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1.
In dark-adapted spinach leaves approximately one third of the Photosystem II (PS II) reaction centers are impaired in their ability to transfer electrons to Photosystem I. Although these inactive PS II centers are capable of reducing the primary quinone acceptor, QA, oxidation of QA occurs approximately 1000 times more slowly than at active centers. Previous studies based on dark-adapted leaves show that minimal energy transfer occurs from inactive centers to active centers, indicating that the quantum yield of photosynthesis could be significantly impaired by the presence of inactive centers. The objective of the work described here was to determine the performance of inactive PS II centers in light-adapted leaves. Measurements of PS II activity within leaves did not indicate any increase in the concentration of active PS II centers during light treatments between 10 s and 5 min, showing that inactive centers are not converted to active centers during light treatment. Light-induced modification of inactive PS II centers did occur, however, such that 75% of these centers were unable to sustain stable charge separation. In addition, the maximum yield of chlorophyll fluorescence associated with inactive PS II centers decreased substantially, despite the lack of any overall quenching of the maximum fluorescence yield. The effect of light treatment on inactive centers was reversed in the dark within 10–20 mins. These results indicate that illumination changes inactive PS II centers into a form that quenches fluorescence, but does not allow stable charge separation across the photosynthetic membrane. One possibility is that inactive centers are converted into centers that quench fluorescence by formation of a radical, such as reduced pheophytin or oxidized P680. Alternatively, it is possible that inactive PS II centers are modified such that absorbed excitation energy is dissipated thermally, through electron cycling at the reaction center.Abbreviations A518 absorbance change at 518 nm, reflecting the formation of an electric field across the thylakoid membrane - AFL1 amplitude of the fast (<100 ms) phase of A518 induced by the first of two saturating, single-turnover flashes spaced 30 ms apart - AFL2 amplitude of the fast (<100 ms) phase of A518 induced by the second of two saturating, single-turnover flashes spaced 50 ms apart - DCBQ 2,6-dichloro-p-benzoquinone - Fo yield of chlorophyll fluorescence when QA is fully oxidized - Fm yield of chlorophyll fluorescence when QA is fully reduced - Fx yield of chlorophyll fluorescence when QA is fully reduced at inactive PS II centers, but fully oxidized at active PS II centers - Pheo pheophytin - P680 the primary donor of Photosystem II - PPFD photosynthetic photon flux density - QA Primary quinone acceptor of PS II - QB secondary quinone acceptor of PS II  相似文献   

2.
When CO2 is abruptly removed from the atmosphere surrounding an illuminated leaf, the primary electron-accepting plastoquinone of photosystem II (QA) (as measured by photochemical quenching, qp) is rapidly reduced and then, after some seconds, becomes more oxidized. The reoxidation of QA is accompanied by an increase in ΔpH (as measured by nonphotochemical quenching, qN) with kinetics consistent with a causal relationship. The fact that, in such circumstances, QA can become more oxidized in the absence of CO2 than in its presence indicates a diminished rate of reduction of QA, consequent upon impaired photosystem II efficacy. Dithiothreitol (DTT) feeding, which does not affect quantum yield or the maximum rate of photosynthesis, inhibits the reoxidation of QA but not the increase in the proton gradient. When leaves are reilluminated in high light following a dark interval of several minutes, DTT also abolishes the separation in time between the first maximum in qP and the first maximum in the rate of O2 evolution. It also diminishes subsequent oscillations. These results are held to demonstrate ΔpH control of photosystem II and are consistent with DTT inhibition of the xanthophyll cycle and hydrogen peroxide reduction. They support the concept that oxygen and hydrogen peroxide are involved, as Hill oxidants, in a pH-related manner, during oscillatory behavior.  相似文献   

