Energy transfer between Photosystem II and Photosystem I in chloroplasts

A model for the photochemical apparatus of photosynthesis is presented which accounts for the fluorescence properties of Photosystem II and Photosystem I as well as energy transfer between the two photosystems. The model was tested by measuring at ?196 °C fluorescence induction curves at 690 and 730 nm in the absence and presence of 5 mM MgCl₂ which presumably changes the distribution of excitation energy between the two photosystems. The equations describing the fluorescence properties involve terms for the distribution of absorbed quanta, α, being the fraction distributed to Photosystem I, and β, the fraction to Photosystem II, and a term for the rate constant for energy transfer from Photosystem II to Photosystem I,k_T(II→I). The data, analyzed within the context of the model, permit a direct comparison of α andk_T(II→I) in the absence (?) and presence (+) of Mg²⁺:α/^?α⁺= 1.2andk/^?_T(II→I)k⁺_T(II→I)= 1.9. If the criterion thatα + β = 1 is applied absolute values can be calculated: in the presence of Mg²⁺,a⁺ = 0.27 and the yield of energy transfer,φ⁺_T(II→I) varied from 0.065 when the Photosystem II reaction centers were all open to 0.23 when they were closed. In the absence of Mg²⁺,α^? = 0.32 andφ_T(II→I) varied from 0.12 to 0.28.The data were also analyzed assuming that two types of energy transfer could be distinguished; a transfer from the light-harvseting chlorophyll of Photosystem II to Photosystem I,k_T(II→I), and a transfer from the reaction centers of Photosystem II to Photosystem I,k_t(II→I). In that caseα/^?α⁺= 1.3,k/^?_T(II→I)k⁺_T(II→I)= 1.3 andk/^?_t(II→I)k⁺_(tII→I)= 3.0. It was concluded, however, that both of these types of energy transfer are different manifestations of a single energy transfer process.     

A demonstration of energy transfer from photosystem II to photosystem I in chloroplasts

Photosystem I activity of Tris-washed chloroplasts was measured at room temperature as the rate of photoreduction of NADP and as the rate of oxygen uptake mediated by methyl viologen in both cases using dichlorophenolindophenol plus ascorbate as the source of electrons for Photosystem I. With both assay systems the rate of electron transport by Photosystem I was stimulated approx. 20 % by the addition of 3-(3,4-dichlorophenyl)-1, 1-dimethylurea which caused the Photosystem II reaction centers to close. Photosystem I activity of chloroplasts was measured at low temperature as the rate of photooxidation of P-700. Chloroplasts suspended in the presence of hydroxylamine and 3-(3,4-dichlorophenyl)-1, 1-dimethylurea were frozen to ?196 °C after adaptation to darkness or after a preillumination at room temperature. The Photosystem II reaction centers of the frozen dark-adapted sample were all open; those of the preilluminated sample were all closed. The rate of photooxidation of P-700 at ?196 °C with the preilluminated sample was approx. 25 % faster than with the dark-adapted sample. We conclude from both the room temperature and the low temperature experiments that there is greater energy transfer from Photosystem II to Photosystem I when the Photosystem II reaction centers are closed and that these results are a direct demonstration of spillover.     

Development of the Primary Photochemical Apparatus of Photosynthesis during Greening of Etiolated Bean Leaves

Seven-day-old dark-grown bean leaves were greened under continuous light. The amount of chlorophyll, the ratio of chlorophyll a to chlorophyll b, the O₂ evolving capacity and the primary photochemical activities of Photosystem I and Photosystem II were measured on the leaves after various times of greening. The primary photochemical activities were measured as the photo-oxidation of P₇₀₀, the photoreduction of C-550, and the photo-oxidation of cytochrome b₅₅₉ in intact leaves frozen to −196 C. The results indicate that the reaction centers of Photosystem I and Photosystem II begin to appear within the first few minutes and that Photosystem II reaction centers accumulate more rapidly than Photosystem I reaction centers during the first few hours of greening. The very early appearances of the primary photochemical activity of Photosystem II was also confirmed by light-induced fluorescence yield measurements at −196 C.     

