The rise time of photoreduction, difference spectrum, and oxidation-reduction potential of P430

Structure and function of bark and wood chloroplasts in a drought-tolerant tree (Fraxinus ornus L.)

Leaves are the most important photosynthetic organs in most woody plants, but chloroplasts are also found in organs optimized for other functions. However, the actual photosynthetic efficiency of these chloroplasts is still unclear. We analyzed bark and wood chloroplasts of Fraxinus ornus L. saplings. Optical and spectroscopic methods were applied to stem samples and compared with leaves. A sharp light gradient was detected along the stem radial direction, with blue light mainly absorbed by the outer bark, and far-red-enriched light reaching the underlying xylem and pith. Chlorophylls were evident in the xylem rays and the pith and showed an increasing concentration gradient toward the bark. The stem photosynthetic apparatus showed features typical of acclimation to a low-light environment, such as larger grana stacks, lower chlorophyll a/b and photosystem I/II ratios compared with leaves. Despite likely receiving very few photons, wood chloroplasts were photosynthetically active and fully capable of generating a light-dependent electron transport. Our data provide a comprehensive scenario of the functional features of bark and wood chloroplasts in a woody species and suggest that stem photosynthesis is coherently optimized to the prevailing micro-environmental conditions at the bark and wood level.

Quantification of energy-converting protein complexes in plant thylakoid membranes

Knowledge about the exact abundance and ratio of photosynthetic protein complexes in thylakoid membranes is central to understanding structure-function relationships in energy conversion. Recent modeling approaches for studying light harvesting and electron transport reactions rely on quantitative information on the constituent complexes in thylakoid membranes. Over the last decades several quantitative methods have been established and refined, enabling precise stoichiometric information on the five main energy-converting building blocks in the thylakoid membrane: Light-harvesting complex II (LHCII), Photosystem II (PSII), Photosystem I (PSI), cytochrome b₆f complex (cyt b₆f complex), and ATPase. This paper summarizes a few quantitative spectroscopic and biochemical methods that are currently available for quantification of plant thylakoid protein complexes. Two new methods are presented for quantification of LHCII and the cyt b₆f complex, which agree well with established methods. In addition, recent improvements in mass spectrometry (MS) allow deeper compositional information on thylakoid membranes. The comparison between mass spectrometric and more classical protein quantification methods shows similar quantities of complexes, confirming the potential of thylakoid protein complex quantification by MS. The quantitative information on PSII, PSI, and LHCII reveal that about one third of LHCII must be associated with PSI for a balanced light energy absorption by the two photosystems.

Adjustments of embryonic photosynthetic activity modulate seed fitness in Arabidopsis thaliana

In this work, we dissect the physiological role of the transient photosynthetic stage observed in developing seeds of Arabidopsis thaliana. By combining biochemical and biophysical approaches, we demonstrate that despite similar features of the photosynthetic apparatus, light absorption, chloroplast morphology and electron transport are modified in green developing seeds, as a possible response to the peculiar light environment experienced by them as a result of sunlight filtration by the pericarp. In particular, enhanced exposure to far-red light, which mainly excites photosystem I, largely enhances cyclic electron flow around this complex at the expenses of oxygen evolution. Using pharmacological, genetic and metabolic analyses, we show that both linear and cyclic electron flows are important during seed formation for proper germination timing. Linear flow provides specific metabolites related to oxygen and water stress responses. Cyclic electron flow possibly adjusts the ATP to NADPH ratio to cope with the specific energy demand of developing seeds. By providing a comprehensive scenario of the characteristics, function and consequences of embryonic photosynthesis on seed vigour, our data provide a rationale for the transient building up of a photosynthetic machinery in seeds.

