Effect of Supra-High Irradiation on the Photosynthetic System of the Cyanophyte Synechocystis PCC 6714

Stability of thylakoid components under supra-high irradiancewas studied with the cyanophyte Synechocystis PCC 6714. Theactivity of overall photosynthesis was quickly inactivated (T_1/2=20min) under supra-high irradiance (300 W m^–2, white light).In parallel with the inactivation of photosynthesis, QA in PSII was also inactivated. Both inactivations were acceleratedby chloramphenicol (CAP) addition. The reactivation of PS IIrequired weak irradiation and was suppressed by CAP. However,PS I measured as P700 was very stable. The level of PS I measuredas P700 was not significantly reduced by the irradiation for12 h even in the presence of CAP while the level of Cyt b₅₅₉,component of PS II, was decreased markedly. The function ofPS I before and after supra-high irradiation with CAP was examinedby comparing sizes of P700 oxidation induced by a short flash,by a continuous light, and by determination of O₂-and ferredoxin-reduction.No difference was observed in PS I actions before and afterthe irradiation treatment. These results indicate that the PSI complex is very tolerant of supra-high irradiation. However,the cells grown under supra-high irradiance contained much fewerPS I and PS II complexes than Cyt b₆–f complexes. Theformer levels were reduced to a half to one fourth of thosebefore growth while the level of Cyt b₆–f complex wasnot reduced so much. A possible mechanism for changes in thylakoidcomposition under supra-high irradiation was discussed. (Received February 16, 1991; Accepted June 12, 1991)     

Regulation of Electron Transport Composition in Cyanobacterial Photosynthetic System: Stoichiometry among Photosystem I and II Complexes and Their Light-Harvesting Antennae and Cytochrome b6/fComplex

This study was done to confirm our previous observation withthe pattern of changes in electron transport composition inducedby an imbalance of the electron transport state. Contents ofphotosystem (PS) I and II complexes and their antennae and Cytb₆/f complex were determined for systems of cyanobacterium SynechocystisPCC 6714 of different PS I/PS II ratios. The results indicatedthat (1) the observed changes in the PS I/PS II ratio are not-dueto regulation of the activities of the respective PS's but tochanges in their contents, (2) the molar ratio between PS IIand Cyt b₆/f complexes was fairly constant when marked changesoccurred in the PS I content, and (3) the PS II and Cyt b₆/fcontents per cell remained fairly constant while the PS I contentchanged markedly. These findings agree with our previous observationwith autotrophic cells of Anacystis nidulans Tx 20 and supportour argument that in cyanobacterial and red algal electron transportsystems, the content of the terminalcomponent(s), such as PSI complex, is regulated in order to maintain a balance betweenthe electron influx by PS II action to the system and the effluxby PS I action from it. (Received June 3, 1987; Accepted September 20, 1987)     

Regulation of Stoichiometry between PSI and PSII in Response to Light Regime for Photosynthesis Observed with Synechocystis PCC 6714: Relationship between Redox State of Cyt b6-f Complex and Regulation of PSI Formation

The effect of the Cyt b₆-f redox state on the PSI formationwas examined with the cyanophyte Synechocystis PCC 6714 by usinga Q-cycle inhibitor, HQNO (2-n-heptyl-4-hydroxyquinoline N-oxide).HQNO inhibited the rapid reduction of flash-oxidized Cyt f,the reaction correlating with the stimulation of PSI formation,on one hand, and accumulated reduced Cyt b₆, on the other, indicatingthat the electron flow in the Q-cycle correlates with regulationof PSI synthesis. HQNO also inhibited the stimulation of PSIformation under PSII light, resulting in a low PSI/PSII ratioeven under PSII light, while the PSI formation under PSI lightwas not suppressed by HQNO. Simultaneous inhibition of Cyt b₆oxidation through the Q-cycle and the stimulated PSI formationby HQNO suggests that an HQNO-sensitive Cyt b₆ oxidation isinvolved in the mechanism of monitoring the state of electrontransport system for regulation of PSI formation. (Received March 3, 1993; Accepted August 9, 1993)     

Chromatic Regulation of Cytochrome Composition and Electron Transfer from Cytochrome b6-f Complex to Photosystem I in Anacystis nidulans (Tx 20)

