Photorespiration is More Effective than the Mehler Reaction in Protecting the Photosynthetic Apparatus against Photoinhibition

I. Isolated intact chloroplasts: Photosystem II, but not photosystem I, of the electron transport chain is rapidly photoinactivated even by very low intensities of red light when no large proton gradient can be formed and the electron transport chain becomes over-reduced in the absence of oxygen and other reducable substrates. Electron acceptors including oxygen provide protection against photoinactivation. Nevertheless, photosystem II is rapidly, and photosystem I more slowly, photoinactivated by high intensities of red light when oxygen is the only electron acceptor available. Increased damage is observed at increased oxygen concentrations although catalase is added to destroy H₂O₂ formed during oxygen reduction in the Mehler reaction. Photoinactivation can be decreased, but not prevented by ascorbate which reduces hydrogen peroxide inside the chloroplasts and increases coupled electron flow. II. Leaves: Simple measurements of chlorophyll fluorescence permit assessment of damage to photosystem II after exposure of leaves to high intensity illumination. In contrast to isolated chloroplasts, chloroplasts suffer more damage in situ at reduced than at elevated oxygen concentrations. The difference in the responses is due to photorespiration which is active in leaves, but not in isolated chloroplasts. After photosynthesis and photorespiration are inhibited by feeding glyceraldehyde to leaves, photoinactivation is markedly increased, although oxygen reduction in the Mehler reaction is not affected by glyceraldehyde. In the presence of reduced CO₂ levels, photorespiratory reactions, but not the Mehler reaction, can prevent the overreduction of the electron transport chain. Over-reduction indicates ineffective control of photosystem II activity. Effective control is needed for protection of the electron transport chain against photoinactivation. It is suggested to be made possible by coupled cyclic electron flow around photosystem I which is facilitated by the redox poising resulting from the interplay between photorespiratory carbohydrate oxidation and the refixation of evolved CO₂.     

Chemical modification studies of chloroplast membranes. Water oxidation inhibition by diazonium-benzenesulfonic acid

The water-soluble chemical modifier, diazonium benzene-sulfonic acid, significantly inhibited photosystem II-dependent water oxidation (oxygen evolution) when the compound was reacted with chloroplast membranes in the light but not in the dark. The photochemistry of photosystem II was not affected by the diazonium treatment, shown by complete restoration of photosystem II-dependent electron flow from the alternate electron donor diphenylcarbazide.Paralleling the inhibition of oxygen evolution the illuminated chloroplasts bound significantly more diazonium reagent than did chloroplasts treated in the dark. Both the inhibition of oxygen evolution and the increased binding of the diazonium to the membranes were dependent on photosystem II electron flux, which could not be replaced by photosystem I cyclic electron flow. A dark base to acid or acid to base transition resulted in a similar inhibition of water oxidation and increased diazonium binding.The results suggest a membrane conformational change associated with photosystem II electron flow that exposes otherwise buried diazo reactive groups at the external grana membrane surface.     

Regulation of photosynthetic electron transport and photophosphorylation in intact chloroplasts and leaves of Spinacia oleracea L.

Oxygen ist reduced by the electron transport chain of chloroplasts during CO₂ reduction. The rate of electron flow to oxygen is low. Since antimycin A inhibited CO₂-dependent oxygen evolution, it is concluded that cyclic photophosphorylation contributes ATP to photosynthesis in chloroplasts which cannot satisfy the ATP requirement of CO₂ reduction by electron flow to NADP and to oxygen. Inhibition of photosynthesis by antimycin A was more significant at high than at low light intensities suggesting that cyclic photophosphorylation contributes to photosynthesis particularly at high intensities. Cyclic electron flow in intact chloroplasts is under the control of electron acceptors. At low light intensities or under far-red illumination it is decreased by substrates which accept electrons from photosystem I such as oxaloacetate, nitrite or oxygen. Obviously, the cyclic electron transport pathway is sensitive to electron drainage. In the absence of electron acceptors, cyclic electron flow is supported by far-red illumination and inhibited by red light. The inhibition by light exciting photosystem II demonstrated that the cyclic electron transport pathway is accessible to electrons from photosystem II. Inhibition can be relieved by oxygen which appears to prevent over-reduction of electron carriers of the cyclic pathway and thus has an important regulatory function. The data show that cyclic electron transport is under delicate redox control. Inhibition is caused both by excessive oxidation and by over-reduction of electron carriers of the pathway.     