Spectral properties of a divinyl chlorophyll a harboring mutant of Synechocystis sp. PCC6803

A divinyl chlorophyll (DV-Chl) a harboring mutant of Synechocystis sp. PCC 6803, in which chlorophyll species is replaced from monovinyl(normal)-Chl a to DV-Chl a, was characterized. The efficiency of light utilization for photosynthesis was decreased in the mutant. Absorption spectra at 77 K and their fourth derivative analyses revealed that peaks of each chlorophyll forms were blue-shifted by 1-2 nm, suggesting lowered stability of chlorophylls at their binding sites. This was also true both in PSI and PSII complexes. On the other hand, fluorescence emission spectra measured at 77 K were not different between wild type and the mutant. This indicates that the mode of interaction between chlorophyll and its binding pockets responsible for emitting fluorescence at 77 K is not altered in the mutant. P700 difference spectra of thylakoid membranes and PSI complexes showed that the spectrum in Soret region was red-shifted by 7 nm in the mutant. This is a characteristic feature of DV-Chl a. Microenvironments of iron-sulfur center of a terminal electron acceptor of PSI complex, P430, were practically the same as that of wild type.

Mitochondrial responses to intracellular pulses of photosynthetic oxygen

When submitting anaerobic algal cells to a series of saturating flashes, transient absorption changes of mitochondrial origin were detected, showing the characteristic flash-number dependence of photosynthetic oxygen evolution. The faster kinetic event is the oxidation of heme a(3) of the cytochrome-c oxidase, which reaches a maximum at [unk]3.5 ms before again being reduced within 20 ms. The oxidation of cytochrome c involves an initial submillisecond lag, and its half-time is [unk]3.3 ms. Another component, probably indicating oxidation of heme a, is seen around 607 nm, with a kinetic behavior similar to that of cytochrome c. The fast time scale of these reactions excludes long-range diffusion and supports a direct intracellular trapping of O(2). It is estimated that, under appropriate conditions, the yield of this process is >30%. The linearity of these responses with respect to the amplitude of the oxygen pulse implies that a single turnover of the cytochrome oxidase is involved. These results suggest that the intracellular oxygen pathway may be of physiological importance in green algae. On the other hand, this technique seems promising both as an alternative to polarographic detection of photosynthetic oxygen and as a means of studying the cytochrome oxidase response in vivo to single-turnover oxygen pulses.

Oxidation-reduction potentials of bound iron-sulfur proteins of photosystem I

Digitonin - fractionated photosystem - I subchloroplasts were titrated potentiometrically between -450 and -610 mV at pH 10. Examination of the titrated subchloroplasts by low-temperature (13 degrees K) electron paramagnetic resonance spectroscopy revealed resonances centered at values of 2.05, 1.94, 1.92, 1.89, and 1.86 on the g-factor scale. The peak heights depended on the potentials at which the chloroplasts were poised. The resonances of at least three iron-sulfur centers can be recognized: one with lines at g = 2.05 and 1.94; one with lines at g = 2.05, 1.92, and 1.89; and one for which only a line at g = 1.86 has been resolved. The midpoint potentials of the iron-sulfur species fall into two distinctly separate regions: the titration profile of the g = 1.94 signal, the first segment of the g = 2.05 plot, and the rise phase of the g = 1.86 signal had a value of -530 +/- 5 mV; the upper segment of the g = 2.05 plot, the decrease phase of the g = 1.86 signal, and the g = 1.89 profile had a midpoint potential estimated to be [unk] -580 mV. The oxidation-reduction reaction of each of the bound iron-sulfur species, as represented by the changes of the electron paramagnetic resonance spectra, was reversible and apparently involved a two-electron change.Titration at pH 9 could only be carried to -560 mV, and essentially only the first half of the titration behavior as found at pH 10 was seen. At any given potential more positive than -560 mV, the part of the iron-sulfur protein that was not reduced electrochemically could be reduced photochemically, but only to the maximum extent reduced electrochemically at -560 mV. Whereas, chloroplasts illuminated at room temperature and then frozen while still being illuminated developed a signal similar to that produced by electrochemical reduction at -610 mV, illumination at 77 degrees K did not bring about photoreduction beyond that accomplished electrochemically at about -560 mV.Dithionite alone in the dark and under anaerobic conditions brought about a partial reduction to the extent of the first electrochemical reduction step. Dithionite plus illumination at room temperature or dithionite plus methyl viologen in the dark produced the maximum signal. Electron paramagnetic resonance spectra due to either light or electrochemically reduced iron-sulfur proteins showed no detectable decay for at least 3 days when samples were stored in the dark at 77 degrees K.

