Analysis of the slow phases of the in vivo chlorophyll fluorescence induction curve. Changes in the redox state of Photosystem II electron acceptors and fluorescence emission from Photosystems I and II

An analysis of the photo-induced decline in the in vivo chlorophyll a fluorescence emission (Kautsky phenomenon) from the bean leaf is presented. The redox state of PS II electron acceptors and the fluorescence emission from PS I and PS II were monitored during quenching of fluorescence from the maximum level at P to the steady state level at T. Simultaneous measurement of the kinetics of fluorescence emission associated with PS I and PS II indicated that the ratio of PS I/PS II emission changed in an antiparallel fashion to PS II emission throughout the induction curve. Estimation of the redox state of PS II electron acceptors at given points during P to T quenching was made by exposing the leaf to additional excitation irradiation and determining the amount of variable PS II fluorescence generated. An inverse relationship was found between the proportion of PS II electron acceptors in the oxidised state and PS II fluorescence emission. The interrelationships between the redox state of PS II electron acceptors and fluorescence emission from PS I and PS II remained similar when the shape of the induction curve from P to T was modified by increasing the excitation photon flux density. The contributions of photochemical and non-photochemical quenching to the in vivo fluorescence decline from P to T are discussed.     

Changes in the distribution of light energy between the two photosystems in spinach leaves

Time courses of chlorophyll fluorescence at room temperature and fluorescence spectra at 77 K were measured to investigate the light-induced changes in the distribution of light energy between the two photosy stems in young spinach leaves. Illumination of the dark adapted leaves with primarily system II light induced typical fluorescence transients at room temperature. Fluorescence spectra at 77 K showed that the intensity of system II fluorescence at 77 K changed nearly in parallel with the fluorescence transients at room temperature within the range from M₁ to T during illumination of the leaf. Illumination of the dark adapted leaves with light I produced an increase of system II fluorescence measured at 77 K. The characteristics of the changes induced by light I or II were different, showing that these two effects are related to different mechanisms. These results suggest that the dark state in spinach leaves is state II, that light I induces a state II to I transition, while light II induces fluorescence changes that are produced by mechanisms other than state I-state II transitions.     

Release and aggregation of the light-harvesting complex in intact leaves subjected to strong CO2 deficit

Tobacco plants were subjected to long-term CO₂ deficit. The stress caused photoinhibition of Photosystem (PS) II photochemistry and the aggregation of the light-harvesting complex of PS II (LHC II). The aggregation was shown by the appearance of the characteristic band at 698–700 nm (F₆₉₉) in 77 K fluorescence emission spectra. LHC II aggregates are considered to quench fluorescence and, therefore, the fluorescence yield was determined to verify their quenching capability. PS II photochemistry, measured as F_V/F_M, was largely depressed during first 4 days of the stress. Unexpectedly, the total fluorescence yield increased in this period. Fitting of emission spectra by Gaussian components approximating emission bands of LHC II, PS II core, PS I and F₆₉₉ revealed that mainly the bands at 680 and 699 nm, representing emission of LHC II aggregates, were responsible for the increase of the fluorescence yield. This shows an interruption of the excitation energy transfer between LHC II and both photosystems and, thus, a physical disconnection of LHC II from photosystems. PS II and PS I emissions were not quenched in this period. Therefore, it was concluded that these LHC II aggregates were accumulated out of PS II antenna, and, thus they cannot be involved in dumping of excess excitation. The total fluorescence yield turned to decrease only after the large depression of PS II photochemistry, when LHC II aggregation was considerably speeded up and the fluorescence yields of PS I and II turned to decline.     

Evidence for the contribution of the S-state transitions of oxygen evolution to the initial phase of fluorescence induction

Fluorescence induction of isolated spinach chloroplasts was measured by using weak continuous light. It is found that the kinetics of the initial phase of fluorescence induction as well as the initial fluorescence level F_j are influenced by the number of preilluminating flashes, and shows damped period 4 oscillation. Evidence is given to show that it is correlated with the S-state transitions of oxygen evolution. Based on the previous observations that the S states can modulate the fluorescence yield of Photosystem II, a simulating calculation suggests that, in addition to the Photosystem II centers inactive in the plastoquinone reduction, the S-state transitions can also make a contribution to the intial phase of fluorescence induction.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl urea - F₀ non-variable fluorescence level emitted when all PS II centers are open - F_i initial fluorescence level immediately after shutter open - F_pt intermediate plateau fluorescence level - F_m maximum fluorescence level emitted when all PS II centers are closed - PS II Photosystem II - Q_A primary quinone acceptor of PS II - Q_B secondary quinone acceptor of PS II     

