14CO2 fixation and glycolate metabolism in the dark in isolated maize (Zea mays L.) bundle sheath strands

Bundle sheath strands capable of assimilating up to 68 μmoles CO₂ per mg chlorophyll per hr in the dark have been isolated from fully expanded leaves of Zea mays L. This dark CO₂-fixing system is dependent on exogenous ribose-5-phosphate, ADP or ATP, and Mg²⁺ for maximum activity. The principal product of dark fixation in this system is 3-phosphoglycerate, indicating that the CO₂-fixing reaction is mediated by ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39). The rate of dark CO₂ uptake in the strands in the presence of saturating levels of ribose-5-phosphate plus ADP is inhibited by oxygen. The inhibitory effect of oxygen is rapidly and completely reversible, and is relieved by increased levels of CO₂. Glycolate is synthesized in this dark system in the presence of [U-¹⁴C]ribose-5-phosphate, ADP, oxygen, and an inhibitor of glycolate oxidase (EC 1.1.3.1). Glycolate formation is completely abolished by heating the strands, and the rate of glycolate synthesis is markedly reduced by either lowering the oxygen tension or increasing the level of CO₂.These results, obtained with intact cells in the absence of light, indicate that the direct inhibitory effect of oxygen on photosynthesis is associated with photosynthetic carbon metabolism, probably at the level of ribulose-1,5-bisphosphate carboxylase, and not with photophosphorylation or photosynthetic electron transport. Furthermore, the findings indicate that the synthesis of glycolate from exogenous substrate can readily occur in the absence of photosynthetic electron transport, an observation consistent with the ribulose-1, 5-bisphosphate “oxygenase” scheme for glycolate formation during photosynthesis.     

16O2/18O2 analysis of oxygen exchange in Dunaliella tertiolecta. Evidence for the inhibition of mitochondrial respiration in the light

A mass spectrometric ¹⁶O₂/¹⁸O₂-isotope technique was used to analyse the rates of gross O₂ evolution, net O₂ evolution and gross O₂ uptake in relation to photon fluence rate by Dunaliella tertiolecta adapted to 0.5, 1.0, 1.5, 2.0 and 2.5 M NaCl at 25°C and pH 7.0.At concentrations of dissolved inorganic carbon saturating for photosynthesis (200 M) gross O₂ evolution and net O₂ evolution increased with increasing salinity as well as with photon fluence rate. Light compensation was also enhanced with increased salinities. Light saturation of net O₂ evolution was reached at about 1000 mol m^-2s^-1 for all salt concentrations tested. Gross O₂ uptake in the light was increased in relation to the NaCl concentration but it was decreased with increasing photon fluence rate for almost all salinities, although an enhanced flow of light generated electrons was simultaneously observed. In addition, a comparison between gross O₂ uptake at 1000 mol photons m^-2s^-1, dark respiration before illumination and immediately after darkening of each experiment showed that gross O₂ uptake in the light paralleled but was lower than mitochondrial O₂ consumption in the dark.From these results it is suggested that O₂ uptake by Dunaliella tertiolecta in the light is mainly influenced by mitochondrial O₂ uptake. Therefore, it appears that the light dependent inhibition of gross O₂ uptake is caused by a reduction in mitochondrial O₂ consumption by light.Abbreviations DCMU 3-(3, 4-dichlorophenyl)-1, 1-dimethylurea - DHAP dihydroxy-acetonephosphate - DIC dissolved inorganic carbon - DR_a rate of dark respiration immediately after illumination - DR_b rate of dark respiration before illumination - E₀ rate of gross oxygen evolution in the light - NET rate of net oxygen evolution in the light - PFR photon fluence rate - RubP rubulose-1,5-bisphosphate - SHAM salicyl hydroxamic acid - U₀ rate of gross oxygen uptake in the light     

