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.     

Cation-mediated regulation of excitation energy distribution in chloroplasts lacking organized Photosystem II complexes

Despite the total loss of Photosystem II activity, thylakoids isolated from the green nuclear maize mutant hcf1-3 contain normal amounts of the light-harvesting chlorophyll $\text{a}\text{b}$ pigment-protein complex (LHC). We interpret the spectroscopic and ultrastructural characteristics of these thylakoids to indicate that the LHC present in these membranes is not associated with Photosystem II reaction centers and thus exists in a ‘free’ state within the thylakoid membrane. In contrast, the LHC found in wild-type maize thylakoids shows the usual functional association with Photosystem II reaction centers. Several lines of evidence suggest that the free LHC found in thylakoids isolated from hcf1-3 is able to mediate cation-dependent changes in both thylakoid appression and energy distribution between the photosystems: (1) Thylakoids isolated from hcf1-3 and wild-type seedlings exhibit a similar Mg²⁺-dependent increase in the short/long wavelength fluorescence emission peak ratio at 77 K. This Mg²⁺ effect is lost following incubation of thylakoids isolated from either source with low concentrations of trypsin. Such treatment results in the partial proteolysis of the LHC in both membrane types. (2) Thylakoids isolated from both hcf1-3 and wild-type seedlings show a similar Mg²⁺ dependence for the enhancement of the maximal yield of room temperature fluorescence and light scattering; both Mg²⁺ effects are abolished by brief incubation of the thylakoids with low concentrations of trypsin (3) Mg²⁺ acts to reduce the relative quantum efficiency of Photosystem I-dependent electron transport at limiting 650 nm light in thylakoids isolated from hcf1-3. (4) The pattern of digitonin fractionation of thylakoid membranes, which is dependent upon structural membrane interactions and upon LHC in the thylakoids, is similar in thylakoids isolated from both hcf1-3 and wild-type seedlings. We conclude that the surface-exposed segment of the LHC, but not the LHC-Photosystem II core association, is necessary for the cation-dependent changes in both thylakoid appression and energy distribution between the two photosystems, and that the LHC itself is able to transfer excitation energy directly to Photosystem I in a Mg²⁺-dependent fashion in the absence of Photosystem II reaction centers. The latter phenomenon is equivalent to a cation-induced change in the absorptive cross-section of Photosystem I.     

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.     

A new mechanism for adaptation to changes in light intensity and quality in the red alga,Porphyra perforata. I. Relation to State 1—State 2 transitions

Time courses of chlorophyll fluorescence and fluorescence spectra at 77 K after various light treatments were measured in the red alga, Porphyra perforata. Photosystem (PS) I or II light (light 1 or 2) induced differences in the fluorescence spectra at 77 K. Light 2 decreased the two PS II fluorescence bands (F-685 and F-695) in parallel, while light 1 preferentially increased F-695. Light 1 and 2 also produced different effects on the activities of PS I and II. Preillumination with light 1 increased PS II activity and decreased PS I activity. However, preillumination with light 2 decreased PS II activity with no effect on PS I activity. These results show that there are at least two mechanisms that can alter the transfer of light energy in P. perforata. The dark state in this alga was found to be State 2 and light 1 induced a State 2-State 1 transition which retarded the transfer of light energy from PS II to PS I. Light 2 induced another change (which we have called a State 2-State 3 transition) that was accompanied by a change only in PS II activity.     

Excitation energy transfer to Photosystem I in filaments and heterocysts of Nostoc punctiforme

Cyanobacteria adapt to varying light conditions by controlling the amount of excitation energy to the photosystems. On the minute time scale this leads to redirection of the excitation energy, usually referred to as state transitions, which involves movement of the phycobilisomes. We have studied short-term light adaptation in isolated heterocysts and intact filaments from the cyanobacterium Nostoc punctiforme ATCC 29133. In N.punctiforme vegetative cells differentiate into heterocysts where nitrogen fixation takes place. Photosystem II is inactivated in the heterocysts, and the abundancy of Photosystem I is increased relative to the vegetative cells. To study light-induced changes in energy transfer to Photosystem I, pre-illumination was made to dark adapted isolated heterocysts. Illumination wavelengths were chosen to excite Photosystem I (708 nm) or phycobilisomes (560 nm) specifically. In heterocysts that were pre-illuminated at 708 nm, fluorescence from the phycobilisome terminal emitter was observed in the 77 K emission spectrum. However, illumination with 560 nm light caused quenching of the emission from the terminal emitter, with a simultaneous increase in the emission at 750 nm, indicating that the 560 nm pre-illumination caused trimerization of Photosystem I. Excitation spectra showed that 560 nm pre-illumination led to an increase in excitation transfer from the phycobilisomes to trimeric Photosystem I. Illumination at 708 nm did not lead to increased energy transfer from the phycobilisome to Photosystem I compared to dark adapted samples. The measurements were repeated using intact filaments containing vegetative cells, and found to give very similar results as the heterocysts. This demonstrates that molecular events leading to increased excitation energy transfer to Photosystem I, including trimerization, are independent of Photosystem II activity.     