Fluorescence emission spectra of Photosystem I,Photosystem II and the light-harvesting chlorophyll a/b complex of higher plants

Fluorescence emission spectra excited at 514 and 633 nm were measured at ?196 °C on dark-grown bean leaves which had been partially greened by a repetitive series of brief xenon flashes. Excitation at 514 nm resulted in a greater relative enrichment of the 730 nm emission band of Photosystem I than was obtained with 633 nm excitation. The difference spectrum between the 514 nm excited fluorescence and the 633 nm excited fluorescence was taken to be representative of a pure Photosystem I emission spectrum at ?196 °C. It was estimated from an extrapolation of low temperature emission spectra taken from a series of flashed leaves of different chlorophyll content that the emission from Photosystem II at 730 nm was 12% of the peak emission at 694 nm. Using this estimate, the pure Photosystem I emission spectrum was subtracted from the measured emission spectrum of a flashed leaf to give an emission spectrum representative of pure Photosystem II fluorescence at ?196 °C. Emission spectra were also measured on flashed leaves which had been illuminated for several hours in continuous light. Appreciable amounts of the light-harvesting chlorophyll a/b protein, which has a low temperature fluorescence emission maximum at 682 nm, accumulate during greening in continuous light. The emission spectra of Photosystem I and Photosystem II were subtracted from the measured emission spectrum of such a leaf to obtain the emission spectrum of the light-harvesting chlorophyll a/b protein at ?196 °C.     

Light-dependent quenching of chlorophyll fluorescence in pea chloroplasts induced by adenosine 5′-triphosphate

Addition of ATP to chloroplasts causes a reversible 25–30% decrease in chlorophyll fluorescence. This quenching is light-dependent, uncoupler insensitive but inhibited by DCMU and electron acceptors and has a half-time of 3 minutes. Electron donors to Photosystem I can not overcome the inhibitory effect of DCMU, suggesting that light activation depends on the reduced state of plastoquinone. Fluorescence emission spectra recorded at ?196°C indicate that ATP treatment increases the amount of excitation energy transferred to Photosystem I. Examination of fluorescence induction curves indicate that ATP treatment decreases both the initial (F_o) and variable (F_v) fluorescence such that the ratio of F_v to the maximum (F_m) yield is unchanged. The initial sigmoidal phase of induction is slowed down by ATP treatment and is quenched 3-fold more than the exponential slow phase, the rate of which is unchanged. A plot of F_v against area above the induction curve was identical plus or minus ATP. Thus ATP treatment can alter quantal distribution between Photosystems II and I without altering Photosystem II-Photosystem II interaction. The effect of ATP strongly resembles in its properties the phosphorylation of the light-harvesting complex by a light activated, ATP-dependent protein kinase found in chloroplast membranes and could be the basis of physiological mechanisms which contribute to slow fluorescence quenching in vivo and regulate excitation energy distribution between Photosystem I and II. It is suggested that the sensor for this regulation is the redox state of plastoquinone.     

Excitation spectra for photosystem I and photosystem II in chloroplasts and the spectral characteristics of the distributions of quanta between the two photosystems.

