The optical cross section and absolute size of a photosynthetic unit

The concepts of a photosynthetic unit (PSU) and of an optical cross section are defined. The various estimates of sizes of photosynthetic units are described, and it is shown how an unambiguous measurement of the size of a unit can be obtained by measurement of its optical cross section via the saturation response to a single turnover light flash. The Emerson-Arnold unit must be divided by the quantum requirement for oxygen to obtain the true size of the unit. The size so obtained is the average number of chlorophylls per trap or reaction center. The effects of escape from open and closed traps are considered and it is shown that when these escape probabilities are equal, their effect on the saturation curve vanishes, leaving the simple cumulative one hit Poisson distribution.     

Estimation of oxygen evolution by marine phytoplankton from measurement of the efficiency of Photosystem II electron flow

The relation between photosynthetic oxygen evolution and Photosystem II electron transport was investigated for the marine algae t Phaeodactylum tricornutum, Dunaliella tertiolecta, Tetraselmis sp., t Isochrysis sp. and t Rhodomonas sp.. The rate of Photosystem II electron transport was estimated from the incident photon flux density and the quantum efficiency of Photosystem II electron transport as determined by chlorophyll fluorescence. The relation between the estimated rate of Photosystem II electron transport and the rate of oxygen evolution was investigated by varying the ambient light intensity. At limiting light intensities a linear relation was found in all species. At intensities approaching light saturation, the relation was found to deviate from linearity. The slope of the line in the light-limited range is species dependent and related to differences in absorption cross-section of Photosystem II. The observed non-linearity at high irradiances is not caused by photorespiration but probably by a Mehler-type of oxygen reduction. The relationship could be modelled by including a redox-state dependent oxygen uptake. In the diatom t Phaeodactylum tricornutum, the photochemical efficiency of dark adapted open Photosystem II centers was found to be temperature-dependent with an optimum near 10°C.     

Light saturation curves show competence of the water splitting complex in inactive Photosystem II reaction centers

Photosystem II complexes of higher plants are structurally and functionally heterogeneous. While the only clearly defined structural difference is that Photosystem II reaction centers are served by two distinct antenna sizes, several types of functional heterogeneity have been demonstrated. Among these is the observation that in dark-adapted leaves of spinach and pea, over 30% of the Photosystem II reaction centers are unable to reduce plastoquinone to plastoquinol at physiologically meaningful rates. Several lines of evidence show that the impaired reaction centers are effectively inactive, because the rate of oxidation of the primary quinone acceptor, Q_A, is 1000 times slower than in normally active reaction centers. However, there are conflicting opinions and data over whether inactive Photosystem II complexes are capable of oxidizing water in the presence of certain artificial electron acceptors. In the present study we investigated whether inactive Photosystem II complexes have a functional water oxidizing system in spinach thylakoid membranes by measuring the flash yield of water oxidation products as a function of flash intensity. At low flash energies (less that 10% saturation), selected to minimize double turnovers of reaction centers, we found that in the presence of the artificial quinone acceptor, dichlorobenzoquinone (DCBQ), the yield of proton release was enhanced 20±2% over that observed in the presence of dimethylbenzoquinone (DMBQ). We argue that the extra proton release is from the normally inactive Photosystem II reaction centers that have been activated in the presence of DCBQ, demonstrating their capacity to oxidize water in repetitive flashes, as concluded by Graan and Ort (Biochim Biophys Acta (1986) 852: 320–330). The light saturation curves indicate that the effective antenna size of inactive reaction centers is 55±12% the size of active Photosystem II centers. Comparison of the light saturation dependence of steady state oxygen evolution in the presence of DCBQ or DMBQ support the conclusion that inactive Photosystem II complexes have a functional water oxidation system.Abbreviations DCBQ 2,6-dichloro-p-benzoquinone - DMBQ 2,5-dimethyl-p-benzoquinone - F_o initial fluorescence level using dark-adapted thylakoids - Inactive reaction centers reaction centers inactive in plastoquinone reduction - PS II Photosystem II - Q_A primary quinone acceptor of Photosystem II - Q_B secondary quinone acceptor of Photosystem II Department of Plant Biology, University of IllinoisDepartment of Physiology & Biophysics, University of Illinois     

Induction kinetics of delayed light emission in spinach chloroplasts

Induction curves of the delayed light emission in spinach chloroplasts were studied by measuring the decay kinetics after each flash of light. This study differs from previous measurements of the induction curves where only the intensities at one set time after each flash of light were recorded. From the decay kinetics after each flash of light, the induction curves of the delayed light emission measured 2 ms after a flash of light were separated into two components: one component due to the last flash only and one component due to all previous flashes before the last one. On comparing the delayed light induction curves of the two components with the fluorescence induction curves in chloroplasts treated with 3-(3,4-dichlorophenyl)-1,1-dimethylurea and in chloroplasts treated with hydroxylamine and 3-(3,4-dichlorophenyl)-1,1-dimethylurea, the component due to the last flash only is found to be dependent on the concentration of open reaction centers and the component due to all previous flashes except the last is dependent on the concentration of closed reaction centers. This implies that the yield of the fast decaying component of the delayed light emission is dependent on the concentration of open reaction centers and the yield of the slow decaying component is dependent on the concentration of closed reaction centers.     

