Photoinactivation of the photosynthetic electron transport chain by accumulation of over-saturating light pulses given to dark adapted pea leaves

The effect of cumulative over-saturating pulses (OSP) of white light (1 s, > 10 000 mu mol photons m(-2) s(-1)), applied every 20 min on pea leaves, was investigated during a complete diurnal cycle of 24 h. In dark-adapted leaves, this treatment leads to a progressive decline of the optimum Photosystem II (PS II) quantum yield. Continuous low background light (except far-red light) had a protective effect against this OSP-induced photoinactivation. The lack of far-red effect could be due to its absorption mainly in PS I and not in PS II, but could be also due to the general low absorption in this wavelength region. The photoinactivation was enhanced in leaves that had been previously infiltrated with chloramphenicol. The quantum yield of CO2 assimilation, but not its maximal capacity, was inhibited by the OSP treatment. The most spectacular effects observed, in addition to an irreversible quenching of Fm, was a strong inhibition of Q A(-) reoxidation revealed by a large increase in the Fs level and consequently by a decrease of DeltaF/Fm'. Under such conditions, we observed that the electron flow deduced from DeltaF/Fm' underestimated the real electron flow to CO2. Time-resolved Chlorophyll a fluorescence measurements showed that the reduced capacity of Q A(-) reoxidation in OSP treated leaves was accompanied by the appearance of a 4.7 ns component attributed to PS II charge recombination. We suggest that a modification at the Q B site may influence the redox potential of Q A/Q A(-), facilitating the reversion of the primary charge separation. In addition, a 1.2 ns fluorescence component accumulated, which appeared to be responsible for the underestimation of PS II electron flow. The observed photoinactivation seemed to be different from the photoinhibition often described in the literature, which occurs under continuous light.