3.
Wise RR  Ort DR 《Plant physiology》1989,90(2):657-664
The response of in situ photophosphorylation in attached cucumber (Cucumis sativus L. cv Ashley) leaves to chilling under strong illumination was investigated. A single-beam kinetic spectrophotometer fitted with a clamp-on, whole leaf cuvette was used to measure the flash-induced electrochromic absorbance change at 518 minus 540 nanometers (ΔA518−540) in attached leaves. The relaxation kinetics of the electric field-indicating ΔA518−540 measures the rate of depolarization of the thylakoid membrane. Since this depolarization process is normally dominated by proton efflux through the coupling factor during ATP synthesis, this technique can be used, in conjuction with careful controls, as a monitor of in situ ATP formation competence. Whole, attached leaves were chilled at 5°C and 1000 microeinsteins per square meter per second for up to 6 hours then rewarmed in the dark at room temperature for 30 minutes and 100% relative humidity. Leaf water potential, chlorophyll content, and the effective optical pathlength for the absorption measurements were not affected by the treatment. Light- and CO2-saturated leaf disc oxygen evolution and the quantum efficiency of photosynthesis were inhibited by approximately 50% after 3 hours of light chilling and by approximately 75% after 6 hours. Despite the large inhibition to net photosynthesis, the measurements of ΔA518−540 relaxation kinetics showed photophosphorylation to be largely unaffected by the chilling and light exposure. The amplitude of the ΔA518-540 measures the degree of energization of the photosynthetic membranes and was reduced significantly by chilling in the light. The cause of the decreased energization was traced to impaired turnover of photosystem II. Our measurements showed that the chilling of whole leaves in the light caused neither an uncoupling of photophosphorylation from photosynthetic electron transport nor any irreversible inhibition of the chloroplast coupling factor in situ. The sizeable inhibition in net photosynthesis observed after chilling in the light cannot, therefore, be attributed to any direct effect on photophosphorylation competence.  相似文献   

4.
Photosystem II complexes of higher plants are structurally and functionally heterogeneous. While the only clearly defined structural difference is that Photosystem II reaction centers are served by two distinct antenna sizes, several types of functional heterogeneity have been demonstrated. Among these is the observation that in dark-adapted leaves of spinach and pea, over 30% of the Photosystem II reaction centers are unable to reduce plastoquinone to plastoquinol at physiologically meaningful rates. Several lines of evidence show that the impaired reaction centers are effectively inactive, because the rate of oxidation of the primary quinone acceptor, QA, is 1000 times slower than in normally active reaction centers. However, there are conflicting opinions and data over whether inactive Photosystem II complexes are capable of oxidizing water in the presence of certain artificial electron acceptors. In the present study we investigated whether inactive Photosystem II complexes have a functional water oxidizing system in spinach thylakoid membranes by measuring the flash yield of water oxidation products as a function of flash intensity. At low flash energies (less that 10% saturation), selected to minimize double turnovers of reaction centers, we found that in the presence of the artificial quinone acceptor, dichlorobenzoquinone (DCBQ), the yield of proton release was enhanced 20±2% over that observed in the presence of dimethylbenzoquinone (DMBQ). We argue that the extra proton release is from the normally inactive Photosystem II reaction centers that have been activated in the presence of DCBQ, demonstrating their capacity to oxidize water in repetitive flashes, as concluded by Graan and Ort (Biochim Biophys Acta (1986) 852: 320–330). The light saturation curves indicate that the effective antenna size of inactive reaction centers is 55±12% the size of active Photosystem II centers. Comparison of the light saturation dependence of steady state oxygen evolution in the presence of DCBQ or DMBQ support the conclusion that inactive Photosystem II complexes have a functional water oxidation system.Abbreviations DCBQ 2,6-dichloro-p-benzoquinone - DMBQ 2,5-dimethyl-p-benzoquinone - Fo initial fluorescence level using dark-adapted thylakoids - Inactive reaction centers reaction centers inactive in plastoquinone reduction - PS II Photosystem II - QA primary quinone acceptor of Photosystem II - QB secondary quinone acceptor of Photosystem II Department of Plant Biology, University of IllinoisDepartment of Physiology & Biophysics, University of Illinois  相似文献   