Delayed fluorescence observed in the nanosecond time region at 77 K originates directly from the photosystem II reaction center

The excited-state dynamics of delayed fluorescence in photosystem (PS) II at 77 K were studied by time-resolved fluorescence spectroscopy and decay analysis on three samples with different antenna sizes: PS II particles and the PS II reaction center from spinach, and the PS II core complexes from Synechocystis sp. PCC 6803. Delayed fluorescence in the nanosecond time region originated from the 683-nm component in all three samples, even though a slight variation in lifetimes was detected from 15 to 25 ns. The relative amplitude of the delayed fluorescence was higher when the antenna size was smaller. Energy transfer from the 683-nm pigment responsible for delayed fluorescence to antenna pigment(s) at a lower energy level was not observed in any of the samples examined. This indicated that the excited state generated by charge recombination was not shared with antenna pigments under the low-temperature condition, and that delayed fluorescence originates directly from the PS II reaction center, either from chlorophyll a(D1) or P680. Supplemental data on delayed fluorescence from spinach PS I complexes are included.     

Photophysics of metalloazurins

The fluorescence lifetimes of Cu(II), Cu(I), Ag(I), Hg(II), Co(II), and Ni(II) azurin Pae from Pseudomonas aeruginosa and Cu(II), Cu(I), and Hg(II) azurin Afe from Alcaligenes faecalis were measured at 295 K by time-correlated single-photon counting. In addition, fluorescence lifetimes of Cu(II) azurin Pae were measured between 30 and 160 K and showed little change in value. Ultraviolet absorption difference spectra between metalloazurin Pae and apoazurin Pae were measured, as were the fluorescence spectra of metalloazurins. These spectra were used to determine the spectral overlap integral required for dipole-dipole resonance calculations. All metalloazurins exhibit a reduced fluorescence lifetime compared to their respective apoazurins. Forster electronic energy transfer rates were calculated for both metalloazurin Pae and metalloazurin Afe derivatives; both enzymes contain a single tryptophyl residue which is located in a different position in the two azurins. These azurins have markedly different fluorescence spectra, and electronic energy transfers occur from these two tryptophyl sites with different distances and orientations and spectral overlap integral values. Intramolecular distances and orientations were derived from an X-ray crystallographic structure and a molecular dynamic simulation of the homologous azurin Ade from Alcaligenes denitrificans, which contains both tryptophyl sites. Assignments were made of metal-ligand-field electronic transitions and of transition dipole moments and directions for tryptophyl residues, which accounted for the observed fluorescence quenching of Hg(II), Co(II), and Ni(II) azurin Pae and Cu(II) and Hg(II) azurin Afe. The fluorescence of azurin Pae is assigned as a 1Lb electronic transition, while that of azurin Afe is 1La. The marked fluorescence quenching of Cu(II) azurin Pae and Cu(I) azurin Pae and Afe is less well reproduced by our calculations, and long-range oxidative and reductive electron transfer, respectively, are proposed as additional quenching mechanisms. This study illustrates the application of Forster electronic energy transfer calculations to intramolecular transfers in structurally well characterized molecular systems and demonstrates its ability to predict observed fluorescence quenching rates when the necessary extensive structural, electronic transition assignment, and spectroscopic data are available. The agreement between Forster calculations and quenching rates derived from fluorescence lifetime measurements suggests there are limited changes in conformation between crystal structure and solution structures, with the exception of the tryptophyl residue of azurin Afe, where a conformation derived from a molecular simulation in water was necessary rather than that found in the crystal structure.     