Femtosecond primary charge separation in Synechocystis sp. PCC 6803 photosystem I

The ultrafast (<100 fs) conversion of delocalized exciton into charge-separated state between the primary donor P700 (bleaching at 705 nm) and the primary acceptor A0 (bleaching at 690 nm) in photosystem I (PS I) complexes from Synechocystis sp. PCC 6803 was observed. The data were obtained by application of pump-probe technique with 20-fs low-energy pump pulses centered at 720 nm. The earliest absorbance changes (close to zero delay) with a bleaching at 690 nm are similar to the product of the absorption spectrum of PS I complex and the laser pulse spectrum, which represents the efficiency spectrum of the light absorption by PS I upon femtosecond excitation centered at 720 nm. During the first approximately 60 fs the energy transfer from the chlorophyll (Chl) species bleaching at 690 nm to the Chl bleaching at 705 nm occurs, resulting in almost equal bleaching of the two forms with the formation of delocalized exciton between 690-nm and 705-nm Chls. Within the next approximately 40 fs the formation of a new broad band centered at approximately 660 nm (attributed to the appearance of Chl anion radical) is observed. This band decays with time constant simultaneously with an electron transfer to A1 (phylloquinone). The subtraction of kinetic difference absorption spectra of the closed (state P700+A0A1) PS I reaction center (RC) from that of the open (state P700A0A1) RC reveals the pure spectrum of the P700+A0- ion-radical pair. The experimental data were analyzed using a simple kinetic scheme: An*-->k1[(PA0)*A1--><100 fs P+A0-A1]-->k2P+A0A1-, and a global fitting procedure based on the singular value decomposition analysis. The calculated kinetics of transitions between intermediate states and their spectra were similar to the kinetics recorded at 694 and 705 nm and the experimental spectra obtained by subtraction of the spectra of closed RCs from the spectra of open RCs. As a result, we found that the main events in RCs of PS I under our experimental conditions include very fast (<100 fs) charge separation with the formation of the P700+A0-A1 state in approximately one half of the RCs, the approximately 5-ps energy transfer from antenna Chl* to P700A0A1 in the remaining RCs, and approximately 25-ps formation of the secondary radical pair P700+A0A1-.

Modelling of the electron transfer reactions in Photosystem I by electron tunnelling theory: The phylloquinones bound to the PsaA and the PsaB reaction centre subunits of PS I are almost isoenergetic to the iron–sulfur cluster FX

Photosystem I is a large macromolecular complex located in the thylakoid membranes of chloroplasts and in cyanobacteria that catalyses the light driven reduction of ferredoxin and oxidation of plastocyanin. Due to the very negative redox potential of the primary electron transfer cofactors accepting electrons, direct estimation by redox titration of the energetics of the system is hampered. However, the rates of electron transfer reactions are related to the thermodynamic properties of the system. Hence, several spectroscopic and biochemical techniques have been employed, in combination with the classical Marcus theory for electron transfer tunnelling, in order to access these parameters. Nevertheless, the values which have been presented are very variable. In particular, for the case of the tightly bound phylloquinone molecule A(1), the values of the redox potentials reported in the literature vary over a range of about 350 mV. Previous models of Photosystem I have assumed a unidirectional electron transfer model. In the present study, experimental evidence obtained by means of time resolved absorption, photovoltage, and electron paramagnetic resonance measurements are reviewed and analysed in terms of a bi-directional kinetic model for electron transfer reactions. This model takes into consideration the thermodynamic equilibrium between the iron-sulfur centre F(X) and the phylloquinone bound to either the PsaA (A(1A)) or the PsaB (A(1B)) subunit of the reaction centre and the equilibrium between the iron-sulfur centres F(A) and F(B). The experimentally determined decay lifetimes in the range of sub-picosecond to the microsecond time domains can be satisfactorily simulated, taking into consideration the edge-to-edge distances between redox cofactors and driving forces reported in the literature. The only exception to this general behaviour is the case of phylloquinone (A(1)) reoxidation. In order to describe the reported rates of the biphasic decay, of about 20 and 200 ns, associated with this electron transfer step, the redox potentials of the quinones are estimated to be almost isoenergetic with that of the iron sulfur centre F(X). A driving force in the range of 5 to 15 meV is estimated for these reactions, being slightly exergonic in the case of the A(1B) quinone and slightly endergonic, in the case of the A(1A) quinone. The simulation presented in this analysis not only describes the kinetic data obtained for the wild type samples at room temperature and is consistent with estimates of activation energy by the analysis of temperature dependence, but can also explain the effect of the mutations around the PsaB quinone binding pocket. A model of the overall energetics of the system is derived, which suggests that the only substantially irreversible electron transfer reactions are the reoxidation of A(0) on both electron transfer branches and the reduction of F(A) by F(X).

