Development and Repair of Photosystem II Activity in Normal and Chloramphenicol-treated Euglena gracilis Cells

Photosystem II activity, development, and organization were studied in membranes from dark-adapted Euglena gracilis Klebs var. Z Pringsheim cells during a modulated greening process of light-dark-light cycles. The results obtained from measurements of overlapping partial photosystem II (PSII) reactions (fluorescence induction parameters, quantum yield, flash yield, and maximal rate of H₂O → 2,6-dichlorophenolindophenol [DCIP] and 1,5-diphenylcarbazide [DPC] → DCIP reactions) during these cycles indicate the formation of active PSII units in the dark. The necessity for proteins from the chloroplast translational machinery for this formation is evidenced by the inhibition of synthesis of the PSII units in chloramphenicol-treated cells. The effect of this drug, both during the dark and second light periods, can be summarized as follows: (a) disruption of the electron transfer connection to the plastoquinone pool, or decrease in the pool size; (b) loss of excitation energy transfer efficiency in the second light period; (c) impairment of the O₂ evolution appratus, as shown by comparison of the efficiency of DCP and H₂O as electron donors.

These conclusions are based on the use of a previously developed method of measurement and analysis of data (Cahen et al. 1976 Plant Physiol 58: 257-267).

     

The effect on photosynthetic electron transport of temperature-dependent changes in the fluidity of the thylakoid membrane in a thermophilic blue-green alga

Various electron transport reactions in cell or isolated thylakoid membranes of the thermophilic blue-green alga, Synechococcus sp. were measured at different temperatures between 72 and 3 degrees C. They are classified into two groups with respect to their temperature dependency. The first group involves cytochrome 553 photooxidation, methyl viologen photoreduction with reduced 2,6-dichlorophenolindophenol as electron donor and 3-(3',4'-dichlorophenyl)-1,1-dimethylurea-resistant ferricyanide photoreduction determined in the presence or absence of silicomolybdate. The Arrhenius plot of these reactions showed a single straight line with the activation energy of about 10 kcal/mol throughout wide temperature ranges studied. Methyl viologen photoreduction with water as electron donor, reduction of flash-oxidized cytochrome 553, ferricyanide photoreduction and photosynthetic O2 evolution form the second group. Their arrhenius plots are characterized by discontinuities or breaks at about 30 and 10 degrees C, which respectively correspond to the upper and lower boundaries of the lateral phase separation of the membrane lipids. The first group reactions represent short spans of electron transport which are mediated either by Photosystem I or Photosystem II alone and not related to plastoquinone, whereas all the reactions of the second group involve plastoquinone. It is concluded therefore that the membrane fluidity affect electron transport specifically at the region of plastoquinone. It is proposed that the reaction center chlorophyll-protein complexes of both Photosystems I and II are closely associated with related electron carrier proteins to form functional supramolecular assemblies so that electron transfer within such a cluster of proteins proceeds independently of the phase changes in the membrane lipids. On the other hand, the role of plastoquinone as a mobile electron carrier mediating electron transfer from the protein assembly of Photosystem II to that of Photosystem I through the fluid hydrophobic matrix of the membranes is highly sensitive to the physical state of the membrane lipids.     

Changes in Photosystem II Account for the Circadian Rhythm in Photosynthesis in Gonyaulax polyedra

Cell-free extracts that show activity in photosynthetic electron flow have been prepared from the unicellular dinoflagellate, Gonyaulax polyedra. Electron flow, as O₂ uptake, was measured through both photo-system I and II from water to methyl viologen, through photosystem I alone from reduced 2,6-dichlorophenol indophenol to methyl viologen which does not include the plastoquinone pool or from duroquinol to methyl viologen which includes the plastoquinone pool. Electron flow principally through photosystem II was measured from water to diaminodurene and ferricyanide, as O₂ evolution. Cultures of Gonyaulax were grown on a 12-hour light:12 hour dark cycle to late log phase, then transferred to constant light at the beginning of a light period. After 3 days, measurements of electron flow were made at the maximum and minimum of the photosynthetic rhythm, as determined from measurements of the rhythm of bioluminescence. Photosynthesis was also measured in whole cells, either as ¹⁴C fixation or O₂ evolution. Electron flow through both photosystems and through photosystem II alone were clearly rhythmic, while electron flow through photosystem I, including or excluding the plastoquinone pool, was constant with time in the circadian cycle. Thus, only changes in photosystem II account for the photosynthesis rhythm in Gonyaulax.     