The parameters listed in the title were determined within the context of a model for the photochemical apparatus of photosynthesis. The fluorescence of variable yield at 750 nm at -196 degrees C is due to energy transfer from Photosystem II to Photosystem I. Fluorescence excitation spectra were measured at -196 degrees C at the minimum, FO, level and the maximum, FM, level of the emission at 750 nm. The difference spectrum, FM-FO, which represents the excitation spectrum for FV is presented as a pure Photosystem II excitation spectrum. This spectrum shows a maximum at 677 nm, attributable to the antenna chlorophyll a of Photosystem II units, with a shoulder at 670 nm and a smaller maximum at 650 nm, presumably due to chlorophyll a and chlorophyll b of the light-harvesting chlorophyll complex. Fluoresence at the FO level at 750 nm can be considered in two parts; one part due to the fraction of absorbed quanta, alpha, which excites Photosystem I more-or-less directly and another part due to energy transfer from Photosystem II to Photosystem I. The latter contribution can be estimated from the ratio of FO/FV measured at 692 nm and the extent of FV at 750 nm. According to this procedure the excitation spectrum of Photosystem I at -196 degrees C was determined by subtracting 1/3 of the excitation spectrum of FV at 750 nm from the excitation spectrum of FO at 750 nm. The spectrum shows a relatively sharp maximum at 681 nm due to the antenna chlorophyll a of Photosystem I units with probably some energy transfer from the light-harvesting chlorophyll complex. The wavelength dependence of alpha was determined from fluorescence measurements at 692 and 750 nm at -196 degrees C. Alpha is constant to within a few percent from 400 to 680 nm, the maximum deviation being at 515 nm where alpha shows a broad maximum increasing from 0.30 to 0.34. At wavelengths between 680 and 700 nm, alpha increases to unity as Photosystem I becomes the dominant absorber in the photochemical apparatus.     

Energy transfer from photosystem II to photosystem I in Porphyridium cruentum

Rates of photooxidation of P-700 by green (560 nm) or blue (438 nm) light were measured in whole cells of Porphyridium cruentum which had been frozen to ?196 °C under conditions in which the Photosystem II reaction centers were either all open (dark adapted cells) or all closed (preilluminated cells). The rate of photooxidation of P-700 at ?196 °C by green actinic light was approx. 80% faster in the preilluminated cells than in the dark-adapted cells. With blue actinic light, the rates of P-700 photooxidation in the dark-adapted and preilluminated cells were not significantly different. These results are in excellent agreement with predictions based on our previous estimates of energy distribution in the photosynthetic apparatus of Porphyridium cruentum including the yield of energy transfer from Photosystem II to Photosystem I determined from low temperature fluorescence measurements.     

Des activites photosynthetiques sans les complexes pigmentaires CP1 et CP2

Photosynthetic activity in the absence of the CP1 and CP2 pigmentary complexesVarious photochemical activities were tested on chloroplasts of Zea mays that received 4 s of light every 4 h during the culture period. Photosystem I and Photosystem II were functioning, as well as the photosynthetic electron transport. These chloroplasts exhibited upon sodium dodecyl sulphate gel electrophoresis neither Complex 1 (M_r 70 000) generally associated with Photosystem I nor Complex 2 M_r 25 000) generally associated with Photosystem II. Chlorophyll is indeed attached to polypeptides of molecular weight 21 000 and 29 000.These results lead us to question the functional role of chloroplast protein-pigment complexes observed by sodium dodecyl sulphate gel electrophoresis.     

A quantitative study of the slow decline of chlorophyll a fluorescence in isolated chloroplasts.

A detailed study of the photo-induced decline in chlorophyll a fluorescence intensity (Kautsky phenomenon) in coupled isolated chloroplasts from a high level (P) to a low stationary level (S) is presented. 1. A linear relationship between P leads to S quenching and intrathylakoid H+ concentration was found. When the light-induced proton gradient was abolished by uncoupling, the fluorescence emission at room temperature was lowered proportionally to increased H+ concentration in the medium. 2. Fluorescence spectra at -196 degrees C of samples frozen at the P and S states showed no significant differences in the Photosystem I/Photosystem II ratio of fluorescence emission. Furthermore, freezing to -196 degrees C reversed the P leads to S quenching. This indicates that the P leads to S quenching is not related to an increase of spillover of excitation energy from Photosystem II to Photosystem I. 3. When Mg2+ was added to thylakoids suspended in a medium free of divalent cations, the inhibition of spillover required lower Mg2+ concentrations (half saturation at 0.6 mM). Increased proton concentration in the medium also inhibited spillover. 4. The results are interpreted in terms of two sites of Mg2+ and H+ effects on excitation deactivation in Photosystem II. One site is located on the outer face of the thylakoid membrane; action of both Mg2+ and H+ at this side diminishes spillover. The second site is located on the inner face of the membrane; as Mg2+ is displaced there by protons, a non-photochemical quenching of Photosystem II fluorescence is induced, which is manifested by the P leads to S decline.     