Xanthophyll-induced aggregation of LHCII as a switch between light-harvesting and energy dissipation systems

The xanthophyll cycle pigments, violaxanthin and zeaxanthin, present outside the light-harvesting pigment-protein complexes of Photosystem II (LHCII) considerably enhance specific aggregation of proteins as revealed by analysis of the 77 K chlorophyll a fluorescence emission spectra. Analysis of the infrared absorption spectra in the Amide I region shows that the aggregation is associated with formation of intermolecular hydrogen bonding between the α helices of neighboring complexes. The aggregation gives rise to new electronic energy levels, in the Soret region (530 nm) and corresponding to the Q spectral region (691 nm), as revealed by analysis of the resonance light scattering spectra. New electronic energy levels are interpreted in terms of exciton coupling of protein-bound photosynthetic pigments. The energy of the Q excitonic level of chlorophyll is not high enough to drive the light reactions of Photosystem II but better suited to transfer excitation energy to Photosystem I, which creates favourable energetic conditions for the state I-state II transition. The lack of fluorescence emission from this energy level, at physiological temperatures, is indicative of either very high thermal energy conversion rate or efficient excitation quenching by carotenoids. Chlorophyll a fluorescence was quenched up to 61% and 34% in the zeaxanthin- and violaxanthin-containing samples, respectively, as compared to pure LHCII. Enhanced aggregation of LHCII, observed in the presence of the xanthophyll cycle pigments, is discussed in terms of the switch between light-harvesting and energy dissipation systems.     

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.     

Photochemical reactions of chlorophyll in dehydrated Photosystem II: two chlorophyll forms (680 and 700 nm)

Lichens and phototolerant poikilohydric mosses differ from spinach leaves, fern fronds or photosensitive mosses in that they show strongly decreased F_o chlorophyll fluorescence after drying. This desiccation-induced fluorescence loss is rapidly reversible under rehydration. Fluorescence emission from Photosystem II at 685 nm was decreased more strongly by dehydration than 720 nm emission. Reaction centers of Photosystem II lose activity on dehydration and regain it on hydration. Heating of desiccated lichens increased F_o chlorophyll fluorescence. The activation energy for the reversible part of the temperature-dependent fluorescence increase was 0.045 eV, which corresponds to the energy difference between the 680 and 697 nm absorption bands. In desiccated chlorolichens such as Parmelia sulcata, heating induces the appearance of positive variable fluorescence related to the reversible reduction of Q_A due to overcoming the energy barrier. This is interpreted to provide information on the mechanism of photoprotection: energy is dissipated by changing Chl680 or P680 into a chlorophyll form, which absorbs at 700 nm and emits light at 720 nm (Chl-720 or P680(700)) with a low quantum yield. Dissipation of light energy in this trap is activated by desiccation.     

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.     

Excitation-energy redistribution in the cryptomonad alga Cryptomonas ovata

We have studied the redistribution of excitation energy in the cryptomonad alga Cryptomonas ovata. Low-temperature fluorescence emission spectra from cells preilluminated with light 1 and light 2 show that preferential excitation of Photosystem II (PS II) leads to decreased fluorescence emission from chlorophyll (Chl) a associated with PS II relative to the emission following the preferential excitation of Photosystem I (PS I). The fluorescence change is indicative of a light-state transition by the cells. However, comparision of measurements of the kinetics of P-700 photooxidation by cells fixed with glutaraldehyde following illumination with light 1 or light 2 shows that the relative activity of PS I is lower in cells fixed in light 2. This is in contrast to the expectation for cells in State 2. Excitation spectra for the fluorescence emission from PS II Chl a show that preferential excitation of PS II leads to a decreased probability for energy transfer from phycoerythrin and Chl c₂ to PS II when compared to cells in which PS I is preferentially excited. This result is in accordance with recent picosecond time-resolved fluorescence studies (Bruce, D., Biggins, J., Charbonneau, S. and Thewalt, M. (1987) in Progress in Photosynthesis Research (Biggins, J., ed.), Vol. II, pp. 777–780, Martinus Nijhoff, Dordrecht) and we, therefore, suggest that C. ovata does not undergo a classical light-state transition. However, preferential excitation of PS II or PS I appears to cause pigment-protein conformational changes which change the probability for energy transfer from phycoerythrin to PS II, and we suggest that this may be a mechanism for photoprotection of PS II. Studies of the kinetics of excitation-energy redistribution, and of the effects of electron-transport inhibitors and uncouplers of photophosphorylation indicate that the mechanism for excitation-energy redistribution in C. ovata and phycobilisome-containing organisms may be similar.     