On the O2 flash yields of two cyanophytes

We explored O₂ flash yield in two cyanophytes, Anacystis nidulans and Agmenellum quadruplicatum. On a rate-measuring electrode, a single flash gave a contour of O₂ evolution with a peak at about 10 ms which was maximum (100) for 680 nm background light. On 625 nm illumination the peak was smaller (62) but was followed by an increased tail of O₂ attributed to enhancement of the background. After a period of darkness, repetitive flashes (5 Hz) gave a highly damped initial oscillation in individual flash yields which finally reached steady state at 94% of the yield for 680 nm illumination. When O₂ of repetitive flashes was measured as an integrated flash yield the results was distinctive and similar to that for a continuous light 1 (680 nm). An apparent inhibition of respiration which persisted into the following dark period was taken as evidence for the Kok effect. With a concentration-measuring electrode, integrated flash yield vs. flash rate showed the same nonlinear behavior as O₂ rate vs. intensity of light 1. We draw three conclusions about the two cyanophytes. (a) The plastoquinone pool is substantially reduced in darkness. (b) Because of a high ratio of reaction centers, reaction center 1 / reaction center 2, for the two photoreactions, saturating flashes behave as light 1. (c) Because repetitive flashes are light 1, they also give a Kok effect which must be guarded against in measurements designed to count reaction centers.     

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.     

Reversible inactivation of photosystem II reaction centers in cation-depleted chloroplast membranes

Isolated pea chloroplasts were washed once in 10 mm NaCl and were then suspended in “low-salt” medium. Approximately one-half of the photosystem II reaction centers of these salt-depleted membranes were found to be photochemically inactive. These units became active in the presence of low concentrations of divalent cations (5–10 mm Mg²⁺) or high concentrations of monovalent cations (150–200 mm Na⁺), as evidenced by a twofold increase in the steady-state flash yield of oxygen evolution under short (~10-μs) saturating repetitive flashes (two per second). The half-maximal increase in flash yield occurred at ~2 mM Mg²⁺ or ~75 mm Na⁺. The flash yield of hydroxylamine oxidation in these low-salt chloroplasts increased twofold after Mg²⁺ addition, indicating that the cation action was close to the reaction-center chlorophyll complex. The relation between flash yield and dark time between flashes was not changed significantly by Mg²⁺, indicating that the rate-limiting step of the overall electron transport (H₂0 —→ ferricyanide) was not affected significantly. When the rate-limiting step was bypassed using silicomolybdate as the photosystem II electron acceptor (in the presence of diuron), the reduction rate doubled in the presence of Mg²⁺, even under continuous, saturating light. In glutaraldehyde-fixed chloroplasts, Mg²⁺ did not increase the flash yield of O² evolution; this suggests that protein conformational changes in the chloroplast membranes were involved in Mg²⁺ activation of photosystem II centers.     

Photosynthetic Unit Size during the Synchronous Life Cycle of Scenedesmus

Apparent size of the photosynthetic unit (chlorophyll/O₂ per flash) was estimated by O₂ yield of repetitive short flashes on cell samples taken at various times from a synchronized culture (14-hour light, 10-hour dark) of Scenedesmus obliquus. Unit size was essentially invariant (< 10% variation) with a mean value of 1750 chlorophyll/O₂ per flash. In contrast, the light-saturated photosynthetic rate per unit chlorophyll, or turnover rate of the photosynthetic unit, varied with the life cycle, rising 40% in the first three hours of the light period and decaying slowly thereafter. The results are taken as evidence that the metabolic machinery is subject to far greater control and adjustment than is the photochemical machinery.     

O(2) uptake in the light in chlamydomonas: evidence for persistent mitochondrial respiration

