicated by the qu enching of chlorophyll fluorescence), zeaxanthin availability and protonati on reactions was investigated in the moss Rhytidiadelphus squarrosus (Hedw. ) Warnst. In the absence of zeaxanthin and actinic illumination, acidificat ion by 20% CO2 in air was incapable of quenching basal, so-called F-0 fluor escence either in the moss or in spinach (Spinacia oleracea L.) leaves. How ever, 1-s light pulses given either every 40, 60 or 200 s increased thermal dissipation as indicated by Fo and F, quenching in the presence of 20% CO2 in air in the moss, but not in spinach while reaction centres of photosyst em II (PSII) were photochemically open. In the moss, a few short light puls es, which were separated by prolonged dark times, were sufficient to raise zeaxanthin levels in the presence of 20% CO2 in air. Simultaneously, quantu m 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 qu enching, one molecule of zeaxanthin per reaction centre of PSII was suffici ent to decrease quantum efficiency of charge separation in PSII by 50%. Thu s, in combination with a protonation reaction, one molecule of zeaxanthin w as as efficient at capturing excitation energy as a photochemically open re action centre. The data are discussed in relation to the interaction betwee n zeaxanthin and thylakoid protonation, which enables effective thermal dis sipation of light energy in the antennae of PSII in the moss but not in hig her plants when actinic illumination is absent.