Coregulation of electron transport through PS I by Cyt b 6 f,excitation capture by P700 and acceptor side reduction. Time kinetics and electron transport requirement

Regulation of electron transport rate through Photosystem I (PS I) was investigated in intact sunflower leaves. The rate constant of electron donation via the cytochrome b ₆ f complex (k_q, s^–1) was obtained from the postillumination P700⁺ reduction rate, measured as the exponential decay of the light-dark difference (D830) of the 830 nm transmission signal. D830 corresponding to maximum oxidisable P700 (D830_m) was obtained by applying white light flashes of different intensity and extrapolating the plot of the quantum yield Y vs. D830 to the axis of abscissae (Y->0). Maximum quantum yield of PS I at completely reduced P700 (Y_m) was obtained by extrapolating the same plot to the axis of ordinates (D830->0). Regulation of k_q, D830_m and Y_m under rate-limiting CO₂ and O₂ concentrations applied after air (21% O₂, 310 ppm CO₂) was investigated. The amplitude of the downregulation of k_q (photosynthetic control) was maximal when electron transport rate (ETR) was limited to about 3 nmol cm^–2 s^–1 and decreased when ETR was higher or lower. Downregulation did not occur in the absence of CO₂ and O₂. These gases acted only as substrates of ribulosebisphosphate carboxylase-oxygenase, no high-affinity reaction of O₂ leading to enhanced photosynthetic control (e.g. Mehler reaction) was detected. After the transition, D830_m at first decreased and then increased again, showing that the reduction of the PS I acceptor side disappeared as a result of the downregulation of k_q. The variation of Y_m had two reasons, PS I acceptor side reduction and variable excitation capture efficiency by P700. It is concluded that electron transport through PS I is coregulated by the rate of plastoquinol oxidation at Cyt b ₆ f, excitation capture efficiency by P700, and by acceptor side reduction.Abbreviations Cyt b ₆ f cytochrome b ₆ f complex - D830 difference of the 830 nm signal from the dark level - ETR electron transport rate - PAD photon absorption density nmol cm^–2 s^–1 - PFD incident photon flux density, nmol cm^–2 s^–1 - PS I Photosystem I - PS II Photosystem II - PQH₂ plastoquinol - P700 Photosystem I donor pigment - Y quantum yield of PS I electron transport, rel. un.     

Effects of inorganic carbon accumulation on photosynthetic oxygen reduction and cyclic electron flow in the cyanobacterium Synechococcus PCC7942

This paper examines the effect of inorganic carbon transport and accumulation in Synechococcus PCC7942 on fluorescence quenching, photosynthetic oxygen reduction and both linear and cyclic electron flow. The data presented support the previous findings of Miller et al. (1991) that the accumulation of Ci by the CO₂ concentrating mechanism is able to stimulate oxygen photoreduction, particularly so when CO₂ fixation is inhibited by PCR cycle inhibitors such as glycolaldehyde. This effect is found with both high and low-Ci grown cells, but the potential for oxygen photoreduction is about two-fold higher in low-Ci grown cells. This greater potential for O₂ photoreduction is also correlated with a higher ability of low-Ci cells to photoreduce H₂O₂. Experiments with a mutant which transports Ci but does not accumulate it internally, indicates that the stimulation of O₂ photoreduction appears to be a direct effect of the internal accumulation of Ci rather than from its participation in the transport process. In the absence of Ci, no specific partial reactions of photosynthetic electron transport appear to be inhibited, and the PS 1 acceptors PNDA and MV as well as the PS 2 acceptor DMQ can all run electron transport at levels approaching those during active CO₂ fixation. Measurements of P700⁺ show that when the cells are depleted of Ci during photosynthesis, P700 becomes more oxidised. This indicates that the resupply of electrons from the intersystem chain is relatively more restricted under conditions of Ci limitation than is the availability of PS 1 electron acceptors. It is proposed that the accumulated Ci pool can directly stimulate the ability of O₂ to act as a PS 1 acceptor and that the ability of PS 1 acceptors, such as O₂, to relieve restrictions on intersystem electron transfer is perhaps a result of a reduction in cyclic electron flow and a subsequent increase in the oxidation state of the plastoquinone pool.Abbreviations BTP 1,3-bis[tris(hydroxymethyl)-methylaminopropane] - CA carbonic anhydrase' - Ci inorganic carbon (CO₂+HCO₃ ^–+CO₃ ^2–) - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DMQ 2,6-dimethylbenzoquinone - EZ ethoxyzolamide or 6-ethoxy-2-benzothiazole-sulfonamide - FCCP carbonyl cyanide p-trifluoro methoxyphenyl-hydrazone - F steady-state chlorophyll fluorescence - F_m chlorophyll fluorescence during a saturating light pulse - F_o chlorophyll fluorescence in the dark, prior to illumination by actinic light - MV methyl viologen or 1,1-dimethyl-4,4-bipyridinium dichloride - PCR cycle photosynthetic carbon reduction cycle - PNDA N,N-dimethyl-p-nitrosoaniline - _{PS 1} the quantum yield of Photosystem 1 - _{PS 2} the quantum yield of Photosystem 2     