The nature of the process responsible for the stationary O₂ uptake occurring in the light under saturating CO₂ concentration in Chlamydomonas reinhardii has been investigated. For this purpose, a mass spectrometer with a membrane inlet system was used to measure O₂ uptake and evolution in the algal suspension. First, we observed that the O₂ uptake rate was constant (about 0.5 micromoles of O₂ per milligram chlorophyll per minute) during a light to dark transition and was not affected by 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Salicylhydroxamic acid had no effect on O₂ uptake in the dark or in the light, but was found to have the same inhibitory effect either in the dark or in the light when added to cyanide-treated algae. The stimulation of the O₂ uptake rate due to the uncoupling effect of carbonyl cyanide m-chlorophenylhydrazone was about the same in the dark or in the light. From these results, we conclude that mitochondrial respiration is maintained during illumination and therefore is not inhibited by high ATP levels. Another conclusion is that in conditions where photorespiration is absent, no other light-dependent O₂ uptake process occurs. If Mehler reactions are involved, in Chlamydomonas, under conditions where both photosynthetic carbon oxidation and reduction cycles cannot operate (as in cyanide-treated algae), their occurrence in photosynthesizing algae either under saturating CO₂ concentration or at the CO₂ compensation point appears very unlikely. The comparison with the situation previously reported in Scenedesmus (R. J. Radmer and B. Kok 1976 Plant Physiol 58: 336-340) suggests that different O₂ uptake processes might be present in these two algal species.     

Gas Exchange Characteristics of the Submerged Aquatic Crassulacean Acid Metabolism Plant, Isoetes howellii

The submerged aquatic plant Isoetes howellii Engelmann possesses Crassulacean acid metabolism (CAM) comparable to that known from terrestrial CAM plants. Infrared gas analysis of submerged leaves showed Isoetes was capable of net CO₂ uptake in both light and dark. CO₂ uptake rates were a function of CO₂ levels in the medium. At 2,500 microliters CO₂ per liter (gas phase, equivalent to 1.79 milligrams per liter aqueous phase), Isoetes leaves showed continuous uptake in both the light and dark. At this CO₂ level, photosynthetic rates were light saturated at about 10% full sunlight and were about 3-fold greater than dark CO₂ uptake rates. In the dark, CO₂ uptake rates were also a function of length of time in the night period. Measurements of dark CO₂ uptake showed that, at both 2,500 and 500 microliters CO₂ per liter, rates declined during the night period. At the higher CO₂ level, dark CO₂ uptake rates at 0600 h were 75% less than at 1800 h. At 500 microliters CO₂ per liter, net CO₂ uptake in the dark at 1800 h was replaced by net CO₂ evolution in the dark at 0600 h. At both CO₂ levels, the overnight decline in net CO₂ uptake was marked by periodic bursts of accelerated CO₂ uptake. CO₂ uptake in the light was similar at 1% and 21% O₂, and this held for leaves intact as well as leaves split longitudinally. Estimating the contribution of light versus dark CO₂ uptake to the total carbon gain is complicated by the diurnal flux in CO₂ availability under field conditions.     

Photosynthetic efficiency of Chlamydomonas reinhardtii in flashing light

Efficient light to biomass conversion in photobioreactors is crucial for economically feasible microalgae production processes. It has been suggested that photosynthesis is enhanced in short light path photobioreactors by mixing‐induced flashing light regimes. In this study, photosynthetic efficiency and growth of the green microalga Chlamydomonas reinhardtii were measured using LED light to simulate light/dark cycles ranging from 5 to 100 Hz at a light‐dark ratio of 0.1 and a flash intensity of 1000 µmol m⁻² s⁻¹. Light flashing at 100 Hz yielded the same photosynthetic efficiency and specific growth rate as cultivation under continuous illumination with the same time‐averaged light intensity (i.e., 100 µmol m⁻² s⁻¹). The efficiency and growth rate decreased with decreasing flash frequency. Even at 5 Hz flashing, the rate of linear electron transport during the flash was still 2.5 times higher than during maximal growth under continuous light, suggesting storage of reducing equivalents during the flash which are available during the dark period. In this way the dark reaction of photosynthesis can continue during the dark time of a light/dark cycle. Understanding photosynthetic growth in dynamic light regimes is crucial for model development to predict microalgal photobioreactor productivities. Biotechnol. Bioeng. 2011;108: 2905–2913. © 2011 Wiley Periodicals, Inc.     