Distribution of light excitation in an intact leaf between the two photosystems of photosynthesis. Changes in absorption cross-sections following state 1-state 2 transitions

Using the photoacoustic technique, state 1-state 2 transitions were studied in an intact leaf by direct monitoring of modulated oxygen evolution, excited by modulated light. States 1 and 2 were characterized by the extent of immediate enhancement of the modulated oxygen evolution — ‘Emerson enhancement’ — and the concomitant fluorescence quenching, resulting from the addition of continuous far-red light (greater than 700 nm), absorbed primarily in Photosystem I (light 1). The extent of Emerson enhancement as well as the saturation curve of this effect by far-red light are very sensitive and quantitative indicators for the ratio of light excitation distributed between Photosystems I and II. The enhancement ratios at 650 nm light, a typical light 2, were in a range 1.4–1.8 in state 1, while values as low as 1.06 were observed in state 2. During the transition from state 2 to state 1, monitored in presence of modulated light 2 and background continuous light 1, the modulated oxygen yield increased considerably, indicating a major increase in excitation flux into Photosystem II. Conversely, with modulated light 2 alone in state 1, the modulated oxygen evolution yield was smaller than in state 2, indicating a decrease of the excitation flux in Photosystem I. In a typical example, of the transition to state 1, the fraction of light absorbed by Photosystem II, β, increased from 0.46 to 0.64, while that absorbed by Photosystem II, α, decreased from 0.43 to 0.36. State 1-state 2 transitions, thus, reflect reciprocal changes in the cross-sections of the two photosystems for light absorption. There is no evidence for the operation of a ‘spill-over’ mechanism. The enhancement effect displayed maxima at 480 and 650 nm, related to chlorophyll-b absorption, as well as another band at 500–550 nm. In a chlorophyll-b-less barley mutant, state 1-state 2 transitions, as monitored by modulated oxygen evolution, were absent, and the resulting enhancement corresponded to state 2. These observations are consistent with the model that the light-harvesting $\text{chlorophyll}\text{-}\text{a}\text{b}$ complex plays a role in regulating the distribution of light to the photosystems. It is probable that this complex migrates reversibly in the thylakoid membrane in such a way that it is mainly associated with Photosystem II in state 1, but is more evenly distributed in the two photosystems in state 2.     

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.     

Spectroscopy of non-photochemical and photochemical quenching of chlorophyll fluorescence in leaves; evidence for a role of the light harvesting complex of Photosystem II in the regulation of energy dissipation

Dissipation of absorbed excitation energy as heat, measured by its effect on the quenching of chlorophyll fluorescence, is induced under conditions of excess light in order to protect the photosynthetic apparatus of plants from light-dependent damage. The spectral characteristics of this quenching have been compared to that due to photochemistry in the Photosystem II reaction centre using leaves of Guzmania monostachia. This was achieved by making measurements at 77K when fluorescence emission bands from each type of chlorophyll protein complex can be distinguished. It was demonstrated that photochemistry and non-photochemical dissipation preferentially quench different emission bands and therefore occur by dissimilar mechanisms at separate sites. It was found that photochemistry was associated with a preferential quenching of emission at 688 nm whereas the spectrum for rapidly reversible non-photochemical quenching had maxima at 683 nm and 698 nm, suggesting selective quenching of the bands originating from the light harvesting complexes of Photosystem II. Further evidence that this was occurring in the light harvesting system was obtained from the fluorescence excitation spectra recorded in the quenched and relaxed states.Abbreviations pH transthylakoid pH gradient - F_o minimum level of chlorophyll fluorescence when Photosystem II reaction centres are open - F_m maximum level of fluorescence when Photosystem II reaction centres are closed - F_v variable fluorescence F_m minus F_o - F'_o F_o in any quenched state - F_m F_m in any quenched state - LHCII light harvesting complexes of Photosystem II - PSI Photosystem I - PS II Photosystem II - qN non-photochemical quenching of chlorophyll fluorescence - qE non-photochemical quenching of chlorophyll fluorescence that occurs in the presence of a pH     

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.     

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.     