Effect of cadmium on photosystem II activity in <Emphasis Type="Italic">Chlamydomonas reinhardtii</Emphasis>: alteration of O–J–I–P fluorescence transients indicating the change of apparent activation energies within photosystem II

In this study, we evaluated how cadmium inhibitory effect on photosystem II and I electron transport may affect light energy conversion into electron transport by photosystem II. To induce cadmium effect on the photosynthetic apparatus, we exposed Chlamydomonas reinhardtii 24 h to 0–4.62 μM Cd²⁺. By evaluating the half time of fluorescence transients O–J–I–P at different temperatures (20–30°C), we were able to determine the photosystem II apparent activation energies for different reduction steps of photosystem II, indicated by the O–J–I–P fluorescence transients. The decrease of the apparent activation energies for PSII electron transport was found to be strongly related to the cadmium-induced inhibition of photosynthetic electron transport. We found a strong correlation between the photosystem II apparent activation energies and photosystem II oxygen evolution rate and photosystem I activity. Different levels of cadmium inhibition at photosystem II water-splitting system and photosystem I activity showed that photosystem II apparent activation energies are strongly dependent to photosystem II donor and acceptor sides. Therefore, the oxido-reduction state of whole photosystem II and I electron transport chain affects the conversion of light energy from antenna complex to photosystem II electron transport.     

Absorption and Transfer of Light and Photoreduction Activities of Spinach Chloroplasts under Calcium Deficiency: Promotion by Cerium

Chloroplasts were isolated from spinach cultured in calcium-deficient, cerium-chloride-administered calcium-present Hoagland’s media or that of calcium-deficient Hoagland’s media and demonstrated the effects of cerium on distribution of light energy between photosystems II and I and photochemical activities of spinach chloroplast grown in calcium-deficient media. It was observed that calcium deprivation significantly inhibited light absorption, energy transfer from LHCII to photosystemII, excitation energy distribution from PSI to PSII, and transformation from light energy to electron energy and oxygen evolution of chloroplasts. However, cerium treatment to calcium-deficient chloroplasts could obviously improve light absorption and excitation energy distribution from photosystem I to photosystem II and increase activity of whole chain electron transport, photosystems II and I DCPIP photoreduction, and oxygen evolution of chloroplasts. The results suggested that cerium under calcium deficiency condition could substitute for calcium in chloroplasts, maintain the stability of chloroplast membrane, and improve photosynthesis of spinach chloroplast, but the mechanisms still need further study.     

Kinetics Studies on the Fluorescence Quencher in Isolated Chloroplasts

The chlorophyll fluorescence yield in isolated chloroplasts without an added electron acceptor is increased by actinic illumination. The decline in the fluorescence yield when the actinic illumination is extinguished can be accurately represented by three, independent, exponential decays with half-times of approximately 0.8, 5, and 30 sec. These results have been interpreted using Duysens' theory of fluorescence quenching by a compound (Q) on the reducing side of photosystem II. This theory states that changes in fluorescence yield are indicative of electron flow through Q. The most rapid decay is eliminated by an EDTA washing of the chloroplasts and the half-time is increased by uncoupling with ammonia and by added electron acceptors in suboptimal concentrations. Thus, this decay may represent electron flow from Q to intermediates on the oxidizing side of photosystem I. The decay with a half-time of 5 sec is affected in the same manner as the decay with the shortest half-time by the same procedures. However, electron donors to photosystem II lengthen the half-time of the 5 sec decay while eliminating the most rapid decay. This 5 sec decay can be interpreted as electron flow from Q to intermediates either on the reducing side of photosystem II or on the oxidizing side of photosystem I. The decay with the longest half-time is affected only by pH and electron donors to photosystem II. Therefore, this decay may indicate electron flow from Q to intermediates on the oxidizing side of photosystem II which may be connected to the regeneration of the oxygen burst.     