Dark reoxidation of the plastoquinone-pool is mediated by the low-potential form of cytochrome b-559 in spinach thylakoids

We have found that in isolated spinach thylakoids, plastoquinone-pool (PQ-pool), after its photoreduction, undergoes dark-reoxidation with the half-time of _1/2 = 43 ± 3 s. To explain the observed rates of PQ-pool reoxidation, a nonenzymatic plastoquinol (PQH₂) autoxidation under molecular oxygen and an enzymatic oxidation by the low-potential form of cytochrome b-559 (cyt. b-559LP), as the postulated PQ-oxidase in chlororespiration, were investigated. It was found that the autoxidation rate of PQH₂ in organic solvents and liposomes was too low to account for the observed oxidation rate of PQH₂ in thylakoids. The rate of cyt. b-559LP autoxidation in isolated Photosystem II was found to be similar (_1/2 = 26 ± 5 s) to that of the PQ-pool. This suggests that the LP form of cyt. b-559 is probably responsible for the PQ-oxidase activity observed during chlororespiration.     

Mechanisms for controlling balance between light input and utilisation in the salt tolerant alga Dunaliella C9AA

The yield of photosynthetic O₂ evolution was measured in cultures of Dunaliella C9AA over a range of light intensities, and a range of low temperatures at constant light intensity. Changes in the rate of charge separation at Photosystem I (PS I) and Photosystem II (PS II) were estimated by the parameters _{PS I} and _{PS II} . _{PS I} is calculated on the basis of the proportion of centres in the correct redox state for charge separation to occur, as measured spectrophotometrically. _{PS II} is calculated using chlorophyll fluorescence to estimate the proportion of centres in the correct redox state, and also to estimate limitations in excitation delivery to reaction centres. With both increasing light intensity and decreasing temperature it was found that O₂ evolution decreased more than predicted by either _{PS I} or _{PS II}. The results are interpreted as evidence of non-assimilatory electron flow; either linear whole chain, or cyclic around each photosystem.Abbreviations F₀ dark level of chlorophyll fluorescence yield (PS II centres open) - F_m maximum level of chlorophyll fluorescence yield (PS II centres closed) - F_v variable fluorescence (F_m-F₀) - PS I Photosystem I - PS II Photosystem II - P₇₀₀ reaction centre chlorophyll(s) of PS I - q_N coefficient of non-photochemical quenching of chlorophyll fluorescence - q_P coefficient of photochemical quenching of fluorescence yield - q_E high-energy-state quenching coefficient - _{PS I} yield of PS I - _{PS II} yield of PS II - _S yield of photosynthetic O₂ evolution - _P intrinsic yield of open PS II centres     

Plastoquinol generates and scavenges reactive oxygen species in organic solvent: Potential relevance for thylakoids

The present work reports reactions of plastoquinol (PQH₂-9) and plastoquinone (PQ-9) in organic solvents and summarizes the literature to understand similar reactions in thylakoids. In thylakoids, PQH₂-9 is oxidized by the cytochrome b₆/f complex (Cyt b₆/f) but some PQH₂-9 is also oxidized by reactions in which oxygen acts as an electron acceptor and is converted to reactive oxygen species (ROS). Furthermore, PQH₂-9 reacts with ROS. Light enhances oxygen-dependent oxidation of PQH₂-9. We examined the oxidation of PQH₂-9 via dismutation of PQH₂-9 and PQ-9 and scavenging of the superoxide anion radical (O₂^?) and hydrogen peroxide (H₂O₂) by PQH₂-9. Oxidation of PQH₂-9 via dismutation to semiquinone was slow and independent of pH in organic solvents and in solvent/buffer systems, suggesting that intramembraneous oxidation of PQH₂-9 in darkness mainly proceeds via reactions catalyzed by the plastid terminal oxidase and cytochrome b₅₅₉. In the light, oxidation of PQH₂-9 by singlet oxygen and by O₂^? formed in PSI contribute significantly. In addition, Cyt b_6/f forms H₂O₂ with a PQH₂-9 dependent mechanism. Measurements of the reaction of O₂^? with PQH₂-9 and PQ-9 in acetonitrile showed that O₂^? oxidizes PQH₂-9, forming PQ-9 and several PQ-9-derived products. The rate constant of the reaction between PQH₂-9 and O₂^? was found to be 10⁴?M^?1?s^?1. H₂O₂ was found to oxidize PQH₂-9 to PQ-9, but failed to oxidize all PQH₂-9, suggesting that the oxidation of PQH₂-9 by H₂O₂ proceeds via deprotonation mechanisms producing PQH^?-9, PQ^2?-9 and the protonated hydrogen peroxide cation, H₃O₂⁺.     

