Changes in the S0 and S1 properties during dark adaptation in oxygen-evolving Photosystem-II-enriched thylakoid membranes

Evidence for intermediate S-states as initial phase in the process of oxygen-evolving complex oxidation

We have analyzed flash-induced period-four damped oscillation of oxygen evolution and chlorophyll fluorescence with the aid of a kinetic model of photosystem II. We have shown that, for simulation of the period-four oscillatory behavior of oxygen evolution, it is essential to consider the so-called intermediate S-state as an initial phase of each of the S(n)-S(n+1), (n = 0, 1, 2, 3) transitions. The intermediate S-states are defined as [S(n)Y(Z)(ox)]-states (n = 0, 1, 2, 3) and are formed with rate constant k(iSn) approximately 1.5 x 10(6) s(-1), which was determined from comparison of theoretical predictions with experimental data. The assumed intermediate S-states shift the equilibrium in reaction P680(+)Y(Z)<-->P680Y(Z)(ox) more to the right and we suggest that kinetics of the intermediate S-states reflects a relaxation process associated with changes of the redox equilibrium in the above reaction. The oxygen oscillation is simulated without the miss and double-hit parameters, if the intermediate S-states, which are not the source of the misses or the double-hits, are included in the simulation. Furthermore, we have shown that the intermediate S-states, together with S(2)Q(A)(-) charge recombination, are prerequisites for the simulation of the period-four oscillatory behavior of the chlorophyll fluorescence.

Flash-induced oxygen evolution in photosynthesis: simple solution for the extended S-state model that includes misses, double-hits, inactivation, and backward-transitions

Flash-induced oxygen evolution in higher plants, algae, and cyanobacteria exhibits damped period-four oscillations. To explain such oscillations, Kok suggested a simple phenomenological S-state model, in which damping is due to empirical misses and double-hits. Here we developed an analytical solution for the extended Kok model that includes misses, double-hits, inactivation, and backward-transitions. The solution of the classic Kok model (with misses and double-hits only) can be obtained as a particular case of this solution. Simple equations describing the flash-number dependence of individual S-states and oxygen evolution in both cases are almost identical and, therefore, the classic Kok model does not have a significant advantage in its simplicity over the extended version considered in this article. Developed equations significantly simplify the fitting of experimental data via standard nonlinear regression analysis and make unnecessary the use of many previously developed methods for finding parameters of the model. The extended Kok model considered here can provide additional insight into the effect of dark relaxation between flashes and inactivation.

Chlorophyll a fluorescence rise induced by high light illumination of dark-adapted plant tissue studied by means of a model of photosystem II and considering photosystem II heterogeneity