Excitation spectra for Photosystem I and Photosystem II in chloroplasts and the spectral characteristics of the distribution of quanta between the two photosystems

The parameters listed in the title were determined within the context of a model for the photochemical apparatus of photosynthesis.

The fluorescence of variable yield at 750 nm at −196 °C is due to energy transfer from Photosystem II to Photosystem I. Fluorescence excitation spectra were measured at −196 °C at the minimum, F_O, level and the maximum, F_M, level of the emission at 750 nm. The difference spectrum, F_M–F_O, which represents the excitation spectrum for F_V is presented as a pure Photosystem II excitation spectrum. This spectrum shows a maximum at 677 nm, attributable to the antenna chlorophyll a of Photosystem II units, with a shoulder at 670 nm and a smaller maximum at 650 nm, presumably due to chlorophyll a and chlorophyll b of the light-harvesting chlorophyll complex.

Fluorescence at the F_O level at 750 nm can be considered in two parts; one part due to the fraction of absorbed quanta, , which excites Photosystem I more-or-less directly and another part due to energy transfer from Photosystem II to Photosystem I. The latter contribution can be estimated from the ratio of F_O/F_V measured at 692 nm and the extent of F_V at 750 nm. According to this procedure the excitation spectrum of Photosystem I at −196 °C was determined by subtracting 1/3 of the excitation spectrum of F_V at 750 nm from the excitation spectrum of F_O at 750 nm. The spectrum shows a relatively sharp maximum at 681 nm due to the antenna chlorophyll a of Photosystem I units with probably some energy transfer from the light-harvesting chlorophyll complex.

The wavelength dependence of was determined from fluorescence measurements at 692 and 750 nm at −196 °C. is constant to within a few percent from 400 to 680 nm, the maximum deviation being at 515 nm where shows a broad maximum increasing from 0.30 to 0.34. At wavelengths between 680 and 700 nm, increases to unity as Photosystem I becomes the dominant absorber in the photochemical apparatus.     

Energy distribution in the photochemical apparatus of Porphyridium cruentum in state I and state II

Fluorescence of Porphyridium cruentum in state I (cells equilibrated in light absorbed predominantly by Photosystem I) and in state II (cells equilibrated in light absorbed appreciably by Photosystem II) was examined to determine how the distribution of excitation energy was altered in the transitions between state I and state II. Low temperature emission spectra of cells frozen in state I and state II confirmed that a larger fraction of the excitation energy is delivered to Photosystem II in state I. Low temperature measurements showed that the yield of energy transfer from Photosystem II to Photosystem I was greater in state II and calculations indicated that the photochemical rate constant for such energy transfer was approximately twice as large in state II. Measurements at low temperature also showed that the cross sections and the spectral properties of the photosystems did not change in the transitions between state I and state II. In agreement with predictions made from the parameters measured at low temperature, the action spectra for oxygen evolution measured at room temperature were found to be the same in state I and state II.     

