Contribuţii Botanice, XLI, (1), 2006 Grădina Botanică “Alexandru Borza” Cluj-Napoca CHLORORESPIRATION AND THE KINETICS OF PLASTOQUINONE POOL REOXIDATION IN THE PRESENCE OF MITOCHONDRIAL RESPIRATION INHIBITORS IN THE ALGA Mougeotia sp. STRAIN AICB 560 Victor BERCEA 1 , Cătălina VASILESCU 1 , Bogdan DRUGĂ 1 , Nicolae DRAGOŞ 2 1 Institutul de Cercetări Biologice, str. Republicii, nr. 48, RO-400015 Cluj-Napoca 2 Universitatea “Babeş-Bolyai”, Facultatea de Biologie şi Geologie, str. Clinicilor, nr. 5-7, RO-400006 Cluj-Napoca e-mail: bercea_victor@yahoo.com Abstract: The involvement of chlororespiration electrons in plastoquinone reoxidation under the action of photosynthetic and mitochondrial electron transporter chain inhibitors was studied in the green algae. Under the action of DCMU, 32.7% of plastoquinol pool was reoxidized. The plastoquinol reoxidation has diminished 25% in the presence of sodium azide. It was established that 72.7% of plastoquinol pool was reoxidized in the presence of salicylhydroxamic acid and n-propyl gallate, while 38.2% in the presence of thenoyltrifluoroacetone. In the presence of dithiothreitol the amount of plastoquinol reoxidized at dark was of 40%. In the condition of plastoquinol reoxidation at dark following light reduction takes place the change in initial fluorescence. The minimum and maximum fluorescence are maintained high, along with the diminishing of the efficiency and the quantum production. Propyl gallate makes an exception, which increased the initial fluorescence, and the salicylhydroxamic acid has inhibited it respectively. The increase of F 0 in light-dark transition results from the accumulation of Q - A as a consequence of plastoquinone reduction at dark. The clear reoxidation of Q - A has diminished due to the reduced state of plastoquinone at dark. Keywords: DCMU, fluorescence kinetic, mitochondrial respiration inhibitors, plastoquinone pool, reoxidation Introduction The photosynthesis reactions at light encompass the transport of photosynthetic electrons between the two photosystems PSI and PSII, involving different carriers at the level of chloroplasts thylakoids. The plastoquinone (PQ), involved in this transport, is reduced by NAD(P)H dehydrogenase and is reoxidized on the account of the molecular oxygen, reaction mediated by a chloroplastic oxidase [1, 2, 3, 4]. The electron transfer from respiratory chain to oxygen via NAD(P)H is elecrogenic, being thus responsible for the permanent membrane potential. Small quantities of chloroplastic homologues of the mitochondrial NADH dehydrogenase were found in thylakoids [14]. The stimulation of oxygen absorption was observed after lighting at Chlamydomonas reinhardtii mutants lacking the PSII reaction centers. This absorption was attributed to the presence of plastoquinol: oxygen oxidoreductase sensitive to n-propyl gallate [7], an inhibitor of alternative mitochondrial oxidases [12]. The chloroplastic complex NAD(P)H-dehydrogenase represents a part of an essential metabolic path in certain conditions of stress [9, 17]. The activity of Ndh complex can be monitored by the transitory increase of chlorophyll fluorescence after light exposure [6, 13, 16]. Research supported by CERES Program nr. 4-10/2004