PARENT SESSION
Posters P2B Light, redox and metabolic regulation: Light Reactions. Abstracts (444-478)

A zeaxanthin-independent non-photochemical quenching mechanism localised in the Photosystem II core complex. Giovanni Finazzi^*,1, Giles Johnson², Luca Dallosto³, Pierre Joliot¹, Francis-André Wollman¹, Roberto Bassi³, ¹ UPR 1261 CNRS, Institut de Biologie Physico Chimique, Paris, France² University of Manchester, School of Biological Sciences, Manchester, United Kingdom³ L.G.B.P. Faculté des Sciences de Luminy, Marseille, France

ABSTRACT- Illumination of dark-adapted barley plants with low light transiently induced a large non-photochemical quenching of chlorophyll fluorescence. This was identified as a form of high energy quenching (qE) associated with the photosystem II (PSII) reaction centre. Its appearance was not accompanied by zeaxanthin synthesis, but was associated to the inactivation of a fraction of PSII centres. Both fluorescence quenching and PSII inactivation relaxed in parallel to the activation of the Calvin cycle. This suggests that the generation of the quenched state is due to transient over-acidification of the thylakoid lumen and that its dissipation depends on the relaxation of both the pH and redox pressure with activation of carbon fixation. Under saturating light, inactivation of PSII was also seen at the onset of illumination. At steady state, however, PSII activity fully recovered while qE remained, being constituted essentially of antenna quenching. Reaction centre quenching seems therefore to be a common transient feature to protect the phoytosynthetic apparatus at the onset of illumination, being replaced by other forms of quenching (photochemical, antenna, photoinhibition), depending on the ratio between the light and the carbon fixation fluxes. Consistent with this hypothesis, illumination of Chlamydomonas reinhardtii cells under physiological conditions did not result in the establishment of qE, very likely because of a fast activation of the Calvin cycle. Fluorescence quenching could only be observed when carbon fixation was inhibited, either by disrupting Rubisco activity, or by slowing down the temperature to ∼0 °C, and remained of much lower amplitude than the one observed in higher plants.

KEY WORDS: photosystem II, non photochemical quenching, fluorescence, photochemistry