Quinone Electron Acceptor Research Articles

PHOTOPHYSIOLOGICAL RESPONSES OF FRAGILARIOPSIS CYLINDRUS (BACILLARIOPHYCEAE) TO NITROGEN DEPLETION AT TWO TEMPERATURES(1).

The photosynthetic efficiency and photoprotective capacity of the sea-ice diatom, Fragilariopsis cylindrus (Grunow) W. Krieg., grown in a matrix of nitrogen repletion and depletion at two different temperatures (-1°C and +6°C) was investigated. Temperature showed no significant effect on photosynthetic efficiency or photoprotection in F. cylindrus. Cultures under nitrogen depletion showed enhanced photoprotective capacity with an increase in nonphotochemical quenching (NPQ) when compared with nitrogen-replete cultures. This phenomenon was achieved at no apparent cost to the photosynthetic efficiency of PSII (FV /FM ). Nitrogen depletion yielded a partially reduced electron transport chain in which maximum fluorescence (FM ) could only be obtained by adding 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). reoxidation curves showed the presence of QB nonreducing PSII centers under nitrogen depletion. Fast induction curves (FICs) and electron transport rates (ETRs) revealed slowing of the electrons transferred from the primary (QA ) to the secondary (QB ) quinone electron acceptors of PSII. The data presented show that nitrogen depletion in F. cylindrus leads to the formation of QB nonreducing PSII centers within the photosystem. On a physiological level, the formation of QB nonreducing PSII centers in F. cylindrus provides the cell with protection against photoinhibition by facilitating the rapid induction of NPQ. This strategy provides an important ecological advantage, especially during the Antarctic spring, maintaining photosynthetic efficiency under high light and nutrient-limiting conditions.

Heterologous expression and biochemical characterization of novel pyranose 2-oxidases from the ascomycetes Aspergillus nidulans and Aspergillus oryzae

A gene encoding a pyranose 2-oxidase (POx; pyranose/oxygen 2-oxidoreductase; glucose 2-oxidase; EC 1.1.3.10) was identified in the genome of the ascomycete Aspergillus nidulans. Attempts to isolate POx directly from A. nidulans cultures or to homologously overexpress the native POx (under control of the constitutive gpdA promoter) in A. nidulans were unsuccessful. cDNA encoding POx was synthesized from mRNA and expressed in Escherichia coli, and the enzyme was subsequently purified and characterized. A putative pyranose 2-oxidase-encoding gene was also identified in the genome of Aspergillus oryzae. The coding sequence was synthetically produced and was also expressed in E. coli. Both purified enzymes were shown to be flavoproteins consisting of subunits of 65 kDa. The A. nidulans enzyme was biochemically similar to POx reported in literature. From all substrates, the highest catalytic efficiency was found with D-glucose. In addition, the enzyme catalyzes the two-electron reduction of 1,4-benzoquinone, several substituted benzoquinones and 2,6-dichloroindophenol. As judged by the catalytic efficiencies (k (cat)/k(m)), some of these quinone electron acceptors are better substrates for pyranose oxidase than oxygen. The enzyme from A. oryzae was physically similar but showed lower kinetic constants compared to the enzyme from A. nidulans. Distinct differences in the stability of the two enzymes may be attributed to a deletion and an insertion in the sequence, respectively.

Redirecting Electron Transfer in Photosystem II from Water to Redox-Active Metal Complexes

A negatively charged region on the surface of photosystem II (PSII) near Q(A) has been identified as a docking site for cationic exogenous electron acceptors. Oxygen evolution activity, which is inhibited in the presence of the herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), is recovered by adding Co(III) complexes. Thus, a new electron-transfer pathway is created with Co(III) as the new terminal electron acceptor from Q(A)(-). This binding site is saturated at ∼2.5 mM [Co(III)], which is consistent with the existence of low-affinity interactions with a solvent-exposed surface. This is the first example of a higher plant PSII in which the electron-transfer pathway has been redirected from the normal membrane-associated quinone electron acceptors to water-soluble electron acceptors. The proposed Co(III) binding site may enable efficient collection of electrons generated from photochemical water oxidation by PSII immobilized on an electrode surface.

