The influence of cytochrome b 559 on the fluorescence yield of chloroplasts at low temperature

Flash-kinetics as a complementary analytical tool in PAM fluorimetry

A new measuring system based on the already existing Multi-Color-PAM Fluorimeter (Schreiber et al. in Photosynth Res 113:127-144, 2012) was developed that in addition to standard PAM measurements enables pump-and-probe flash measurements and allows simultaneous measurements of the changes in chlorophyll fluorescence yield (F) during application of saturating flashes (ST). A high-power Chip-on-Board LED array provides ST flashes with close to rectangular profiles at wide ranges of widths (0.5 µs to 5 ms), intensities (1.3 mmol to 1.3 mol 440 nm quanta m-2 s-1) and highly flexible repetition times. Using a dedicated rising-edge profile correction, sub-µs time resolution is obtained for assessment of initial fluorescence and rise kinetics. At maximal to moderate flash intensities the flash-kinetics (changes of F during course of ST, STK) are strongly affected by 'High Intensity Quenching' (HIQ), consisting of Car-triplet quenching, TQ, and donor-side-dependent quenching, DQ. The contribution of TQ is estimated by application of a second ST after 20 µs dark-time. Upon application of flash trains (ST sequences with defined repetition times) typical period-4 oscillations in dark fluorescence yield (F0) and ST-induced fluorescence yield, FmST, are obtained which can be measured in vivo both with suspensions and from the surface of leaves. Examples of application with dilute suspensions of Chlorella and an intact dandelion leaf are presented. It is shown that weak far-red light (730-740 nm) advances the S-state distribution of the water-splitting system by one step, resulting in substantial lowering of FmST and also of the I1-level in the polyphasic rise of fluorescence yield induced by a multiple-turnover flash (MT). Based on comparative measurements of STK and the polyphasic rise kinetics with the same Chlorella sample, it is concluded that the generally observed lower values of maximal fluorescence yields using ST-protocols compared to MT-protocols are due to a higher extent of HIQ (mainly DQ) and the contribution of variable PSI fluorescence to FmST.

Photosynthesis: basics, history and modelling

BACKGROUND

With limited agricultural land and increasing human population, it is essential to enhance overall photosynthesis and thus productivity. Oxygenic photosynthesis begins with light absorption, followed by excitation energy transfer to the reaction centres, primary photochemistry, electron and proton transport, NADPH and ATP synthesis, and then CO2 fixation (Calvin-Benson cycle, as well as Hatch-Slack cycle). Here we cover some of the discoveries related to this process, such as the existence of two light reactions and two photosystems connected by an electron transport 'chain' (the Z-scheme), chemiosmotic hypothesis for ATP synthesis, water oxidation clock for oxygen evolution, steps for carbon fixation, and finally the diverse mechanisms of regulatory processes, such as 'state transitions' and 'non-photochemical quenching' of the excited state of chlorophyll a.

SCOPE

In this review, we emphasize that mathematical modelling is a highly valuable tool in understanding and making predictions regarding photosynthesis. Different mathematical models have been used to examine current theories on diverse photosynthetic processes; these have been validated through simulation(s) of available experimental data, such as chlorophyll a fluorescence induction, measured with fluorometers using continuous (or modulated) exciting light, and absorbance changes at 820 nm (ΔA820) related to redox changes in P700, the reaction centre of photosystem I.

CONCLUSIONS

We highlight here the important role of modelling in deciphering and untangling complex photosynthesis processes taking place simultaneously, as well as in predicting possible ways to obtain higher biomass and productivity in plants, algae and cyanobacteria.