5.
Thermoluminescence (TL) signals were recorded from grana stacks, margins, and stroma lamellae from fractionated, dark-adapted thylakoid membranes of spinach (Spinacia oleracea L.) in the absence and in the presence of 2,6-dichlorphenylindophenol (DCMU). In the absence of DCMU, the TL signal from grana fractions consisted of a homogenous B-band, which originates from recombination of the semi-quinone QB with the S2 state of the water-splitting complex and reflects active photosystem II (PSII). In the presence of DCMU, the B-band was replaced by the Q-band, which originates from an S2QA recombination. Margin fractions mainly showed two TL-bands, the B- and C-bands, at approximately 50°C in the absence of DCMU, and Q- and C-bands in the presence of DCMU. The C-band is ascribed to a TyrD+-QA recombination. In the absence of DCMU, the fractions of stromal lamellae mainly gave rise to a TL emission at 42°C. The intensity of this band was independent of the number of excitation flashes and was shifted to higher temperatures (52°C) after the addition of DCMU. Based on these observations, this band was considered to be a C-band. After photoinhibitory light treatment of uncoupled thylakoid membranes, the TL intensities of the B- and Q-bands decreased, whereas the intensity at 45°C (C-band) slightly increased. It is proposed that the 42 to 52°C band that was observed in marginal and stromal lamellae and in photoinhibited thylakoid membranes reflects inactive PSII centers that are assumed to be equivalent to inactive PSII QB-nonreducing centers.  相似文献   

6.
It is well known that two photosystems, I and II, are needed to transfer electrons from H2O to NADP+ in oxygenic photosynthesis. Each photosystem consists of several components: (a) the light-harvesting antenna (L-HA) system, (b) the reaction center (RC) complex, and (c) the polypeptides and other co-factors involved in electron and proton transport. First, we present a mini review on the heterogeneity which has been identified with the electron acceptor side of Photosystem II (PS II) including (a) L-HA system: the PS II and PS II units, (b) RC complex containing electron acceptor Q1 or Q2; and (c) electron acceptor complex: QA (having two different redox potentials QL and QH) and QB (QB-type; Q'B type; and non-QB type); additional components such as iron (Q-400), U (Em,7=–450 mV) and Q-318 (or Aq) are also mentioned. Furthermore, we summarize the current ideas on the so-called inactive (those that transfer electrons to the plastoquinone pool rather slowly) and active reaction centers. Second, we discuss the bearing of the first section on the ratio of the PS II reaction center (RC-II) and the PS I reaction center (RC-I). Third, we review recent results that relate the inactive and active RC-II, obtained by the use of quinones DMQ and DCBQ, with the fluorescence transient at room temperature and in heated spinach and soybean thylakoids. These data show that inactive RC-II can be easily monitored by the OID phase of fluorescence transient and that heating converts active into inactive centers.Abbreviations DCBQ 2,5 or 2,6 dichloro-p-benzoquinone - DMQ dimethylquinone - QA primary plastoquinone electron acceptor of photosystem II - QB secondary plastoquinone electron acceptor of photosystem II - IODP successive fluorescence levels during time course of chlorophyll a fluorescence: O for origin, I for inflection, D for dip or plateau, and P for peak  相似文献   

