JXB Advance Access originally published online on March 21, 2006
Journal of Experimental Botany 2006 57(6):1211-1223; doi:10.1093/jxb/erj104
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REVIEW ARTICLE |
Conservation and dissipation of light energy as complementary processes: homoiohydric and poikilohydric autotrophs
1Julius von Sachs Institute of Biosciences, University of Würzburg, D-97082 Würzburg, Germany
2Institute of Basic Biological Problems, Russian Academy of Sciences, Pushchino-na-Oke, Moscow Region, and Laboratory of Biophysics, Belozersky Institute of Chemical and Physical Biology, Moscow State University, Moscow 119992, Russia
* To whom correspondence should be addressed. E-mail: heber{at}botanik.uni-wuerzburg.de
The relationship between photosynthetic energy conservation and thermal dissipation of light energy is considered, with emphasis on organisms which tolerate full desiccation without suffering photo-oxidative damage in strong light. As soon as water becomes available to dry poikilohydric organisms, they resume photosynthetic water oxidation. Only excess light is then thermally dissipated in mosses and chlorolichens by a mechanism depending on the protonation of a thylakoid protein and availability of zeaxanthin. Upon desiccation, another mechanism is activated which requires neither protonation nor zeaxanthin although the zeaxanthin-dependent mechanism of energy dissipation remains active, provided desiccation occurs in the light. Increased thermal energy dissipation under desiccation finds expression in the loss of variable, and in the quenching of, basal chlorophyll fluorescence. Spectroscopical analysis revealed the activity of photosystem II reaction centres in the absence of water. Oxidized ß-carotene (Car+) and reduced chlorophyll (Chl–), perhaps ChlD1 next to P680 within the D1 subunit, accumulates reversibly under very strong illumination. Although recombination between Car+ and Chl– is too slow to contribute significantly to thermal energy dissipation, a much faster reaction such as the recombination between P680+ and the neighbouring Chl– is suggested to form the molecular basis of desiccation-induced energy dissipation in photosystem II reaction centres. Thermal dissipation of absorbed light energy within a picosecond time domain deactivates excited singlet chlorophyll, thereby preventing triplet accumulation and the consequent photo-oxidative damage by singlet oxygen.
Key words: Chlorophyll fluorescence, energy dissipation, lichens, mosses, photoprotection, photosystem II, reaction centre, zeaxanthin
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