Development of fluorescence quenching in Chlamydomonas reinhardtii upon prolonged illumination at 77 K

Low-temperature fluorescence measurements are frequently used in photosynthesis research to assess photosynthetic processes. Upon illumination of photosystem II (PSII) frozen to 77 K, fluorescence quenching is observed. In this work, we studied the light-induced quenching in intact cells of Chlamydomonas reinhardtii at 77 K using time-resolved fluorescence spectroscopy with a streak camera setup. In agreement with previous studies, global analysis of the data shows that prolonged illumination of the sample affects the nanosecond decay component of the PSII emission. Using target analysis, we resolved the quenching on the PSII-684 compartment which describes bulk chlorophyll molecules of the PSII core antenna. Further, we quantified the quenching rate constant and observed that as the illumination proceeds the accumulation of the quencher leads to a speed up of the fluorescence decay of the PSII-684 compartment as the decay rate constant increases from about 3 to 4 ns^- 1. The quenching on PSII-684 leads to indirect quenching of the compartments PSII-690 and PSII-695 which represent the red chlorophyll of the PSII core. These results explain past and current observations of light-induced quenching in 77 K steady-state and time-resolved fluorescence spectra.

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.

Systems approach to excitation-energy and electron transfer reaction networks in photosystem II complex: model studies for chlorophyll a fluorescence induction kinetics

Photosystem II (PS II) is a protein complex which evolves oxygen and drives charge separation for photosynthesis employing electron and excitation-energy transfer processes over a wide timescale range from picoseconds to milliseconds. While the fluorescence emitted by the antenna pigments of this complex is known as an important indicator of the activity of photosynthesis, its interpretation was difficult because of the complexity of PS II. In this study, an extensive kinetic model which describes the complex and multi-timescale characteristics of PS II is analyzed through the use of the hierarchical coarse-graining method proposed in the authors׳ earlier work. In this coarse-grained analysis, the reaction center (RC) is described by two states, open and closed RCs, both of which consist of oxidized and neutral special pairs being in quasi-equilibrium states. Besides, the PS II model at millisecond scale with three-state RC, which was studied previously, could be derived by suitably adjusting the kinetic parameters of electron transfer between tyrosine and RC. Our novel coarse-grained model of PS II can appropriately explain the light-intensity dependent change of the characteristic patterns of fluorescence induction kinetics from O-J-I-P, which shows two inflection points, J and I, between initial point O and peak point P, to O-J-D-I-P, which shows a dip D between J and I inflection points.

Modeling chlorophyll a fluorescence transient: relation to photosynthesis

To honor Academician Alexander Abramovitch Krasnovsky, we present here an educational review on the relation of chlorophyll a fluorescence transient to various processes in photosynthesis. The initial event in oxygenic photosynthesis is light absorption by chlorophylls (Chls), carotenoids, and, in some cases, phycobilins; these pigments form the antenna. Most of the energy is transferred to reaction centers where it is used for charge separation. The small part of energy that is not used in photochemistry is dissipated as heat or re-emitted as fluorescence. When a photosynthetic sample is transferred from dark to light, Chl a fluorescence (ChlF) intensity shows characteristic changes in time called fluorescence transient, the OJIPSMT transient, where O (the origin) is for the first measured minimum fluorescence level; J and I for intermediate inflections; P for peak; S for semi-steady state level; M for maximum; and T for terminal steady state level. This transient is a real signature of photosynthesis, since diverse events can be related to it, such as: changes in redox states of components of the linear electron transport flow, involvement of alternative electron routes, the build-up of a transmembrane pH gradient and membrane potential, activation of different nonphotochemical quenching processes, activation of the Calvin-Benson cycle, and other processes. In this review, we present our views on how different segments of the OJIPSMT transient are influenced by various photosynthetic processes, and discuss a number of studies involving mathematical modeling and simulation of the ChlF transient. A special emphasis is given to the slower PSMT phase, for which many studies have been recently published, but they are less known than on the faster OJIP phase.