Charge accumulation and recombination in Photosystem II studied by thermoluminescence. I. Participation of the primary acceptor Q and secondary acceptor B in the generation of thermoluminescence of chloroplasts

In the glow curves of chloroplasts excited by a series of flashes at +1°C the intensity of the main thermoluminescence band appearing at +30°C (B band; B, secondary acceptor of Photosystem II) exhibits a period-4 oscillation with maxima on the 2nd and 6th flashes indicating the participation of the S₃ state of the water-splitting system in the radiative charge recombination reaction. After long-term dark adaptation of chloroplasts (6 h), when the major part of the secondary acceptor pool (B pool) is oxidized, a period-2 contribution with maxima occurring at uneven flash numbers appears in the oscillation pattern. The B band can even be excited at ?160°C as well as by a single flash in which case the water-splitting system undergoes only one transition (S₁ → S₂). The experimental observations and computer simulation of the oscillatory patterns suggest that the B band originates from charge recombination of the S₂B^? and S₃B^? redox states. The half-time of charge recombination responsible for the B band is 48 s. When a major part of the plastoquinone pool is reduced due to prolonged excitation of the chloroplasts by continuous light, a second band (Q band; Q, primary acceptor of Photosystem II) appears in the glow curve at +10°C which overlaps with the B band. In chloroplasts excited by flashes prior to DCMU addition only the Q band can be observed showing maxima in the oscillation pattern at flash numbers 2, 6 and 10. The Q band can also be induced by flashes after DCMU addition which allows only one transition of the water-splitting system (S₁ → S₂). In the presence of DCMU, electrons accumulate on the primary acceptor Q, thus the Q band can be ascribed to the charge recombination of either the S₂Q^? or S₃Q^? states depending on whether the water-splitting system is in the S₂ or the S₃ state. The half-time of the back reaction of Q^? with the donor side of PS II (S₂ or S₃ states) is 3 s. It was also observed that in a sequence of flashes the peak positions of the Q and B bands do not depend on the advancement of the water-splitting system from the S₂ state to the S₃ state. This result implies that the midpoint potential of the water-splitting system remains unmodified during the S₂ → S₃ transition.     

The photochemical and fluorescence properties of whole cells,spheroplasts and spheroplast particles from the blue-green alga Phormidium luridum

The photochemical activities and fluorescence properties of cells, spheroplasts and spheroplast particles from the blue-green alga Phormidium luridum were compared. The photochemical activities were measured in a whole range of wavelengths and expressed as quantum yield spectra (quantum yield vs. wavelength). The following reactions were measured: Photosynthesis (O₂ evolution) in whole cells; Hill reaction (O₂ evolution) with Fe(CN)₆^3? and NADP as electron acceptors (Photosystem II and Photosystem II+Photosystem I reactions); electron transfer from reduced 2,6-dichlorophenolindophenol to diquat (Photosystem I reaction). The fluorescence properties were emission spectra, quantum yield spectra and the induction pattern.On the basis of comparison between the quantum yield spectra and the pigments compositions the relative contribution of each pigment to each photosystem was estimated. In normal cells and spheroplasts it was found that Photosystem I (Photosystem II) contains about 90 % (10 %) of the chlorophyll a, 90 % (10 %) of the carotenoids and 15 % (85 %) of the phycocyanin. In spheroplast particles there is a reorganization of the pigments: they loose a certain fraction (about half) of the phycocyanin but the remaining phycocyanin attaches itself exclusively to Photosystem I (!). This is reflected by the loss of Photosystem II activity, a flat quantum yield vs. wavelength dependence and a loss of the fluorescence induction.The fluorescence quantum yield spectra conform qualitatively to the above conclusion. More quantitative estimation shows that only a fraction (20–40 %) of the chlorophyll of Photosystem II is fluorescent. Total emission spectrum and the ratio of variable to constant fluorescence are in agreement with this conclusion.The fluorescence emission spectrum shows characteristic differences between the constant and variable components. The variable fluorescence comes exclusively from chlorophyll a; the constant fluorescence is contributed, in addition to chlorophyll a, by phycocyanine and an unidentified long wavelength component.The variable fluorescence does not change in the transition from whole cells to spheroplasts. However, the constant fluorescence increases considerably. This indicates the release of a small fraction of pigments from the photosynthetic photochemical apparatus which then become fluorescent.     