Cytochrome composition of the cyanobacterial photosyntheticsystem was studied with Anacystis nidulans (Tx 20) in relationto the chromatic regulation of photosystem composition. Comparisonof cytochrome compositions in cells with a high PS I/II ratio(3.0, grown under weak orange light) and with a low ratio (1.6,grown under weak red light) indicated that cytochrome compositionwas also changed in the chromatic regulation of photosystemcomposition. Two types of cytochrome change were observed: 1)contents of cytochromes C₅₅₃ and c₅₄₈ were changed in parallelwith the changes in PS I content, and 2) cytochrome b₅₅₃ andcytochrome b₆-f complex were held at a constant molar ratioto PS II. The molar ratio, PS II : cytochrome b₅₅₉ : cytochromeb₆-f complex : cytochrome c₅₅₃ : PS I : cytochrome C₅₄₈, inthe red-grown cells was 1 : 2.5 : 1.3 : 0.17 : 1.6 : 0.67, andthe ratio in the orange-grown cells, 1:2.4:0.9:0.32:3.0:1.2.In both types of cells, almost all cytochrome f in the cytochromeb₆-f complex was rapidly oxidized after multiple flash activation,indicating that all cytochrome b₆-f complexes in cells of bothtypes are functionally connected to PS I, even when the molarratio to PS I is largely changed. The content of cytochromeC₅₅₃ was at most 0.14 of PS I, suggesting that the cytochrometurns over several times per one turnover of PS I. ¹Present address: Department of Biology, Faculty of Science,Tokyo Metropolitan University, Fukazawa 2-1-1, Setagaya, Tokyo158, Japan. (Received January 20, 1986; Accepted March 17, 1986)     

Steady State of Photosynthetic Electron Transport in Cells of the Cyanophyti Synechocystis PCC 6714 Having Different Stoichiometry between PS I and PS II: Analysis of Flash-Induced Oxidation-Reduction of Cytochrome f and P700 under Steady State of Photosynthesis

The steady state of photosynthetic electron transport drivenby two photosystems was studied with cells of the cyanophyteSynechocystis PCC 6714 by analyzing the flash-induced oxidation-reductionof Cyt f and P700 under continuous background illumination.We first analyzed the spectra and the kinetics of flash-inducedabsorption changes in the 400 to 440 nm wavelength region anddefined the absorption changes due to oxidation-reduction ofCyt f and P700. Results indicated that the flash-induced absorptionchanges at 420 and 435 nm are due to the oxidation-reductionof Cyt f and P700, respectively. Determination of the steadystate of Cyt f (420 nm) and P700 (435 nm) was made for the cellsgrown under a weak orange light exciting mainly PS II (PS IIlight) and having a high ratio of PS I to PS II (PS I/PS II),and those grown under a weak red light exciting preferentiallyPS I (PS I light) and having a low PS I/PS II. The steady stateof electron transport in cells of the two types were comparedunder PS I and PS II lights. The results indicated that: (1)under the light conditions used for growth (both red and orangelight), the intermediate electron pool between the two photosystemsremained in a redox state so as to keep both photosystems inthe open state. (2) When shifted to PS I light, the intermediatepool and PS I in cells of high PS I/PS II became extremely electron-poor,and so most of the PS I reaction centers were closed. (3) Theintermediate pool in cells of low PS I/PS II became extremelyelectron-rich when shifted to PS II light, and most of the PSII reaction centers were closed. The electron transport stateis released from such biased states by regulation of PS I/PSII. Results supported our previously proposed hypothesis thatthe stoichiometry between PS I and PS II is regulated so asto keep the two photosystems in the open state. The relationshipbetween the steady state of electron transport and the regulationof PS I/PS II is discussed. (Received August 2, 1990; Accepted December 10, 1990)     

Developmental Changes in Amounts of Thylakoid Components in Plastids of Barley Leaves

Changes in the amounts of several components of the photosyntheticelectron-transport system during greening of etiolated barleyleaves were studied on a "per plastid" basis. P700 and Q_A, whichwere initially absent from etioplasts, appeared 2 h after thestart of illumination in complete complexes of PS I and PS II,respectively. From 6 h, they increased rapidly in amount witha constant stoichiometric ratio of 1:1. Amounts of Cyt f, Cytb₆, Cyt b-559 and FeS, initially present in etioplasts at levelsthat were one-third to half of those in mature chloroplasts,also increased rapidly after 6 h of illumination. The molarratio of Cyt f, Cyt b₆ and Cyt b-559 was the same in etioplastsand in mature chloroplasts, namely 1:2:2. After 4 h of illumination,levels of FeS increased at nearly the same rate as those ofthe PS I complex. The increase in levels of all components wasmarked after 6 h of illumination, probably due to the energysupplied by developing plastids that had just become photosyntheticallycompetent. The results are discussed in relation to the timeof appearance of chlorophyll-protein complexes and photochemicalactivities. ¹ Present address: Department of Botany, Faculty of Science,Kyoto University, Kyoto, 606-01 Japan.     

