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

Substituted quinones retard the photooxidation of the primary electron donor of Photosystem I. David Joly^*,1, Rajagopal Subramanyam², Elena Egorova³, Nikolai Bukhov³, Robert Carpentier¹, ¹ Groupe de Recherche en Énergie et Information Biomoléculaires, Trois-Rivières, Québec, Canada² Photosynthesis Center, Tempe, Arizona, USA³ K.A. Tamiriazev Institute of Plant Physiology, Moscow, Russia

ABSTRACT- Substituted quinones such as 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), 2,6 dichloro-p-benzoquinone (DCBQ), and tetramethyl-p-benzoquinone (duroquinone) were used in the present study to test their ability to quench the excited states of chlorophyll (Chl) molecules in photosystem I (PSI). Addition of these quinones exogenously to PSI submembrane fractions resulted in the quenching of Chl fluorescence emission. A gradual increase in the concentrations of these quinones progressively enhanced the fluorescence quenching as indicated by Stern-Volmer plots. Besides, the quenching PSI fluorescence emission strongly correlated with the loss of the basal level of Chl fluorescence (Fo) in thylakoid membranes. As a consequence of the quenching of Chl excited states in PSI submembrane fractions by these quinones, the oxidation kinetics of P700 (the primary electron donor of PSI) markedly retarded even at limiting actinic irradiances. This unambiguously suggests the ability of these substituted quinones to decrease the photochemical efficiency of the absorbed quanta. These experimental data were used to identify the possible role of non-photochemical quenching of the excited Chl states by oxidized phylloquinones, the electron acceptors of PSI, and oxidized plastoquinones, the mobile electron carriers between PSII and the cytochrome b6/f complex, in controlling the photochemical activity of PSI. The above will be discussed in detail.

KEY WORDS: Photooxidation of P700, Photosystem I, Non-photochemical quenching, Plastoquinone