Redox potentials of primary electron acceptor quinone molecule (Q A ) − and conserved energetics of photosystem II in cyanobacteria with chlorophyll a and chlorophyll d

In a previous study, we measured the redox potential of the primary electron acceptor pheophytin (Phe) a of photosystem (PS) II in the chlorophyll d-dominated cyanobacterium Acaryochloris marina and a chlorophyll a-containing cyanobacterium, Synechocystis. We obtained the midpoint redox potential (E(m)) values of -478 mV for A. marina and -536 mV for Synechocystis. In this study, we measured the redox potentials of the primary electron acceptor quinone molecule (Q(A)), i.e., E(m)(Q(A)/Q(A)(-)), of PS II and the energy difference between [P680·Phe a(-)·Q(A)] and [P680·Phe a·Q(A)(-)], i.e., ΔG(PhQ). The E(m)(Q(A)/Q(A)(-)) of A. marina was determined to be +64 mV without the Mn cluster and was estimated to be -66 to -86 mV with a Mn-depletion shift (130-150 mV), as observed with other organisms. The E(m)(Phe a/Phe a(-)) in Synechocystis was measured to be -525 mV with the Mn cluster, which is consistent with our previous report. The Mn-depleted downshift of the potential was measured to be approximately -77 mV in Synechocystis, and this value was applied to A. marina (-478 mV); the E(m)(Phe a/Phe a(-)) was estimated to be approximately -401 mV. These values gave rise to a ΔG(PhQ) of -325 mV for A. marina and -383 mV for Synechocystis. In the two cyanobacteria, the energetics in PS II were conserved, even though the potentials of Q(A)(-) and Phe a(-) were relatively shifted depending on the special pair, indicating a common strategy for electron transfer in oxygenic photosynthetic organisms.

High and low potential forms of the Q A quinone electron acceptor in Photosystem II of Thermosynechococcus elongatus and spinach

The redox potential of Q A in Photosystem II (PSII) from Thermosynechococcus elongatus was titrated monitoring chlorophyll fluorescence. A high potential form ( E m = +60 ± 25 mV) was found in the absence of Mn 4Ca, the active site for water oxidation. The low potential form ( E m = −60 ± 48 mV), which is difficult to measure in conventional titration experiments, could be “locked in” by cross-linking the active enzyme. This indicates that the presence of Mn 4Ca is relayed to the quinone site by significant structural changes in the protein. The presence of high and low potential forms agrees with what has been seen in plants, algae from our lab and in T. elongatus (Shibamoto et al., Biochemistry 48 (2009) 10682–10684). In the latter work, the potentials of Q A were shifted to lower potentials compared to other measurements. The redox potential of Q A in Mn-depleted PSII from spinach was titrated in the presence of redox mediators and the midpoint potential was shifted by 80 mV towards a more negative value compared to titrations without mediators. The lower values of the midpoint potential of the ( Q A / Q A - ) redox couple in the literature could be due to a perturbation due to a specific mediator.

Characterization of photosystem II photochemistry in transgenic tobacco plants with lowered Rubisco activase content