7.
The nature of Cu2+ inhibition of photosystem II (PSII) photochemistry in pea (Pisum sativum L.) thylakoids was investigated monitoring Hill activity and light emission properties of photosystem II. In Cu2+-inhibited thylakoids, diphenyl carbazide addition does not relieve the loss of Hill activity. The maximum yield of fluorescence induction restored by hydroxylamine in Tris-inactivated thylakoids is markedly reduced by Cu2+. This suggests that Cu2+ does not act on the donor side of PSII but on the reaction center of PSII or on components beyond. Thermoluminescence and delayed luminescence studies show that charge recombination between the positively charged intermediate in water oxidation cycle (S2) and negatively charged primary quinone acceptor of pSII (QA) is largely unaffected by Cu2+. The S2QB charge recombination, however, is drastically inhibited which parallels the loss of Hill activity. This indicates that Cu2+ inhibits photosystem II photochemistry primarily affecting the function of the secondary quinone electron acceptor, QB. We suggest that Cu2+ does not block electron flow between the primary and secondary quinone acceptor but modifies the QB site in such a way that it becomes unsuitable for further photosystem II photochemistry.  相似文献   

8.
Disulfiram (tetraethylthiuram disulfide), a metal chelator, inhibits photosynthetic electron transport in broken chloroplasts. A major site of inhibition is detected on the electron-acceptor side of photosystem II between QA, the first plastoquinone electron-acceptor, and the second plastoquinone electron-acceptor, QB. This site of inhibition is shown by a severalfold increase in the half-time of QA oxidation, as monitored by the decay of the variable chlorophyll a flourescence after an actinic flash. Another site of inhibition is detected in the functioning of the reaction center of photosystem II; disulfiram is observed to quench the room temperature variable chlorophyll a fluorescence, as well as the intensity of the 695 nm peak, relative to the 685 nm peak, in the chlorophyll a fluorescence spectrum at 77 K. Electron transport from H2O to the photosystem II electron-acceptor silicomolybdate is also inhibited. Disulfiram does not inhibit electron flow before the site(s) of donation by exogenous electron donors to photosystem II, and no inhibition is detected in the partial reactions associated with photosystem I.  相似文献   

9.
When the photosystem II quinone acceptor complex has been singly reduced to the state QAQ?B, there is a 22 s half-time back-reaction of Q?B with an oxidized photosystem II donor (S2), directly measured here for the first time. From the back-reaction kinetics with and without inhibitors, kinetic and equilibrium parameters have been estimated. We suggest that the state QAQ?B of the complex is formed by a second-order reaction of vacant reaction centers in the state Q?A with plastoquinone from the pool, and discuss the physico-chemical parameters involved.  相似文献   

10.
We have measured the electrochromic response of the bacteriopheophytin, BPh, and bacteriochlorophyll, BChl, cofactors during the QA QB QAQB electron transfer in chromatophores of Rhodobacter (Rb.) capsulatus and Rb. sphaeroides. The electrochromic response rises faster in chromatophores and is more clearly biexponential than it is in isolated reaction centers. The chromatophore spectra can be interpreted in terms of a clear kinetic separation between fast electron transfer and slower non-electron transfer events such as proton transfer or protein relaxation. The electrochromic response to electron transfer exhibits rise times of about 4 µs (70%) and 40 µs (30%) in Rb. capsulatus and 4 µs (60%) and 80 µs (40%) in Rb. sphaeroides. The BPh absorption band is shifted to nearly equivalent positions in the QA and nascent QB states, indicating that the electrochromic perturbation of BPh absorption from the newly formed QB state is comparable to that of QA . Subsequently, partial attenuation of the QB electrochromism occurs with a time constant on the order of 200 µs. This can be attributed to partial charge compensation by H+ (or other counter ion) movement into the QB pocket. Electron transfer events were found to be slower in detergent isolated RCs than in chromatophores, more nearly monoexponential, and overlap H+ transfer, suggesting that a change in rate-limiting step has occurred upon detergent solubilization.  相似文献   