Chlorophyll a fluorescence induction: a personal perspective of the thermal phase, the J-I-P rise

The fast (up to 1 s) chlorophyll (Chl) a fluorescence induction (FI) curve, measured under saturating continuous light, has a photochemical phase, the O-J rise, related mainly to the reduction of Q(A), the primary electron acceptor plastoquinone of Photosystem II (PSII); here, the fluorescence rise depends strongly on the number of photons absorbed. This is followed by a thermal phase, the J-I-P rise, which disappears at subfreezing temperatures. According to the mainstream interpretation of the fast FI, the variable fluorescence originates from PSII antenna, and the oxidized Q(A) is the most important quencher influencing the O-J-I-P curve. As the reaction centers of PSII are gradually closed by the photochemical reduction of Q(A), Chl fluorescence, F, rises from the O level (the minimal level) to the P level (the peak); yet, the relationship between F and [Q(A) (-)] is not linear, due to the presence of other quenchers and modifiers. Several alternative theories have been proposed, which give different interpretations of the O-J-I-P transient. The main idea in these alternative theories is that in saturating light, Q(A) is almost completely reduced already at the end of the photochemical phase O-J, but the fluorescence yield is lower than its maximum value due to the presence of either a second quencher besides Q(A), or there is an another process quenching the fluorescence; in the second quencher hypothesis, this quencher is consumed (or the process of quenching the fluorescence is reversed) during the thermal phase J-I-P. In this review, we discuss these theories. Based on our critical examination, that includes pros and cons of each theory, as well mathematical modeling, we conclude that the mainstream interpretation of the O-J-I-P transient is the most credible one, as none of the alternative ideas provide adequate explanation or experimental proof for the almost complete reduction of Q(A) at the end of the O-J phase, and for the origin of the fluorescence rise during the thermal phase. However, we suggest that some of the factors influencing the fluorescence yield that have been proposed in these newer theories, as e.g., the membrane potential ΔΨ, as suggested by Vredenberg and his associates, can potentially contribute to modulate the O-J-I-P transient in parallel with the reduction of Q(A), through changes at the PSII antenna and/or at the reaction center, or, possibly, through the control of the oxidation-reduction of the PQ-pool, including proton transfer into the lumen, as suggested by Rubin and his associates. We present in this review our personal perspective mainly on our understanding of the thermal phase, the J-I-P rise during Chl a FI in plants and algae.

On the primary nature of fluorescence yield changes associated with photosynthesis

Absorbance changes of C-550 and cytochrome b(559), and fluorescence-yield changes were measured during irradiation of chloroplasts at -196 degrees . The photo-reduction of C-550 proceeded more rapidly than the photo-oxidation of cytochrome b(559), and the fluorescence-yield change had similar kinetics to the cytochrome b(559) change. The fluorescence yield of chloroplasts exposed to a 16-musec flash at -196 degrees did not increase during the flash, but increased in the dark after the flash. Both of these experiments indicate that the fluorescence yield follows the dark reduction of the primary electron donor of Photosystem II, not the photoreduction of the acceptor. This explanation would also account for the recent results of Mauzerall [Proc. Nat. Acad. Sci. USA (1972) 69, 1358-1362] showing that the fluorescence yield of chloroplasts at room temperature requires about 20 musec to reach a maximum after a very brief flash.

Inhibition of oxygen evolution in Photosystem II by Cu(II) ions is associated with oxidation of cytochrome b559

We have found that elevated copper concentrations, apart from the inhibition of oxygen evolution, changed the initial states distribution of the oxygen-evolving complex. Already at low concentrations, copper ions oxidized the low-potential form of cytochrome b (559) and also its high-potential form at higher concentrations at which fluorescence quenching was observed. We suggest that the primary target sites in Photosystem II for copper is tyrosine(z), both cytochrome b (559) forms and chlorophyll(z), and that these sites are the source of the copper-induced fluorescence quenching and oxygen evolution inhibition in Photosystem II.