Rubisco activase plays an important role in the regulation of CO 2 assimilation. However, it is unknown how activase regulates photosystem II (PSII) photochemistry. To investigate the effects of Rubisco activase on PSII photochemistry, we obtained transgenic tobacco ( Nicotiana tabacum) plants with 50% (i7), 25% (i28), and 5% (i46) activase levels as compared to wild type plants by using a gene encoding tobacco activase for the RNAi construct. Both CO 2 assimilation and PSII activity were significantly reduced only in transgenic i28 and i46 plants, suggesting that activase deficiency led to decreased PSII activity. Flash-induced fluorescence kinetics indicated that activase deficiency resulted in a slow electron transfer between Q A (primary quinine electron acceptor of PSII) and Q B (secondary quinone electron acceptor of PSII). Thermoluminescence measurements revealed that activase deficiency induced a shift of S 2Q A − and S 2Q B − recombinations to higher temperatures in parallel, and a decrease in the intensities of the thermoluminescence emissions. Activase deficiency also dampened the period-four oscillation of the thermoluminescence B-band. Protein gel blot analysis showed that activase deficiency resulted in a significant decrease in the content of D1, D2, CP43, CP47, and PsbO proteins. Transmission electron microscopy analysis demonstrated that activase deficiency induced a significant decrease in the number of grana stacks per chloroplast and discs per grana stack. Our results suggest that activase plays an important role in maintaining PSII function and chloroplast development.

Species-dependence of the redox potential of the primary quinone electron acceptor Q A in photosystem II verified by spectroelectrochemistry

The redox potentials E m(Q A/ Q A − ) of the primary quinone electron acceptor Q A in oxygen-evolving photosystem II complexes of three species were determined by spectroelectrochemistry. The E m(Q A/ Q A − ) values were experimentally found to be −162 ± 3 mV for a higher plant spinach, −171 ± 3 mV for a green alga Chlamydomonas reinhardtii and −104 ± 4 mV vs. SHE for a red alga Cyanidioschyzon merolae. On the basis of possible deviations for the experimental values, as estimated to differ by 9–29 mV from each true value, plausible causes for such remarkable species-dependence of E m(Q A/ Q A − ) are discussed, mainly by invoking the effects of extrinsic subunits on the delicate structural environment around Q A.

Spectroscopic and Functional Characterizations of Cyanobacterium Synechocystis PCC 6803 Mutants on and near the Heme Axial Ligand of Cytochrome b559 in Photosystem II

The functional role of cytochrome (cyt) b(559) in photosystem II (PSII) was investigated in H22K alpha and Y18S alpha cyt b(559) mutants of the cyanobacterium Synechocystis sp. PCC6803. H22K alpha and Y18S alpha cyt b(559) mutant carries one amino acid substitution on and near one of heme axial ligands of cyt b(559) in PSII, respectively. Both mutants grew photoautotrophically, assembled stable PSII, and exhibited the normal period-four oscillation in oxygen yield. However, both mutants showed several distinct chlorophyll a fluorescence properties and were more susceptible to photoinhibition than wild type. EPR results indicated the displacement of one of the two axial ligands to the heme of cyt b(559) in H22K alpha mutant reaction centers, at least in isolated reaction centers. The maximum absorption of cyt b(559) in Y18S alpha mutant PSII core complexes was shifted to 561 nm. Y18S alpha and H22K alpha mutant PSII core complexes contained predominately the low potential form of cyt b(559). The findings lend support to the concept that the redox properties of cyt b(559) are strongly influenced by the hydrophobicity and ligation environment of the heme. When the cyt b(559) mutations placed in a D1-D170A genetic background that prevents assembly of the manganese cluster, accumulation of PSII is almost completely abolished. Overall, our data support a functional role of cyt b(559) in protection of PSII under photoinhibition conditions in vivo.

Different enhancement of senescence induced by metabolic products of Alternaria alternata in tobacco leaves of different ages