Action of UV-B radiation on photosynthetic primary reactions in spinach chloroplasts

Spinach ( Spinacia oleracea L. cv. Matador) chloroplasts were irradiated with several levels of UV-B radiation. Measurements which reflect characteristic steps of photosynthetic electron transport were made to localize the site of impairment of photosynthesis by UV-B radiation.
Variable fluorescence, the μs-kinetics of the 320 nm absorption changes and also oxygen evolution were substantially reduced in chloroplasts irradiated with UV-B. It was not possible to restore the amplitude of the 320 nm absorption changes nor the signal of the transmembrane electric field measured at 520 nm by adding the photo-system II donor couple hydroquinone/ascorbate to UV-B treated chloroplast samples. This indicates that impairment of photosystem II activity is not caused by selective inhibition of the water-splitting enzyme system Y, but rather is due to blockage of photosystem II reaction centers. Photosystem 1 is inferred to be highly resistant to UV-B radiation.
These results suggest that the reaction centers of photosystem II are transformed into dissipative sinks for excitation energy by action of UV-B radiation.     

Effects of oxygen on the electron transport chain of photosynthesis

Summary Oxygen was taken up by both intact and broken chloroplasts when catalase was posioned. In confirmation of other work we found that oxygen enters the electron transport chain of isolated chloroplasts by oxidizing the primary photoreductant of system I. In isolated intact chloroplasts this reaction proceeds in addition to oxygen evolution by PGA reduction. The reductant produced by photosystem II does not react with oxygen at a significant rate.In normal leaves oxygen depresses chlorophyll fluorescence. However, this depression does not take place in DCMU poisoned leaves or in a mutant having a nonfunctional photosystem II; furthermore, another mutant with a weakly functioning photosystem I gave only a very small fluorescence depression with oxygen. This shows that the site of interaction of oxygen is at the reducing end of the electron transport chain. This view is supported by the extent of the fluorescence depression in leaves as a function of oxygen concentration which is very similar to the oxygen dependence of oxygen uptake by isolated chloroplasts.An oxygen requirement of isolated intact chloroplasts reducing PGA and nitrate was indicated by lower reaction rates and faster decay of activity under nitrogen than under air.Dedicated to Prof. Harder on his eightieth birthday.     

Studies on the ADRY agent-induced mechanism of the discharge of the holes trapped in the photosynthetic waterspliting enzyme system Y

The effect of 2-(3-chloro-4-trifluoromethyl)anilino-3,5-dinitrothiophene (ANT 2p) on the oxygen evolution, fluorescence and delayed light emission of spinach chloroplasts has been investigated. It was found that;

1. 1. ANT 2p strongly accelerates the deactivation of states S₂ and S₃ of the water-splitting enzyme system Y.

2. 2. In DCMU-poisoned chloroplasts ANT 2p prevents the back reaction of the electrons located at the primary acceptor, Q, with the holes (positive charges) stored in the water-splitting enzyme system Y.

3. 3. In chloroplast suspensions without artificial electron acceptors, the fluorescence rise in weak actinic light vanishes in the presence of ANT 2p. The fluorescence yield in DCMU-inhibited chloroplasts is not significantly changed by ANT 2p.

4. 4. The intensity of the delayed light emitted after excitation with one short flash is remarkably decreased by ANT 2p.

5. 5. In weak actinic light the reduction rate of the artificial electron acceptor methyl viologen is suppressed in the presence of ANT 2p.

From these experimental results it is concluded that ANT 2p induces a cycle within the electron transport chain, leading to a dissipative recombination of the holes stored in the water-splitting enzyme Y with the electrons of an as yet unknown donor.

Two possibilities for the mode of action of this cycle are discussed.     

Oxygen Evolution and Oxygen Uptake by Isolated Chloroplasts of Wheat Irradiated with Monochromatic Light, without the Addition of an Oxidant