Photochemical efficiency of photosystem II, photon yield of O2 evolution, photosynthetic capacity, and carotenoid composition during the midday depression of net CO2 uptake in Arbutus unedo growing in Portugal

During the "midday depression" of net CO2 exchange in the mediterranean sclerophyllous shrub Arbutus unedo, examined in the field in Portugal during August of 1987, several parameters indicative of photosynthetic competence were strongly and reversibly affected. These were the photochemical efficiency of photosystem (PS) II, measured as the ratio of variable to maximum chlorophyll fluorescence, as well as the photon yield and the capacity of photosynthetic O2 evolution at 10% CO2, of which the apparent photon yield of O2 evolution was most depressed. Furthermore, there was a strong and reversible increase in the content of the carotenoid zeaxanthin in the leaves that occurred at the expense of both violaxanthin and β-carotene. Diurnal changes in fluorescence characteristics were interpreted to indicate three concurrent effects on the photochemical system. First, an increase in the rate of radiationless energy dissipation in the antenna chlorophyll, reflected by changes in 77K fluorescence of PSII and PSI as well as in chlorophyll a fluorescence at ambient temperature. Second, a state shift characterized by an increase in the proportion of energy distributed to PSI as reflected by changes in PSI fluorescence. Third, an effect lowering the photon yield of O2 evolution and PSII fluorescence at ambient temperature without affecting PSII fluorescence at 77K which would be expected from a decrease in the activity of the water splitting enzyme system, i.e. a donor side limitation.

Comparative thermoluminescence study of autotrophically and photoheterotrophically cultivated Chlamydobotrys stellata

Thermoluminescence (TL) from autotrophically and photoheterotrophically cultivated Chlamydobotrys stellata was measured. Strong TL was emitted at 30°C after acetatenutrition of the alga. DCMU enhanced this band, as also did ferricyanide. It also appeared after poisoning of the alga with NH2OH or ANT-2p. These observations suggest that an alternative donor to photosystem II and not the water-splitting system is responsible for the TL + 30 band.

Tetranitromethane modification of photosystem 2

Inhibition of photosystem 2 by the peptide-modification reagent, tetranitromethane, has been investigated with spinach digitonin particles. In the presence of tetranitromethane, (1) the initial fluoresence yield is suppressed with a concomitant elimination of the variable component of fluorescence; (2) the optical absorption transient at 820 nm, attributed to P680(+), is greatly attenuated; (3) diphenylcarbazide-supported photoreduction of dichlorophenol indophenol is abolished; and (4) electron spin resonance Signal 2f and Signal 2s are eliminated. These results are consistent with multiple sites of modification in photosystem 2 by tetranitromethane, and suggest further that this reagent can inhibit charge stabilization in the reaction center.

Electron donors and acceptors in photosynthetic reaction centers

A review is given of primary and associated electron transport reactions in various division of photosynthetic bacteria and in the two photosystems of plant photosynthesis. Two types of electron acceptor chains are distinguished: type 'Q', found in purple bacteria, Chloroflexus and system II of oxygenic photosynthesis and type 'F', found in green sulfur bacteria, Heliobacterium and photosystem I. Secondary donor reactions are discussed in relation to plant photosystem II.

Modification of oxygen evolving center by Tris-washing

Tris-washing inhibits the O2-evolving center of chloroplasts and their particles specifically and reversibly, and it was applied to many investigations on O2-evolving center and PS II reaction center. In this review are introduced the various photosynthetic investigations in which Tris-washing was applied and are also discussed briefly on the site and the mechanism of Tris-inactivation, properties of P680 and Z, characteristic change in fluorescence and delayed light emission, and reactivation of O2-evolving center by DCPIP.H2-treatment and photo-reactivation of Tris-washed chloroplasts and their particles.