Fluorescence detected magnetic resonance of the primary donor and inner core antenna chlorophyll in Photosystem I reaction centre protein: Sign inversion and energy transfer

The Photosystem I reaction centre protein CP1, isolated from barley using polyacrylamide gel electrophoresis showed an EPR (Electron Paramgnetic Resonance) spectrum with the polarisation pattern AEEAAE, typical of the primary donor triplet state ³P700, created via radical pair formation and recombination. ³P700 could also be detected by Fluorescence Detected Magnetic Resonance (FDMR) at _f > 700 nm even in the presence of a large number of chlorophyll antennae. Its zero field splitting parameters, D=282.5×10^-4 cm^-1 and E=38.5×10^-4 cm^-1, were independent of the detection wavelength, and agreed with ADMR (Absorption Detected Magnetic Resonance) and EPR values. The signs of the ³P700 D+E and D-E transitions were positive (increase in fluorescence intensity on applying a resonance microwave field). In contrast, in the emission band 685 < _f < 700 nm FDMR spectra with negative D+E and D-E transitions were detected, and the D value was wavelength-dependent. These FDMR results support an excitation energy transfer model for CP1, derived from time-resolved fluorescence studies, in which two chlorophyll antenna forms are distinguished, with fluorescence at 685 < _f < 700 nm (inner core antennae, F690), and _f > 700 nm (low energy antenna sites, F720), in addition to the P700. The FDMR spectrum in F690 emission can be interpreted as that of ³P700, observed via reverse singlet excitation energy transfer and added to the FDMR spectrum of the antenna triplet states generated via intramolecular intersystem crossing. This would indicate that reversible energy transfer between F690 and P700 occurs even at 4.2 K.Abbreviations Chl chlorophyll - CP1 core chlorophyll protein of Photosystem I - EPR electron paramagnetic resonance - F690, F720 chlorophyll forms having fluorescence maximum at 690–695 and 720 nm, respectively - F(A)(O)DMR fluorescence (absorption) (optical) detected magnetic resonance - FF fluorescence fading - ISC intramolecular intersystem crossing - _f fluorescence emission wave-length - LHC I light harvesting chlorophyll a/b protein of Photosystem I - P700 primary donor of Photosystem I - PS I Photosystem I - RC reaction centre - RP radical pair - SDS sodium dodecyl sulphate - ZFS zero field splitting     

Divergent pathways of photosynthetic electron transfer: The autonomous oxygenic and anoxygenic photosystems

The aim of this article is to assemble and integrate, from a personal perspective of a research participant, seldom examined evidence that is incompatible with some basic tenets of photosynthetic electron transport, the cornerstone of which is the Z scheme. The nonconforming evidence pertaining to the mode of ferredoxin reduction and the role of the copper redox protein, plastocyanin, indicates that contrary to the Z scheme ferredoxin is reduced in two experimentally distinguishable ways: oxygenically by PS II (renamed the oxygenic photosystem), without the participation of PS I, and anoxygenically by PS I (renamed the anoxygenic photosystem). It also indicates that plastocyanin is not only, as the Z scheme asserts, the electron donor to the reaction center chlorophyll of PS I (P700) but also to the reaction center chlorophyll of PS II (P680). Other unconventional findings include evidence that the fully functional oxygenic photosystem, when operating separately from the anoxygenic photosystem, reduces plastoquinone to plastoquinol and subsequently oxidizes plastoquinol by two pathways acting in concert: one being the universally recognized DBMIB-sensitive pathway via the Rieske iron-sulfur center of the cytochrome bf complex and the other, a hitherto unrecognized, DBMIB-insensitive electron transport pathway around P680 that centers on cytochrome b-559. These nonconforming findings form the basis of an alternate hypothesis of photosynthetic electron transport that modifies and complements the Z scheme.Abbreviations PS photosystem - PQ oxidized plastoquinone - PQH₂ reduced plastoquinone (plastoquinol) - Q_A and Q_B specialized membrane-bound forms of PQ - PC plastocyanin - Fd ferredoxin - BISC F_AF_B, membrane-bound iron-sulfur centers of PS I - DBM1B 2,5-dibromo-3-methyl-6-isopropyl-n-benzoquinone (dibromothymoquinone) - DNP-INT dinitrophenol ether of iodonitrothymol - NADP⁺ NADPH, oxidized and reduced forms of nicotinamide adenine dinucleotide phosphate - FCCP carbonylcyanide-p-trifluoromethoxyphenyl-hydrazone - CCCP carbonyl cyanide-3-chlorophenylhydrazone - SF 6847 2,6,-di-(t-butyl)-4-(2,2-dicyanovinyl) phenol - diuron (DCMU) 3-(3,4-dichlorophenyl)-1,1-dimethylurea - EPR electron paramagnetic resonance - DCIP 2,6-dichloro-phenolindophenol - UHDBT 5-(n-undecyl)-6-hydroxy-4-7-dioxobenzothiazole; cytochrome b-559_HP-cytochrome b-559_LP, high- and low potential states of cytochrome b-559 - oxygenic reductions reductions in which water is the electron donor - BBY PS II preparation made according to Berthold et al. (1981) Dedicated to Professor Achim Trebst on his 65th birthday.Based in part on lecture in Advanced Course on Trends in Photosynthesis Research, Palma de Mallorca, Spain, September 18, 1990.     