Intactness of the oxygen-evolving system in thylakoids and Photosystem II particles

Photosystem II activity of oxygen-evolving membranes can be quantified by their capacity to do charge separation or their capacity to transport electrons. In this study using flash excitation of saturating intensity, charge separation is measured by absorption changes in the ultraviolet region of the spectra associated with primary-quinone reduction, and electron transport is measured by oxygen flash yield. These methods are applied to thylakoids and three different types of Photosystem II particles. In thylakoids electron-transport activity is 75–85% of charge separation activity. In Photosystem II particles this percentage is 60–70%, except for the BBY type (Berthold, D.A., Babcock, G.T. and Yocum, C.F. (1981) FEBS Lett. 135, 231–234), in which it is only 29%. These estimates of non-functional oxygen-evolving centers agree within experimental error, except for the BBY particle, with the quantum requirement for oxygen evolution measured under light-limited conditions. These reaction centers that are non-functional in oxygen evolution occur during sample preparation and are not a result of inhibition by ferricyanide or quinone acceptor systems. In thylakoids on the first flash, absorption changes at 325 nm do not show significant contributions from oxygen evolution S-state transitions. In the presence of ferricyanide the absorption change at 325 nm does have a significant contribution from Q₄₀₀ in thylakoids, but considerably less in Photosystem II particles.     

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.     

Effects of Growth Irradiance and Nitrogen Limitation on Photosynthetic Energy Conversion in Photosystem II

Photosynthetic energy conversion was investigated in five species of marine unicellular algae, (Dunaliella tertiolecta, Thalassiosira pseudonana, T. weisflogii, Skeletorema costatum, Isochrysis galbana) representing three phylogenetic classes, which were grown under steady state conditions with either light or inorganic nitrogen as a limiting factor. Using a pump and probe fluorescence technique we measured the maximum change in variable fluorescence yields, the flash intensity saturation curves for the change in fluorescence yields and the kinetics of the decay in fluorescence yields. Under all growth irradiance levels nutrient replete cells exhibited approximately the same changes in fluorescence yields and similar fluorescence decay kinetics. The apparent relative absorption cross-section of photosystem II, calculated from the slope of the flash intensity saturation curves, generally increased as cells shade adapted. The decay kinetics of the fluorescence yield following a saturating pump flash can be expressed as the sum of three exponential components, with half-times of 160 and 600 microseconds and 30 to 300 milliseconds. The relative contribution of each component did not change significantly with growth irradiance. As cells became more nitrogen limited, however, the maximum change in fluorescence yield decreased, and was accompanied by a decrease in the proportion of a 160 microsecond fluorescence decay component, which corresponds to the transfer of electrons from Q_a⁻ to Q_b. Changes in fluorescence yields were also accompanied by changes in the levels of D1, a protein which is integral in reaction center II, and CP47, a chlorophyll protein forming part of the core of photosystem II. These results are consistent with a loss of functional photosystem II reaction centers. Moreover, in spite of losses of total cellular chlorophyll, which invariably accompanied nitrogen limitation, the apparent absorption cross-sections of photosystem II increased. Our results suggest that nitrogen limitation leads to substantial decreases in photosynthetic energy conversion efficiency.     

Energy transfer in the photochemical apparatus of flashed bean leaves

Fluorescence and energy transfer properties of bean leaves greened by brief, repetitive xenon flashes were studied at −196 °C. The bleaching of P-700 has no influence on the yield of fluorescence at any wavelength of emission. The light-induced fluorescence yield changes which are observed in both the 690 and 730 nm emission bands in the low temperature fluorescence spectra are due to changes in the state of the Photosystem II reaction centers. The fluorescence yield changes in the 730 nm band are attributed to energy transfer from Photosystem II to Photosystem I. Such energy transfer was also confirmed by measurements of the rate of photooxidation of P-700 at −196 °C in leaves in which the Photosystem II reaction centers were either all open or all closed. It is concluded that energy transfer from Photosystem II to Photosystem I occurs in the flashed bean leaves which lack the light-harvesting chlorophyll a/b protein.     