The purpose of this study was to explore the mechanisms by which Alternaria alternata damages tobacco (Nicotiana tabacum) leaves. Treatment with A. alternata metabolic products enhanced senescence in leaves of different ages, as indicated by the significant decrease in chlorophyll, soluble protein, photosynthetic O(2) evolution and catalase (CAT, EC 1.11.1.6) activity as well as an increase in H(2)O(2) content. The induction of senescence by A. alternata metabolic products increased as the age of the leaves increased. A. alternata metabolic products greatly influenced the behavior of photosystem II (PSII) in the leaves: oxygen evolving complex (OEC) activity and electron transport from primary quinone electron acceptor of PS II (Q(A)) to secondary quinone electron acceptor of PS II (Q(B)) were both significantly inhibited. This inhibition also became more pronounced in older leaves. In vitro experiments revealed that, without the influence of natural senescence, the A. alternata metabolic products directly inhibited the activity of a commercial CAT solution and inhibited photosynthetic O(2) evolution, which resulted in excess PSII excitation pressure and an overaccumulation of H(2)O(2) in leaf segments. These results suggest that the significant declines in photosynthesis and CAT activity induced by the metabolic products of A. alternata were important contributors to the overaccumulation of reactive oxygen species (ROS), which accelerated senescence in tobacco leaves. The fact that the enhancement of senescence was getting more pronounced with the age of tobacco leaves might be related to the fact that older leaves already had higher H(2)O(2) levels and less antioxidant activity as reflected in lower CAT activity.

2P289 1A1325 種々の生物における光化学系II電子受容体キノンの酸化還元電位の分光電気化学計測(光生物-光合成,第48回日本生物物理学会年会)

2P289 1A1325 種々の生物における光化学系II電子受容体キノンの酸化還元電位の分光電気化学計測(光生物-光合成,第48回日本生物物理学会年会)

Structural Coupling of a Tyrosine Side Chain with the Non-Heme Iron Center in Photosystem II As Revealed by Light-Induced Fourier Transform Infrared Difference Spectroscopy

The non-heme iron is located between the quinone electron acceptors, QA and QB, in photosystem II (PSII), and together with its bicarbonate ligand, it regulates the electron and proton transfer reactions of quinone acceptors. In this study, we have investigated the structural coupling of a nearby Tyr residue with the non-heme iron center using Fourier transform infrared (FTIR) spectroscopy. Light-induced Fe2+/Fe3+ FTIR difference spectra of PSII core complexes from unlabeled and [4-13C]Tyr-labeled Thermosynechococcus elongatus revealed that the CO stretching (nuCO) bands of a Tyr side chain are located at 1253 and 1241 cm(-1) in the Fe2+ and Fe3+ states, respectively. Upon deuteration, both nuCO bands were upshifted by 11-12 cm(-1). Taking into account the criteria for determining the hydrogen bond structure of a Tyr side chain from infrared bands reported previously [Takahashi, R., and Noguchi, T. (2007) J. Phys. Chem. B 111, 13833-13844] and the results of DFT calculations of model complexes of p-cresol hydrogen-bonded with bicarbonate, we interpreted the observed nuCO bands and their deuteration effects as indicating that one Tyr side chain with a hydrogen bond donor-acceptor form is strongly coupled to the non-heme iron. From the X-ray structures of PSII core complexes, it is proposed that either D1-Y246 or D2-Y244 provides a hydrogen bond to the oxygen of the bicarbonate ligand but the other Tyr does not directly interact with bicarbonate. The Tyr residue coupled to the non-heme iron may play a key role in the regulatory function of the iron-bicarbonate center by stabilizing the bicarbonate ligand and forming a rigid hydrogen bond network around the non-heme ion.

1P-232 QA再構成とFTIR解析によるPSIIにおける第一電子受容体QAの分子間相互作用の解明(光生物-光合成,第47回日本生物物理学会年会)

1P-232 QA再構成とFTIR解析によるPSIIにおける第一電子受容体QAの分子間相互作用の解明(光生物-光合成,第47回日本生物物理学会年会)

Riboflavin Is an Active Redox Cofactor in the Na+-pumping NADH:Quinone Oxidoreductase (Na+-NQR) from Vibrio cholerae