Variable fluorescence of photosystem I particles and its application to the study of the structure and function of photosystem I

Chlorophyll a fluorescence in Photosystem I (PSI) particles isolated according to the method of Bengis and Nelson [J. Biol. Chem. 252, 4564-4569 (1977)] was found to be dependent on the redox state of both P700 and X (an acceptor on the reducing side of PSI). Addition of dithionite plus neutral red to PSI caused an increase in fluorescence intensity and a shift of the main fluorescence peak from 689 to 674 nm. Addition of electron acceptors such as ferredoxin and methyl viologen decreased the fluorescence yield when added to PSI incubated under anaerobic conditions in the presence of excess dichlorophenol indophenol (DCIPH2). The Km for ferredoxin agreed with that determined from direct measurements of ferredoxin reduction, showing that X is a quencher of fluorescence. P700 was also found to be a quencher of fluorescence, since electron donors such as DCIPH2, TMPD, and plastocyanin decreased fluorescence with Km's nearly identical to those observed for P700+ reduction. Chemical modification of PSI (with ethylene diamine + a water-soluble carbodiimide) to make it positively charged increased the fluorescence yield and shifted the 689-nm peak to 674 nm. The Km's for DCIPH2 and ferredoxin were decreased. In contrast, modification of PSI with succinic anhydride, which increased the net negative charge, increased the Km for ferredoxin. Salts affected the interaction of methyl viologen with PSI. Both anion and cation selectivity were observed. Limited proteolysis increased the Km for both methyl viologen and ferredoxin, indicating that their binding site on PSI was altered. These results suggest that the binding site for ferredoxin is on either the 70- or the 20-kDa subunit of PSI.

Comparative study of the fluorescence yield and of the C550 absorption change at room temperature

The C550 absorption change and the fluorescence yield were studied at room temperature in chloroplasts in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, and under conditions in which contributions of P-700 and of the electrochromic effect were neglible. 1. The C550 difference spectrum is a typical band shift with an isobestic point close to 550 nm. 2. The maximum amplitude of C550 absorption change is reached upon the first flash of a series of saturating flashes, unlike the maximum fluorescence yield which is attained after several flashes. 3. The comparison of the induction curves of the C550 change and the fluorescence yield in weak light shows that the fluorescence yield is controlled by two quenchers: one of them (Q1), the redox state of which C550 is a probe, is responsible for the major part of the quenching; the other one (Q2), which is less concentrated and less efficient becomes predominant at the end of the fluorescence induction. 4. Quencher Q2 back-reacts faster than quencher Q1. 5. Two alternative models are discussed in which Q1 and Q2 belong either to the same Photosystem II center or to two different photocenters.

Variable chlorophyll a fluorescence from P-700 enriched photosystem I particles dependent on the redox state of the reaction centre

1. Photosystem I particles enriched in P-700 prepared by Triton X-100 treatment of chloroplasts show a light-induced increase in fluorescence yield of more than 100% in the presence of dithionite but not in its absence. 2. Steady state light maintains the P-700, of these particles, in the oxidised state when ascorbate is present but in the presence of dithionite only a transient oxidation occurs. 3 EPR data show that, in these particles, the primary electron acceptor (X) is maintained in the reduced state by light at room temperature only when the dithionite is also present. In contrast, the secondary electron acceptors are reduced in the dark by dithionite. 4. Fluorescence emission and excitation spectra and fluorescence lifetime measurements for the constant and variable fluorescence indicate a heterogeneity of the chlorophyll in these particles. 5. It is concluded that the variable fluorescence comes from those chlorophylls which can transfer their energy to the reaction centre and that the states PX and P+X are more effective quenchers of chlorophyll fluorescence than PX-, where P is P-700.

Interactions between photosystem II components in chloroplast membranes. A correlation between the existence of a low potential species of cytochrome b-559 and low chlorophyll fluorescence in inhibited and developing chloroplasts