11.
Photoinhibition under aerobic and anaerobic conditions was analyzed in O2-evolving and in Tris-treated PS II-membrane fragments from spinach by measuring laser-flash-induced absorption changes at 826 nm reflecting the transient P680+ formation and the chlorophyll fluorescence lifetime. It was found that anaerobic photoinhibitory treatment leads in both types of samples to the appearence of two long-lived fluorescence components with lifetimes of 7 ns and 16 ns, respectively. The extent of these fluorescence kinetics depends on the state of the reaction center (open/closed) during the fluorescence measurements: it is drastically higher in the closed state. It is concluded that this long-lived fluorescence is mainly emitted from modified reaction centers with singly reduced QA(QA -). This suggests that the observation of long-lived fluorescence components cannot necessarily be taken as an indicator for reaction centers with missing or doubly reduced and protonated QA (QAH2). Time-resolved measurements of 826 nm absorption changes show that the rate of photoinhibition of the stable charge separation (P680*QA P680+QA -), is nearly the same in O2-evolving and in Tris-treated PS II-membrane fragments. This finding is difficult to understand within the framework of the QAH2-mechanism for photoinhibition of stable charge separation because in that case the rate of photoinhibition should strongly depend on the functional integrity of the donor side of PS II. Based on the results of this study it is inferred, that several processes contribute to photoinhibition within the PS II reaction center and that a mechanism which comprises double reduction and protonation of QA leading to QAH2 formation is only of marginal – if any – relevance for photoinhibition of PS II under both, aerobic and anaerobic, conditions.  相似文献   

12.
Inhibition of electron transport and damage to the protein subunits by ultraviolet-B (UV-B, 280–320 nm) radiation have been studied in isolated reaction centers of the non-sulfur purple bacterium Rhodobacter sphaeroides R26. UV-B irradiation results in the inhibition of charge separation as detected by the loss of the initial amplitude of absorbance change at 430 nm reflecting the formation of the P+(QAQB) state. In addition to this effect, the charge recombination accelerates and the damping of the semiquinone oscillation increases in the UV-B irradiated reaction centers. A further effect of UV-B is a 2 fold increase in the half- inhibitory concentration of o-phenanthroline. Some damage to the protein subunits of the RC is also observed as a consequence of UV-B irradiation. This effect is manifested as loss of the L, M and H subunits on Coomassie stained gels, but not accompanied with specific degradation products. The damaging effects of UV-B radiation enhanced in reaction centers where the quinone was semireduced (QB ) during UV-B irradiation, but decreased in reaction centers which lacked quinone at the QB binding site. In comparison with Photosystem II of green plant photosynthesis, the bacterial reaction center shows about 40 times lower sensitivity to UV-B radiation concerning the activity loss and 10 times lower sensitivity concerning the extent of reaction center protein damage. It is concluded that the main effect of UV-B radiation in the purple bacterial reaction center occurs at the QAQB quinone acceptor complex by decreasing the binding affinity of QB and shifting the electron equilibration from QAQB to QA QB. The inhibitory effect is likely to be caused by modification of the protein environment around the QB binding pocket and mediated by the semiquinone form of QB. The UV-resistance of the bacterial reaction center compared to Photosystem II indicates that either the QAQB acceptor complex, which is present in both types of reaction centers with similar structure and function, is much less susceptible to UV damage in purple bacteria, or, more likely, that Photosystem II contains UV-B targets which are more sensitive than its quinone complex.Abbreviations Bchl bacteriochlorophyll - P Bchl dimer - QA primary quinone electron acceptor - QB secondary quinone electron acceptor - RC reaction center - UV-B ultraviolet-B  相似文献   