Evidence for two active branches for electron transfer in photosystem I

All photosynthetic reaction centers share a common structural theme. Two related, integral membrane polypeptides sequester electron transfer cofactors into two quasi-symmetrical branches, each of which incorporates a quinone. In type II reaction centers [photosystem (PS) II and proteobacterial reaction centers], electron transfer proceeds down only one of the branches, and the mobile quinone on the other branch is used as a terminal acceptor. PS I uses iron-sulfur clusters as terminal acceptors, and the quinone serves only as an intermediary in electron transfer. Much effort has been devoted to understanding the unidirectionality of electron transport in type II reaction centers, and it was widely thought that PS I would share this feature. We have tested this idea by examining in vivo kinetics of electron transfer from the quinone in mutant PS I reaction centers. This transfer is associated with two kinetic components, and we show that mutation of a residue near the quinone in one branch specifically affects the faster component, while the corresponding mutation in the other branch specifically affects the slower component. We conclude that both electron transfer branches in PS I are active.

Decay kinetics and quantum yields of fluorescence in photosystem I from Synechococcus elongatus with P700 in the reduced and oxidized state: are the kinetics of excited state decay trap-limited or transfer-limited?

Transfer and trapping of excitation energy in photosystem I (PS I) trimers isolated from Synechococcus elongatus have been studied by an approach combining fluorescence induction experiments with picosecond time-resolved fluorescence measurements, both at room temperature (RT) and at low temperature (5 K). Special attention was paid to the influence of the oxidation state of the primary electron donor P700. A fluorescence induction effect has been observed, showing a approximately 12% increase in fluorescence quantum yield upon P700 oxidation at RT, whereas at temperatures below 160 K oxidation of P700 leads to a decrease in fluorescence quantum yield ( approximately 50% at 5 K). The fluorescence quantum yield for open PS I (with P700 reduced) at 5 K is increased by approximately 20-fold and that for closed PS I (with P700 oxidized) is increased by approximately 10-fold, as compared to RT. Picosecond fluorescence decay kinetics at RT reveal a difference in lifetime of the main decay component: 34 +/- 1 ps for open PS I and 37 +/- 1 ps for closed PS I. At 5 K the fluorescence yield is mainly associated with long-lived components (lifetimes of 401 ps and 1.5 ns in closed PS I and of 377 ps, 1.3 ns, and 4.1 ns in samples containing approximately 50% open and 50% closed PS I). The spectra associated with energy transfer and the steady-state emission spectra suggest that the excitation energy is not completely thermally equilibrated over the core-antenna-RC complex before being trapped. Structure-based modeling indicates that the so-called red antenna pigments (A708 and A720, i.e., those with absorption maxima at 708 nm and 720 nm, respectively) play a decisive role in the observed fluorescence kinetics. The A720 are preferentially located at the periphery of the PS I core-antenna-RC complex; the A708 must essentially connect the A720 to the reaction center. The excited-state decay kinetics turn out to be neither purely trap limited nor purely transfer (to the trap) limited, but seem to be rather balanced.