Reconstitution of plastoquinone in the D1/D2/cytochrome b-559 photosystem II reaction centre complex

Reconstitution of plastoquinone in the photosystem II D1/D2/cytochrome b-559 reaction centre complex, in the presence of the detergent Triton X-100, is reported. Illumination of the reconstituted system results in the reduction of cytochrome b-559, the process being partly herbicide-sensitive. In addition, the reconstitution of plastoquinone results in the ability of the isolated reaction centre to catalyse the photoreduction of 2,6-dichlorophenolindophenol in the presence of the exogenous electron donor diphenylcarbazide.     

Temperature-dependent changes in Photosystem II heterogeneity support a cycle of Photosystem II during photoinhibition

High light treatments were given to attached leaves of pumpkin (Cucurbita pepo L.) at room temperature and at 1°C where the diffusion- and enzyme-dependent repair processes of Photosystem II are at a minimum. After treatments, electron transfer activities and fluorescence induction were measured from thylakoids isolated from the treated leaves. When the photoinhibition treatment was given at 1°C, the Photosystem II electron transfer assays that were designed to require electron transfer to the plastoquinone pool showed greater inhibition than electron transfer from H₂O to paraphenyl-benzoquinone, which measures all PS II centers. When the light treatment was given at room temperature, electron transfer from H₂O to paraphenyl-benzoquinone was inhibited more than whole-chain electron transfer. Variable fluorescence measured in the presence of ferricyanide decreased only during room-temperature treatments. These results suggest that reaction centers of one pool of Photosystem II, non-Q_B-PS II, replace photoinhibited reaction centers at room temperature, while no replacement occurs at 1°C. A simulation of photoinhibition at 1°C supports this conclusion.Abbreviations BSA bovine serum albumin - Chl chlorophyll - DCMU 3-(3,4,-dichlorophenyl)-1,1,-dimethylurea - DCPIP dichlorophenol-indophenol (2,6-dichloro-4((4-hydroxyphenyl)imino)-2,5-cyclohexadien-1-one) - DPC diphenyl carbazide (2,2-diphenylcarbonic dihydrazide) - FeCN ferricyanide (hexacyanoferrate(III)) - _app apparent quantum yield of photosynthetic oxygen evolution - MV methyl viologen (1,1-dimethyl-4,4-bipyridinium dichloride) - PPBQ phenyl-p-benzoquinone - PPFD photosynthetic photon flux density - PQ pool plastoquinone - Q_B secondary quinone acceptor of PS II - RT room temperature - WC whole chain electron transfer     

On the nature of the photosynthetic energy storage monitored by photoacoustic spectroscopy

The photosynthetic energy storage yield of uncoupled thylakoid membranes was monitored by photoacoustic spectroscopy at various measuring beam intensities. The energy storage rate as evaluated by the half-saturation measuring beam intensity (i₅₀) was inhibited by 3-(3,4-dichlorophenyl)-1,1 dimethylurea, by heat inactivation or by artificial electron acceptors specific for photosystem I or photosystem II; and was activated by electron donors to photosystem I. The reactions involving both photosystems were all characterized by a similar maximal energy storage yield of 16±2 percent. The data could be interpreted if we assumed that the energy storage elicited by the photosystems at 35 Hz is detected at the level of the plastoquinone pool.Abbreviations PS photosystem - Tes N-Tris [hydroxymethl] methyl-2-aminoethanesulfonic acid - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DCIP 2,6-dichlorophenolindophenol - FeCN potassium ferricyanide - DCBQ 2,5-dichlorobenzoquinone - TMPD N,N,N-tetramethyl-p-phenilenediamine     