Regulation of cyanobacterial photosynthesis determined from variable fluorescence yields of photosystem II

Abstract The cyanobacteria Fremyella diplosiphon 7601 and Synechocystis 6701 were grown in continuous cultures with monochromatic red light (680 nm). The distribution of light energy over photosystem I and II was determined from changes in PS II fluorescence at 685 nm. In both organisms, wavelengths absorbed primarily by chlorophyll a caused the high fluorescent state of PS II (State 1), while wavelengths absorbed by the phycobilisome led to low PS II fluorescence (State 2). Superimposing continuous light 2 on the excitation light yielded State 2 fluorescence patterns for Synechocystis 6701, while F. diplosiphon 7601 showed fluorescence patterns similar to state 1 → 2 transitions and changes in fluorescence yield were related to the intensity of the background light. Some ecological implications of energy (re)distribution in cyanobacterial photosynthesis are discussed.     

Fluorescence spectroscopic analysis of calpain II interactions with calcium and calmodulin antagonists

1. The intrinsic fluorescence of epoxysuccinyl-inhibited calpain II undergoes a Ca2(+)-dependent decrease which contrasts with the increase observed for calmodulin. 2. Calpain II was inhibited by the calmodulin antagonist toluidinylnaphthalenesulfonate (TNS), and a Ca2(+)-dependent increase in TNS fluorescence intensity was observed for epoxysuccinyl-inhibited calpain II. 3. The calmodulin antagonists calmidazolium CDZ and felodipine both caused decreases in the intrinsic fluorescence of epoxysuccinyl-inhibited calpain II. 4. Increasing concentrations of Ca2+ caused an increase in the fluorescence intensity of the inhibited enzyme in the presence of (CDZ), and a decrease in the presence of felodipine. 5. It is concluded from these studies that Ca2+ and calmodulin antagonists induce conformational changes in calpain II, and that changes occur in regions other than the Ca2(+)-binding domains.     

On the relationship between the non-photochemical quenching of the chlorophyll fluorescence and the Photosystem II light harvesting efficiency. A repetitive flash fluorescence induction study

Plants respond to excess light by a photoprotective reduction of the light harvesting efficiency. The notion that the non-photochemical quenching of chlorophyll fluorescence can be reliably used as an indicator of the photoprotection is put to a test here. The technique of the repetitive flash fluorescence induction is employed to measure in parallel the non-photochemical quenching of the maximum fluorescence and the functional cross-section (sigma(PS II)) which is a product of the photosystem II optical cross-section a(PS II) and of its photochemical yield Phi(PS II) (sigma (PS II) = a(PS II) Phi(PS II)). The quenching is measured for both, the maximum fluorescence found in a single-turnover flash (F(M) (ST)) and in a multiple turnover light pulse (F(M) (MT)). The experiment with the diatom Phaeodactylum tricornutum confirmed that, in line with the prevalent model, the PS II functional cross-section sigma (PS II) is reduced in high light and restored in the dark with kinetics and amplitude that are closely matching the changes of the F(M) (ST) and F(M) (MT) quenching. In contrast, a poor correlation between the light-induced changes in the PS II functional cross-section sigma (PS II) and the quenching of the multiple-turnover F(M) (MT) fluorescence was found in the green alga Scenedesmus quadricauda. The non-photochemical quenching in Scenedesmus quadricauda was further investigated using series of single-turnover flashes given with different frequencies. Several mechanisms that modulate the fluorescence emission in parallel to the Q(A) redox state and to the membrane energization were resolved and classified in relation to the light harvesting capacity of Photosystem II.     

Synthesis of Zn(II)-cloxacillin sodium complex and study of its interaction with calf thymus DNA

In this paper, a solid complex of cloxacillin sodium (CS) with Zn(II) was prepared by coprecipitation and characterized by UV, fluorescence, IR, and thermal spectra. Furthermore, the nature of the complex has been studied by ¹H-NMR and ¹³C-NMR spectroscopy. The influence of Zn(II) on the combination of CS and calf thymus DNA (CT DNA) was studied using fluorescence spectrophotometry, and the formation of binary CS-Zn(II) and CS-CT DNA complexes and ternary CS-Zn(II)-CT DNA complex was studied. The results show that the fluorescence intensity of CS can be quenched in the presence of Zn(II) or DNA. In the presence of Zn(II), the fluorescence quenching action of DNA on CS was strongly enhanced. Based on the fluorescence intensity, the formation constants of CS-Zn(II) and CS-CT DNA complexes were calculated, and the mechanism of interaction between CS, Zn(II), and DNA is discussed. Published in Russian in Biokhimiya, 2007, Vol. 72, No. 2, pp. 184–193.     

