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Changes in Photosystem II fluorescence in Chlamydomonas reinhardtii exposed to increasing levels of irradiance in relationship to the photosynthetic response to light
Authors:Stefan Falk  Jerry W Leverenz  Göran Samuelsson  Gunnar Öquist
Institution:(1) Department of Plant Physiology, University of Umeå, S-901 87 Umeå, Sweden
Abstract:The effects of a 60 min exposure to photosynthetic photon flux densities ranging from 300 to 2200 mgrmol m–2s–1 on the photosynthetic light response curve and on PS II heterogeneity as reflected in chlorophyll a fluorescence were investigated using the unicellular green alga Chlamydomonas reinhardtii. It was established that exposure to high light acts at three different regulatory or inhibitory levels; 1) regulation occurs from 300 to 780 mgrmol m–2s–1 where total amount of PS II centers and the shape of the light response curve is not significantly changed, 2) a first photoinhibitory range above 780 up to 1600 mgrmol m–2s–1 where a progressive inhibition of the quantum yield and the rate of bending (convexity) of the light response curve can be related to the loss of QB-reducing centers and 3) a second photoinhibitory range above 1600 mgrmol m–2s–1 where the rate of light saturated photosynthesis also decreases and convexity reaches zero. This was related to a particularly large decrease in PS IIagr centers and a large increase in spill-over in energy to PS I.Abbreviations Chl chlorophyll - DCMU 3,(3,4-dichlorophenyl)-1,1-dimethylurea - FM maximal fluorescence yield - Fpl intermediate fluorescence yield plateau level - F0 non-variable fluorescence yield - Fv total variable fluorescence yield (FM-F0) - phiv initial slope to the light response curve, used as an estimate of initial quantum yield - theta convexity (rate of bending) of the light response curve of photosynthesis - LHC light-harvesting complex - Pmax maximum rate of photosynthesis - PQ plastoquinone - Q photosynthetically active photon flux density (400–700 nm, mgrmol m–2s–1) - PS photosystem - QA and QB primary and secondary quinone electron acceptor of PS II
Keywords:Chlamydomonas  convexity  chlorophyll a fluorescence  Photosystem II heterogeneity  photoinhibition  photosynthesis  PS II repair cycle
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