Shikonin isovalerate: An inhibitor of photosystem II in Chlorogloeopsis fritschi

Shikonin isovalerate, extracted from the roots of the desert plant Arnebia decumbens, was tested for its effect on photosynthetic electron transport system of Chlorogloeopsis fritschii. The ferricyanide-Hill reaction with water and DPC as electron donors was inhibited completely with 10^-5 M shikonin isovalerate. The photoreduction of DCPIP through photosystem II was only slightly inhibited. Photosystem I from durohydroquinone to methyl viologen was not affected using 10^-6 M shikonin isovalerate. The same concentration caused 49% inhibition of cyclic photophosphorylation. These results suggest that shikonin isovalerate inhibits photosynthetic electron flow at the plastoquinone pool.Abbreviations DCMU 3-(3,4-dichlorophenyl)-N,N-dimethyl urea - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-P-benzoquinone - DCPIP 2–6-dichlorophenolindophenol - DPC Diphenylcarbazide - Tricine N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine     

Effect of photosystem II reaction center closure on nanosecond fluorescence relaxation kinetics

The fluorescence decay of chlorophyll in spinach thylakoids was measured as a function of the degree of closure of Photosystem II reaction centers, which was set for the flowed sample by varying either the preillumination by actinic light or the exposure of the sample to the exciting pulsed laser light. Three exponential kinetic components originating in Photosystem II were fitted to the decays; a fourth component arising from Photosystem I was determined to be negligible at the emission wavelength of 685 nm at which the fluorescence decays were measured. Both the lifetimes and the amplitudes of the components vary with reaction center closure. A fast (170–330 ps) component reflects the trapping kinetics of open Photosystem II reaction centers capable of reducing the plastoquinone pool; its amplitude decreases gradually with trap closure, which is incompatible with the concept of photosynthetic unit connectivity where excitation energy which encounters a closed trap can find a different, possibly open one. For a connected system, the amplitude of the fast fluorescence component is expected to remain constant. The slow component (1.7–3.0 ns) is virtually absent when the reaction centers are open, and its growth is attributable to the appearance of closed centers. The middle component (0.4–1.7 ns) with approximately constant amplitude may originate from centers that are not functionally linked to the plastoquinone pool. To explain the continuous increase in the lifetimes of all three components upon reaction center closure, we propose that the transmembrane electric field generated by photosynthetic turnover modulates the trapping kinetics in Photosystem II and thereby affects the excited state lifetime in the antenna in the trap-limited case.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - HEPES 4-(2-hydroxyethyl)-1-piperazineethane sulfonic acid - PQ plastoquinone - PSI and PSII Photosystem I and II - Q_A and Q_B primary and secondary quinone acceptor of PSII     

Development of the photosynthetic apparatus during light-dependent greening of a mutant of Chlorella fusca

The formation of chlorophyll, cytochrome f, P-700, ribulose bisphosphate carboxylase as well as photosynthesis and Hill reaction activities were tested during the light-dependent greening process of the Chlorella fusca mutant G 10. Neither chlorophyll nor protochlorophyllide was detected in the darkgrown cells. When transferred to light the mutant cells developed chlorophyll and established its photosynthetic capacity after a short lag phase. In the in vivo absorption spectra a spectral shift of the red absorption peak position from 674 to 680 nm was indicated during the first 3 h of greening. Cytochrome f was already present in the dark-grown cells, but during the greening phase a threefold increase in the cytochrome f content could be seen. At the early stages of greening a characteristic primary oscillation in the content of cytochrome f was observed. P-700 was lacking in the dark and during the first 30 min of illumination. From the first to the second h of light a forced synthesis of P-700 took place and the time-course curve for the ratios of P-700/chlorophyll rose to a sharp maximum. The synthesis of P-700 started together with photosystem I activity and showed similar kinetics. We found the simultaneous appearance of photosystem II, photosystem I, and photosynthetic activities 30 min after the beginning of the illumination. Based on chlorophyll content they attained maximum activity after 2 h of light, but at this time photosystem I capacity proved to be remarkably higher than photosynthetic and photosystem II activities. Highest carboxylase activity existed in darkgrown cells. During the greening process the activity of the enzyme decreased continuously. After 2 h of illumination chlorophyll synthesis partially served to increase the size of the photosynthetic unit, which consequently led to a decrease in the light energy needed to saturate photosynthesis and also to a decrease of photosynthetic rate based on chlorophyll content.Abbreviations Chl chlorophyll - Cyt f cytochrome f - DPIP 2,6-dichlorophenolindophenol - EDTA ethylenediaminetetraacetic acid - GSH glutathione - LH light-harvesting - PS photosystem - RuBP ribulose bisphosphate     