On the nature of the oxygen uptake in the light by Chondrus crispus. Effects of inhibitors,temperature and light intensity

The nature of the different processes of O₂ uptake involved in the light in the red macroalga Chondrus crispus Stackhouse (Rhodophyta, Gigartinales) was investigated. At limiting CO₂, INH (2.5 mM) did not alter the O₂ uptake rate. Glycolate was not excreted and did not accumulate within the cells. KCN reduced the rate of O₂ uptake in the light by 76% at limiting CO₂ and by 43% at saturating CO₂, but caused > 95% inhibition of O₂ evolution. DCMU (5 μM) totally blocked the photosynthetic electron transport chain, but allowed a residual O₂ uptake of 3.0±0.6 μmol O₂ .h^?1.g^?1 FW, irrespective of the CO₂ concentration. In saturating CO₂, a high light intensity pretreatment significantly stimulated the rate of O₂ uptake compared to net O₂ evolution, suggesting the persistence, in the light, of mitochondrial respiration. Irrespective of the CO₂ concentration, the optimum temperature for O₂ evolution was 17°C whereas dark O₂ uptake increased linearly with temperature. In contrast, O₂ uptake in the light showed an optimum at 17°C in limiting CO₂, and 21–25° C in saturating CO₂; its Q₁₀ was 2.4 at limiting CO₂, a value close to that of RuBP oxygenase, and 3.1 at saturating CO₂, a value close to that of dark respiration. It is concluded that: 1) mitochondrial respiration and Mehler reaction are both involved at all CO₂ concentrations, 2) RuBP oxygenase activity cannot account for more than 45%, and Mehler reaction for less than 20%, of the total O₂ uptake observed in the light at limiting CO₂.     

Light control of the respiration of exogenous glycerol in the red macroalga Grateloupia doryphora

Heterotrophic activity in macroalgae has been little studied, but the red macroalga Grateloupia doryphora is known to grow in light at a higher rate in a glycerol-containing medium than in seawater. The effects of 0·1 M exogenous glycerol in seawater (SW90-gly) on the respiration rate of G. doryphora and the role played by light were investigated. The algae pretreated for 2 h in the light and in SW90-gly evolved oxygen and fixed carbon dioxide (H¹⁴CO₃ ^?), but also evolved radioactive ¹⁴CO₂ from [¹⁴C]glycerol. The rate of oxygen evolution was lower than that of samples in seawater, due to a high respiration rate and/or a partial inhibition of photosynthesis induced by glycerol. In contrast, the rate of inorganic carbon fixation was higher in SW90-gly than in control samples in seawater, suggesting that non-photosynthetic patterns were operating. In darkness, after pretreatment in the light in SW90-gly, samples showed a high oxygen uptake rate just after the light was turned off. Twenty minutes of darkness were enough to decrease this high respiration rate to that of samples in seawater. The oxygen uptake observed in all experiments with glycerol was mitochondrial as it was inhibited by potassium cyanide and salicylhydroxamic acid (SHAM). Pretreatment of samples in the light in SW90-gly with the photosynthetic inhibitor DCMU did not inhibit ensuing dark respiration, thus providing evidence for a non-photosynthetic effect of the light. The highest dark respiration rate was observed after the samples were pretreated in monochromatic blue light in glycerol-containing media.     

The primary structure of D1 near the QB pocket influences oxygen evolution

Photosystem II (PS II) is the site of oxygen evolution. Activation of dark adapted samples by a train of saturating flashes produces oxygen with a yield per flash which oscillates with a periodicity of four. Damping of the oxygen oscillations is accounted for by misses and double hits. The mechanisms hidden behind these parameters are not yet fully understood. The components which participate in charge transfer and storage in PS II are believed to be anchored to the heterodimer formed by the D1 and D2 proteins. The secondary plastoquinone acceptor Q_B binds on D1 in a loop connecting the fourth and fifth helices (the Q_B pocket). Several D1 mutants, mutated in the Q_B binding region, have been studied over the past ten years.In the present report, our results on nine D1 mutants of Synechocystis PCC 6714 and 6803 are analyzed. When oxygen evolution is modified, it can be due to a change in the electron transfer kinetics at the level of the acceptor side of PS II and also in some specific mutants to a long ranging effect on the donor side of PS II. The different properties of the mutants enable us to propose a classification in three categories. Our results can fit in a model in which misses are substantially determined by the fraction of centers which have Q_A ^- before each flash due to the reversibility of the electron transfer reactions. This idea is not new but was more thoroughly studied in a recent paper by Shinkarev and Wraight (1993). However, we will show in the discussion that some doubts remain as to the true origin of misses and double hits.Abbreviations BQ p-benzoquinone - Chl chlorophyll - D1 and D2 proteins of the core of PS II - DCMU 3-(3,4-dichlorophenyl)-1,1 dimethyl urea - OEC oxygen evolving complex - P₆₈₀ chlorophyll center of PS II acting as the primary donor - PS II Photosystem II - Q_A and Q_B primary and secondary quinone electron acceptor - TL thermoluminescence     