Control of the photosynthetic electron transport by PQ diffusion microdomains in thylakoids of higher plants

We investigate the role of plastoquinone (PQ) diffusion in the control of the photosynthetic electron transport. A control analysis reveals an unexpected flux control of the whole chain electron transport by photosystem (PS) II. The contribution of PSII to the flux control of whole chain electron transport was high in stacked thylakoids (control coefficient, CJ(PSII) =0.85), but decreased after destacking (CJ(PSII)=0.25). From an 'electron storage' experiment, we conclude that in stacked thylakoids only about 50 to 60% of photoreducable PQ is involved in the light-saturated linear electron transport. No redox equilibration throughout the membrane between fixed redox groups at PSII and cytochrome (cyt) bf complexes, and the diffusable carrier PQ is achieved. The data support the PQ diffusion microdomain concept by Lavergne et al. [J. Lavergne, J.-P. Bouchaud, P. Joliot, Biochim. Biophys. Acta 1101 (1992) 13-22], but we come to different conclusions about size, structure and size distribution of domains. From an analysis of cyt b6 reduction, as a function of PSII inhibition, we conclude that in stacked thylakoids about 70% of PSII is located in small domains, where only 1 to 2 PSII share a local pool of a few PQ molecules. Thirty percent of PSII is located in larger domains. No small domains were found in destacked thylakoids. We present a structural model assuming a hierarchy of specific, strong and weak interactions between PSII core, light harvesting complexes (LHC) II and cyt bf. Peripheral LHCII's may serve to connect PSII-LHCII supercomplexes to a flexible protein network, by which small closed lipid diffusion compartments are formed. Within each domain, PQ moves rapidly and shuttles electrons between PSII and cyt bf complexes in the close vicinity. At the same time, long range diffusion is slow. We conclude, that in high light, cyt bfcomplexes located in distant stromal lamellae (20 to 30%) are not involved in the linear electron transport.

Spectroscopic resolution of the picosecond reduction kinetics of the secondary electron acceptor A1 in photosystem I

Forward electron transfer in photosystem I from Synechocystis sp. PCC 6803 has been studied in the picosecond time range with transient absorption spectroscopy in the blue and near-UV spectral regions. From the direct measurement, at 380-390 nm, of the reduction kinetics of the phylloquinone secondary acceptor A1 and from the absence of spectral evolution between 100 ps and 2 ns, we conclude that electron transfer, from the chlorophyll a primary acceptor A0, to A1 occurs directly and completely with a time constant of about 30 ps.

Spectral and kinetic characterization of electron acceptor A1 in a Photosystem I core devoid of iron-sulfur centers F X, F B and F A

The kinetic and spectroscopic properties of the secondary electron acceptor A1 were determined by flash absorption spectroscopy at room and cryogenic temperatures in a Photosystem I (PS I) core devoid of the iron-sulfur clusters FX, FB and FA. It was shown earlier (Warren, P.V., Golbeck, J.H. and Warden, J.T. (1993) Biochemistry 32: 849-857) that the majority of the flash-induced absorbance increase at 820 nm, reflecting formation of P700(+), decays with a t1/2 of 10 μs due to charge recombination between P700(+) and A1 (-). Following A1 (-) directly around 380 nm, where absorbance changes due to the formation of P700(+) are negligible, two major decay components were resolved in this study with t1/2 of ≈ 10 μs and 110 μs at an amplitude ratio of ≈ 2.5:1. The difference spectra between 340 and 490 nm of the two kinetic phases are highly similar, showing absorbance increases from 340 to 400 nm characteristic of the one-electron reduction of the phylloquinone A1. When measured at 10 K, the flash-induced absorbance changes around 380 nm can be fitted with two decay phases of t1/2 ≈ 15 μs and 150 μs at an amplitude ratio ≈ 1:1. The difference spectra of both kinetic phases from 340 to 400 nm are similar to those determined at 298 K and are therefore attributed to charge recombination in the pair P700(+)A1 (-). These results indicate that the backreaction between P700(+) and A1 (-) is multiphasic when FX, FB and FA are removed, and only slightly temperature dependent in the range of 298 K to 10 K.