Chlorophyll a fluorescence transient as an indicator of active and inactive Photosystem II in thylakoid membranes

Upon illumination, a dark-adapted photosynthetic sample shows time-dependent changes in chlorophyll (Chl) a fluorescence yield, known as the Kautsky phenomenon or the OIDPS transient. Based on the differential effects of electron acceptors such as 2,5-dimethyl-p-benzoquinone (DMQ) and 2,6-dichloro-p-benzoquinone (DCBQ) on Chl a fluorescence transients of spinach thylakoids, we suggest that the OID phase reflects the reduction of the electron acceptor QA to QA- in the inactive PS II (see Graan, T. and Ort, D. (1986) Diochim. Biophys. Acta 852, 320-330). In spinach thylakoids, heat-induced increase of the Chl a fluorescence yield is also differentially sensitive to the addition of DMQ and DCBQ suggesting that this increase is mainly on the 'I' level, and thus heating is suggested to convert active PS II to inactive PS II centers. The kinetics of decay of QA-, calculated from variable Chl a fluorescence, was analyzed into three exponential components (365-395 microseconds; 6-7 ms; and 1.4-1.7 s). In heated samples, the decay rate of variable Chl a fluorescence is slower than the normal back-reaction rate; there is a preponderance of the slow component that may be due, partly, to the active centers undergoing slow back reaction between QA- and the S2 state of the oxygen-evolving complex.     

Cobalt induced changes in photosystem activity in Synechocystis PCC 6803: Alterations in energy distribution and stoichiometry

Adaptive responses to excess (supraoptimal) level of cobalt supplied to the growth medium were studied in the cyanobacterium Synechocystis PCC 6803. Growth of cells in the medium containing 10 M CoCl₂ led to a large stimulation (50%) in O₂-evolution and an overall increase (30%) in the photosynthetic electron transport rates. Analysis of variable Chl a fluorescence yield of PS II and immuno-detection of Photosystem II (PS II) reaction-center protein D1, showed a small increase (15–20%) in the number of PS II units in cobalt-grown cells. Cobalt-grown cells, therefore, had a slightly elevated PS II/PS I ratio compared to control.We observed alteration in the extent of energy distribution between the two photosystems in the eobalt grown cells. Energy was preferentially distributed in favour of PS II accompanied by a reduction in the extent of energy transfer from PS II to PS I in cobalt-grown cells. These cells also showed a smaller PS I absorption cross-section and a smaller size of intersystem electron pool than the control cells. Thus, our results suggest that supplementation of 10 M CoCl₂, to the normal growth medium causes multiple changes involving small increase in PS II to PS I ratio, enhanced funneling of energy to PS II and an increase in PS I electron transport, decrease PS I cross section and reduction in intersystem pool size. The cumulative effects of these alterations cause stimulation in electron transport and O₂ evolution.Abbreviations BCIP 5-bromo-4-chloro-3-indolylphosphate - Chl a Chlorophyll a - Cyt blf Cytochrome blf - DCBQ 2,6-dichlorobenzoquinone - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl urea - DCPIP 2,6-dichlorophenol indophenol - DPC Diphenyl carbazide - F_o fluorescence when all reaction centers are open - F_M fluorescence yield when all reaction centers are closed - F_v variable chlorophyll fluorescence - HEPES N-2-hydroxyethyl piperazine-N'-2-ethanesulphonic acid - MV methyl viologen - NBT nitro-blue tetrazolium - pBQ para-benzoquinone - PB somes phycobilisomes - PC Phycocyanin - PQ plastoquinone - PS I Photosystem I - PS II Photosystem II - P700 reaction center Chl a of PS 1 - ST-and MT-flash single turnover and multiple turnover flash     