Regulation of the Stoichiometry of Thylakoid Components in the Photosynthetic System of Cyanophytes: Model Experiments Showing that Control of the Synthesis or Supply of Chl a can Change the Stoichiometric Relationship between the Two Photosystems

Regulation of the assembly of the photosystem I (PS I) complexin response to the light regime in the photosynthetic systemof cyanophytes was studied in Synechocystis PCC 6714. The relationshipbetween the assembly of the PS I complex and synthesis of Chla was examined by model experiments in which synthesis of Chla was controlled by two inhibitors, gabaculine (GAB) and 2,2'-dipyridyl(DP). Both inhibitors caused a change to a lower ratio of PSI to PS II even under light that normally induces a high ratioof PS I to PS II. The change in stoichiometry induced by theseinhibitors was suppressed when protein synthesis was inhibitedby chloram-phenicol, similarly to the change in the stoichiometryinduced by light that excites mainly PS I (PS I light). Comparisonof the levels of PS I, PS II and Cyt b₆-f complexes per cellindicated that a selective suppression of the assembly of thePS I complex was induced by the inhibitors: the stoichiometricrelationship among PS I, PS II and Cyt b₆-f complexes was identicalto that induced by PS I light or white light of high intensity.GAB induced a decrease in size of the phycobilisome also, whileDP did not, similarly to PS I light. The results indicate thatthe ratio of PS I to PS II can be changed by the control ofsynthesis of Chl a. They also suggest that control of the synthesisor supply of Chl a probably exerted at site(s) in or after theprocess of the Mg-protoporphyrin branch, is involved in themechanism of regulation of the assembly of the PS I complexin cyanophytes. (Received September 7, 1989; Accepted November 20, 1989)     

Cyt b-559 (Fd) Participating in Cyclic Electron Transport in Spinach Chloroplasts: Evidence for Kinetic Connection with the Cyt b6/f Complex

A small fraction of low potential Cyt b-559, amounting to only13% of total Cyt b-559 in spinach chloroplasts, is analyzedwith the help of a highly selective, computer-controlled spectrophotometer,which simultaneously applies 16 pulse modulated narrow bandmeasuring beams with wavelengths in the cytochrome -band (500–600nm) for recordings of time resolved difference spectra. ThisCyt b-559 fraction remains oxidized upon dark incubation withascorbate and is reduced upon illumination. It can be reducedby cyclic PSI in an antimycin A-sensitive reaction or in thecourse of antimycin A-insensitive linear electron transportvia the Cyt b₆/f complex. Reduction by NADPH in the dark requiresferredoxin. Simultaneous recordings of Cyt b-563 and Cyt f revealclose kinetic connection between this Cyt b-559 fraction andthe low potential chain of the Cyt b₆/f complex. These resultsconfirm and extend previous observations of Miyake et al. 1995(Plant Cell Physiol. 36: 743) in maize mesophyll thylakoids,which led to the hypothesis that Cyt b-559 (Fd) occupies theposition of the postulated ferredoxin-plastoquinone reductase(FQR) in cyclic electron transport. (Received March 9, 1999; Accepted May 21, 1999)     

Changes in Stoichiometry among PSI, PSII and Cyt b6-f Complexes in Response to Chromatic Light for Cell Growth Observed with the Red Alga Porphyra yezoensis

Stoichiometry among 3 thylakoid components, PSI and PSII andCyt b6-f complexes, was determined with the red alga Porphyrayezoensis with special reference to the regulation of PSI/PSIIstoichiometry in response to light regime. The ratio of PSIto PSII abundance was four times greater in thalli grown underorange light which excites mainly phycobilisome, thus PSII,than that under red light which excites preferentially Chl a,thus PSI. Cyt b₆-f abundance remained almost constant. The PSIand PSII content was regulated separately under the two growthlight conditions as was also observed with the red alga Porphyridiumcruentum by Cunningham et al. [(1990) Plant Physiol. 93: 888].This differs from the cyanophyte Synechocystis PCC 6714 whereadjustment occurs only in the PSI content [(1987) Plant CellPhysiol. 28: 1547]. However, results on the marine cyanophyteSynechococcus NIBB 1071 indicate that changes in the PSI/PSIIsoichiometry is similar to red algae. In this species, as inthe red algae, more than one PSII is associated with each phycobilisome.The light regime also induced changes in the phycobiliproteincomposition in Porphyra yezoensis. Under PSII light, phycoerythrinincreased, and phycocyanin decreased, while under PSI lightthe response was reversed. The change suggests an occurrenceof complementary chromatic adaptation. (Received April 8, 1994; Accepted June 1, 1994)     