Photosynthesis, Photorespiration and Respiration of Detached Spruce Twigs as Influenced by Oxygen Concentration and Light Intensity

The effects of oxygen concentration and light intensity on the rates of apparent photosynthesis, true photosynthesis, photorespiration and dark respiration of detached spruce twigs were determined by means of an infra-red carbon dioxide analyzer (IRCA). A closed circuit system IRCA was filled with either 1 per cent of oxygen in nitrogen, air (21 % O₂) or pure oxygen (100 % O₂). Two light intensities 30 × 10³ erg · cm ^?2· s^?1 and 120 × 10³ erg · cm^?2· s^?1 were applied. It has been found that the inhibitory effect of high concentration of oxygen on the apparent photosynthesis was mainly a result of a stimulation of the rate of CO₂ production in light (photorespiration). In the atmosphere of 100 % O₂, photorespiration accounts for 66–80 per cent of total CO₂ uptake (true photosynthesis). Owing to a strong acceleration of photorespiration by high oxygen concentrations, the rate of true photosynthesis calculated as the sum of apparent photosynthesis and photorespiration was by several times less inhibited by oxygen than the rate of apparent photosynthesis. The rates of dark respiration were essentially unaffected by the oxygen concentrations used in the experiments. An increase in the intensity of light from 30 × 10³ erg · cm^?3· s^?1 to 120 · 10³ erg · cm^?2· s^?1 enhanced the rate of photorespiration in the atmospheres of 21 and 100 % oxygen but not in 1 % O₂. The rate of apparent photosynthesis, however, was little affected by light intensity in an atmosphere of 1 % oxygen.     

Compared thermoluminescence characteristics of pea thylakoids studied in vitro and in situ (in leaves). The effect of photoinhibitory treatments

Characteristics of thermoluminescence (TL) glow curves were studied in thylakoids (isolated from pea leaves) or in intact pea leaves after an exposure to very high light for 2 min in the TL device. The inhibition of photosynthesis was detected as decreases of oxygen evolution rates and/or of variable fluorescence.In thylakoids exposed to high light, then dark adapted for 5 min, a flash regime induced TL glow curves which can be interpreted as corresponding to special B bands since: 1) they can be fitted by a single B band (leaving a residual band at –5°C) with a lower activation energy and a shift of the peak maximum by –5 to –6°C and, 2) the pattern of oscillation of their amplitudes was normal with a period of 4 and maxima on flashes 2 and 6. During a 1 h dark adaptation, no recovery of PS II activity occurred but the shift of the peak maximum was decreased to –1 to –2°C, while the activation energy of B bands increased. It is supposed that centers which remained active after the photoinhibitory treatment were subjected to reversible and probably conformational changes.Conversely, in intact leaves exposed to high light and kept only some minutes in the dark, TL bands induced by a flash regime were composite and could be deconvoluted into a special B band peaking near 30°C and a complex band with maximum at 2–5°C. In the case of charging bands by one flash, this low temperature band was largely decreased in size after a 10 min dark adaptation period; parallely, an increase of the B band type component appeared. Whatever was the flash number, bands at 2–5°C were suppressed by a short far red illumination given during the dark adaptation period and only remained a main band a 20°C; therefore, the origin of the low temperature band was tentatively ascribed to recombinations in centers blocked in state S₂Q_A ^–Q_B ^2–. In vivo, the recovery of a moderately reduced state in the PQ pool, after an illumination, would be slow and under the dependence of a poising mechanism, probably involving an electron transfer between cytosol and chloroplasts or the so-called chlororespiration process.Abbreviations E_a- activation energy - FR- far-red - MV- methylviologen - pBQ- p-benzoquinone - PQ- plastoquinone - PS II- Photosystem II - Q_A- primary quinone electron acceptor of PS II - Q_B- secondary quinone electron acceptor of PS II - TL- thermoluminescence     

Temporal separation of nitrogen fixation and photosynthesis in the filamentous,non-heterocystous cyanobacterium Oscillatoria sp.

