Characterization of the Photosynthetic Activity of Platinized Chloroplasts: A Study Using Fluorescence and Photoacoustic Techniques

In this report, the effect of platinization on the photosynthetic activity of the chloroplast membranes is studied. Oxygen evolution, fluorescence emission and thermal de-activation processes are modified after platinization. It is shown that photosystem II activity is affected by the hydrogen purging involved in the platinization procedure as seen by the reduced rates of oxygen evolution and a decrease in variable fluorescence. Depletion of bicarbonate from photosystem II during purging is suggested to be responsible partly for the decreased electron transfer rates and for a lower half-saturation light intensity required for energy storage as measured by photoacoustic spectroscopy. On the other hand, the electron sink created by the reduction of hydrogen at the acceptor side of photosystem I is shown to reoxidize efficiently the plas-toquinone pool of photosystem II.     

Transformation of the Photoacoustic Signal after Treatment of Barley Leaves with Methylviologen or High Temperatures

Abstract— The photoacoustic (PA) signal at the modulation frequency of 35 Hz was studied in MV-treated barley leaves or leaves preheated at different temperatures. Saturating illumination enhanced the magnitude of the PA signal in MV-treated leaves in contrast with the opposite result usually found in untreated intact leaves where saturating illumination abolishes the photobaric component of the PA signal due to oxygen evolution and thus decreases the total PA signal. A linear relationship was found between the changes induced by continuous background light in the negative response of PA signal to saturating light in intact leaves and in the positive response in MV-treated leaves. A linear relationship was also observed in MV-treated leaves between the positive changes in the PA signal and the changes in the rate of electron transport through photosystem II (PSII) calculated from chlorophyll fluorescence data. The conclusion was drawn that only the thermal component contributes to the PA signal measured at low modulated frequency in MV-treated leaves because the enhanced O2 uptake provides a zero net oxygen exchange by superimposing with O2 evolution. The leaves preheated at temperatures above 43°C demonstrated the positive response of the PA signal to saturating light at 35 Hz. In leaves preheated at 41.5°C, the first and second saturating pulses induced the enhancement of PA signal, whereas other pulses decreased the PA signal due to onset of oxygen evolution. The energy storage activity measured in the absence of oxygen evolution in heat-treated leaves is proposed to be associated with cyclic electron transport activities around PSII and PSI.     

Reconstruction of the Water-Oxidizing Complex in Manganese-Depleted Photosystem II Preparations Using Mononuclear Manganese Complexes

Abstract— The water-oxidizing complex of chloroplast photosystem II is composed of a cluster of four manganese atoms that can accumulate four oxidizing redox equivalents. Depletion of manganese from the water-oxidizing complex fully inhibits oxygen evolution. However, the complex can be reconstituted in the presence of exogenous manganese in a process called photoactivation. In the present study, mononuclear manganese complexes with ligands derived from either nitrosonaphthol and ethylenediamine (Niten) or from diaminohexane and salicylaldehyde (Salhxn) are used in photoactivation experiments. Measurements of photoinduced changes of chlorophyll fluorescence yield, thermal dissipation using photoacoustic spectroscopy, photoreduction of 2,6-dichorophenolindophenol and oxygen evolution in manganese-depleted and in reconstituted photosystem II preparations demonstrate that photoactivation is more efficient when Niten and Salhxn complexes are used instead of MnCl₂. It is inferred that the aromatic ligands facilitate the interaction of the manganese atoms with photosystem II. The addition of CaCl₂ and of the extrinsic polypeptide of 33 kDa known as the manganese-stabilizing protein during photoactivation further enhances the recovery of electron transport and oxygen evolution activities. It is proposed that mononuclear manganese complexes are able to contribute to re-constitution of the water-oxidizing complex by sequential addition of single ions similarly to the current model for assembly of the tetranuclear manganese cluster and that these complexes constitute suitable model systems to study the assembly of the water-oxidizing complex.     