Cyclic electron flow around Photosystem II in vivo

The oxygen flash yield (Y_O2) and photochemical yield of PS II (_{PS II}) were simultaneously detected in intact Chlorella cells on a bare platinum oxygen rate electrode. The two yields were measured as a function of background irradiance in the steady-state and following a transition from light to darkness. During steady-state illumination at moderate irradiance levels, Y_O2 and _{PS II} followed each other, suggesting a close coupling between the oxidation of water and Q_A reduction (Falkowski et al. (1988) Biochim. Biophys. Acta 933: 432–443). Following a light-to-dark transition, however, the relationship between Q_A reduction and the fraction of PS II reaction centers capable of evolving O₂ became temporarily uncoupled. _{PS II} recovered to the preillumination levels within 5–10 s, while the Y_O2 required up to 60 s to recover under aerobic conditions. The recovery of Y_O2 was independent of the redox state of Q_A, but was accompanied by a 30% increase in the functional absorption cross-section of PS II (_{PS II}). The hysteresis between Y_O2 and the reduction of Q_A during the light-to-dark transition was dependent upon the reduction level of the plastoquinone pool and does not appear to be due to a direct radiative charge back-reaction, but rather is a consequence of a transient cyclic electron flow around PS II. The cycle is engaged in vivo only when the plastoquinone pool is reduced. Hence, the plastoquinone pool can act as a clutch that disconnects the oxygen evolution from photochemical charge separation in PS II.Abbreviations ADRY acceleration of the deactivation reactions of the water-splitting enzyme (agents) - Chl chlorophyll - cyt cytochrome - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - F_O minimum fluorescence yield in the dark-adapted state - F_I minimum fluorescence yield under ambient irradiance or during transition from the light-adapted state - F_M maximum fluorescence yield in the dark-adapted state - F_M maximum fluorescence yield under ambient irradiance or during transition from light-adapted state - F_V, F_V variable fluorescence (F_V=F_M–F_O ; F_V=F_M–F_I) - FRR fast repetition rate (fluorometer) - _{PS II} quantum yield of Q_A reduction (_{PS II}=(F_M – F_O)/F_M or _{PS II})=(F_M= – F_I=)/F_M=) - LHCII Chl a/b light harvesting complexes of Photosystem II - OEC oxygen evolving complex of PS II - P680 reaction center chlorophyll of PS II - PQ plastoquinone - POH₂ plastoquinol - PS I Photosystem I - PS II Photosystem II - RC II reaction centers of Photosystem II - PS II the effective absorption cross-section of PHotosystem II - TL thermoluminescence - Y_O2 oxygen flash yield The US Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

Photoinhibition of Photosystem I electron transfer activity in isolated Photosystem I preparations with different chlorophyll contents

Photoinhibition of the light-induced Photosystem I (PS I) electron transfer activity from the reduced dichlorophenol indophenol to methyl viologen was studied. PS I preparations with Chl/P700 ratios of about 180 (PS I-180), 100 (PS I-100) and 40 (PS I(HA)-40) were isolated from spinach thylakoid membranes by the treatments with Triton X-100, followed by sucrose density gradient centrifugation and hydroxylapatite column chromatography. White light irradiation (1.1 × 10⁴E m^–2 s^–1) of PS I-180 for 2 hours bleached 50% of the chlorophyll and caused a 58% decrease in the electron transfer activity with virtually no loss of the primary donor, P700. The flash-induced absorbance change showed the decay phase with a half time of about 10 s that was attributed to the P700 triplet, suggesting that the photoinhibitory light treatment caused the destruction of the PS I acceptor(s), F_x and possibly A₁. PS I-100 was similarly photobleached by the irradiation and the electron transfer activity decreased. There was, however, no apparent photoinhibition of the electron transport activity in PS I(HA)-40. Photoinhibition similar to that seen in PS I-180 also occurred in membrane fragments that were isolated without any detergent from a PS II-deficient mutant strain of the cyanobacterium Synechocystis sp. PCC 6803. PS I-180 was not photoinhibited under anaerobic conditions. The production of superoxide and fatty acid hydroperoxide during white light irradiation was significantly greater in PS I-180 than in PS I(HA)-40. The mechanism of photoinhibition in PS I preparations is discussed in relation to the formation of toxic oxygen molecules.Abbreviations A₀,A₁ primary and secondary electron acceptors of PS I - CD circular dichroism - DCPIP 2,6-dichlorophenol indophenol - F_A, F_B, F_X iron-sulfur centers A, B, X - HA hydroxylapatite - LHCI lightharvesting complex of PS I - MDA malondialdehyde - MV methyl viologen - Na-Asc sodium L-ascorbate - P700 primary electron donor of PS I - PFD photon flux density - PS I-A and PS I-B psaA and psaB gene products - TBA thiobarbituric acid     