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.     

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     

Trapping and annihilation in the antenna system of Photosystem I

The primary charge separation in Photosystem I of pea chloroplasts was measured as a photovoltage in the pico- and nanosecond time range by applying laser flashes at 532 nm of variable energy and different duration (12 ns and 30 ps, respectively). Contributions to the photovoltage from Photosystem II was eliminated by addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea and preillumination. The dependence of the photovoltage amplitude on the excitation energy could be described by an exponential saturation law when the excitation flash had a duration of 12 ns. Nearly the same dependence was found when the excitation source was the train of a mode-locked laser (approx. ten 30-ps flashes spaced by 7 ns; highest energy of a single flash, 80 μJ / cm⁻²). Even with single 30-ps flashes the photovoltage was only slightly smaller than the one elicited by 12-ns flashes of the same energy. These findings demonstrate that trapping of excitation energy by the reaction center of Photosystem I is much more effective than losses by annihilation and other loss processes. The photovoltage yield was nearly independent of the fraction of closed traps, thus demonstrating that the absorption cross section of Photosystem I is not altered by the closing of its reaction centers. By recording the rise time of the photovoltage with our highest time resolution we found that the trapping rate of the excitation energy in Photosystem I depended on the energy of the 30-ps flashes: at low excitation energies (less than 10¹⁴ photons / cm² per pulse) trapping occurred within 90 ± 15 ps and at high excitation energy (10¹⁵ photons / cm² per pulse) trapping and charge stabilization occurred within the time resolution of the apparatus, i.e., up to 50 ps. The trapping rate at low energies is in agreement with the one determined by fluorescence decay kinetics. Up to 50 ns there was no further detectable electrogenic phase (neither forward nor backward reactions). This demonstrates that all the electrogenicity, produced by the charge separation, takes place in less than 50 ps.     

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     

Evidence for a double hit process in photosystem II based on fluorescence studies

1. The amplitudes of the fast (0-20 microseconds) and slow (20 microseconds-2 ms) fluorescence rise induced by a 2 microseconds flash have been measured as a function of the energy of the flash in chloroplasts inhibited by 3(3,4-dichlorophenyl)-1, 1-dimethylurea. The saturation curve for the slow rise shows a characteristic lag which is not observed for the fast fluorescence rise. This lag indicates that Photosystem II centers undergo a double hit process which implies that (a), each photocenter includes two acceptors Q1 and Q2; (B), after the first hit, oxidized chlorophyll Chl+ is reduced by a secondary acceptor Y in a time shor compared to the duration of the flash; (c), after the second hit, Chl+ is reduced by another secondary donor, D. 2. According to Den Haan et al. (1974) Biochim. Biophys. Acta 368, 409-421), hydroxylamine destroys the secondary donor responsible for the fast reduction of Chl+. In the presence of 3 mM hydroxylamine, only the secondary donor D is functional and a flash induses mainly a single hit process. 3. The saturation curves for the fast and the slow rises have been studied in the presence of 3(3,4-dichlorophenyl)-1, 1-dimethylurea for a second actinic flash given 2.5 s after a first saturating one. The large decrease in the half-saturating energy indicates the existence of efficient energy transfer occuring between potosynthetic units. 4. Two alternate hypotheses are discussed (a) in which D is an auxiliary donor and (b) in which D is included in the main electron transfer chain.     

A seconds range component of the reoxidation of the primary Photosystem II acceptor,Q. Effects of bicarbonate depletion in chloroplasts

Broken chloroplasts depleted of bicarbonate (HCO^?₃) show 30–50% inhibition of the Hill reaction in low-intensity light. Also, photoreactions excited by repetitive flashes measured by oxygen evolution, ESR signal IIvf, and absorption changes at 680 and 334 nm show inhibition of 30–50%. An effect of HCO^?₃ was sought to explain these phenomena. The decay of chlorophyll a fluorescence yield in the millisecond and seconds range, following a single flash, was observed to be multiphasic with a very slow component of 1–2 s half-time. In HCO^?₃ -depleted samples this component is enhanced 2- or 3-fold. Since this occurs even after one flash, it is suggested that HCO^?₃ affects the Q^? B → QB^? reaction. In this work it is shown that 40% inhibition of oxygen flash yield is relieved to a great extent if the excitation flash rate is decreased from 2 to 0.33 Hz. A measurement of 520 nm absorption change in the presence of ferricyanide, which is proportional to Photosystem II charge separation, shows a similar inhibition that is dependent on flash rate. The maximum amplitude of variable fluorescence yield and 520 nm absorption change after a single flash are unaffected by HCO^?₃ depletion. The dark distribution of oxygen-evolution S-states is found to be shifted to a more reduced configuration in depleted samples. It is concluded that normal charge separation occurs in HCO^?₃ -depleted Photosystem II reaction centers but that a large fraction of Q^? decays so slowly that not all Q^? is reoxidized between flashes given at a rate of 1 or 2 Hz. Thus, a portion of the Photosystem II centers would be closed to photochemistry. There is a reversible effect of HCO^?₃ depletion on the oxygen-evolution system that is observed as a shift in the dark distribution of S-states.     