Chromatic Regulation in Chlamydomonas reinhardtii: Time Course of Photosystem Stoichiometry Adjustment Following a Shift in Growth Light Quality

Photosystem stoichiometry adjustments in Chlamydomonas reinhardtiiwere induced upon a sudden shift in the light quality duringcell growth. Reversible changes in the PSI/PSII ratio were acompensation response to changes in the balance of light absorptionby the two photosystems. Quantitations of PSII, Cyt b₆-f complexand PSI revealed a constancy in the cellular content of PSIIand the Cyt b₆-f complex, and variable amounts of PSI in C.reinhardtii. These results strengthen the notion that PSI isthe thyla-koid component subject to chromatic regulation andresponsible for the adjustment and optimization of the PSI/PSII ratio in the thylakoid of oxygenic photosynthesis. Additionalresults, obtained upon the use of protein biosynthesis translationinhibitors (chloramphenicol and cyclohex-imide), suggested thata chromatically-induced lowering of the PSI/PSII ratio in C.reinhardtii occurs by suppression of de novo biosynthesis ofPSI components and, therefore, by dilution of the PSI complexin the thylakoid membrane, rather than by active degradationof assembled PSI in chlo-roplasts. (Received November 8, 1996; Accepted December 6, 1996)     

Effects of CaCl2-washing on EPR Signals of Cytochrome b559 in Photosystem II Preparation from Spinach Chloroplasts

Washing of PS II preparation by 1 M CaCl₂ inactivates oxygenevolution without loss of bound manganese [Ono and Inoue (1983)FEBS Lett. 164: 255]. Most of the high-potential Cyt b₅₅₀, whichamounts to about a half of the total Cyt b₅₅₉ in untreated preparation,was converted to its low-potential form by CaCl₂-washing. Theeffect was similar to that of Tris-washing. The peak positionof the g_s band of the EPR spectrum of the CaCl₂-washed preparation(g=2.95) was the same as that of the low potential form of untreatedpreparation but was slightly different from that of the Tris-washedor heat-treated preparation (g=2.98). ¹ Present address: Department of Biology, Faculty of Science,Tokyo Metropolitan University, Fukazawa 2-1-1, Setagaya, Tokyo158, Japan. (Received November 14, 1984; Accepted January 30, 1985)     

Photosynthetic Units and Carbon Assimilation in Leaves of Grain Sorghum under Different Light Intensities

The Cyt f and P700 contents in leaves of three Sorghum, varietieswere measured, in relation to their carbon assimilation, underdifferent light intensities during growth. At the maximum irradiationused (1,800 µE m^–2 s^–1) the ratio of P700to Cyt f was close to unity, whereas under low irradiation (450µE m^–2 s^–1) the ratio of P700 to Cyt f rangedfrom two to three. A strikingly positive correlation existedbetween the P700 contents of the leaves and their rates of carbondioxide fixation, dry matter production and Cyt f contents,only when the plants were grown under high light intensities.The P700 content of the leaves in plants grown under low irradiationwas unrelated to the contents of Cyt f. Thus, at a high lightintensity there is a close relationship between the Cyt f andP700 levels, but at low intensities the amounts of electroncarriers and the reaction centre are independent. (Received March 7, 1983; Accepted August 24, 1983)     

Molecular Structure of a High-Potential Form of Thylakoid Cytochrome b-559 as Determined by Radiation Inactivation

Cytochrome b-559 in photosystem II can be characteristicallyconverted from a high- to a low-potential form. Taking thisresponse of Cyt b-559 as evidence for the denaturation of proteinmolecules, the sizes of the structures that stabilize the high-potentialform of Cyt b-559 in PS II membranes and thylakoids from spinachwere determined by radiation inactivation. When a target of26 kDa was inactivated in PS II membranes, Cyt b-559 was convertedto the low-potential form. The size was consistent with a molecularweight of Cyt b-559 in a proposed tetrameric structure thatconsists of two sets of 9.2-kDa and 4.3-kDa subunits [Widgeret al. (1985) FEBS Lett. 191: 186–190]. In contrast tothe functional size of 26 kDa in the PS II membranes, the functionalsize was 116 kDa in thylakoid membranes. The results suggestthe presence of an extra 90-kDa electron carrier between a redoxtitrator outside the membranes and the Cyt b-559, which maynot expose its active site to the surface of the thylakoids. (Received March 9, 1989; Accepted June 23, 1989)     