1. Chloroplasts inhibited by incubation with hydroxylamine in the light exhibit a low fluorescence yield upon illumination in the presence of dithionite sufficient to completely reduce the primary acceptor, Q. In the absence of magnesium ions, the fluorescence yield is the same as in control chloroplasts, suggesting that the reason for the low yield is a defect in the mechanism by which Mg2+ enhances the fluorescence. These chloroplasts were previouly shown to contain only low potential (Em7.8 = +80 mV) cytochrome b-559 (Horton, P. and Croze, E (1977) Biochim. Biophys. Acta 462, 86-101). 2. In Photosystem II particles, in heat-treated chloroplasts and in trypsin-digested chloroplasts, high potential cytochrome b-559 is absent and the variable fluorescence yield is again low. 3. Peas grown under intermittent light contain only one-fifth of the content of high potential cytochrome b-559 seen in fully greened plants, yet show high rates of water to methyl viologen electron transport. Aquisition of the high potential cytochrome b-559 accompanies synthesis of chlorophyll b, the onset of Mg-stimulated fluorescence and an increased variable yield of fluorescence. A similar correlation was seen during greening of dark-grown barley. 4. It is proposed that the high potential state of cytochrome b-559 is due to the same membrane properties which allow cation enhanced variable fluorescence, so that the presence of low potential cytochrome b-559 is accompanied by a decrease in variable fluorescence yield.

Heat-induced changes of chlorophyll fluorescence in isolated chloroplasts and related heat-damage at the pigment level

The heat-induced changes of chlorophyll fluorescence excitation and emission properties were studied in isolated chloroplasts of Larrea divaricata Cav. An analysis of the temperature dependency of fluorescence, under Fo and Fmax conditions, of temperature-jump fluorescence induction kinetics, and of 77 degrees K emission spectra of preheated chloroplasts revealed two major components in the heat-induced fluorescence changes: (1) a fluorescence rise, reflecting the block of Photosystem II reaction centers; and (2) a fluorescence decrease, caused by the functional separation of light-harvesting pigment protein complex from the rest of the pigment system. Preferential excitation of chlorophyll a around 420 nm, produced a predominant fluorescence rise. Preferential excitation of chlorophyll b, at 480 nm, gives a predominant fluorescence decrease. It is proposed that the overlapping of the fluorescence decrease on the somewhat faster fluorescence rise, results in the biphasic fluorescence rise kinetics observed in isolated chloroplasts. Both the rise component and the decay component are affected by the thermal stability of the chloroplasts, acquired during growth of the plants in different thermal environments. Mg2+ enhances the stability against heat-damage expressed in the decrease component, but has no effect on the rise component. Heat pretreatment leads to a decrease of the variable fluorescence in the light-induced 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) rise curve, but no change in half-rise time is observed. It is concluded that the block of Photosystem II reaction centers precedes the loss of the light-harvesting pigment protein complex. However, the approximately antiparallel heat-induced Fmax decrease and Fo increase suggest a common cause for the two events. A heat-induced perturbation of the thylakoid membrane is discussed.

Evidence for a double hit process in photosystem II based on fluorescence studies

1. The amplitudes of the fast (0-20 microseconds) and slow (20 microseconds-2 ms) fluorescence rise induced by a 2 microseconds flash have been measured as a function of the energy of the flash in chloroplasts inhibited by 3(3,4-dichlorophenyl)-1, 1-dimethylurea. The saturation curve for the slow rise shows a characteristic lag which is not observed for the fast fluorescence rise. This lag indicates that Photosystem II centers undergo a double hit process which implies that (a), each photocenter includes two acceptors Q1 and Q2; (B), after the first hit, oxidized chlorophyll Chl+ is reduced by a secondary acceptor Y in a time shor compared to the duration of the flash; (c), after the second hit, Chl+ is reduced by another secondary donor, D. 2. According to Den Haan et al. (1974) Biochim. Biophys. Acta 368, 409-421), hydroxylamine destroys the secondary donor responsible for the fast reduction of Chl+. In the presence of 3 mM hydroxylamine, only the secondary donor D is functional and a flash induses mainly a single hit process. 3. The saturation curves for the fast and the slow rises have been studied in the presence of 3(3,4-dichlorophenyl)-1, 1-dimethylurea for a second actinic flash given 2.5 s after a first saturating one. The large decrease in the half-saturating energy indicates the existence of efficient energy transfer occuring between potosynthetic units. 4. Two alternate hypotheses are discussed (a) in which D is an auxiliary donor and (b) in which D is included in the main electron transfer chain.