Delayed fluorescence observed in the nanosecond time region at 77 K originates directly from the photosystem II reaction center

The excited-state dynamics of delayed fluorescence in photosystem (PS) II at 77 K were studied by time-resolved fluorescence spectroscopy and decay analysis on three samples with different antenna sizes: PS II particles and the PS II reaction center from spinach, and the PS II core complexes from Synechocystis sp. PCC 6803. Delayed fluorescence in the nanosecond time region originated from the 683-nm component in all three samples, even though a slight variation in lifetimes was detected from 15 to 25 ns. The relative amplitude of the delayed fluorescence was higher when the antenna size was smaller. Energy transfer from the 683-nm pigment responsible for delayed fluorescence to antenna pigment(s) at a lower energy level was not observed in any of the samples examined. This indicated that the excited state generated by charge recombination was not shared with antenna pigments under the low-temperature condition, and that delayed fluorescence originates directly from the PS II reaction center, either from chlorophyll a_D1 or P680. Supplemental data on delayed fluorescence from spinach PS I complexes are included.     

A kinetic study of the reconstitution of azurin from Cu(II) and the apoprotein.

The kinetics of reconstitution of Pseudomonas aeruginosa azurin from its apoprotein and copper(II) salts have been studied using absorbance at 625 nm and fluorescence emission at 308 nm as monitors of the process. At low Cu(II) concentrations the rates of both absorbance and fluorescence changes are linearly dependent on Cu(II) concentration. At higher Cu(II) concentrations the rate of absorbance change is independent of Cu(II) concentration. The rates of both absorbance and fluorescence changes as a function of pH suggest that the titration of a single ionizable group is important for the Cu(II)-dependent reaction. Overall analysis of the kinetics suggests that the fluorescence change and the absorbance change are associated with at least two steps in the overall pathway of the formation of the metal-protein complex, and that the copper(II) and tryptophan environments in this protein, though perhaps spatially close, may be distinct.     

Energy transfer and quantum yield in Photosystem II

The photoreduction and dark reoxidation of Q_α and Q_β, the primary electron acceptors of Photosystems (PS) IIα and IIβ, respectively, in the presence of 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea (DCMU) were studied in tobacco chloroplasts by means of fluorescence and absorbance measurements. The magnitude of a correction for an absorbance change by the oxidizing side of PS II needed in our previous study of the quantum yield of Q reduction (Biochim. Biophys. Acta 635 (1981), 111–120) has been determined. The absorbance change occurs in PS IIα mainly. The maximum fluorescence yield was found to be the same as in the mutant Su/su, which has a 3-fold higher reaction center concentration and a lower PS IIα to PS IIβ ratio. The kinetics of the light-induced fluorescence increase were measured after various pretreatments and the corresponding kinetics of the integrated fluorescence deficit were analyzed into their α and β components. From the results the contribution to the minimum fluorescence level, the degree of energy transfer between units, and the quantum efficiency of Q reduction were calculated for both types of PS II. This led to the following conclusions. The absence of energy between PS IIβ antennae is confirmed. Fluorescence quenching in PS IIα was adequately described by the matrix model, except for a decrease in the energy transfer between units during photoreduction of Q_α, possibly due to the formation of ‘islets’ of closed centers. PS II reaction centers in which Q is reduced do not significantly quench fluorescence. The ratio of variable to maximum fluorescence, 0.77 in PS IIα and 0.92 in PS IIβ, multiplied by the fraction of Q remaining in the reduced state after one saturating flash, 0.88 in PS IIα and greater than 0.95 in PS IIβ, leads to a net quantum efficiency of Q reduction in the presence of DCMU and NH₂OH of 0.68 in PS IIα and about 0.90 in PS IIβ. These values are in good agreement with the measured overall quantum efficiency of Q reduction.     