Effects of UV-B radiation on Chl fluorescence of greening barley (Hordeum vulgare L.) seedling

Effects of UV-B radiation on the developing chloroplast of barley (Hordeum vulgare L.) seedling during greening were determined by Chl contents, Fo, Fv and fluorescence quenching coefficients. In greening of etiolated barley seedling, the value of Fo was greatly increased after the initiation of greening. However Fv and Fv/Fo were gradually increased. In greening with the additional irradiation of UV-B radiation, the value of Fo was strikingly decreased than that of the control after the initiation of greening, but Fv was gradually decreased from than that of the control during the greening period. These results suggest that the function of light-harvesting Chl are more sensitive than photosynthetic electron transport system by UV-B. Chl contents, Fv/Fo, qP and qNP, were decreased from than that of the control during the 72 h greening, especially, qR was strikingly decreased, but qE was slightly decreased by UV-B. These suggest that the sites of inhibition by UV-B are PSII and all sites of photosynthetic electron transport system. But PQ pool seems to be slightly inhibited by UV-B.     

Functional sits of plastoquinone in photosynthetic electron transport system

Mild extraction of lyophilized chloroplasts with hexane eliminatedHill activity with 2,6-dichlorophenolindophenol (DCIP) as anelectron acceptor, and most of the activity was restored byreconstitution with plastoquinone A. The same extraction didnot affect the activity of Photosystem II, determined by thephotoreduction of DCIP supported with an artificial electrondoneor, 1,5-diphenylcarbazied. The fluorescence yield changesof extracted chloroplasts indicated that the electron transportchain between Photosystems I and II was also blocked. The resultssuggest that plastoquinone functions at both sides of PhotosystemII; at the reductive side it acts as an electron carrier, andat the oxidative side as a structural element of the thylakoidmembrance necessary for a component to be active in the oxygen-evolutionsystem. (Received August 22, 1973; )     

Variable thermal dissipation in a Photosystem I submembrane fraction

Photoacoustic spectroscopy was used to study the thermal deactivation processes in a Photosystem I submembrane fraction isolated from spinach. A large part of the thermal dissipation was variable. The yield of this variable thermal emission depended on the redox state of the Photosystem. It increased with the measuring modulated light intensity coinciding with the gradual closure of the reaction centers. Thermal deactivation was maximal when the reaction centers were closed by a saturating illumination. Extrapolation of the data at zero light intensity indicated that the yield of non-variable thermal emission represented about 37% of the maximal emission. The presence of methylviologen as artificial electron acceptor decreased the yield of variable thermal emission whereas inhibition following heat stress treatments increased it. The significance of the variable and non-variable components of thermal dissipation is discussed and the measured energy storage is suggested to originate from the reduction of the plastoquinone pool during cyclic electron transport around Photosystem I.Abbreviations Chl chlorophyll - DCIP 2,6-dichlorophenolindophenol - MV methylviologen - Pheo pheophytin - PA photoacoustic - PS I Photosystem I - PS II Photosystem II - Tes [N-tris (hydroxymethyl)] methyl-2-aminoethanesulfonic acid     

Time-dependent changes in the in vitro effects of sodium chloride on photosynthetic electron transport of isolated chloroplasts

The effects of a wide concentration range of NaCl and sorbitol on three photosynthetic electron transport reactions of Pisum sativum L. cv. Feltham First chloroplasts were examined as a function of time from thylakoid membrane isolation. Rates of electron flow from water to diaminodurene (DAD) and ferricyanide were determined polarographically, whilst photoreduction of 2,6-dichlorophenolindophenol (DCPIP) was monitored spectrophotometrically. Assay of thylakoids immediately after isolation showed that the rate of photoreduction of all three electron acceptors decreased with increasing salt concentration. However, 100 min after leaf homogenisation the response pattern of ferricyanide and DCPIP photoreduction to increasing NaCl, but not increasing sorbitol concentration, became significantly modified. This was not the case for DAD photoreduction. The results are discussed in relation to the assessment of the possible effect of salinity on photosynthetic electron transport in vivo.     