13.
A fraction (usually in the range of 10–25%) of PS II centers is unable to transfer electrons from the primary quinone acceptor QA to the secondary acceptor QB. These centers are inactive with respect to O2 evolution since their reopening after photochemical charge separation to the S2OA - state involves predominantly a back reaction to S1QA in the few seconds time range (slower phases are also occurring). Several properties of these centers are analyzed by fluorescence and absorption change experiments. The initial rise phase Fo-Fpl of fluorescence induction under weak illumination reflects both the closure of inactive centers and the modulation of the fluorescence yield by the S-states of the oxygen-evolving system: We estimate typical relative amplitudes of these contributions as, respectively, 65 and 35% of the Fo-Fpl amplitude. The half-rise time of this phase is significantly shorter than for the fluorescence induction in the presence of DCMU (in which all centers are involved). This finding is shown to be consistent with inactive centers sharing the same light-harvesting antenna as normal centers, a view which is also supported by comparing the dependence of the fluorescence yield on the amount of closed active or inactive centers estimated through absorption changes. It is argued that the exponential kinetics of the Fo-Fpl phase does not indicate absence of excitation energy transfer between the antennas of inactive and active centers. We show that the acceptor dichlorobenzoquinone does not restore electron transfer in inactive centers, in disagreement with previous suggestions. We confirm, however, the enhancement of steady-state electron flow caused by this quinone and suggest that it acts by relieving a blocking step involved in the reoxidation of a fraction of the plastoquinone pool. Part of the discrepancies between the present results and those from previous literature may arise from the confusion of inactive centers characterized on a single turnover basis and PS II centers that become blocked under steady-state conditions because of deficient reoxidation of their secondary acceptors.Abbreviations DCBQ 2,6-dichloro-p-benzoquinone - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DMQ 2,5-dimethyl-p-benzoquinone - PS photosystem  相似文献   

14.
The temperature dependence of the electric field-induced chlorophyll luminescence in photosystem II was studied in Tris-washed, osmotically swollen spinach chloroplasts (blebs). The system II reaction centers were brought in the state Z+P+-QA -QB - by preillumination and the charge recombination to the state Z+PQAQB - was measured at various temperatures and electrical field strengths. It was found that the activation enthalpy of this back reaction was 0.16 eV in the absence of an electrical field and diminished with increasing field strength. It is argued that this energy is the enthalpy difference between the states IQA - and I-QA and accounts for about half of the free energy difference between these states. The redox state of QB does not influence this free energy difference within 150 s after the photoreduction of QA. The consequences for the interpretation of thermodynamic properties of QA are discussed.Abbreviations DCMU 3(3,4-dichlorophenyl)-1,1-dimethylurea - I intermediary electron acceptor - Mops 3-(N-morpholino)propanesulphonic acid - P (P680) primary electron donor - PS II photosystem II - QA and QB first and second quinone electron acceptors - Tricine N-tris(hydroxymethyl)methylglycine - Tris tris-(hydroxymethyl)aminomethane - Z secondary electron donor Dedicated to Professor L.N.M. Duysens on the occasion of his retirement  相似文献   

15.
The action of the environmental toxic Pb2+ on photosynthetic electron transport was studied in thylakoid membranes isolated from spinach leaves. Fluorescence and thermoluminescence techniques were performed in order to determine the mode of Pb2+ action in photosystem II (PSII). The invariance of fluorescence characteristics of chlorophyll a (Chl a) and magnesium tetraphenylporphyrin (MgTPP), a molecule structurally analogous to Chl a, in the presence of Pb2+ confirms that Pb cation does not interact directly with chlorophyll molecules in PSII. The results show that Pb interacts with the water oxidation complex thus perturbing charge recombination between the quinone acceptors of PSII and the S2 state of the Mn4Ca cluster. Electron transfer between the quinone acceptors QA and QB is also greatly retarded in the presence of Pb2+. This is proposed to be owing to a transmembrane modification of the acceptor side of the photosystem.  相似文献   