Spectroscopic characterization of PS I core complexes from thermophilic Synechococcus sp. Identical reoxidation kinetics of A1- before and after removal of the iron-sulfur-clusters FA and FB

Monomeric and trimeric PS I complexes missing the three stromal subunits E,C and D (termed PS I core complexes) were prepared from the thermophilic cyanobacterium Synechococcus sp. by incubation with urea. The subunits E,C and D are sequentially removed. In the monomeric PS I the subunit C is removed with a half life of approx. 5 min. This is about eight times faster than in the trimeric PS I complex. In parallel with the removal of the FA/B containing subunit C the reduction kinetics of P700+ changed from a half life of about 25 ms to about 750 microseconds. The partner of P700+ in the 750 microseconds charge recombination was identified to be FX by the difference spectrum of this phase. There are some minor differences in the spectra of trimeric and monomeric PS I core complexes. At 77K the forward electron transfer from A1- to FX is blocked in the major fraction of the PS I core complexes and P700+ A1- recombines with a half life of about 220 microseconds. In the remaining fraction P700+FX- is formed and decays with a half life of approx. 10 ms at 77 K. The kinetics of the forward electron transfer from A1- to the iron-sulfur-clusters was measured in the native PS I and the corresponding core complexes. The reoxidation kinetics of A1- are identical in both cases (t1/2 = 180 ns). We conclude that FX is an obligatory intermediate in the normal forward electron transfer.

Electron transfer from cytochrome f to photosystem I in green algae

The time course of P700(+) reduction and cytochrome f oxidation following a single-turnover flash excitation of photosystem I was measured under various conditions in different strains of green algae. P700(+) was reduced with a half-time of 4 μs. The rate of cytochrome f oxidation was found to depend widely on physiological factors. Reversible transitions are described from a 'slow-oxidation' state (t 1/2=500 μs) to a 'fast-oxidation' state (t 1/2=80 μs). The addition of ionophore strongly favours and stabilizes the 'fast-oxidation' state. We suggest that these transitions reflect either reversible association between the cytochrome bf complex and the reaction center of photosystem I or changes in the mobility of oxidized plastocyanin. The transitions might be under the control of the membrane potential or the intracellular ATP content. The relation of these reversible transitions with the 'light state' transitions, and their possible involvement in a switch from linear to cyclic electron transfer, are discussed.

Electron donors and acceptors in photosynthetic reaction centers

A review is given of primary and associated electron transport reactions in various division of photosynthetic bacteria and in the two photosystems of plant photosynthesis. Two types of electron acceptor chains are distinguished: type 'Q', found in purple bacteria, Chloroflexus and system II of oxygenic photosynthesis and type 'F', found in green sulfur bacteria, Heliobacterium and photosystem I. Secondary donor reactions are discussed in relation to plant photosystem II.

Primary photochemistry in photosystem-I

In this review, the main research developments that have led to the current simplified picture of photosystem I are presented. This is followed by a discussion of some conflicting reports and unresolved questions in the literature. The following points are made: (1) the evidence is contradictory on whether P700, the primary donor, is a monomer or dimer of chlorophyll although at this time the balacnce of the evidence points towards a monomeric structure for P700 when in the triplet state; (2) there is little evidence that the iron sulfur centers FA and FB act in series as tertiary acceptors and it is as likely that they act in parallel under physiological conditions; (3) a role for FX, probably another iron sulfur centrer, as an obligatory electron carrier in forward electron transfer has not been proven. Some evidence indicates that its reduction could represent a pathway different to that involving FA and FB; (4) the decay of the acceptor 'A2 (-)' as defined by optical spectroscopy corresponds with 700(+) % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOramaaBa% aaleaadaqdaaqaaiaadIfaaaaabeaaaaa!37D1!\[F_{\overline X } \] recombination under some circumstances but under other conditions it probably corresponds with P700(+) A1 (-) recombination; (5) P700(+) A1 (-) recombination as originally observed by optical spectroscopy is probably due to the decay of the P700 triplet state; (6) the acceptor A1 (-) as defined by EPR may be a special semiquinone molecule; (7) A0 is probably a chlorophyll a molecule which acts as the primary acceptor. Recombination of P700(+) A0 (-) gives rise to the P700 triplet state.A working model for electron transfer in photosystem I is presented, its general features are discussed and comparisons with other photosystems are made.