When growing in laternating light-dark cycles, nitrogenase activity (acetylene reduction) in the filamentous, non-heterocystous cyanobacterium Oscillatoria sp. strain 23 (Oldenburg) is predominantly present during the dark period. Dark respiration followed the same pattern as nitrogenase. Maximum activities of nitrogenase and respiration appeared at the same time and were 3.6 mol C₂H₄ and 1.4 mg O₂ mg Chl a ^-1·h^-1, respectively. Cultures, adapted to light-dark cycles, but transferred to continuous light, retained their reciprocal rhythm of oxygenic photosynthesis and nitrogen fixation. Moreover, even in the light, oxygen uptake was observed at the same rate as in the dark. Oxygen uptake and nitrogenase activity coincided. However, nitrogenase activity in the light was 6 times as high (22 mol C₂H₄ mg Chl a ^-1·h^-1) as compared to the dark activity. Although some overlap was observed in which both oxygen evolution and nitrogenase activity occurred simultaneously, it was concluded that in Oscillatoria nitrogen fixation and photosynthesis are separated temporary. If present, light covered the energy demand of nitrogenase and respiration very probably fulfilled a protective function.     

Primary Events, Energy Transfer, and Reactions in Photosynthetic Units: Quantum Accumulation in Photosynthetic Oxygen Evolution

Three independent methods have been used to determine the size of the quantum accumulation unit in green plant photosynthesis. This unit is defined as that group of pigment molecules within which quantal absorption acts must take place leading to the evolution of a single O₂ molecule. All three methods take advantage of the nonlinearity of oxygen yield with light dose at very low dosages. The experimental values of this unit size, based on an assumed model for the charge cooperation in O₂ evolution, ranging from 800 to 1600, suggest that there is either limited energy transfer between energy-trapping units or chemical cooperation among oxygen precursors formed in several neighboring energy-trapping units. Widely diffusible essential precursors to molecular oxygen are ruled out by these results. Inhibition studies show that O₂ evolution is blocked when 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) is added to chloroplasts after two preliminary flashes and before a third flash which would have yielded O₂ in the absence of DCMU. This experiment is interpreted as evidence that the site of DCMU inhibition is on the oxidizing side of system II. Pretreatment of chloroplasts with large concentrations of Tris, previously believed to destroy O₂ evolution by blocking an essential reaction in the electron chain between water and system II, may be alternately interpreted as promoting the dark reversal of the system II light-induced electron transfer.     

Electron acceptors of bacterial photosynthetic reaction centers II. H+ binding coupled to secondary electron transfer in the quinone acceptor complex

The photoreduction of ubiquinone in the electron acceptor complex (Q₁Q₁₁) of photosynthetic reaction centers from Rhodopseudomonas sphaeroides, R26, was studied in a series of short, saturating flashes. The specific involvement of H⁺ in the reduction was revealed by the pH dependence of the electron transfer events and by net H⁺ binding during the formation of ubiquinol, which requires two turnovers of the photochemical act. On the first flash Q₁₁ receives an electron via Q₁ to form a stable ubisemiquinone anion (Q?^?₁₁); the second flash generates Q?^?₁. At low pH the two semiquinones rapidly disproportionate with the uptake of 2 H⁺, to produce Q₁₁H₂. This yields out-of-phase binary oscillations for the formation of anionic semiquinone and for H⁺ uptake. Above pH 6 there is a progressive increase in H⁺ binding on the first flash and an equivalent decrease in binding on the second flash until, at about pH 9.5, the extent of H⁺ binding is the same on all flashes. The semiquinone oscillations, however, are undiminished up to pH 9. It is suggested that a non-chromophoric, acid-base group undergoes a pK shift in response to the appearance of the anionic semiquinone and that this group is the site of protonation on the first flash. The acid-base group, which may be in the reaction center protein, appears to be subsequently involved in the protonation events leading to fully reduced ubiquinol. The other proton in the two electron reduction of ubiquinone is always taken up on the second flash and is bound directly to Q?^?₁₁. At pH values above 8.0, it is rate limiting for the disproportionation and the kinetics, which are diffusion controlled, are properly responsive to the prevailing pH. Below pH 8, however, a further step in the reaction mechanism was shown to be rate limiting for both H⁺ binding electron transfer following the second flash.     