Evidence for a double hit process in Photosystem II based on fluorescence studies

1. The amplitudes of the fast (0–20 μs) and slow (20 μs–2 ms) fluorescence rise induced by a 2 μs flash have been measured as a function of the energy of the flash in chloroplasts inhibited by 3(3,4-dichlorophenyl)-1,1-dimethylurea. The saturation curve for the slow rise shows a characteristic lag which is not observed for the fast fluorescence rise. This lag indicates that Photosystem II centers undergo a double hit process which implies that (a), each photocenter includes two acceptors Q₁ and Q₂; (b), after the first hit, oxidized chlorophyll Chl⁺ is reduced by a secondary acceptor Y in a time short compared to the duration of the flash; (c), after the second hit, Chl⁺ is reduced by another secondary donor, D.

2. According to Den Haan et al. ((1974) Biochim. Biophys. Acta 368, 409–421), hydroxylamine destroys the secondary donor responsible for the fast reduction of Chl⁺. In the presence of 3 mM hydroxylamine, only the secondary donor D is functional and a flash induces mainly a single hit process.

3. The saturation curves for the fast and the slow rises have been studied in the presence of 3(3,4-dichlorophenyl)-1,1-dimethylurea for a second actinic flash given 2.5 s after a first saturating one. The large decrease in the half-saturating energy indicates the existence of efficient energy transfer occuring between photosynthetic units.

4. Two alternate hypotheses are discussed (a) in which D is an auxiliary donor and (b) in which D is included in the main electron transfer chain.     

Delayed fluorescence induction in chloroplasts. Irradiance dependence

We have studied the delayed fluorescence in spinach chloroplasts produced 0.5 ms after each of a pair of (sub)-microsecond flashes. We observe an increase in the delayed fluorescence from the second flash relative to that produced by the first. This increase is proportional to the product of the first and second flash irradiances, appearing as an I² dependence if both flashes are increased together. The enhancement is observable at very weak flash levels (roughly 1 photon absorbed/100 PS II centers). If the irradiance of the first flash is increased, but the irradiance of the second held constant, the delayed fluorescence from the second flash is observed to increase, but then to saturate well below the first flash irradiance at which the delayed fluorescence from the first flash itself saturates. For most experiments, the dark time between flashes was 30 ms. If the dark time is varied, the enhancement changes, reaching a half-maximal value for a dark time of approx. 300 μs. The enhancement is stopped by hydroxylamine, but not by gramicidin, valinomycin, DCMU, or mild heating. These experiments are consistent with the notion that there are two different types of Photosystem II centers if we assume that only one type is responsible for the induction we see and has an optical cross-section about 4-times the size of the other type of center.     

Evidence for slow turnover in a fraction of Photosystem II complexes in thylakoid membranes

We used two different techniques to measure the recovery time of Photosystem II following the transfer of a single electron from P-680 to Q_A in thylakoid membranes isolated from spinach. Electron transfer in Photosystem II reaction centers was probed first by spectroscopic measurements of the electrochromic shift at 518 nm due to charge separation within the reaction centers. Using two short actinic flashes separated by a variable time interval we determined the time required after the first flash for the electrochromic shift at 518 nm to recover to the full extent on the second flash. In the second technique the redox state of Q_A at variable times after a saturating flash was monitored by measurement of the fluorescence induction in the absence of an inhibitor and in the presence of ferricyanide. The objective was to determine the time required after the actinic flash for the fluorescence induction to recover to the value observed after a 60 s dark period. Measurements were done under conditions in which (1) the electron donor for Photosystem II was water and the acceptor was the endogenous plastoquinone pool, and (2) Q₄₀₀, the Fe²⁺ near Q_A, remained reduced and therefore was not a participant in the flash-induced electron-transfer reactions. The electrochromic shift at 518 nm and the fluorescence induction revealed a prominent biphasic recovery time for Photosystem II reaction centers. The majority of the Photosystem II reaction centers recovered in less than 50 ms. However, approx. one-third of the Photosystem II reaction centers required a half-time of 2–3 s to recover. Our interpretation of these data is that Photosystem II reaction centers consist of at least two distinct populations. One population, typically 68% of the total amount of Photosystem II as determined by the electrochromic shift, has a steady-state turnover rate for the electron-transfer reaction from water to the plastoquinone pool of approx. 250 e⁻ / s, sufficiently rapid to account for measured rates of steady-state electron transport. The other population, typically 32%, has a turnover rate of approx. 0.2 e⁻ / s. Since this turnover rate is over 1000-times slower than normally active Photosystem II complexes, we conclude that the slowly turning over Photosystem II complexes are inconsequential in contributing to energy transduction. The slowly turning over Photosystem II complexes are able to transfer an electron from P-680 to Q_A rapidly, but the reoxidation of Q⁻_A is slow (t1/2 = 2 s). The fluorescence induction measurements lead us to conclude that there is significant overlap between the slowly turning over fraction of Photosystem II complexes and PS II_β reaction centers. One corollary of this conclusion is that electron transfer from P-680 to Q_A in PS II_β reaction centers results in charge separation across the membrane and gives rise to an electrochromic shift.     