Assignment of the low-temperature fluorescence in oxygen-evolving Photosystem II

Low-temperature absorption and fluorescence spectra of fully active cores and membrane-bound PS II preparations are compared. Detailed temperature dependence of fluorescence spectra between 5 and 70 K are presented as well as 1.7-K fluorescence line-narrowed (FLN) spectra of cores, confirming that PS II emission is composite. Spectra are compared to those reported for LHCII, CP43, CP47 and D1/D2/cytit b₅₅₉ subunits of PS II. A combination of subunit spectra cannot account for emission of active PS II. The complex temperature dependence of PS II fluorescence is interpretable by noting that excitation transfer from CP43 and CP47 to the reaction centre is slow, and strongly dependent on the precise energy at which a ‘slow-transfer’ pigment in CP43 or CP47 is located within its inhomogeneous distribution. PS II fluorescence arises from CP43 and CP47 ‘slow-transfer’ states, convolved by this dependence. At higher temperatures, thermally activated excitation transfer to the PS II charge-separating system bypasses such bottlenecks. As the charge-separating state of active PS II absorbs at >700 nm, PS II emission in the 680–700 nm region is unlikely to arise from reaction centre pigments. PS II emission at physiological temperatures is discussed in terms of these results.     

Characterization of the internal calcium(II) binding sites in dissolved insulin hexamer using europium(III) fluorescence

The fluorescence of Eu(III) is used to study the nature of the Ca(II) binding sites in the central cavity of the two-zinc(II) insulin hexamer. The dependence of the Eu(III) fluorescence lifetime upon Eu(III) stoichiometry indicates that there are three identical Eu(III) binding sites present in the two-zinc(II) insulin hexamer in solution. Addition of excess Ca(II) causes a decrease in the Eu(III) fluorescence intensity, confirming that Ca(II) competes for the observed Eu(III) sites. The solvent dependence of the Eu(III) fluorescence lifetime (H2O vs. D2O) indicates that four OH groups are coordinated to each Eu(III) in the hexamer. Substitution of Co(II) for Zn(II) causes a decrease in the Eu(III) fluorescence lifetime. Calculations based on F?rster energy-transfer theory predict that the Co(II) [or Zn(II) in vivo] and Eu(III) [or Ca(II) in vivo] binding sites are separated by 9.6 +/- 0.5 A. Variation of the metal stoichiometries indicates that all three Eu(III) [or Ca(II) in vivo] sites are equidistant from the Zn(II) sites. We conclude that these sites are identical with the three central Zn(II) sites present in insulin hexamer crystals soaked in excess Zn(II) [Emdin, S. O., Dodson, G., Cutfield, J. M., & Cutfield, S. M. (1980) Diabetologia 19, 174-182] and suggest that these central sites are occupied by Ca(II) in vivo.     

Spectral properties of the CP43-deletion mutant of Synechocystis sp. PCC 6803

Spectral properties, particularly fluorescence spectra and their time-dependent behavior, were investigated for a mutant of the cyanobacterium Synechocystis sp. PCC 6803 lacking the 43 kDa chlorophyll-protein (CP43, PsbC). Lack of CP43 was confirmed by a size shift of the corresponding gene and by Western blotting. The CP43-deletion mutant grown under heterotrophic conditions accumulated a small amount of photosystem (PS) II, but virtually no PS II fluorescence was observed. A 686-nm fluorescence band was clearly observed by phycocyanin excitation, coming from the terminal pigments of phycobilisomes. In contrast, no PS I fluorescence was detected by phycocyanin excitation when accumulation of PS II components was not proved by a fluorescence excitation spectrum, indicating that energy transfer to PS I chlorophyll a was mediated by PS II chlorophyll a. Direct connection of phycobilisomes with PS I was not suggested. Based on these fluorescence properties, the energy flow in the CP43-deletion mutant cells is discussed.     

Light-induced fluorescence quenching in the light-harvesting chlorophyll a/b protein complex

Summary Irradiation of the principal photosystem II light-harvesting chlorophyll-protein antenna complex, LHC II, with high light intensities brings about a pronounced quenching of the chlorophyll fluorescence. Illumination of isolated thylakoids with high light intensities generates the formation of quenching centres within LHC II in vivo, as demonstrated by fluorescence excitation spectroscopy. In the isolated complex it is demonstrated that the light-induced fluorescence quenching: a) shows a partial, biphasic reversibility in the dark; b) is approximately proportional to the light intensity; c) is almost independent of temperature in the range 0–30°C; d) is substantially insensitive to protein modifying reagents and treatments; e) occurs in the absence of oxygen. A possible physiological importance of the phenomenon is discussed in terms of a mechanism capable of dissipating excess excitation energy within the photosystem II antenna.Abbreviations chla chlorophyll a - chlb chlorophyll b - F₀ fluorescence yield with reaction centers open - F_m fluorescence yield with reaction centres closed - F_i fluorescence at the plateau level of the fast induction phase - LHC II light-harvesting chlorophyll a/b protein complex II - PS II photosystem II - PSI photosystem I - Tricine N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine     