Electron transport to oxygen mitigates against the photoinactivation of Photosystem II in vivo

The role of electron transport to O₂ in mitigating against photoinactivation of Photosystem (PS) II was investigated in leaves of pea (Pisum sativum L.) grown in moderate light (250 mol m^–2 s^–1). During short-term illumination, the electron flux at PS II and non-radiative dissipation of absorbed quanta, calculated from chlorophyll fluorescence quenching, increased with increasing O₂ concentration at each light regime tested. The photoinactivation of PS II in pea leaves was monitored by the oxygen yield per repetitive flash as a function of photon exposure (mol photons m^–2). The number of functional PS II complexes decreased nonlinearly with increasing photon exposure, with greater photoinactivation of PS II at a lower O₂ concentration. The results suggest that electron transport to O₂, via the twin processes of oxygenase photorespiration and the Mehler reaction, mitigates against the photoinactivation of PS II in vivo, through both utilization of photons in electron transport and increased nonradiative dissipation of excitation. Photoprotection via electron transport to O₂ in vivo is a useful addition to the large extent of photoprotection mediated by carbon-assimilatory electron transport in 1.1% CO₂ alone.Abbreviations Fm, Fo, Fv- maximal, initial (corresponding to open PS II traps) and variable chlorophyll fluorescence yield, respectively - NPQ- non-photochemical quenching - PS- photosystem - Q_A- primary quinone acceptor - qP- photochemical quenching coefficient     

Characterization of a Synechococcus sp. strain PCC 7002 mutant lacking Photosystem I. Protein assembly and energy distribution in the absence of the Photosystem I reaction center core complex

A Synechococcus sp. strain PCC 7002 psaAB::cat mutant has been constructed by deletional interposon mutagenesis of the psaA and psaB genes through selection and segregation under low-light conditions. This strain can grow photoheterotrophically with glycerol as carbon source with a doubling time of 25 h at low light intensity (10 E m^–2 s^–1). No Photosystem I (PS I)-associated chlorophyll fluorescence emission peak was detected in the psaAB::cat mutant. The chlorophyll content of the psaAB::cat mutant was approximately 20% that of the wild-type strain on a per cell basis. In the absence of the PsaA and PsaB proteins, several other PS I proteins do not accumulate to normal levels. Assembly of the peripheral PS I proteins PsaC,PsaD, PsaE, and PsaL is dependent on the presence of the PsaA and PsaB heterodimer core. The precursor form of PsaF may be inserted into the thylakoid membrane but is not processed to its mature form in the absence of PsaA and PsaB. The absence of PS I reaction centers has no apparent effect on Photosystem II (PS II) assembly and activity. Although the mutant exhibited somewhat greater fluorescence emission from phycocyanin, most of the light energy absorbed by phycobilisomes was efficiently transferred to the PS II reaction centers in the absence of the PS I. No light state transition could be detected in the psaAB::cat strain; in the absence of PS I, cells remain in state 1. Development of this relatively light-tolerant strain lacking PS I provides an important new tool for the genetic manipulation of PS I and further demonstrates the utility of Synechococcus sp. PCC 7002 for structural and functional analyses of the PS I reaction center.Abbreviations ATCC American type culture collection - Chl chlorophyll - DCMU 3-(3,4-dichlorophyl)-1,1-dimethylurea - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - HEPES N-[2-hydroxyethyl]piperazine-N-[2-ethanesulfonic acid] - PCC Pasteur culture collection - PS I Photosystem I - PS II Photosystem II - SDS sodium dodecyl sulfate     

Two sites of photoinhibition of the electron transfer in oxygen evolving and Tris-treated PS II membrane fragments from spinach