Chlorophyll triplet states associated with Photosystem I and Photosystem II in thylakoids of the green alga Chlamydomonas reinhardtii

The analysis of FDMR spectra, recorded at multiple emission wavelengths, by a global decomposition technique, has allowed us to characterise the triplet populations associated with Photosystem I and Photosystem II of thylakoids in the green alga Chlamydomonas reinhardtii. Three triplet populations are observed at fluorescence emissions characteristic of Photosystem II, and their zero field splitting parameters have been determined. These are similar to the zero field parameters for the three Photosystem II triplets previously reported for spinach thylakoids, suggesting that they have a widespread occurrence in nature. None of these triplets have the zero field splitting parameters characteristic of the Photosystem II recombination triplet observed only under reducing conditions. Because these triplets are generated under non-reducing redox conditions, when the recombination triplet is undetectable, it is suggested that they may be involved in the photoinhibition of Photosystem II. At emission wavelengths characteristic of Photosystem I, three triplet populations are observed, two of which are attributed to the P₇₀₀ recombination triplet frozen in two different conformations, based on the microwave-induced fluorescence emission spectra and the triplet minus singlet difference spectra. The third triplet population detected at Photosystem I emission wavelengths, which was previously unresolved, is proposed to originate from the antenna chlorophyll of the core or the unusually blue-shifted outer antenna complexes of this organism.     

Photosystem II energy coupling in chloroplasts with H2O2 as electron donor

NH₂OH-treated, non-water-splitting chloroplasts can oxidize H₂O₂ to O₂ through Photosystem II at substantial rates (100–250 μequiv · h^?1 · mg^?1 chlorophyll with 5 mM H₂O₂) using 2,5-dimethyl-p-benzoquinone as an electron acceptor in the presence of the plastoquinone antagonist dibromothymoquinone. This H₂O₂ → Photosystem II → dimethylquinone reaction supports phosphorylation with a $\text{P}\text{e}{}_2$ ratio of 0.25–0.35 and proton uptake with $\text{H}{}^{\mathrm{+}}\text{e}$ values of 0.67 (pH 8)–0.85 (pH 6). These are close to the $\text{P}\text{e}{}_2$ value of 0.3–0.38 and the $\text{H}{}^{\mathrm{+}}\text{e}$ values of 0.7–0.93 found in parallel experiments for the H₂O → Photosystem II → dimethylquinone reaction in untreated chloroplasts. Semi-quantitative data are also presented which show that the donor → Photosystem II → dibromothymoquinone (→O₂) reaction can support phosphorylation when the donor used is a proton-releasing reductant (benzidine, catechol) but not when it is a non-proton carrier (I^?, ferrocyanide).     

The kinetics of light-induced changes of C-550, cytochrome b559 and fluorescence yield in chloroplasts at low temperature

The kinetics of the photoreduction of C-550, the photooxidation of cytochrome b₅₅₉ and the fluorescence yield changes during irradiation of chloroplasts at ?196 °C were measured and compared. The photoreduction of C-550 proceeded more rapidly than the photooxidation of cytochrome b₅₅₉ and the fluorescence yield increase followed the cytochrome b₅₅₉ oxidation. These results suggest that fluorescence yield under these conditions indicates the dark reduction of the primary electron donor to Photosystem II, P680⁺, by cytochrome b₅₅₉ rather than the photoreduction of the primary electron acceptor.The photoreduction of C-550 showed little if any temperature dependence over the range of ?196 to ?100 °C. The amount of cytochrome b₅₅₉ photooxidized was sensitive to temperature decreasing from the maximal change at temperatures between ?196 to ?160 °C to no change at ?100 °C. To the extent that the reaction occurred at temperatures between ?160 and ?100 °C the rate was largely independent of temperature. The rate of the fluorescence increase was dependent on temperature over this range being 3–4 times more rapid at ?100 than at ?160 °C. At ?100 °C the light-induced fluorescence increase and the photoreduction of C-550 show similar kinetics. The temperature dependence of the fluorescence induction curve is attributed to the temperature dependence of the dark reduction of P680⁺.The intensity dependence of the photoreduction of C-550 and of the photooxidation of cytochrome b₅₅₉ are linear at low intensities (below 200 μW/cm²) but fall off at higher intensities. The failure of reciprocity in the photoreduction of C-550 at the higher intensities is not explained by the simple model proposed for the Photosystem II reaction centers.     