Effects of desiccation on the excitation energy distribution from phycoerythrin to the two photosystems in the red alga Porphyra perforata

Low temperature (77°K) fluorescence emission and excitation spectra were recorded for wet and desiccated thalli of Porphyra perforata . The photosystem I (F730) and photosystem II (F695) fluorescence emission kinetics during photosystem II trap closure were also recorded at 77°K. Desiccation induced a lowering of the fluorescence yield over the whole emission spectrum but the decrease was most pronounced for the photosystem II fluorescence bands, F688 and F695. It was shown that the desiccation-induced changes of the phycoerythrin sensitized emission spectrum were due to 1) a decrease in the fluorescence yield of the photosystem I antenna, 2) an even stronger decrease in the fluorescence of photosystem II, which was mediated by an increased spillover (k_T(II→I)) of excitation to photosystem I and an increase in the absorption cross section, α, for photosystem I. We hypothesize that the increase of both k_T(II→I) and α are part of a mechanism by which the desiccation-tolerant, high light exposed, Porphyra can avoid photodynamic damage to photosystem II, when photosynthesis becomes inhibited as a result of desiccation during periods of low tide.     

Chlorophyll fluorescence at 680 and 730 nm and leaf photosynthesis

Chlorophyll fluorescence constitutes a simple, rapid, and non-invasive means to assess light utilization in Photosystem II (PS II). This study examines aspects relating to the accuracy and applicability of fluorescence for measurement of PS II photochemical quantum yield in intact leaves. A known source of error is fluorescence emission at 730 nm that arises from Photosystem I (PS I). We measured this PS I offset using a dual channel detection system that allows measurement of fluorescence yield in the red (660 nm < F < 710 nm) or far red (F > 710 nm) region of the fluorescence emission spectrum. The magnitude of the PS I offset was equivalent to 30% and 48% of the dark level fluorescence F₀ in the far red region for Helianthus annuus and Sorghum bicolor, respectively. The PS I offset was therefore subtracted from fluorescence yields measured in the far red spectral window prior to calculation of PS II quantum yield. Resulting values of PS II quantum yield were consistently higher than corresponding values based on emission in the red region. The basis for this discrepancy lies in the finite optical thickness of the leaf that leads to selective reabsorption by chlorophyll of red fluorescence emission originating in deeper cell layers. Consequently, red fluorescence measurements preferentially sense emission from chloroplasts in the uppermost layer of the leaf where levels of photoprotective nonphotochemical quenching are higher due to increased photon density. It is suggested that far red fluorescence, corrected for the PS I offset, provides the most reliable quantitative basis for calculation of PS II quantum yield because of reduced sensitivity of these measurements to gradients in leaf transmittance and quenching capacity. This revised version was published online in August 2006 with corrections to the Cover Date.     

Characterization of the photosystem II centers inactive in plastoquinone reduction by fluorescence induction

In order to characterize the photosystem II (PS II) centers which are inactive in plastoquinone reduction, the initial variable fluorescence rise from the non-variable fluorescence level F_o to an intermediate plateau level F_i has been studied. We find that the initial fluorescence rise is a monophasic exponential function of time. Its rate constant is similar to the initial rate of the fastest phase (-phase) of the fluorescence induction curve from DCMU-poisoned chloroplasts. In addition, the initial fluorescence rise and the -phase have the following common properties: their rate constants vary linearly with excitation light intensity and their fluorescence yields are lowered by removal of Mg⁺⁺ from the suspension medium. We suggest that the inactive PS II centers, which give rise to the fluorescence rise from F_o to F_i, belong to the -type PS II centers. However, since these inactive centers do not display sigmoidicity in fluorescence, they thus do not allow energy transfer between PS II units like PS II.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl urea - DMQ 2,5-dimethyl-p-benzoquinone - F_o initial non-variable fluorescence yield - F_m maximum fluorescence yield - F_i intermediate fluorescence yield - PS II photosystem II - Q_A primary quinone acceptor of PS II - Q_B secondary quinone acceptor of PS II