Photoinhibition was analyzed in O₂-evolving and in Tris-treated PS II membrane fragments by measuring flash-induced absorption changes at 830 nm reflecting the transient P680⁺ formation and oxygen evolution. Irradiation by visible light affects the PS II electron transfer at two different sites: a) photoinhibition of site I eliminates the capability to perform a stable charge separation between P680⁺ and Q_A ^- within the reaction center (RC) and b) photoinhibition of site II blocks the electron transfer from Y_Z to P680⁺. The quantum yield of site I photoinhibition (2–3×10^-7 inhibited RC/quantum) is independent of the functional integrity of the water oxidizing system. In contrast, the quantum yield of photoinhibition at site II depends strongly on the oxygen evolution capacity. In O₂-evolving samples, the quantum yield of site II photoinhibition is about 10^-7 inhibited RC/quantum. After selective elimination of the O₂-evolving capacity by Tris-treatment, the quantum yield of photoinhibition at site II depends on the light intensity. At low intensity (<3 W/m²), the quantum yield is 10^-4 inhibited RC/quantum (about 1000 times higher than in oxygen evolving samples). Based on these results it is inferred that the dominating deleterious effect of photoinhibition cannot be ascribed to an unique target site or a single mechanism because it depends on different experimental conditions (e.g., light intensity) and the functional status of the PS II complex.Abbreviations A₈₃₀ absorption change at 830 nm - P680 primary electron donor of PS II - PS II photosystem II - Mes 2(N-morpholino)ethansulfonic acid - Q_A, Q_B primary and secondary acceptors of PS II - DCIP 2,6-dichlorophenolindophenol - DPC 1,5-diphenylcarbohydrazide - FWHM fullwidth at half maximum - Ph-p-BQ phenyl-p-benzoquinone - PFR photon fluence rate - Pheo pheophytin - RC reaction center     

The mechanisms contributing to photosynthetic control of electron transport by carbon assimilation in leaves

Photosynthetic control describes the processes that serve to modify chloroplast membrane reactions in order to co-ordinate the synthesis of ATP and NADPH with the rate at which these metabolites can be used in carbon metabolism. At low irradiance, optimisation of the use of excitation energy is required, while at high irradiance photosynthetic control serves to dissipate excess excitation energy when the potential rate of ATP and NADPH synthesis exceed demand. The balance between pH, ATP synthesis and redox state adjusts supply to demand such that the [ATP]/[ADP] and [NADPH]/[NADP⁺] ratios are remarkably constant in steady-state conditions and modulation of electron transport occurs without extreme fluctuations in these pools.Abbreviations FBPase Fructose-1,6-bisphosphatase - PS I Photosystem I - PS II Photosystem II - Pi inorganic phosphate - PGA glycerate 3-phosphate - PQ plastoquinone - Q_A the bound quinone electron acceptor of PS II - q_P Photochemical quenching of chlorophyll fluorescence associated with the oxidation of Q_A - q_N non-photochemical quenching of chlorophyll fluorescence - q_E non-photochemical quenching associated with the high energy state of the membrane - RuBP ribulose-1,5-bisphosphate - TP triose phosphate - intrinsic quantum yield of PS II - quantum yield of electron transport - quantum yield of CO₂ assimilation     

Deciphering the 820 nm signal: redox state of donor side and quantum yield of Photosystem I in leaves

By recording leaf transmittance at 820 nm and quantifying the photon flux density of far red light (FRL) absorbed by long-wavelength chlorophylls of Photosystem I (PS I), the oxidation kinetics of electron carriers on the PS I donor side was mathematically analyzed in sunflower (Helianthus annuus L.), tobacco (Nicotiana tabacum L.) and birch (Betula pendula Roth.) leaves. PS I donor side carriers were first oxidized under FRL, electrons were then allowed to accumulate on the PS I donor side during dark intervals of increasing length. After each dark interval the electrons were removed (titrated) by FRL. The kinetics of the 820 nm signal during the oxidation of the PS I donor side was modeled assuming redox equilibrium among the PS I donor pigment (P700), plastocyanin (PC), and cytochrome f plus Rieske FeS (Cyt f + FeS) pools, considering that the 820 nm signal originates from P700⁺ and PC⁺. The analysis yielded the pool sizes of P700, PC and (Cyt f + FeS) and associated redox equilibrium constants. PS I density varied between 0.6 and 1.4 μmol m⁻². PS II density (measured as O₂ evolution from a saturating single-turnover flash) ranged from 0.64 to 2.14 μmol m⁻². The average electron storage capacity was 1.96 (range 1.25 to 2.4) and 1.16 (range 0.6 to 1.7) for PC and (Cyt f + FeS), respectively, per P700. The best-fit electrochemical midpoint potential differences were 80 mV for the P700/PC and 25 mV for the PC/Cyt f equilibria at 22 °C. An algorithm relating the measured 820 nm signal to the redox states of individual PS I donor side electron carriers in leaves is presented. Applying this algorithm to the analysis of steady-state light response curves of net CO₂ fixation rate and 820 nm signal shows that the quantum yield of PS I decreases by about half due to acceptor side reduction at limiting light intensities before the donor side becomes oxidized at saturating intensities. Footnote: This revised version was published online in August 2006 with corrections to the Cover Date.     

