The slow component of Photosystem II luminescence. A process with distributed rate constant?

Jean Lavorel (1928-2021): a great scientist, a model of integrity

This paper is a tribute to a great scientist and an authentic "honest man" that was Jean Lavorel (1928-2021). He was a pioneer in research on the primary events of photosynthesis in algae, plants, and photosynthetic bacteria. He focused his attention on chlorophyll fluorescence and luminescence, and also on oxygen evolution, both experimentally (with laboratory-built refined apparatus) and theoretically. Many of his results are classical now. Besides a survey of his main achievements, most of them obtained by him alone, different reminiscences on the researcher and the person he was illustrate the rich personality of Jean Lavorel.

Impact of energy limitations on function and resilience in long-wavelength Photosystem II

Photosystem II (PSII) uses the energy from red light to split water and reduce quinone, an energy-demanding process based on chlorophyll a (Chl-a) photochemistry. Two types of cyanobacterial PSII can use chlorophyll d (Chl-d) and chlorophyll f (Chl-f) to perform the same reactions using lower energy, far-red light. PSII from Acaryochloris marina has Chl-d replacing all but one of its 35 Chl-a, while PSII from Chroococcidiopsis thermalis, a facultative far-red species, has just 4 Chl-f and 1 Chl-d and 30 Chl-a. From bioenergetic considerations, the far-red PSII were predicted to lose photochemical efficiency and/or resilience to photodamage. Here, we compare enzyme turnover efficiency, forward electron transfer, back-reactions and photodamage in Chl-f-PSII, Chl-d-PSII, and Chl-a-PSII. We show that: (i) all types of PSII have a comparable efficiency in enzyme turnover; (ii) the modified energy gaps on the acceptor side of Chl-d-PSII favour recombination via P_D1⁺Phe^- repopulation, leading to increased singlet oxygen production and greater sensitivity to high-light damage compared to Chl-a-PSII and Chl-f-PSII; (iii) the acceptor-side energy gaps in Chl-f-PSII are tuned to avoid harmful back reactions, favouring resilience to photodamage over efficiency of light usage. The results are explained by the differences in the redox tuning of the electron transfer cofactors Phe and Q_A and in the number and layout of the chlorophylls that share the excitation energy with the primary electron donor. PSII has adapted to lower energy in two distinct ways, each appropriate for its specific environment but with different functional penalties.

Delayed fluorescence in photosynthesis

Photosynthesis is a very efficient photochemical process. Nevertheless, plants emit some of the absorbed energy as light quanta. This luminescence is emitted, predominantly, by excited chlorophyll a molecules in the light-harvesting antenna, associated with Photosystem II (PS II) reaction centers. The emission that occurs before the utilization of the excitation energy in the primary photochemical reaction is called prompt fluorescence. Light emission can also be observed from repopulated excited chlorophylls as a result of recombination of the charge pairs. In this case, some time-dependent redox reactions occur before the excitation of the chlorophyll. This delays the light emission and provides the name for this phenomenon-delayed fluorescence (DF), or delayed light emission (DLE). The DF intensity is a decreasing polyphasic function of the time after illumination, which reflects the kinetics of electron transport reactions both on the (electron) donor and the (electron) acceptor sides of PS II. Two main experimental approaches are used for DF measurements: (a) recording of the DF decay in the dark after a single turnover flash or after continuous light excitation and (b) recording of the DF intensity during light adaptation of the photosynthesizing samples (induction curves), following a period of darkness. In this paper we review historical data on DF research and recent advances in the understanding of the relation between the delayed fluorescence and specific reactions in PS II. An experimental method for simultaneous recording of the induction transients of prompt and delayed chlorophyll fluorescence and decay curves of DF in the millisecond time domain is discussed.

Connectivity of photosystem II is the physical basis of retrapping in photosynthetic thermoluminescence

Energy transfer between photosystem II (PSII) centers is known from previous fluorescence studies. We have studied the theoretical consequences of energetic connectivity of PSII centers on photosynthetic thermoluminescence (TL) and predict that connectivity affects the TL Q band. First, connectivity is expected to make the Q band wider and more symmetric than an ideal first-order TL band. Second, the presence of closed PSII centers in an energetically connected group of PSII centers is expected to lower the probability that an exciton originating in a recombination reaction becomes retrapped. The latter effect would shift the Q band toward lower temperature, and the shift would be greater the higher the percentage of closed PSII centers at the beginning of the measurement. These effects can be generalized as second-order effects, as they make the Q band resemble the second-order TL bands obtained from semiconducting solids. We applied the connected-units model of chlorophyll fluorescence to derive equations for quantifying the second-order effects in TL. To test the effect of the initial proportion of closed reaction centers, we measured the Q band with different intensities of the excitation flash and found that the peak position changed by 2.5 degrees C toward higher temperature when the flash intensity was lowered from saturating to 0.39% of saturating. The result shows that energy transfer between reaction centers of PSII forms the physical basis of retrapping in photosynthetic TL. The second-order effects partially explain the deviation of the form of the Q band from ideal first-order TL.