The oxygen exchange obtained when isolated chloroplasts of wheat are irradiated, without the addition of a Hill oxidant, has been investigated. Depending on the wavelength, two types of oxygen exchange are obtained. In light absorbed by both photosystems an oxygen gush appears directly upon irradiation. This oxygen evolving reaction is soon replaced by an oxygen uptake which is present until the end of the irradiation period. In light absorbed mainly in photosystem I, no oxygen gush can be observed, instead an oxygen uptake appears directly upon irradiation. An oxygen evolving process can also be observed in irradiations performed with photo-system I light, but this process appears after 10–15 seconds of irradiation. The influence of various external factors on the oxygen gush and the oxygen uptake, e.g. different wavelengths, light intensity, length of the dark periods between irradiations, was studied. The results show that the oxygen evolving reaction appearing upon irradiation with light absorbed by photosystem II and I, reflect the reduction of an oxidant, probably plasto-quinone, in the electron transport chain between the two photosystems. The reoxidation of this oxidant can be brought about after irradiating with light absorbed in photosystem I, or by prolonging the dark period between irradiations, or through some unknown process connected to photosystem II. The oxygen uptake which consists of two components, one appearing directly upon irradiation and the other one appearing after about 10 seconds of irradiation, confirms earlier observations that oxygen can be reduced in photosystem I. The electrons for the oxygen uptake appearing directly upon irradiation, are obtained from the reduced intermediates in the electron transport chain between the two photosystems. The electrons for the other oxygen uptake process are obtained from a reductant in the chloroplasts with access to the carrier chain between the photosystems. Whether the two oxygen uptake reactions reflect two sites of interaction of oxygen with the electron transport chain or only one site is discussed.     

Computer simulation study of pH-dependent regulation of electron transport in chloroplasts

A mathematical model is presented that describes the key steps of photosynthetic electron transport and transmembrane proton transfer in chloroplasts. Numerical modeling has been performed with due regard for regulatory processes at the donor and acceptor parts of photosystem (PS) I. The influence of pH-dependent activation of the Calvin cycle enzymes and energy dissipation in PS II (nonphotochemical quenching of chlorophyll fluorescence) on the light-induced redox transients of P₇₀₀, plastoquinone, and NADP as well as on the changes in intrathylakoid pH and ATP level is examined. It is demonstrated that pH-dependent regulatory processes alter the distribution of electron fluxes on the acceptor side of PS I and the total rate of electron flow between PS II and PS I. The light-induced activation of the Calvin cycle leads to significant enhancement of the electron flow from PS I to NADP⁺ and attenuation of the electron flow to molecular oxygen.     

Effects of Some Chemicals on the Oxygen Evolution and Oxygen Uptake in Isolated Wheat Chloroplasts Irradiated without the Addition of an Oxidant

The oxygen exchange, obtained when isolated chloroplasts of Triticum aestivum, wheat, are irradiated without the addition of a Hill oxidant has been investigated using an oxygen electrode. Ascorbate, catalase, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone(DBMIB), diethyldithio-carbamate (DEDT), dichlorophenylmethylurea (DCMU), and potassium cyanide were added to the Chloroplasts in order to investigate the oxygen exchange. At least two oxygen uptake reactions, one sensitive to catalase and one catalase-insensitive, appeared upon irradiation. Hydrogen peroxide was the product of the oxygen uptake in the former process, and water was the reductant. The formation of hydrogen peroxide was probably associated with photosystem I. The other oxygen consuming reaction was found to be insensitive to both catalase and potassium cyanide. After the chloroplasts had been treated with DCMU, it was possible to show that the catalase-insensitive oxygen uptake was localized in photosystem I, and that a cyclic electron transport system or some endogenous reductant (-s) acted in the oxygen uptake. Addition of ascorbate or DEDT to the chloroplasts led to an enhanced oxygen uptake in 710 nm light. This was probably due to the effect of these compounds on the superoxide radical ion formed in photosystem I. The stimulated oxygen uptake was only weakly affected by catalase, indicating that hydrogen peroxide was not a product of this oxygen uptake. Addition of DEDT and potassium cyanide inhibited (strongly respectively weakly) the oxygen uptake when photosystem II was functioning. The effect of these compounds was probably due to an inhibition of the electron transport at the plastocyanin. DBMIB inhibited the oxygen uptake reactions and the cooperation between the two photosystems. The cooperation between the photosystems was also studied in DCMU-treated chloroplasts. The reactions in photosystem II, measured as oxygen evolution, were more inhibited than the coupling between the photosystems. The oxygen “gush” appearing upon irradiation in light of 650 nm was not affected by a DBMIB-treatment, showing that the oxygen evolution was due to the reduction of plastoquinone. The reoxidation in the dark of the plastoquinone pool was stimulated by DBMIB and potassium cyanide indicating that an oxygen uptake could be associated with plastoquinone. The sites of interaction of oxygen with the electron transport pathways in chloroplasts, and the different reductants for the oxygen consuming reactions are discussed.     