The rise in chlorophyll a fluorescence yield and decay in delayed light emission in tris-washed chloroplasts in the 6-100 microseconds time range after an excitation flash

Parallel measurements of the rise in chlorophyll a fluorescence yield and delayed light emission decay, after a 10 ns saturating excitation flash, have been made in tris (hydroxymethyl)aminomethane-washed chloroplasts. Various electron donor systems (Mn2+; ascorbate; reduced phenylenediamine and benzidine) were used in conjuction with different preillumination regimes to alter [P+-680], the oxidized form of the Photosystem II reaction center chlorophyll a. Conditions giving rise to high [p+ -680] resulted in only a small rise in fluorescence yield, an inhibition of a 6 microseconds component of delayed light emission. These results confirm the hypothesis that P+-680 acts as a quencher of fluorescence and that delayed light emission in the microsecond time range is due to the back reaction of P+-680 and Q-. (Q is the first "stable" electron acceptor of Photosystem II.) Two preillumination flashes are required before the full effect of Tris washing is observed in the delayed light emission decay and fluorescence yield rise; this suggests that a capacity to hold two charges exists between the Tris block and P+-680. Tris washing has no direct effect on the movement of electrons from Z (the first electron donor to P+-680. Finally, Mn2+ donates electrons to P+-680 via Z.

Kinetics of chlorophyll fluorescence at 77K in Chlorella and chloroplasts. Effects of CCCP, ferricyanide and DCMU

The kinetics of chlorophyll fluorescence at 77K were studied in Chlorella cells and spinach chloroplasts. During a first illumination, the rise is polyphasic with at least three phases. The slowest one is irreversible and corresponds to the cytochrome oxidation. The dark regeneration of half the variable fluorescence is biphasic, the fast phase being inhibited by 3-(3,4-dichlorophenyl)-1, 1-dimethylurea (DCMU) both in Chlorella and chloroplasts. The fluorescence rise during a second illumination is still biphasic. Carbonyl cyanide m-chlorophenylhydrazone (CCCP) slows down the fluorescence rise in Chlorella but has no effect on the dark regeneration. It does not affect the fluorescence of chloroplasts. Ferricyanide which oxidizes cytochrome beta-559 at room temperature produces a quenching of the variable fluorescence and an acceleration of the fluorescence rise during the first illumination. Our results fit the idea of the heterogeneity of the Photosystem II centers at low temperature.

Photooxidation of chlorophyll in spinach chloroplasts between 10 and 180 K

Electron paramagnetic resonance (EPR) and optical absorbance difference spectra and kinetics upon illumination by saturating flashes and continuous light of spinach chloroplasts frozen under various conditions were measured between 10 and 180 K. 1. At 100 K illumination with continuous light caused an EPR signal which decayed during the light in about 30 ms. This change is probably due to the reduction of P+-680, the oxidized primary electron donor of Photosystem II, by a secondary electron donor, cytochrome b-559. Flash illumination yielded the previously observed rapid (2 ms) transient. This transient has been ascribed to a back-reaction of the two primary reagents of Photosystem II (Malkin, R. and Bearden, A.J. (1975) Biochim. Biophys. Acta 396, 250-259; Visser, J.W.M. (1975) Thesis, Leiden). 2. Between 10 and 40 K, illumination with continuous light showed a transient which decayed in about 500 ms. The extent decreased with increasing temperature. However, the half time appeared to be temperature independent. This signal was also attributed to P+-680. 3. At 180 K it appeared to be impossible to observe the 2 and 30 ms components in dark frozen chloroplasts. However, they could be observed again if two short saturating flashes were given shortly before freezing. These changes seem to be dependent on the S-state of the reaction center. 4. After oxidizing the sample with ferricyanide (Eh = 540 mV), the light induced absorbance difference spectrum showed a bleaching near 676 nm. This change is ascribed to the irreversible oxidation of a dimeric chlorophyll molecule which acts as electron donor to P+-680 under these conditions. 5. Titration curves of the irreversible light-induced absorbance change at 676 nm and the irreversible light-induced EPR change near g = 2.00 provide strong evidence that these two changes reflect the same compound. Finally, a model is given to explain the observed reactions of Photosystem II at 10-180 K. The model involves three different ultimate and one intermediate electron donor to P+-680 at these temperatures.