16.
Based on the electron-transport properties on the reducing side of the reaction center, photosystem II (PS II) in green plants and algae occurs in two distinct forms. Centers with efficient electron-transport from QA to plastoquinone (QB-reducing) account for 75% of the total PS II in the thylakoid membrane. Centers that are photochemically competent but unable to transfer electrons from QA to QB (QB-nonreducing) account for the remaining 25% of total PS II and do not participate in plastoquinone reduction. In Dunaliella salina, the pool size of QB-nonreducing centers changes transiently when the light regime is perturbed during cell growth. In cells grown under moderate illumination intensity (500 E m-2s-1), dark incubation induces an increase (half-time 45 min) in the QB-nonreducing pool size from 25% to 35% of the total PS II. Subsequent illumination of these cells restores the steady-state concentration of QB-nonreducing centers to 25%. In cells grown under low illumination intensity (30 µE m–2s–1), dark incubation elicits no change in the relative concentration of QB-nonreducing centers. However, a transfer of low-light grown cells to moderate light induces a rapid (half-time 10 min) decrease in the QB-nonreducing pool size and a concomitant increase in the QB-reducing pool size. These and other results are explained in terms of a pool of QB-nonreducing centers existing in a steady-state relationship with QB-reducing centers and with a photochemically silent form of PS II in the thylakoid membrane of D. salina. It is proposed that QB-nonreducing centers are an intermediate stage in the process of damage and repair of PS II. It is further proposed that cells regulate the inflow and outflow of centers from the QB-nonreducing pool to maintain a constant pool size of QB-nonreducing centers in the thylakoid membrane.Abbreviations Chl chlorophyll - PS photosystem - QA primary quinone electron acceptor of PS II - QB secondary quinone electron acceptor of PS II - LHC light harvesting complex - Fo non-variable fluorescence yield - Fpl intermediate fluorescence yield plateau level - Fmax maximum fluorescence yield - Fi mitial fluorescence yield increase from Fo to Fpl(Fpl-Fo) - Fv total variable fluorescence yield (Fmax-Fo) - DCMU dichlorophenyl-dimethylurea  相似文献   

17.
Kinetics of the dark relaxation of variable chlorophyll fluorescence, Fv, were studied after brief illumination of dark-adapted barley leaves in order to understand the rapid reversibility of pulse-induced fluorescence increases, which is observed even when fast linear electron transport to an external electron acceptor is not possible. Four kinetically distinct components were observed which reveal complexity in the oxidation of the reduced primary quinone acceptor of Photosystem II, QA : the slowest component accounted for 4–5% of maximal Fv and had a life-time of several seconds. It is suggested to represent a minor population of inactive Photosystem II centers. The other three components displayed first-order kinetics with half-time of 6–8 ms (`fast' component), 60–80 ms (`middle' component) and 650–680 ms (`slow' component). The fast component dominated Fv when methyl viologen or far-red light accelerated oxidation of plastohydroquinone. It shows rapid oxidation of QA during electron flow to plastoquinone commensurate with maximum linear electron flow through the electron transport chain. The other two components were observed under conditions of restricted electron flow and excessive reduction of electron carriers. Unexpectedly, the slow component, which is interpreted to reflect the recombination between QA and an intermediate on the oxidizing side of Photosystem II, saturated already at low irradiances of actinic light when plastoquinone was not yet strongly reduced suggesting that dark-adaptation of leaves results not only in the loss of activity of light-regulated enzymes of the carbon cycle but affects also electron flow from QA to plastoquinone. KCN poisoning or high temperature treatment of leaves produced a nonexponential pattern of slow Fv relaxation. This effect was largely (heat treatment) or even completely (KCN) abolished by far-red light. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

18.
Ryo Nagao  Sho Kitazaki  Takumi Noguchi 《BBA》2018,1859(2):129-136
Light-induced Fourier transformed infrared (FTIR) difference spectroscopy is a powerful method to study the structures and reactions of redox cofactors involved in the photosynthetic electron transport chain. So far, most of the FTIR studies of the reactions of oxygenic photosynthesis have been performed using isolated photosystem I (PSI) and photosystem II (PSII) preparations, which, however, could be modified during isolation procedures. In this study, we developed a methodology to evaluate the photosynthetic activities of thylakoids using FTIR spectroscopy. FTIR difference spectra upon successive flashes using thylakoids from spinach exhibited signals typical of the S-state cycle at the Mn4CaO5 cluster and QB reactions in PSII with period-four and -two oscillations, respectively. Similar measurement in the presence of an artificial quinone as an exogenous electron acceptor showed features specific to the S-state cycle. Simulations of the oscillation patterns provided the quantum efficiencies of the S-state cycle and electron transfer in PSII. Moreover, FTIR measurement under continuous illumination on thylakoids in the presence of DCMU showed signals due to QA reduction and P700 oxidation simultaneously. From the relative amplitudes of marker bands of QA? and P700+, the molar ratio of photoactive PSII and PSI centers in thylakoids was estimated. FTIR analyses of the photo-reactions in thylakoids, which are more intact than isolated photosystems, will be useful in investigations of the photosynthetic mechanism especially by genetic modification of photosystem proteins.  相似文献   