The effect of outer antenna complexes on the photochemical trapping rate in barley thylakoid Photosystem II

We have investigated the previous suggestions in the literature that the outer antenna of Photosystem II of barley does not influence the effective photosystem primary photochemical trapping rate. It is shown by steady state fluorescence measurements at the F₀ fluorescence level of wild type and the chlorina f2 mutant, using the chlorophyll b fluorescence as a marker, that the outer antenna is thermally equilibrated with the core pigments, at room temperature, under conditions of photochemical trapping. This is in contrast with the conclusions of the earlier studies in which it was suggested that energy was transferred rapidly and irreversibly from the outer antenna to the Photosystem II core. Furthermore, the effective trapping time, determined by single photon counting, time-resolved measurements, was shown to increase from 0.17±0.017 ns in the chlorina Photosystem II core to a value within the range 0.42±0.036-0.47±0.044 ns for the wild-type Photosystem II with the outer antenna system. This 2.5-2.8-fold increase in the effective trapping time is, however, significantly less than that expected for a thermalised system. The data can be explained in terms of the outer antenna increasing the primary charge separation rate by about 50%.     

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.     

Modulation of photosynthetic electron transport in the absence of terminal electron acceptors: Characterization of the rbcL deletion mutant of tobacco

Tobacco rbcL deletion mutant, which lacks the key enzyme Rubisco for photosynthetic carbon assimilation, was characterized with respect to thylakoid functional properties and protein composition. The ΔrbcL plants showed an enhanced capacity for dissipation of light energy by non-photochemical quenching which was accompanied by low photochemical quenching and low overall photosynthetic electron transport rate. Flash-induced fluorescence relaxation and thermoluminescence measurements revealed a slow electron transfer and decreased redox gap between Q_A and Q_B, whereas the donor side function of the Photosystem II (PSII) complex was not affected. The 77 K fluorescence emission spectrum of ΔrbcL plant thylakoids implied a presence of free light harvesting complexes. Mutant plants also had a low amount of photooxidisible P700 and an increased ratio of PSII to Photosystem I (PSI). On the other hand, an elevated level of plastid terminal oxidase and the lack of F₀ ‘dark rise’ in fluorescence measurements suggest an enhanced plastid terminal oxidase-mediated electron flow to O₂ in ΔrbcL thylakoids. Modified electron transfer routes together with flexible dissipation of excitation energy through PSII probably have a crucial role in protection of PSI from irreversible protein damage in the ΔrbcL mutant under growth conditions. This protective capacity was rapidly exceeded in ΔrbcL mutant when the light level was elevated resulting in severe degradation of PSI complexes.     

Light- and temperature-induced changes in the distribution of excitation energy between Photosystem I and Photosystem II in spinach leaves

The influence of light quality and temperature on the distribution of the absorbed quanta between Photosystem I (PS I) and Photosystem II (PS II) in spinach leaves has been studied from the characteristics of chlorophyll fluorescence at 77 K. Leaves were preilluminated at different temperatures with either PS I light (to establish State 1) or with PS II light (to establish State 2), then cooled to 77 K and measured for fluorescence. In State 1, energy distribution appeared to be unaffected by temperature. A transition to State 2 resulted in an increase in PS I fluorescence and a decrease in the PS II fluorescence, indicating that a larger fraction of energy becomes redistributed to PS I. However, the extent of this redistribution varied: it was only small at 5°C to 20°C, but it largely increased at temperatures exceeding 20°C. This variation in the extent was related to a change in the mechanism of the state transition: at 15°C only the ‘initial’ distribution of energy was affected, while at 35°C an additional increase in the spill-over constant, k_{T (II → I)}, was included. It is assumed that under physiological conditions k_{T (II → I)} is under the control of temperature rather than of light quality, whereby in leaves adapted to high physiological temperatures, the probability of energy spill-over from closed PS II centres to PS I is enhanced. In darkened leaves, the spill-over constant has been manipulated by preincubation at different temperatures. Then, the light-induced ‘energization’ of thylakoid membranes has been tested by measuring the light-induced electrochromic absorbance change at 515 nm (and light-induced light-scattering changes) in these leaves. The flash-induced 515 nm signal as well as the initial peak during a 1 s illumination were not affected by energy distribution. However, the amplitude of the pseudo-steady-state signal (as established during 1 s illumination) was considerably enhanced in leaves in which a larger fraction of the absorbed energy is distributed to PS I at the expense of PS II excitation. The results have been interpreted in such a way that an increase in energy spill-over from PS II to PS I favours a cyclic electron transport around PS I. It is discussed that changes in energy distribution (via spill-over) may serve to maintain a suitable balance between non-cyclic and cyclic electron transport in vivo.