Energy transfers from Photosystem II to Photosystem I in Cryptomonas rufescens (Cryptophyceae)

In Cryptomonas rufescens (Cryptophyceae), phycoerythrin located in the thylakoid lumen is the major accessory pigment. Oxygen action spectra prove phycoerythrin to be efficient in trapping light energy.The fluorescence excitation spectra at ?196°C obtained by the method of Butler and Kitajima (Butler, W.L. and Kitajima, M. (1975) Biochim. Biophys. Acta 396, 72–85) indicate that like in Rhodophycease, chlorophyll a is the exclusive light-harvesting pigment for Photosystem I.For Photosystem II we can observe two types of antennae: (1) a light-harvesting chlorophyll complex connected to Photosystem II reaction centers, which transfers excitation energy to Photosystem I reaction centers when all the Photosystem II traps are closed. (2) A light-harvesting phycoerythrin complex, which transfers excitation energy exclusively to the Photosystem II reaction complexes responsible for fluorescence at 690 nm.We conclude that in Cryptophyceae, phycoerythrin is an efficient light-harvesting pigment, organized as an antenna connected to Photosystem II centers, antenna situated in the lumen of the thylakoid. However, we cannot afford to exclude that a few parts of phycobilin pigments could be connected to inactive chlorophylls fluorescing at 690 nm.     

Probing the kinetics of Photosystem I and Photosystem II fluorescence in pea chloroplasts on a picosecond pulse fluorometer

Picosecond fluorescence kinetics of pea chloroplasts have been investigated at room temperature using a pulse fluorometer with a resolution time of 10^?11 s. Fluorescence has been excited by both a ruby and neodymium-glass mode-locked laser and has been recorded within the 650 to 800 nm spectral region.We have found three-component kinetics of fluorescence from pea chloroplasts with lifetimes of 80, 300 and 4500 ps, respectively. The observed time dependency of the fluorescence of different components on the functional state of the photosynthetic mechanism as well as their spectra enabled us to conclude that Photosystem I fluoresces with a lifetime of 80 ps (τ_I) and Photosystem II fluoresces with a lifetime of 300 ps (τ_II). Fluorescence with a lifetime of 4500 ps (τ_III) may be interpreted as originating from chlorophyll monomeric forms which are not involved in photosynthesis.It was determined that the rise time of Photosystem I and Photosystem II fluorescence after 530 nm photoexcitation is 200 ps, which corresponds to the time of energy migration to them from carotenoids.     

Inhibitory effect of p-nitrothiophenol in the light on the Photosystem II activity of spinach chloroplasts

The treatment of spinach chloroplasts with p-nitrothiophenol in the light at acidic and neutral pH's caused specific inhibition of the Photosystem II activity, whereas the same treatment in the dark did not affect the activity at all. The photosystem I activity was not inhibited by p-nitrothiophenol both in the light and in the dark. The inhibition was accompanied by changes of fluorescence from chloroplasts. As observed at room temperature, the 685-nm band was lowered by the p-nitrothiophenol treatment in the light and, at liquid nitrogen temperature, the relative height of the 695-nm band to the 685-nm band increased and the 695-nm band shifted to longer wavelengths. The action spectra for these effects of p-nitrothiophenol on the activity and fluorescence showed a peak at 670 nm with a red drop at longer wavelengths. It was concluded that the light absorbed by Photosystem II is responsible for the chemical modification of chloroplasts with p-nitrothiophenol to causing the specific inhibition of Photosystem II.