Reconciliation of the absorption change at 325 nm and other flash-yield determinations of concentrations of active photosystem II centers

The concentration of photosystem II was determined in thylakoids of dwarf peas by the use of the following methods: absorption change at 325 nm; atrazine binding; and flash yields of oxygen evolution (Emerson-Arnold method), of protons from oxidation of water, and of reduction of DCIP. For the first time all of the flash-yield measurements have been done on the same sample and give equivalent values for the concentration of photosystem II. Agreement of the absorption change measurement at 325 nm with the other measurements was accomplished by the introduction of important improvements to the methods of Melis and co-workers [Proc. Natl. Acad. Sci. USA (1980) 77, 4712-4716]. The atrazine-binding method gave photosystem II values that were twice as large as any of the other photosystem II measurements. Possible reasons are discussed for this discrepancy in terms of the secondary acceptor (Q400) of Ikegami and Katoh [Plant Cell Physiol. (1973) 14, 829-836]. The concentration of photosystem I was measured by absorption change at 705 nm. From the concentration values of photosystem II and I the system II/I stoichiometry was calculated.

Electron and proton transfers from P-430 to ferredoxin-NADP-reductase in Chlorella cells

After blocking Photosystem II on whole Chlorella cells, we measured the absorption changes between 0 degrees C and -10 degrees C. The absorption changes measured 2 mus after the beginning of a Xenon Flash are the sum of changes due to P(+)-700 and changes due to P(-)-430 (after the subtraction of the carotenoid triplet change and of the electrochromic effect). The reduction of P(-)-430 is not resolved by our technique. Its reoxidation presents a half-time around 1 mus at 0 degrees C and around 2 mus at -10 degrees C. The reduction and protonation of ferredoxin-NADP-reductase to its neutral semi-quinoid form FNRH present a half-time of about 3 mus at 0 degrees C and 6 mus at -10 degrees C. The presence of only one photoreducible ferredoxin-NADP-reductase per Photosystem I center is confirmed, but is an acceptor X' the differential extinction coefficients of which are weak or null from 420 nm to 480 nm. Tentative explanations which would reconcile these results with what was already known about ferredoxin are proposed.

The relative absorption cross-sections of photosystem I and photosystem II in chloroplasts from three types of Nicotiana tabacum

In the present study we used three types of Nicotiana tabacum, cv John William's Broad Leaf (the wild type and two mutants, the yellow-green Su/su and the yellow Su/su var. Aurea) in order to correlat functional properties of Photosystem II and Photosystem I with the structural organization of their chloroplasts. The effective absorption cross-section of Photosystem II and Photosystem I centers was measured by means of the rate constant of their photoconversion under light-limiting conditions. In agreement with earlier results (Okabe, K., Schmid, G.H. and Straub, J. (1977) Plant Physiol. 60, 150--156) the photosynthetic unit size for both System II and System I in the two mutants was considerably smaller as compared to the wild type. We observed biphasic kinetics in the photoconversion of System II in all three types of N. tabacum. However, the photoconversion of System I occurred with monophasic and exponential kinetics. Under our experimental conditions, the effective cross-section of Photosystem I was comparable to that of the fast System II component (alpha centers). The relative amplitude of the slow System II component (beta centers) varied between 30% in the wild type to 70% in the Su/su var. Aurea mutant. The increased fraction of beta centers is correlated with the decreased fraction of appressed photosynthetic membranes in the chloroplasts of the two mutants. As a working hypothesis, it is suggested that beta centers are located on photosynthetic membranes directly exposed to the stroma medium.