The effects of digitonin on photochemical activities of isolated chloroplasts

The addition of digitonin to chloroplasts stimulated the rate of oxygen evolution followed by a gradual inhibition. The effect of digitonin was dependent on the digitonin to chlorophyll ratio and on temperature and time. The initial stimulation of oxygen evolution appeared to be a result of uncoupling as digitonin did not stimulate oxygen evolution by uncoupled chloroplasts. The stimulatory effect occurred more rapidly at high digitonin to chlorophyll ratios but the extent of stimulation was low and inhibition occurred soon after addition of the detergent. The inhibition of electron flow by digitonin was due to a site of action near photosystem II which resembled the inhibition reported for tris buffer and resulted in photobleaching. However, digitonin inhibition could not be recovered by washing with reducing agents and was only partially recovered by the addition of artificial electron donors to photosystem II. Electron flow mediated by photosystem I was unaffected by the addition of digitonin but was decreased when the chloroplasts were separated by subsequent centrifuging. This suggested that digitonin solubilizes photosystem I components which remain active in the soluble form.     

Localization of the Meler's reaction with ethanol catalase trap in the chain of photosynthetic electron transport]

The common view of photosystem I as the action site of catalase and ethanol at oxygen uptake in chloroplasts are based on indirect data on this reaction. That is why the question on Mehler reaction localization in electron transport chain with ethanolcatalase trap has been investigated anew. It has been demonstrated that oxygen uptake with catalase and ethanol does not decrease in presence of dibromothymoquinone (2,5-dibromo-3-methyl-6 isopropyl-p-benzoquinone--DBTQ) which blocks electron transfer to photosystem I at plastoquinones level. The summation of oxygen uptake activities is observed on the combined action of catalase and ethanol with any of the Mehler reagents functioning in photosystem I (methylviologen,FMN, epinephrine, ferredoxin). Catalase and ethanol in contrast to methylviologen have no effect on photooxidation rate of reduced dichlorphenolindophenol in photosystem I. The quatum yield of oxygen uptake with catalase and ethanol versus wave length of actinic light shows a distinct maximum in the photosystem II absorption area and a "red drop" in the longware area. The obtained data show that the Mehler reaction with catalase and ethanol takes place in photosystem II only.     

Studies on thylakoid phosphorylation and noncyclic electron transport

The effect of thylakoid phosphorylation on noncyclic electron transport in spinach chloroplasts was investigated by measuring both the reduction of nicotinamide adenine dinucleotide phosphate (NADP) and the steady-state redox level of the primary electron acceptor quinone of photosystem II (Q) during electron flow to NADP. These data are compared with the theoretical predictions for an electron transport model which relates both the redox levels of Q and the photosystem II optical cross section to the overall velocity of noncyclic electron flow. It is demonstrated that transfer of 15-20% of the photosystem II antenna to photosystem I may stimulate electron flow to NADP only if Q is less than 60-70% oxidized (this condition exists with our thylakoids, even at extremely low absorption fluxes, when the illumination is not specifically enriched in photosystem I absorbed wavelengths); in phosphorylated thylakoids the steady-state redox level Q is substantially shifted to a more oxidized one (measurements of this parameter using light of different wavelengths quantitatively support the idea that thylakoid phosphorylation leads to increased photosystem I and decreased photosystem II cross sections); thylakoid phosphorylation leads to stimulated noncyclic electron flow to NADP only when the increased photosystem I antenna is able to bring about large increases in the steady-state level of oxidized Q.     

Studies on the Mechanism of Photoinhibition in Higher Plants: I. EFFECTS OF HIGH LIGHT INTENSITY ON CHLOROPLAST ACTIVITIES IN CUCUMBER ADAPTED TO LOW LIGHT

Cucumber plants (Cucumis sativus L.), grown at low quantum flux density (120-150 microeinsteins per square meter per second) were photoinhibited by a three-hour exposure in air to ten times the light intensity experienced during growth. Chloroplasts were isolated from photoinhibited and control leaves and the following activities determined: O₂ evolution in the presence of ferricyanide, photosystem I activity, noncyclic and cyclic photophosphorylation, and light-induced proton uptake. Chlorophyll and chloroplast absorbance spectra, and chloroplast fluorescence were also measured. It was found that photosystem II electron transport and non-cyclic photophosphorylation were inhibited by about 50%, while cyclic photophosphorylation was less inhibited and photosystem I electron transport and light-induced proton uptake were unaffected. Electron transport to methylviologen could not be fully restored by electron donation to photosystem II. Chloroplast fluorescence induction at room temperature was strongly reduced following photoinhibition. There was no difference in the absorption spectra of the extracted chlorophylls from control and photoinhibited chloroplasts, but an increase of the absorption in the blue wavelength region was observed in the photoinhibited chloroplasts. It is suggested that high light stress does not result in alteration of the membrane properties, as is the case in low-temperature stress for example, but affects directly the photosynthetic reaction centers, primarily of photosystem II.     