Here we present new evidence that riboflavin is present as one of four flavins in Na+-NQR. In particular, we present conclusive evidence that the source of the neutral radical is not one of the FMNs and that riboflavin is the center that gives rise to the neutral flavosemiquinone. The riboflavin is a bona fide redox cofactor and is likely to be the last redox carrier of the enzyme, from which electrons are donated to quinone. We have constructed a double mutant that lacks both covalently bound FMN cofactors (NqrB-T236Y/NqrC-T225Y) and have studied this mutant together with the two single mutants (NqrB-T236Y and NqrC-T225Y) and a mutant that lacks the noncovalently bound FAD in NqrF (NqrF-S246A). The double mutant contains riboflavin and FAD in a 0.6:1 ratio, as the only flavins in the enzyme; noncovalently bound flavins were detected. In the oxidized form, the double mutant exhibits an EPR signal consistent with a neutral flavosemiquinone radical, which is abolished on reduction of the enzyme. The same radical can be observed in the FAD deletion mutant. Furthermore, when the oxidized enzyme reacts with ubiquinol (the reduced form of the usual electron acceptor) in a process that reverses the physiological direction of the electron flow, a single kinetic phase is observed. The kinetic difference spectrum of this process is consistent with one-electron reduction of a neutral flavosemiquinone. The presence of riboflavin in the role of a redox cofactor is thus far unique to Na+-NQR.

Identification of the First Steps in Charge Separation in Bacterial Photosynthetic Reaction Centers of Rhodobacter sphaeroides by Ultrafast Mid-Infrared Spectroscopy: Electron Transfer and Protein Dynamics

Time-resolved visible pump/mid-infrared (mid-IR) probe spectroscopy in the region between 1600 and 1800 cm −1 was used to investigate electron transfer, radical pair relaxation, and protein relaxation at room temperature in the Rhodobacter sphaeroides reaction center (RC). Wild-type RCs both with and without the quinone electron acceptor Q A, were excited at 600 nm (nonselective excitation), 800 nm (direct excitation of the monomeric bacteriochlorophyll (BChl) cofactors), and 860 nm (direct excitation of the dimer of primary donor ( P) BChls ( P L/ P M)). The region between 1600 and 1800 cm −1 encompasses absorption changes associated with carbonyl (C O) stretch vibrational modes of the cofactors and protein. After photoexcitation of the RC the primary electron donor P excited singlet state ( P*) decayed on a timescale of 3.7 ps to the state P + B L − (where B L is the accessory BChl electron acceptor). This is the first report of the mid-IR absorption spectrum of P + B L − ; the difference spectrum indicates that the 9-keto C O stretch of B L is located around 1670–1680 cm −1. After subsequent electron transfer to the bacteriopheophytin H L in ∼1 ps, the state P + H L − was formed. A sequential analysis and simultaneous target analysis of the data showed a relaxation of the P + H L − radical pair on the ∼20 ps timescale, accompanied by a change in the relative ratio of the P L + and P M + bands and by a minor change in the band amplitude at 1640 cm −1 that may be tentatively ascribed to the response of an amide C O to the radical pair formation. We conclude that the drop in free energy associated with the relaxation of P + H L − , is due to an increased localization of the electron hole on the P L half of the dimer and a further consequence is a reduction in the electrical field causing the Stark shift of one or more amide C O oscillators.

Herbicide effect on the hydrogen-bonding interaction of the primary quinone electron acceptor QA in photosystem II as studied by Fourier transform infrared spectroscopy