Chromatographic separation of photosystems I and II from the thylakoid membrane isolated from a thermophilic blue-green alga

The thylakoid membrane of a thermophilic blue-green alga, Synechococcussp., was separated into four chlorophyll-containing fractionsby a single chromatographic manipulation with a diethylaminoethyl-cellulosecolumn after digitonin treatment. Photosystems I and II, orchlorophyll a forms, were unevenly distributed among the fourfractions, which were designated F-1, F-2, F-3 and F-4 in theorder of elution from the column. F-1 has a simple composition of the chlorophyll a form and totallylacks photochemical activity. This fraction may be an antennachlorophyll a-protein in the blue-green alga. F-2 is rich inshorter wavelength chlorophyll a forms and shows the three-bandedfluorescence emission spectrum characteristic of photosystemII at liquid nitrogen temperature. This fraction is highly activein 2,6-dichloroindophenol photoreduction and contains one photooxidizablecytochrome b₅₅₉ per 50–100 chlorophyll a, whereas theP-700 content is as low as one P-700 per 2,000 chlorophyll a.Thus, F-2 represents photosystem II in a highly purified state.F-3 is rich in photosystem I, since this fraction is inactivein 2,6-dichloroindophenol photoreduction, and contains one P-700per 200 chlorophyll a and smaller amounts of cytochrome b₅₅₉.Longer wavelength chlorophyll a forms are abundant and a peakat 730 nm is the most prominent in the low-temperature fluorescencespectrum in this fraction. F-4, which consists of larger membranefragments shows spectral and photochemical features similarto those of F-3. (Received August 8, 1979; )     

Chromatic Regulation of Photosystem Composition in the Photosynthetic System of Red and Blue-Green Algae

A chromatic adaptation in the photosynthetic quantum yield forthe light mainly absorbed by chlorophyll a (Chl a light) firstfound by Yocum (1951) was studied with one red and three blue-greenalgal strains. When the cells were grown under a weak Chl alight, the quantum yield in all the strains increased. Comparisonof photosystem (PS) compositions, including phycobilin (PBP)and Chl a antennae, reaction centers I and II, in the cellsgrown under the light mainly absorbed by PBP and Chl a revealedthat changes in quantum yield could be attributed to changesin the ratio of PS I/II; PS I/II becomes larger than 1 underPBP light but decreases to 1 in most cases under Chl a light.The change in the PS I/II ratio is due solely to the changesin the PS I population in the cell; PS II remains constant.These results are similar to the intensity-dependent responsein PS composition. A common hypothesis for both the chromatic and intensity-inducedregulation of PS composition was proposed based on the ideaof balance between the electron flow from H₂O to NADP drivenby PS I and II and the cyclic one driven by PS I. (Received May 16, 1985; Accepted September 4, 1985)     

Expression of protein complexes and individual proteins upon transition of etioplasts to chloroplasts in pea (Pisum sativum)

The protein complexes of pea (Pisum sativum L.) etioplasts,etio-chloroplasts and chloroplasts were examined using 2D BlueNative/SDS–PAGE. The most prominent protein complexesin etioplasts were the ATPase and the Clp and FtsH proteasecomplexes which probably have a crucial role in the biogenesisof etioplasts and chloroplasts. Also the cytochrome b₆f (Cytb₆f) complex was assembled in the etioplast membrane, as wellas Rubisco, at least partially, in the stroma. These complexesare composed of proteins encoded by both the plastid and nucleargenomes, indicating that a functional cross-talk exists betweenpea etioplasts and the nucleus. In contrast, the proteins andprotein complexes that bind chlorophyll, with the PetD subunitand the entire Cyt b₆f complex as an exception, did not accumulatein etioplasts. Nevertheless, some PSII core components suchas PsbE and the luminal oxygen-evolvong complex (OEC) proteinsPsbO and PsbP accumulated efficiently in etioplasts. After 6h de-etiolation, a complete PSII core complex appeared with40% of the maximal photochemical efficiency, but a fully functionalPSII was recorded only after 24 h illumination. Similarly, thecore complex of PSI was assembled after 6 h illumination, whereasthe PSI–light-harvesting complex I was stably assembledonly in chloroplasts illuminated for 24 h. Moreover, a batteryof proteins responsible for defense against oxidative stressaccumulated particularly in etioplasts, including the stromaland thylakoidal forms of ascorbate peroxidase, glutathione reductaseand PsbS.     