Coupling of ion transport in green cells ofAtriplex spongiosa leaves to energy sources in the light and in the dark

Summary The coupling of ion transport to energy sources in the light and in the dark in green cells ofAtriplex spongiosa leaves was investigated using light of different qualities, an inhibitor of electron transport (dichlorophenyl dimethyl urea), and an uncoupler (p-CF₃O-carbonyl cyanide phenylhydrazone). Two different mechanisms of ion uptake were, distinguished. (1) A light-dependent Cl^– pump which is linked to light-dependent K⁺ uptake. The energy for this pump is probably derived from photosynthetic electron transport or from nicotinamide adenine dinucleotide phosphate, reduced form. This mechanism is dichlorophenyl dimethyl urea-sensitive and enhanced by uncouplers. (2) A mechanism independent of light, which operates at the same rate in the light and in the dark. This mechanism is sensitive to uncouplers. It is probably aK–Na exchange mechanism since K⁺ and Cl^– uptake and a small net uptake of H⁺ are balanced by Na⁺ loss.     

Electron transport to oxygen mitigates against the photoinactivation of Photosystem II in vivo

The role of electron transport to O₂ in mitigating against photoinactivation of Photosystem (PS) II was investigated in leaves of pea (Pisum sativum L.) grown in moderate light (250 mol m^–2 s^–1). During short-term illumination, the electron flux at PS II and non-radiative dissipation of absorbed quanta, calculated from chlorophyll fluorescence quenching, increased with increasing O₂ concentration at each light regime tested. The photoinactivation of PS II in pea leaves was monitored by the oxygen yield per repetitive flash as a function of photon exposure (mol photons m^–2). The number of functional PS II complexes decreased nonlinearly with increasing photon exposure, with greater photoinactivation of PS II at a lower O₂ concentration. The results suggest that electron transport to O₂, via the twin processes of oxygenase photorespiration and the Mehler reaction, mitigates against the photoinactivation of PS II in vivo, through both utilization of photons in electron transport and increased nonradiative dissipation of excitation. Photoprotection via electron transport to O₂ in vivo is a useful addition to the large extent of photoprotection mediated by carbon-assimilatory electron transport in 1.1% CO₂ alone.Abbreviations Fm, Fo, Fv- maximal, initial (corresponding to open PS II traps) and variable chlorophyll fluorescence yield, respectively - NPQ- non-photochemical quenching - PS- photosystem - Q_A- primary quinone acceptor - qP- photochemical quenching coefficient     

RELATIONSHIP BETWEEN PHOTOSYNTHETIC OXYGEN CYCLING AND CARBON ASSIMILATION IN SYNECHOCOCCUS WH7803 (CYANOPHYTA)1

Mass spectrometric analysis of oxygen uptake and evolution in the light by marine Synechococcus WH7803 indicated that the respiration rate was near zero at low irradiance levels but increased significantly at high irradiances. The light intensity (I_r) at which oxygen uptake began to increase with increasing light intensity depended on the growth irradiance of the culture. In each case, I_r coincided with the minimum light intensity for saturation of carbon assimilation (I_k). At irradiances >I_r, net oxygen evolution rates paralleled carbon assimilation rates. Oxygen uptake at high light intensities was inhibited by DCMU, indicating that oxygen uptake was due to Mehler reaction activity. The onset of Mehler activity at I_k supports the idea that oxygen becomes an alternative sink for electrons from photosystem I when NADPH turnover is limited by the capacity of the dark reactions to utilize reductant.