The redox potential of Q(A) in photosystem II (PSII) is known to be lower by approximately 100 mV in the presence of phenolic herbicides compared with the presence of DCMU-type herbicides. In this study, the structural basis underlying the herbicide effects on the Q(A) redox potential was studied using Fourier transform infrared (FTIR) spectroscopy. Light-induced Q(A)(-)/Q(A) FTIR difference spectra of Mn-depleted PSII membranes in the presence of DCMU, atrazine, terbutryn, and bromacil showed a strong CO stretching peak of Q(A)(-) at 1,479 cm(-1), while binding of phenolic herbicides, bromoxynil and ioxynil, induced a small but clear downshift by approximately 1 cm(-1). The CO peak positions and the small frequency difference were reproduced in the S(2)Q(A)(-)/S(1)Q(A) spectra of oxygen-evolving PSII membranes with DCMU and bromoxynil. The relationship of the CO frequency with herbicide species correlated well with that of the peak temperatures of thermoluminescence due to S(2)Q(A)(-) recombination. Density functional theory calculations of model hydrogen-bonded complexes of plastoquinone radical anion showed that the small shift of the CO frequency is consistent with a change in the hydrogen-bond structure most likely as a change in its strength. The Q(A)(-)/Q(A) spectra in the presence of bromoxynil, and ioxynil, which bear a nitrile group in the phenolic ring, also showed CN stretching bands around 2,210 cm(-1). Comparison with the CN frequencies of bromoxynil in solutions suggested that the phenolic herbicides take a phenotate anion form in the Q(B) pocket. It was proposed that interaction of the phenolic C-O(-) with D1-His215 changes the strength of the hydrogen bond between the CO of Q(A) with D2-His214 via the iron-histidine bridge, causing the decrease in the Q(A) redox potential.

Site-directed mutations at D1-His198 and D1-Thr179 of photosystem II in Synechocystis sp. PCC 6803: deciphering the spectral properties of the PSII reaction centre

Site-directed mutations were constructed in photosystem II of Synechocystis sp. PCC6803 in which the axial ligand, D1-His198, of special pair chlorophyll PD1 was replaced with Gln and where D1-Thr179, which overlies monomeric chlorophyll ChlD1, was replaced with His. The D1-His198Gln mutation produces a 3nm displacement to the blue of the bleaching minimum in the Soret and in the Qy region of the (P+QA--PQA) absorbance difference spectrum. To a first approximation, the bleaching can be assigned to the low-energy exciton transition of the special pair chlorophylls PD1/PD2. The D1-Thr179His mutation produces a 2nm displacement to the red of the bleaching minimum in the Qy region of the (3P-1P) absorbance difference spectrum. Analysis of the flash-induced (P+QA--PQA) and (3P-1P) absorbance difference spectra of both mutants compared with wild-type at 80K indicate that the cation of the oxidized donor P+ is predominantly localized on the chlorophyll PD1 of the special pair and that the reaction centre triplet state, produced upon charge recombination from 3[P+Pheo-], when the primary quinone electron acceptor QA is doubly reduced, is primarily localized on ChlD1.

Chlorophyll a fluorescence transient during freezing and recovery in winter wheat

Chlorophyll (Chl) a fluorescence measurements as evaluators of plant freezing tolerance are frequently insufficiently sensitive to detect the early metabolic changes that are initiated following exposure to freezing temperatures. Using cold-acclimated winter wheat, I analysed the polyphasic transience (from 50 µs to 1 s) of Chl a fluorescence. This enabled detailed studies of the progressive energy flows and efficiencies within the photosystem 2 (PS2) complex that ensue following initial exposure to freezing temperatures right through to the plant recovery stage. The initial consequences of mild frosts that may cause primary damage involve a disturbance to the energy transfer subsequent to QA (the primary quinone electron acceptor of PS2). Lower freezing temperatures, on the other hand, may deter energy flow between the PS2 reaction centre (RC), Chl, and QA. All primary damage could only be repaired partially. Further freezing-triggered dysfunction of the electron transfer between the PS2 RCs and QA was connected with secondary damage that could lead to PS2 deactivation. Both primary and secondary freezing damages were reflected in decreased PIABS, the Performance Index based on equal absorption that characterizes all energy bifurcations in PS2. PIABS also differentiated cultivars with contrasting freezing-tolerance either subsequent to the onset of freezing or during the recovery stage. In contrast, the potential quantum yield of PS2 (Fv/Fm), which characterizes efficiency of energy trapping in the PS2 RCs, was only different in cultivars with contrasting freezing-tolerance during the recovery stage.