INHIBITION OF OXYGEN EVOLUTION IN CHLOROPLAST PHOTOSYSTEM II BY THE PROTEIN-MODIFYING AGENT TETRANITROMETHANE

Abstract— The protein-modifying agent tetranitromethane (TNM) reacts with tyrosine residues and -SH groups. It was found to inhibit photo synthetic electron transport on the water splitting side of photosystem II (P. V. Sane and U. Johanningmeier, Z. Naturforsch. 35c, 293–297, 1979). In the present work the inhibition by TNM is studied in detail using photosystem II submembrane fractions. It is shown that the action of TNM with membrane-bound proteins could imply the modification of tyrosine residues. At concentrations below 30 μ M and with short incubation periods (<2 min), TNM produces the release of the extrinsic polypeptides involved in the stabilization of the water-splitting complex, this being correlated with inhibition of electron transport at a site prior to H₂O₂ electron donation even though the inhibition cannot be prevented by the addition of Cl or Ca²⁺, which are known cofactors for oxygen evolution. As the incubation period or the concentration of TNM is increased, photosynthetic pigments are bleached, starting with aggregates absorbing at relatively long wavelengths. The inhibition by low concentrations of TNM differs from the effect of most of the previously reported inhibitors acting at the oxygen-evolving complex of photosystem II.     

高浓度LaCl3抑制黄瓜(Cucumis sativus Linn)光系统Ⅱ(PS Ⅱ)活性

研究了高浓度LaCl3对黄瓜（Cucumis sativus Linn）光系统Ⅱ（PSⅡ）光诱导荧光动力学参数、低温荧光光谱和放氧活性的影响。结果表明，随着黄瓜体内LaCl3浓度的升高、其荧光量子产率、PSⅡ最大光化学效率、放氧活性和电子传递速率都明显降低。低温荧光分析表明，低浓度LaCl3引起激发能更多的分配给PSⅡ。高浓度LaCl3对黄瓜幼茁的抑制作用表现在对类囊体膜结构的破坏。进而导致PSⅡ光合活性下降，并最终抑制黄瓜生长。     

Kinetics of chloroplast-mediated photoxidation of diketogluonate

Abstract— Illuminated chloroplasts can mediate a photoxidation of diketogulonic acid (DKGA) with rates of oxygen uptake equivalent to rates of Hill reactions with ferricyanide or quinone. The photoxidation of DKGA is sensitive to dichlorophenyl dimethylurea (DCMU) and exhibits the drop in quantum yield at long wavelengths characteristic of photosystem II. Still, the reaction is only partially inactivated by heating chloroplasts at 50° for 10 min (which destroys oxygen evolution). The photoxidation is inhibited by copper and detergents; and is stimulated by added flavin (or methyl viologen) and manganous ions. A model system containing Mn³⁺ (as manganipyrophosphate) and DKGA, mimics the chloroplast system. Pre-illuminated chloroplast suspensions can be substituted for Mn³⁺ in the model dark reaction. It seems that a light-dependent oxidation of Mn²⁺ to Mn³⁺ by photosystem II is the essential contribution of the chloroplasts. Electrons from Mn²⁺ move through the electron transport system to ferricyanide or to photosystem I where, via flavin (or methyl viologen), oxygen is reduced to H₂O₂.     

Differential effects of plasma membrane electric excitation on H+ fluxes and photosynthesis in characean cells

Cells of characean algae exposed to illumination arrange plasma-membrane H(+) fluxes and photosynthesis in coordinated spatial patterns (bands). This study reveals that H(+) transport and photosynthesis patterns in these excitable cells are affected not only by light conditions but also by electric excitation of the plasma membrane. It is shown that generation of action potential (AP) temporally eliminates alkaline bands, suppresses O(2) evolution, and differentially affects primary reactions of photosystem II (PSII) in different cell regions. The quantum yield of PSII electron transport decreased after AP in the alkaline but not in acidic cell regions. The effects of electric excitation on fluorescence and the PSII electron flow were most pronounced at light-limiting conditions. Evidence was obtained that the shift in chlorophyll fluorescence after AP is due to the increase in DeltapH at thylakoid membranes. It is concluded that the AP-triggered pathways affecting ion transport and photosynthetic energy conversion are linked but not identical.     