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 degrees 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 degrees 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.     

Target theory and the photoinactivation of Photosystem II

Application of target theory to the photoinactivation of Photosystem II in pea leaf discs (Park et al. 1995, 1996a,b) reveals that there is a critical light dosage below which there is complete photoprotection and above which there is photoinactivation (i.e a light-induced loss of oxygen flash yield). The critical dosage is about 3 mol photons m^–2 for medium and high light-grown leaves and 0.36 mol photons m^–2 for low light-grown leaves. Photoinactivation is a one-hit process with an effective cross-section of 0.045 m² mol^–1 photons which does not vary with growth irradiance, unlike the cross-section for oxygen evolution which increases with decreasing growth irradiance. The cross-section for oxygen evolution increased by about 20% following exposure to 6.8 mol photons m^–2 which may be due to energy transfer from photoinactivated units to functional Photosystem II units. We propose that the photoinactivation of PS II begins when a small group of PS II pigment molecules whose structure is uninfluenced by growth irradiance, becomes uncoupled energetically from the rest of the photosynthetic unit and thus no longer transfers excitions to P680. De-excitation of this group of pigment molecules provides the energy which leads to the damage of Photosystem II. Treatment of pea leaves with dithiothreitol, an inhibitor of the xanthophyll cycle, decreases the critical dosage i.e. decreases photoprotection but has no effect on the PS II photoinactivation cross-section. Treatment with 1 M nigericin increased the photoinactivation cross-section of PS II as did exposure to lincomycin which inhibits D1 protein synthesis and thus the repair of PS II reaction centres.Abbreviations DTT- dithiothreitol - PS II- Photosystem II - Fm- maximum fluorescence - Fv- variable fluorescence - LHCIIb- main light harvesting pigment-protein complex of PS II - D1 protein- psbA gene product - P680- reaction centre chlorophyll of Photosystem II - Qa- first quinone electron acceptor of Photosystem II - (o₂)- cross-section for oxygen evolution - (pi)- cross-section for photoinactivation     

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 degrees C fluorescence induction curves at 690 and 730 nm in the absence and presence of 5mMMgCl2 which presumably changes the distrubution of excitation energy between the two photosystems. The equations describing the fluorescence properties involve terms for the distribution of absorbed quanta, alpha, being the fraction distributed to Photosystem I, and beta, the fraction to Photosystem II to Photosystem I, KT(II yields I). The data, analyzed within the context of the model, permit a direct comparison of alpha and kt(II yields I) in the absence (minus) and presence (+) of Mg-2+ :alpha minus/alpha-+ equals 1.2 and k-minus t)II yields I)/K-+T(II yields I) equal to 1.9. If the criterion that alpha + beta equal to 1 is applied absolute values can be calculated: in the presence of Mg-2+, alpha-+ equal to 0.27 and the yield of energy transfer, phi-+ t(II yields I) varied the presence of Mg-2+, alpha-+ equal to 0.27 and the yield of energy transfer, phi-+ t(II yields 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 Mg-2+, alpha-minus equal to 0.32 and phi t(II yields 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-harvesting chlorophyll of Photosystem II to Photosystem I, kt(II yields I), and a transfer from the reaction centers of Photosystem II to Photosystem I, kt(II yields I). In that case alpha-minus/alpha+ equal to 1.3, k-minus t(II yields I)/k+ t(II yields I)equal to 1.3 and k-minus t(II yields I) equal to 3.0. It was concluded, however, that both of these types of energy transfer are different manifestations of a single energy transfer process.