19.
Photochemical efficiencies of photosystem I (PSI) and photosystem II (PSII) were studied in dry thalli of the lichen Hypogymnia physodes and during their re-hydration. In dry thalli, PSII reaction centers are photochemically inactive, as evidenced by the absence of variable chlorophyll (Chl) fluorescence, whereas the primary electron donor of PSI, P700, exhibits irreversible oxidation under continuous light. Upon application of multiple- and, particularly, single-turnover pulses in dry lichen, P700 oxidation partially reversed, which indicated recombination between P700+ and the reduced acceptor FX of PSI. Re-wetting of air-dried H. physodes initiated the gradual restoration of reversible light-induced redox reactions in both PSII and PSI, but the recovery was faster in PSI. Two slow components of P700+ reduction occurred after irradiation of partially and completely hydrated thalli with strong white light. In contrast, no slow component was found in the kinetics of re-oxidation of QA, the reduced primary acceptor of PSII, after exposure of such thalli to white light. This finding indicated the inability of PSII in H. physodes to provide the reduction of the plastoquinone pool to significant levels. It is concluded that slow alternative electron transport routes may contribute to the energetics of photosynthesis to a larger extent in H. physodes than in higher plants.Abbreviations A0 and A1 Primary acceptor chlorophyll and secondary electron acceptor phylloquinone - Chl a Chlorophyll a - Fm Maximal level of chlorophyll fluorescence when all PSII centers are closed - Fo Minimal level of fluorescence when all PSII centers are open after dark adaptation - FR Far-red - Fv Variable fluorescence (=FmFo) - FX, FA, and FB Iron–sulfur centers - MT pulse Multiple-turnover pulse - PS Photosystem - P700 Reaction center chlorophyll of PSI - QA Primary quinone acceptor of PSII - QB Secondary quinone acceptor of PSII - ST pulse Single-turnover pulse  相似文献   

20.
Several mutant strains of Synechocystis sp. PCC 6803 with large deletions in the D-E loop of the photosystem II (PSII) reaction center polypeptide D1 were subjected to high light to investigate the role of this hydrophilic loop in the photoinhibition cascade of PSII. The tolerance of PSII to photoinhibition in the autotrophic mutant ΔR225-F239 (PD), when oxygen evolution was monitored with 2,6-dichloro-p-benzoquinone and the equal susceptibility compared with control when monitored with bicarbonate, suggested an inactivation of the QB-binding niche as the first event in the photoinhibition cascade in vivo. This step in PD was largely reversible at low light without the need for protein synthesis. Only the next event, inactivation of QA reduction, was irreversible and gave a signal for D1 polypeptide degradation. The heterotrophic deletion mutants ΔG240-V249 and ΔR225-V249 had severely modified QB pockets, yet exhibited high rates of 2,6-dichloro-p-benzoquinone-mediated oxygen evolution and less tolerance to photoinhibition than PD. Moreover, the protein-synthesis-dependent recovery of PSII from photoinhibition was impaired in the ΔG240-V249 and ΔR225-V249 mutants because of the effects of the mutations on the expression of the psbA-2 gene. No specific sequences in the D-E loop were found to be essential for high rates of D1 polypeptide degradation.  相似文献   

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