A study on the dynamic features of photosystem stoichiometry: Accomplishments and problems for future studies

Our study on the regulation by light of photosystem stoichiometry in cyanophytes is briefly reviewed here. It can be summarized as follows: Adjustment of photosystem stoichiometry results in optimization of photosynthetic efficiency under the prevailing light-growth conditions. Photosystem (PS) I is primarily affected, increasing or decreasing, relative to PS II. The regulation of PS I synthesis appears to be at the translation or the post-translational level. The regulation is probably governed by changes in the redox level of the electron transport components, most probably at Cyt b₆in the Cyt b₆–f complex. Our results to date are at the physiological level, but they have raised many questions and provided suggestions for future directions in exploring the mechanism of regulation, some of which are discussed in this perspective.     

Leaf Development in Lolium temulentum: Photosynthesis and Photosynthetic Proteins in Leaves Senescing under Different Irradiances

Changes in carbon fixation rate and the levels of photosyntheticproteins were measured in fourth leaves of Lolium temulentumgrown until full expansion at 360 µmol quanta m^–2s^–1 and subsequently at the same irradiance or shadedto 90 µmol m^–2 s^–1. Ribulose-1,5-bisphosphatecarboxylase/oxygenase (Rubisco), light-harvesting chlorophylla/b protein of photosystem II (LHCII), 65 kDa protein of photosystemI (PSI), cytochrome f (Cytf) and coupling factor 1 (CF₁) declinedsteadily in amount throughout senescence in unshaded leaves.In shaded leaves, however, the decrease in LHCII and the 65kDa protein was delayed until later in senescence whereas theamount of Cyt f protein decreased rapidly following transferto shade and was lower than that of unshaded leaves at the earlyand middle stages of senescence. Decreases in the Rubisco andCF₁ of shaded leaves occurred at slightly reduced rates comparedwith unshaded leaves. These results indicate that chloroplastproteins in fully-expanded leaves are controlled individually,in a direction appropriate to acclimate photosynthesis to agiven irradiance during senescence. (Received August 20, 1992; Accepted January 5, 1993)     

Range of photosynthetic control of postillumination P700+ reduction rate in sunflower leaves

The kinetics of the postillumination reduction of P700⁺ which reflects the rate constant for plastoquinol (PQH₂) oxidation was recorded in sunflower leaves at different photon absorption densities (PAD), CO₂ and O₂ concentrations. The P700 oxidation state was calculated from the leaf transmittance at 830 nm logged at 50 s intervals. The P700⁺ dark reduction kinetics were fitted with two exponents with time constants of 6.5 and about 45 ms at atmospheric CO₂ and O₂ concentrations. The time constant of the fast component, which is the major contributor to the linear electron transport rate (ETR), did not change over the range of PADs of 14.5 to 134 nmol cm^-2 s^-1 in 21% O₂, but it increased up to 40 ms under severe limitation of ETR at low O₂ and CO₂. The acceptor side of Photosystem I (PS I) became reduced in correlation with the downregulation of the PQH₂ oxidation rate constant. It is concluded that thylakoid pH-related downregulation of the PQH₂ oxidation rate constant (photosynthetic control) is not present under normal atmospheric conditions but appears under severe limitation of the availability of electron acceptors. The measured range of photosynthetic control fits with the maximum variation of ETR under natural stress in C₃ plants. Increasing the carboxylase/oxygenase specificity would lead to higher reduction of the PS I acceptor side under stress.Abbreviations Cyt b ₆ f cytochrome b ₆ f complex - C_w cell-wall CO₂ concentration, M - ETR electron transport rate - Fd ferredoxin - FNR ferredoxin-NADP reductase - FRL far-red light - PC plastocyanin - PAD photon absorption density nmol cm^-2 s^-1 - PFD photon flux density nmol cm^-2 s^-1 - PS I Photosystem I complex - PQ plastoquinon - PQH₂ plastoquinol - PS II Photosystem II complex - P700 Photosystem I donor pigment, reduced - S830 830 nm signal (D830, difference of S830 from the dark level) - WL white light - Y_l maximum quantum yield of PS I electron transport, rel. un