Photoinhibition of photosynthesis in chilled potato leaves is not correlated with a loss of Photosystem-II activity

When 23 °C-grown potato leaves (Solanum tuberosum L.) were irradiated at 23 °C with a strong white light, photosynthetic electron transport and Photosystem-II (PS II) activity were inhibited in parallel. When the light treatment was given at a low temperature of 3 °C, the photoinhibition of photosynthesis was considerably enhanced, as expected. Surprisingly, no such stimulation of photoinhibition was observed with respect to the PS II function. A detailed functional analysis of the photosynthetic apparatus, using in-vivo fluorescence, absorbance, oxygen and photoacoustic measurements, and artificial electron donors/acceptors, showed a pronounced alteration of PS I activity during light stress at low temperature. More precisely, it was observed that both the pool of photooxidizeable reaction center pigment (P₇₀₀) of PS I and the efficiency of PS I to oxidize P₇₀₀ were dramatically reduced. Loss of P₇₀₀ activity was shown to be essentially dependent on atmospheric O₂ and to require a continued flow of electrons from PS II, suggesting the involvement of the superoxide anion radical which is produced by the interaction of O₂ and the photosynthetic electron-transfer chain through the Mehler reaction. Mass spectrometric measurements of O₂ exchange by potato leaves under strong illumination did not reveal, however, any stimulation of the Mehler reaction at low temperature, thus leading to the conclusion that O₂ toxicity mainly resulted from a chilling-induced inhibition of the scavenging system for O₂-radicals. Support for this interpretation was provided by the light response of potato leaves infiltrated with an inhibitor (diethyldithiocarbamate) of the chloroplastic Cu-Zn superoxide dismutase. It was indeed possible to simulate the differential inhibition of the PS II photochemical activity and the linear electron transport observed during light stress at low temperature by illuminating at 23 °C diethyldithiocarbamate-poisoned leaves. The experimental data presented here suggests that (i) the previously reported resistance of PS I to photoinhibition damage in-vivo is not an intrinsic property of PS I but results from efficient protective systems against O₂ toxicity, (ii) PS I is photoinhibited in chilled potato leaf due to the inactivation of this PS I defence system and (iii) PS I is more sensitive to superoxide anion radicals than PS II.Abbreviations PS - Photosystem - E - Emerson enhancement - _open ^p and ^P maximal and actual quantum yields of PS II photochemistry - DDC - diethyldithiocarbamate - Q_A and Q_B - primary and secondary (quinone) electron acceptors of PS II - P₆₈₀ and P₇₀₀ - reaction center pigments of PS II and PS I, respectively - SOD - superoxide dismutase     

Evaluation of the role of State transitions in determining the efficiency of light utilisation for CO2 assimilation in leaves

Wheat leaves were exposed to light treatments that excite preferentially Photosystem I (PS I) or Photosystem II (PS II) and induce State 1 or State 2, respectively. Simultaneous measurements of CO₂ assimilation, chlorophyll fluorescence and absorbance at 820 nm were used to estimate the quantum efficiencies of CO₂ assimilation and PS II and PS I photochemistry during State transitions. State transitions were found to be associated with changes in the efficiency with which an absorbed photon is transferred to an open PS II reaction centre, but did not correlate with changes in the quantum efficiencies of PS II photochemistry or CO₂ assimilation. Studies of the phosphorylation status of the light harvesting chlorophyll protein complex associated with PS II (LHC II) in wheat leaves and using chlorina mutants of barley which are deficient in this complex demonstrate that the changes in the effective antennae size of Photosystem II occurring during State transitions require LHC II and correlate with the phosphorylation status of LHC II. However, such correlations were not found in maize leaves. It is concluded that State transitions in C₃ leaves are associated with phosphorylation-induced modifications of the PS II antennae, but these changes do not serve to optimise the use of light absorbed by the leaf for CO₂ assimilation.Abbreviations Fm, Fo, Fv maximal, minimal and variable fluorescence yields - Fm, Fv maximal and variable fluorescence yields in a light adapted state - LHC II light harvesting chlorophyll a/b protein complex associated with PS II - q_P photochemical quenching - A₈₂₀ light-induced absorbance change at 820 nm - _{PS I}, _{PS II} relative quantum efficiencies of PS I and PS II photochemistry - _{CO 2} quantum yield of CO₂ assimilation     

Singlet oxygen scavenging activity of plastoquinol in photosystem II of higher plants: Electron paramagnetic resonance spin-trapping study

Singlet oxygen (¹O₂) scavenging activity of plastoquinol in photosystem II (PSII) of higher plants was studied by electron paramagnetic resonance (EPR) spin-trapping technique. It is demonstrated here that illumination of spinach PSII membranes deprived of intrinsic plastoquinone results in ¹O₂ formation, as monitored by TEMPONE EPR signal. Interestingly, the addition of exogenous plastoquinol (PQH₂-1) to PQ-depleted PSII membranes significantly suppressed TEMPONE EPR signal. The presence of exogenous plastoquinols with a different side-chain length (PQH₂-n, n isoprenoid units in the side chain) caused a similar extent of ¹O₂ scavenging activity. These observations reveal that plastoquinol exogenously added to PQ-depleted PSII membranes serves as efficient scavenger of ¹O₂.     