Chlorophyll a fluorescence rise (FLR) measured in vivo in dark-adapted plant tissue immediately after the onset of high light continuous illumination shows complex O-K-J-I-P transient. The steps typically appear at about 400 micros (K), 2 ms (J), 30 ms (I), and 200 - 500 ms (P) and a transient decrease of fluorescence to local minima (dips D) can be observed after the K, J, and I steps. As the FLR reflects a function of photosystem II (PSII) and to more understand the FLR, a PSII reactions model was formulated comprising equilibrium of excited states among all light harvesting and reaction centre pigments and P680, reversible radical pair formation and the donor and acceptor side functions. Such a formulated model is the most detailed and complex model of PSII reactions used so far for simulations of the FLR. By varying of selected model parameters (rate constants and initial conditions) several conclusions can be made as for the origin of and changes in shape of the theoretical FLR and compare them with in-literature-reported results. For homogeneous population of PSII and using standard in-literature-reported values of the model parameters, the simulated FLR is characterized by reaching the minimal fluorescence F(0) at about 3 ns after the illumination is switched on lasting to about 1 micros, followed by fluorescence rise to a plateau located at about 2 ms and subsequent fluorescence rise to a global maximum that is reached at about 60 ms. Varying of the values of rate constants of fast processes that can compete for utilization of the excited states with fluorescence emission does not change qualitatively the shape of the FLR. However, primary photochemistry of PSII (the charge separation, recombination and stabilization), non-radiative loss of excited states in light harvesting antennae and excited states quenching by oxidized plastoquisnone (PQ) molecules from the PQ pool seem to be the main factors controlling the maximum quantum yield of PSII photochemistry as expressed by the F(V)/F(M) ratio. The appearance of the plateau at about 2 ms in the FLR is affected by several factors: the height of the plateau in the FLR increases when the fluorescence quenching by oxidized P680(+) is not considered in the simulations or when the electron transfer from Q(A)(-) to Q(B)((-)) is slowed down whereas the height of the plateau decreases and its position is shifted to shorter times when OEC is initially in higher S state. The plateau at about 2 ms is changed into the local fluorescence maximum followed by a dip when the fluorescence quenching by oxidized PQ molecules or the charge recombination between P680(+) and Q(A)(-) is not considered in the simulations or when all OEC is initially in the S(0) state or when the S -state transitions of OEC are slowed down. Slowing down of the S -state transitions of OEC as well as of the electron transfer from Q(A)(-) to Q(B)((-)) also causes a decrease of maximal fluorescence level. In the case of full inhibition of the S -state transitions of OEC as well as in the case of full inhibition of the electron donation to P680(+) by Y(Z), the local fluorescence maximum becomes the global fluorescence maximum. Assuming homogeneous PSII population, theoretical FLR curve that only far resembles experimentally measured O-J-I-P transient at room temperature can be simulated when slowly reducing PQ pool is considered. Assuming heterogeneous PSII population (i.e. the alpha/beta and the Q(B) -reducing/Q(B)-non-reducing heterogeneity and heterogeneity in size of the PQ pool and rate of its reduction) enables to simulate the FLR with two steps between minimal and maximal fluorescence whose relative heights are in agreement with the experiments but not their time positions. A cause of this discrepancy is discussed as well as different approaches to the definition of fluorescence signal during the FLR.

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.

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

Flash-induced absorption changes at 515 nm observed as a function of flash number are examined in relation to the flash-induced fluorescence yields in inside-out thylakoids. After partial dissipation of the delocalized transmembrane electric field by adding gramicidin, the analysis of period 4 oscillations and of the kinetics in the 10 ms-1 s range suggest that the variation of the absorption changes at 515 nm as a function of flash number is the result of at least two processes:1) an electric field increase related to the S2 state and 2) the fact that the field generated by the water protons inside the membrane decreases when these protons are released outside the membrane. The former field correlates with the flash-induced fluorescence yield increase induced by the donor side of Photosystem II. Both measurements show similar oscillations as a function of flash number, with maxima on the 1st, 5th and 9th flash. These oscillations, after a shift of two flashes, appear to be different from those of the O2 yield observed under similar conditions. It is proposed that, in a population of centers the electric field during the S2 state reflects the presence of a stabilized positive equivalent in the protein close to the Mn complex.

Absorbance difference spectra of the S-state transitions in Photosystem II core particles

Redox changes of the oxygen evolving complex in PS II core particles were investigated by absorbance difference spectroscopy in the UV-region. The oscillation of the absorbance changes induced by a series of saturating flashes could not be explained by the minimal Kok model (Kok et al. 1970) consisting of a 4-step redox cycle, S0 → S1 → S2 → S3 → S0, although the values of most of the relevant parameters had been determined experimentally. Additional assumptions which allow a consistent fit of all data are a slow equilibration of the S3 state with an inactive state, perhaps related to Ca(2+)-release, and a low quantum efficiency for the first turnover after dark-adaptation. Difference spectra of the successive S-state transitions were determined. At wavelengths above 370 nm, they were very different due to the different contribution of a Chl bandshift in each spectrum. At shorter wavelengths, the S1 → S2 transition showed a difference spectrum similar to that reported by Dekker et al. 1984b and attributed to an Mn(III) to Mn(IV) oxidation. The spectrum of absorbance changes associated with the S2 → S3 transition was similar to that reported by Lavergne 1991 for PS II membranes. The S0 → S1 transition was associated with a smaller but still substantial absorbance increase in the UV. Differences with the spectra reported by Lavergne 1991 are attributed to electrostatic effects on electron transfer at the acceptor side associated with the S-state dependence of proton release in PS II membranes.