Photosynthetic activity in winter needles of the evergreen tree Taxus cuspidata at low temperatures

Photosystems harvest light energy, yet this energy cannot be efficiently employed for CO(2) assimilation at the below-freezing temperatures to which plants are typically exposed during winter in the temperate and boreal zones. To elucidate the mechanisms whereby this energy is dissipated, I evaluated performance of photosystems in winter needles of the evergreen tree Taxus cuspidata Sieb. et Zucc. Chloroplasts were localized adjacent to plasma membranes in needle cells in summer, whereas they congregated together in the centers of the cells during winter. When winter needles were acclimated to a temperature of 20 degrees C, their chloroplasts gradually dispersed to the edges of the cells, as in the summer. Acclimation-dependent relocalization coincided with changes in CO(2) uptake. Examination of photosystem II fluorescence kinetics in winter needles indicated that the quinone electron acceptor (Q(A)) reduction rate exceeded the Q(A) oxidation rate at low temperatures. The majority of Q(A) remained reduced even when winter needles were subjected to a temperature of -5 degrees C at low irradiance.

Inhibition of Photosynthetic Electron Transport by UV-A Radiation Targets the Photosystem II Complex¶

We have studied the inhibition of photosynthetic electron transport by UV-A (320–400 nm) radiation in isolated spinach thylakoids. Measurements of Photosystem II (PSII) and Photosystem I activity by Clark-type oxygen electrode demonstrated that electron flow is impaired primarily in PSII. The site and mechanism of UV-A induced damage within PSII was assessed by flash-induced oxygen and thermoluminescence (TL) measurements. The flash pattern of oxygen evolution showed an increased amount of the S0 state in the dark, which indicate a direct effect of UV-A in the water-oxidizing complex. TL measurements revealed the UV-A induced loss of PSII centers in which charge recombination between the S2 state of the water oxidizing complex and the semireduced QA− and QB− quinone electron acceptors occur. Flash-induced oscillation of the B TL band, originating from the S2QB− recombination, showed a decreased amplitude after the second flash relative to that after the first one, which is consistent with a decrease in the amount of QB− relative to QB in dark adapted samples. The efficiency of UV-A light in inhibiting PSII electron transport exceeds that of visible light 45-fold on the basis of equal energy and 60-fold on the basis of equal photon number, respectively. In conclusion, our data show that UV-A radiation is highly damaging for PSII, whose electron transport is affected both at the water oxidizing complex, and the binding site of the QB quinone electron acceptor in a similar way to that caused by UV-B radiation.

Changes of Photosystem II Electron Transport in the Chlorophyll‐deficient Oilseed Rape Mutant Studied by Chlorophyll Fluorescence and Thermoluminescence

AbstractThe photosystem II (PSII) complex of photosynthetic membranes comprises a number of chlorophyll‐binding proteins that are important to the electron flow. Here we report that the chlorophyll b‐deficient mutant has decreased the amount of light‐harvesting complexes with an increased amount of some core polypeptides of PSII, including CP43 and CP47. By means of chlorophyll fluorescence and thermoluminescence, we found that the ratio of Fv/Fm, qP and electron transport rate in the chlorophyll b‐deficient mutant was higher compared to the wild type. In the chlorophyll b‐deficient mutant, the decay of the primary electron acceptor quinones (QA−) reoxidation was decreased, measured by the fluorescence. Furthermore, the thermoluminescence studies in the chlorophyll b‐deficient mutant showed that the B band (S2/S3QB−) decreased slightly and shifted up towards higher temperatures. In the presence of dichlorophenyl‐dimethylurea, which is inhibited in the electron flow to the second electron acceptor quinines (QB) at the PSII acceptor side, the maximum of the Q band (S2QA−) was decreased slightly and shifted down to lower temperatures, compared to the wild type. Thus, the electron flow within PSII of the chlorophyll b‐deficient mutant was down‐regulated and characterized by faster oxidation of the primary electron acceptor quinine QA− via forward electron flow and slower reduction of the oxidation S states.(Handling editor: Li‐Xin Zhang)