Effect of the redox state of QB on electric field-induced charge recombination in Photosystem II

Electric field-induced charge recombination in Photosystem II (PS II) was studied in osmotically swollen spinach chloroplasts (blebs) by measurement of the concomitant chlorophyll luminescence emission (electroluminescence). A pronounced dependence on the redox state of the two-electron gate Q_B was observed and the earlier failure to detect it is explained. The influence of the Q_B/Q_B ^– oscillation on electroluminescence was dependent on the redox state of the oxygen evolving complex; at times around one millisecond after flash illumination a large effect was observed in the states S₂ and S₃, but not in the state S₄ (actually Z⁺S₃). The presence of the oxidized secondary electron donor, tyrosine Z⁺, appeared to prevent expression of the Q_B/Q_B ^– effect on electroluminescence, possibly because this effect is primarily due to a shift of the redox equilibrium between Z/Z⁺ and the oxygen evolving complex.Abbreviations BSA bovine serum albumin - EDTA ethylene-diaminetetraacetic acid - EL electroluminescence - FCCP carbonylcyanide p-trifluoromethyloxyphenyl-hydrazone - HEPESI 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid - I primary electron acceptor - MOPS 3-(N-morpholino) propane sulfonic acid - P680 primary electron donor of Photosystem II - P700 primary electron donor of Photosystem I - Q_A and Q_B secondary and tertiary electron acceptors of Photosystem II - Z secondary electron donor (D1 Tyr 161)     

The consequences of chlorophyll deficiency for photosynthetic light use efficiency in a single nuclear gene mutation of cowpea

The light harvesting and photosynthetic characteristics of a chlorophyll-deficient mutant of cowpea (Vigna unguilata), resulting from a single nuclear gene mutation, are examined. The 40% reduction in total chlorophyll content per leaf area in the mutant is associated with a 55% reduction in pigment-proteins of the light harvesting complex associated with Photosystem II (LHC II), and to a lesser extent (35%) in the light harvesting complex associated with Photosystem I (LHC I). No significant differences were found in the Photosystem I (PS I) and Photosystem II (PS II) contents per leaf area of the mutant compared to the wildtype parent. The decreases in the PS I and PS II antennae sizes in the mutant were not accompanied by any major changes in quantum efficiencies of PS I and PS II in leaves at non-saturating light levels for CO₂ assimilation. Although the chlorophyll deficiency resulted in an 11% decrease in light absorption by mutant leaves, their maximum quantum yield and light saturated rate of CO₂ assimilation were similar to those of wildtype leaves. Consequently, the large and different decreases in the antennae of PS II and PS I in the mutant are not associated with any loss of light use efficiency in photosynthesis.Abbreviations LHC I, LHC II light harvesting chlorophyll a/b protein complexes associated with PS I and PS II - A₈₂₀ light-induced absorbance change at 820 nm - ø_{PS I}, ø_{PS II} relative quantum efficiencies of PS I and PS II photochemistry     

Calculation of absolute photosystem I absorption cross-sections from P700 photo-oxidation kinetics

A procedure is described which permits determination of the absolute absorption cross-section of a photosynthetic unit from the kinetics of reaction center photo-oxidation under weak, continuous actinic illumination. The method was first tested on a simple model compound of known absorption cross-section. We then applied the technique to absorption cross-section and functional antenna size measurements in photosystem I (PS I). A kinetic model is presented that can be used to fit P700 photo-oxidation measurements and extract the effective photochemical rate constant. The procedure is shown to properly correct for sample scattering and for the presence of heterogeneous absorbers (pigments not functionally coupled to P700). The relevance of these corrections to comparisons of antenna size using techniques that measure relative absorption cross-sections is discussed. Measurements on pea thylakoids in the presence and absence of 5 mM MgCl₂ show a 45% increase in PS I absorption cross-section in unstacked thylakoids. Analysis of detergent-isolated native PS I preparations (200 chlorophyll a+b/P700) clearly indicate that the preparation contains a broad distribution of antenna sizes. Finally, we confirm that Chlamydomonas reinhardtii strain LM3-A4d contains a PS I core antenna complex which binds only 60 chlorophyll a/P700, about half the functional size of the wild type complex. Limitations associated with calculation of functional antenna size from cross-section measurements are also discussed.Abbreviations PS photosystem - PS I-200 detergent-isolated photosystem I preparation containing about 200 Chl a+b/P700 - A _xxx absorbance at xxx nm - absolute absorption cross-section - I _a rate of light absorption - In _o incident actinic light intensity - _p quantum yield of photochemistry - k _eff effective rate constant for P700 photo-oxidation measured under conditions of limiting actinic intensity - k _r rate constant for P700⁺ reduction