Interaction of Zn2+ with the donor side of Photosystem II

The inhibitory effect of Zn²⁺ on photosynthetic electron transport was investigated in native and CaCl₂-treated (depleted in extrinsic polypeptides) Photosystem II (PS II) submembrane preparations. Inhibition of 2,6-dichlorophenolindophenol photoreduction by Zn²⁺ was much stronger in protein-depleted preparations in comparison to the native form. It was found that Ca²⁺ significantly reduced the inhibition in the native PS II preparations, as did Mn²⁺ in a combination with H₂O₂ in the protein-depleted counterparts. No other tested monovalent or divalent cations could replace Ca²⁺ or Mn²⁺ in the respective experiments. Diphenylcarbazide could partially relieve (40–45%) the inhibition in both types of preparations. The above indicates the presence of an active Zn²⁺ inhibitory site on the donor side of PS II. However, neither Ca²⁺ nor Mn²⁺ could completely prevent inhibition by high concentrations of Zn²⁺ (>1 mM). We propose that elevated levels of Zn²⁺ strongly perturb the conformation of the PS II core complex and might also affect the acceptor side of the photosystem.Abbreviations PMSF phenylmethanesulfonyl fluoride - MES 2-(N-morpholino)ethane sulphonic acid - Chl chlorophyll - PS II Photosystem II - DCIP 2,6-dichlorophenolindophenol - DPC sym-diphenylcabazide - DCBQ 2,5-dichlorobenzoquinone     

Genetic variation in soybean photosynthetic electron transport capacity is related to plastocyanin concentration in the chloroplast

Fifteen ancestral genotypes of United States soybean cultivars were screened for differences in photosynthetic electron transport capacity using isolated thylakoid membranes. Plants were grown in controlled environment chambers under high or low irradiance conditions. Thylakoid membranes were isolated from mature leaves. Photosynthetic electron transport was assayed as uncoupled Hill activity using 2,6-dichlorophenolindophenol (DCIP). Soybean electron transport activity was dependent on genotype and growth irradiance and ranged from 6 to 91 mmol DCIP reduced [mol chlorophyll]^–1 s^–1. Soybean plastocyanin pool size ranged from 0.1 to 1.3 mol plastocyanin [mol Photosystem I]^–1. In contrast, barley and spinach electron transport activities were 140 and 170 mmol DCIP reduced [mol chlorophyll]^–1 s^–1, respectively, with plastocyanin pool sizes of 3 to 4 mol plastocyanin [mol Photosystem I]^–1. No significant differences in the concentrations of Photosystem II, plastoquinone, cytochrome b₆f complexes, or Photosystem I were observed. Thus, genetic differences in electron transport activity were correlated with plastocyanin pool size. The results suggested that plastocyanin pool size can vary significantly and may limit photosynthetic electron transport capacity in certain species such as soybean. Soybean plastocyanin consisted of two isoforms with apparent molecular masses of 14 and 11 kDa, whereas barley and spinach plastocyanins each consisted of single polypeptides of 8 and 12 kDa, respectively.Abbreviations DAP days after planting - DCIP 2,6-dichlorophenolindophenol - LiDS lithium dodecyl sulfate - PPFD photosynthetic photon flux density (mol photons m^–2 s^–1) - PS I Photosystem I - PS II Photosystem II - P700 reaction center of Photosystem I The US Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

Response of senescing rice leaves to flooding stress

Flooding stress (FS) induced changes in pigment and protein contents and in photochemical efficiency of thylakoid membranes of chloroplasts were investigated during senescence of primary leaves of rice seedlings. Leaf senescence was accompanied by loss in 2,6-dichlorophenolindophenol (DCPIP) photoreduction, rate of oxygen evolution, quantum yield of photosystem 2 with an increase in MDA accumulation, and non-photochemical quenching (NPQ) of chlorophyll fluorescence. These changes were further aggravated when the leaves during this period experienced FS. The increase in NPQ value under stress may indicate photosynthetic adaptation to FS.     