Light Modulation of Glyceraldehyde-3-phosphate Dehydrogenase and Glucose-6-phosphate Dehydrogenase by Photosynthetic Electron Flow in Pea Chloroplasts

Light activation of NADP-linked glyceraldehyde-3-P dehydrogenase (EC 1.2.1.13) and light inactivation of glucose-6-P dehydrogenase (EC 1.1.1.49) appear to be modulated within pea leaf chloroplasts by mediators which are reduced by photosynthetic electron flow from the photosystem I reaction center. Dichlorophenyl-1, 1-dimethylurea inhibition of this modulation can be completely reversed by ascorbate plus 2,6-dichlorophenolindophenol in broken chloroplasts, but not in intact chloroplasts. Intact chloroplasts are impermeable to 2,6-dichlorophenolindophenol at pH 7.5. Studies on the effect of light in reconstituted chloroplasts with photosystem I-enriched particles in the place of whole thylakoids revealed that photosystem I participates in the light modulation of NADP-linked glyceraldehyde-3-P dehydrogenase and of glucose-6-P dehydrogenase.     

Action Spectrum for Photoactivation of the Water-splitting System in Plastids of Intermittently Illuminated Wheat Leaves

The chloroplasts from wheat leaves developed under intermittent illumination (1 ms light + 12 min dark) were able to photoreduce DPIP with DPC as electron donor but unable to photoreduce DPIP with water as electron donor. On exposure of these leaves to continuous light, the Hill activity with water as electron donor was rapidly induced. The photoactivation was sensitive to the treatment with DCMU prior to exposure to continuous light. The action spectrum for the photoactivation showed a sharp band at 680 nm with a distinct shoulder at 650 nm, and was similar to the absorption spectrum of photosytem-2 particles. These data suggest that the electron transfer driven by photosystem 2 is essential for the activation of the water-splitting system in the chloroplasts of intermittently illuminated leaves.     

Oxygenic photoreduction of methyl viologen and nicotinamide adenine dinucleotide phosphate without the involvement of photosystem I during plastid development

Studies on the appearance of various electron transport functions were followed during greening of etiolated cucumber cotyledons. Appearance of dichlorodimethoxy-p-benzoquinone, dimethyl quinone, tetramethyl-p-phenylenediamine, dichlorophenol indophenol and ferricyanide Hill reactions were observed after 8h of greening. However, photoreduction of methyl viologen (MV) and nicotinamide adenine dinucleotide phosphate (NADP) was observed from 2h of greening. Variable fluorescence, which is a direct indication of water-splitting function, was observed from 2h of greening in cotyledons, thylakoid membranes and photosystem II (PSII) particles. The decrease in variable fluorescence in the presence of MV (due to rapid reoxidation of Q-) observed from early stages of greening confirmed the photoreduction of MV by PSII. The early development of water-splitting function was further confirmed by the abolition of variable fluorescence in thylakoid membranes and PSII particles by heat treatment and concomittant loss of light dependent oxygen uptake in the presence of MV in heat treated chloroplasts. However, the photoreduction of MV and NADP was insensitive to intersystem electron transport inhibitors, dichlorophenyl dimethylurea or dibromomethyl isopropyl-p-benzoquinone till 8h of greening. Though the oxidation of intersystem electron carrier cytochrome f was observed from early stages of greening, the reduction of cytochrome f was not observed till 8h of greening. All these observations confirm that during early stages of greening MV and NADP are photoreduced by PSII without the involvement of intersystem electron carriers or the collaboration of PSI. Since these observations are at variance with the currently prevalent concept (Z-Scheme) of the photosynthetic generation of reducing power, which requires definite collaboration of the two photosystems, an alternate electron flow pathway is proposed.