Quenching of excited chlorophyll A in vivo by nitrobenzene

Nitrobenzene exerts a dual effect on the excitation of chlorophyll a(Chl a) in vivo. (a) A 3(3,4-dichlorophenyl)-1,1-dimethylurea-inhibited quenching that manifests as a partial inhibition of variable chloroplast fluorescence and of 2,6-dichlorophenol indophenol (DCPIP) photoreduction and saturates at ca. 5-10 muM. Since nitrobenzene is not a Hill oxidant, this effect is attributed to a catalyzed back flow of electrons from intersystem intermediates to pre-photosystem II oxidants. (b) A direct quenching of the excited Chl a in vivo. This effect has a threshold of ca. 100 muM nitrobenzene; at higher concentrations it leads to almost complete suppression of chloroplast fluorescence and DCPIP photoreduction. Tris-washed chloroplast enriched in the photosystem II reaction center species Z+Q- and ZQ- are nearly four times more sensitive to nitrobenzene quenching than those enriched in Z+Q. On the other hand, normal chloroplasts are about 10 to the fourth times more sensitive. Hence, it is argued that the extreme sensitivity of normal chloroplast fluorescence is not due to a preferential association of nitrobenzene with a particular redox species of the reaction center.

Fluorescence quenching in photosystem II of chloroplasts

A simple photochemical model for the photosynthetic units of Photosystem II based on first-order rate constants for de-excitation of excited chlorophyll molecules is presented in the form of equations which predict the yields of fluorescence (i.e. at the FO level, at the maximal FM level and the fluorescence of variable yield, FV equals FM minus FO). Two types of quenching mechanisms are recognized: (1) increasing nonradiative decay processes in the bulk chlorophyll by creating quenching centers which complete with the reaction centers for the excitation energy (this mechanism quenches both FO and FV) and (2) increasing nonradiative decay of the excited reaction center chlorophyll (this mechanism quenches FV but not FO). Quenching in the bulk chlorophyll preserves the relationship that Fv/FM is equal to the maximum yield of photochemistry; quenching at the reaction center chlorophyll decreases FV/FM substantially (since FV is quenched specifically) but may have very little effect on the yield of photochemistry. Estimates are made of the relative magnitudes of the rate constants for de-excitation of the excited reaction center chlorophyll by photochemistry, kp, by nonradiative decay processes, kd, and by energy transfer back to the bulk chlorophyll, kt. Fluorescence is assumed to emanate only from the bulk chlorophyll. Energy transfer from Photosystem II to Photosystem I may occur from either the excited bulk chlorophyll or from the excited reaction center chlorophyll. The model is valid for any degree of energy transfer between Photosystem II units.

Primary photochemical reactions in chloroplast photosynthesis

Photosynthesis begins with the absorption of light energy and this absorbed energy is transferred to special sites, termed reaction centres. At these sites, the light energy is transformed into chemical products through an oxidation-reduction reaction that generates the primary reactants, an oxidized pigment molecule (P+) and a reduced electron acceptor (A–) (Clayton, 1972). The subsequent reactions of these species in the dark ultimately results in the formation of chemical products required for the fixation of CO2. In this essay we will discuss the nature of the primary reactants generated in the light reactions of chloroplast photosynthesis, stressing recent advances in the identification and characterization of such reactants.