Importance of the oxidised plastoquinone molecules on the thermal phase of chlorophyll-a fluorescence

摘要

In photosynthetic organisms, the emission of variable Chla fluorescence is composed of two distinct parts, the photochemical and thermal phases (Samson et al., 1999). The photochemical phase, which represents about 50 percent of the total variable fluorescence in dark-adapted intact samples, is related to the reduction of PSII primary quinone electron acceptor Q_A. It can be induced by single turnover flash (STF) or by a strong continuous illumination under which the photoreduction rate of PSII electron acceptors exceeds several times its re-oxidation rate. On the other hand, the thermal phase requires multiple PSII turnovers and it corresponds to the slower phase of fluorescence kinetics induced by intense illumination (Neubauer and Schreiber, 1987). It was recently showed that the low yield of Chla fluorescence induced by STF is due to a non-photochemical quenching localised in the PSII antenna (Vasil' ev and Bruce, 1999). One of the most plausible cause of this non-photochemical quenching is the oxidised PQ molecules. Since the quenching effect of the PQ pool on the steady state Chla fluorescence level observed in presence of DCMU generally accounts for only 10 percent of Fm (Vernotte et al., 1979), it was suggested that the presence of an oxidised PQ molecule bound at the Q_B site of PSII could also be responsible for the quenching corresponding to the thermal phase (Samson et al., 1999). In this context, the main objective of this study was to verify the relative contributions of the freeoxidised PQ molecules and those bound to the Q_B site to the thermal phase of Chla fluorescence.