Events near the reaction center in O2 evolving PS II enriched thylakoid membranes: The presence of an electric field during the S2 state in a population of centers

The Oxygen quantum yield in diverse algae and cyanobacteria is controlled by partitioning of flux between linear and cyclic electron flow within photosystem II

We have measured flash-induced oxygen quantum yields (O2-QYs) and primary charge separation (Chl variable fluorescence yield, Fv/Fm) in vivo among phylogenetically diverse microalgae and cyanobacteria. Higher O2-QYs can be attained in cells by releasing constraints on charge transfer at the Photosystem II (PSII) acceptor side by adding membrane-permeable benzoquinone (BQ) derivatives that oxidize plastosemiquinone QB(-) and QBH2. This method allows uncoupling PSII turnover from its natural regulation in living cells, without artifacts of isolating PSII complexes. This approach reveals different extents of regulation across species, controlled at the QB(-) acceptor site. Arthrospira maxima is confirmed as the most efficient PSII-WOC (water oxidizing complex) and exhibits the least regulation of flux. Thermosynechococcus elongatus exhibits an O2-QY of 30%, suggesting strong downregulation. WOC cycle simulations with the most accurate model (VZAD) show that a light-driven backward transition (net addition of an electron to the WOC, distinct from recombination) occurs in up to 25% of native PSIIs in the S2 and S3 states, while adding BQ prevents backward transitions and increases the lifetime of S2 and S3 by 10-fold. Backward transitions occur in PSIIs that have plastosemiquinone radicals in the QB site and are postulated to be physiologically regulated pathways for storing light energy as proton gradient through direct PSII-cyclic electron flow (PSII-CEF). PSII-CEF is independent of classical PSI/cyt-b6f-CEF and provides an alternative proton translocation pathway for energy conversion. PSII-CEF enables variable fluxes between linear and cyclic electron pathways, thus accommodating species-dependent needs for redox and ion-gradient energy sources powered by a single photosystem.

Measurements of variable chlorophyll fluorescence using fast repetition rate techniques: defining methodology and experimental protocols

We present a methodology, called fast repetition rate (FRR) fluorescence, that measures the functional absorption cross-section (sigmaPS II) of Photosystem II (PS II), energy transfer between PS II units (p), photochemical and nonphotochemical quenching of chlorophyll fluorescence, and the kinetics of electron transfer on the acceptor side of PS II. The FRR fluorescence technique applies a sequence of subsaturating excitation pulses ('flashlets') at microsecond intervals to induce fluorescence transients. This approach is extremely flexible and allows the generation of both single-turnover (ST) and multiple-turnover (MT) flashes. Using a combination of ST and MT flashes, we investigated the effect of excitation protocols on the measured fluorescence parameters. The maximum fluorescence yield induced by an ST flash applied shortly (10 &mgr;s to 5 ms) following an MT flash increased to a level comparable to that of an MT flash, while the functional absorption cross-section decreased by about 40%. We interpret this phenomenon as evidence that an MT flash induces an increase in the fluorescence-rate constant, concomitant with a decrease in the photosynthetic-rate constant in PS II reaction centers. The simultaneous measurements of sigmaPS II, p, and the kinetics of Q-A reoxidation, which can be derived only from a combination of ST and MT flash fluorescence transients, permits robust characterization of the processes of photosynthetic energy-conversion.

Regulation of thermal dissipation of absorbed excitation energy and violaxanthin deepoxidation in the thylakoids of lactuca sativa. Photoprotective mechanism of a population of photosystem II centers

Non-photochemical quenching of chlorophyll a fluorescence is thought to be mainly associated with thermal dissipation of excitation energy taking place within the antenna and reaction center of PS II. In this report, non-photochemical fluorescence quenching was investigated in the fluorescence yields induced by a series of short and high-energy flashes after dark adaptation. The observation of period four fluorescence oscillations with increasing flash number indicates functioning O2 evolving centers. It was found that these PS II centers could not be identical to all the O2 evolving centers. Appreciable differences in antenna size and the number of centers were observed between the PS II centers contributing to fluorescence oscillations and the PS II centers that evolve the flash-induced steady-state O2 yield. Direct evidence for non-photochemical fluorescence quenching was provided by the numerical fitting of the fluorescence oscillations. This procedure revealed that a proportion of the centers exhibiting oscillating fluorescence yields, converted into quenching centers after each flash of a series (7% in February; 17% in June). The observed quenching could not be related to a dissipative process inside the reaction center. Instead, it was attributed to a change in the organization of some PS II centers in the membrane, possibly a conversion of PS II dimers into PS II monomers, resulting in a decreased absorption cross-section for these centers. Quenching resulting from energy de-excitation in the antenna was also observed. This was a light-initiated process, but the modification of the antenna occurred in the dark on a time scale of a few minutes. After this dark period and only on the first flash of a series, antenna quenching was revealed by a smaller absorption cross-section of the PS II centers involved in fluorescence oscillations. This process was reversed on the following flashes. The same period of darkness after illumination was necessary to allow maximum zeaxanthin formation to occur in the dark at a higher pH than the pH for optimum violaxanthin deepoxidation in the absence of preillumination. To explain this effect, comparable to that referred to as light activation for non-photochemical quenching (Ruban and Horton, Aust. J. Plant Physiol. 22 (1995) 221-230), we propose that upon preillumination (before darkness), the protons released in response to a net positive charge in these PS II centers, have access to proton binding groups acting in a cooperative way in LHC II. This accounts for the proton cooperativity as can be deduced from the pH dependence of the rate constant of violaxanthin deepoxidation (Hill coefficient n from 2 to 6). Copyright 1998 Elsevier Science B.V.