Interaction of exogenous quinones with membranes of higher plant chloroplasts: modulation of quinone capacities as photochemical and non-photochemical quenchers of energy in Photosystem II during light-dark transitions

Light modulation of the ability of three artificial quinones, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), 2,6-dichloro-p-benzoquinone (DCBQ), and tetramethyl-p-benzoquinone (duroquinone), to quench chlorophyll (Chl) fluorescence photochemically or non-photochemically was studied to simulate the functions of endogenous plastoquinones during the thermal phase of fast Chl fluorescence induction kinetics. DBMIB was found to suppress by severalfold the basal level of Chl fluorescence (F_o) and to markedly retard the light-induced rise of variable fluorescence (F_v). After irradiation with actinic light, Chl fluorescence rapidly dropped down to the level corresponding to F_o level in untreated thylakoids and then slowly declined to the initial level. DBMIB was found to be an efficient photochemical quencher of energy in Photosystem II (PSII) in the dark, but not after prolonged irradiation. Those events were owing to DBMIB reduction under light and its oxidation in the dark. At high concentrations, DCBQ exhibited quenching behaviours similar to those of DBMIB. In contrast, duroquinone demonstrated the ability to quench F_v at low concentration, while F_o was declined only at high concentrations of this artificial quinone. Unlike for DBMIB and DCBQ, quenched F_o level was attained rapidly after actinic light had been turned off in the presence of high duroquinone concentrations. That finding evidenced that the capacity of duroquinone to non-photochemically quench excitation energy in PSII was maintained during irradiation, which is likely owing to the rapid electron transfer from duroquinol to Photosystem I (PSI). It was suggested that DBMIB and DCBQ at high concentration, on the one hand, and duroquinone, on the other hand, mimic the properties of plastoquinones as photochemical and non-photochemical quenchers of energy in PSII under different conditions. The first model corresponds to the conditions under which the plastoquinone pool can be largely reduced (weak electron release from PSII to PSI compared to PSII-driven electron flow from water under strong light and weak PSI photochemical capacity because of inactive electron transport on its reducing side), while the second one mimics the behaviour of the plastoquinone pool when it cannot be filled up with electrons (weak or moderate light and high photochemical competence of PSI).     

Differential inhibition by plastoquinone analogues of photoreduction of cytochrome b-559 in chloroplasts

The high-potential form of cytochrome b-559 (b-559 HP) is closely linked to the oxygenic photosystem (photosystem II) but its relation to other redox components of the photosynthetic apparatus, including plastoquinone, is still obscure. We investigated the photoreduction of cytochrome b-559 HP by isolated chloroplasts in the presence of 3 antagonists of plastoquinone, of which, DBMIB (dibromothymoquinone) and DNP-INT (dinitrophenyl ether of iodonitrothymol) are known to inhibit the oxidation of the plastoquinone pool (PQ) by the FeS-cytochrome ƒ/b₆ complex and one, UHDBT (5-n-undecyl-6-hydroxy-4,7-dioxobenzothiazole) is known to inhibit the reduction of PQ by Q_B.Q_B is a protein-bound plastoquinone that serves as a two-electron gate for the reduction of PQ. We found that DBMIB and DNP-INT did not inhibit but low concentrations of UHDBT severely inhibited the photoreduction of cytochrome b-559 HP. These results suggest that the electron donor for the reduction of cytochrome b-559 HP was either Q_B or a portion of the PQ pool that was oxidized by a new pathway free of binding sites for DBMIB and DNP-INT.