Comparative study of the photochemistry of chloroplast membranes and photosystem II particles

A comparative study of the photoreducing potentials of spinach thylakoid membranes and spinach photosystem II particles has been made. Hexachloroplatinate ions have been used as electron acceptors in a Hill-like assay for oxygen evolution measurements with both thylakoid membranes and photosystem II particles. However, unlike other Hill acceptors, such as ferricyanide, hexachloroplatinate can be fully reduced to metallic platinum that is catalytically active for hydrogen evolution. This is experimentally confirmed in the ability of chloroplast membranes to photoprecipitate platinum and photoproduce molecular hydrogen. Although similar experiments with photosystem II particles resulted in hexachloroplatinate-supported oxygen evolution, hydrogen evolution was not observed. Moreover, photosystem II particles coupled to ferredoxin and hydrogenase resulted in neither hydrogen nor oxygen evolution—a distinct contrast to the results obtained with chloroplast membranes.

     

Functional in situ evaluation of photosynthesis-protecting carotenoids in mutants of the cyanobacterium Synechocystis PCC6803

Cyanobacteria possess different carotenoids as scavengers of reactive oxygen species. In Synechocystis PCC6803, zeaxanthin, echinenone, beta-carotene and myxoxanthophyll are synthesized. By disruption of the ketolase and hydroxylase genes, it was possible to obtain mutants devoid of either zeaxanthin, echinenone, or a combination of both carotenoids. With these mutants, their function in protecting photosynthetic electron transport under high light stress as well as chlorophyll and carotenoid degradation after initiation of singlet oxygen or radical formation was analyzed. Wild type Synechocystis is very well protected against high light-mediated photooxidation. Absence of echinenone affects photosynthetic electron transport to only a small extent. However, complete depletion of zeaxanthin together with a modification of myxoxanthophyll resulted in strong photoinhibition of overall photosynthetic electron transport as well as the photosystem II reaction. In the double mutant lacking both carotenoids the effects were additive. The light saturation curves of photosynthetic electron transport of the high light-treated mutants exhibited not only a lower saturation value but also smaller slopes. Using methylviologen or methylene blue as a radical or singlet oxygen generators, respectively, massive degradation of chlorophyll and carotenoids, indicative of photooxidative destruction of the photosynthetic apparatus, was observed, especially in the mutants devoid of zeaxanthin.     

Different kinetics of photoinactivation of photosystem I-mediated electron transport and P700 in isolated thylakoid membranes

Photoinactivation kinetics of photosystem I (PSI)-mediated electron transport rate was compared to that of P700 content at room (22 degrees C) and low (4 degrees C) temperatures in isolated spinach thylakoid membranes. The high light treatment was carried out under aerobic and anaerobic conditions. At 22 degrees C the decrease of electron transport rate showed first order exponential kinetics. The amount of P700 decreased linearly, being less affected in the first hours of illumination. During photoinhibition at 4 degrees C in the presence of oxygen, the kinetics of inactivation of PSI photochemical activity and the content of P700 were different. It was found that 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) had different protective effect on the electron transport rate and on P700 content at both temperatures. Treatment with high light intensity under N(2) atmosphere had no effect on the electron transport rate or P700 content. The possible degradation of PSI reaction centre proteins was determined using immunoblot methods. In the presence of linear electron transport at 22 degrees C correlation between formation of toxic hydroxyl radicals and inhibition of oxygen uptake was observed.     

Stabilization of the structure and unctions of a photosystem i submembrane fraction by immobilization in an albumin-glutaraldehyde matrix

The effect of immobilization in an albumin-glutaraldehyde crosslinked matrix on the structure and activity of a photosystem I submembrane fraction has been studied. The photosynthetic activity recovered after immobilization was between 35 and 45% of the oxygen-uptake rates of the native material. Resulting oxygen uptake activities found in immobilized photosystem I preparations with methylviologen as acceptor were as high as 270 μmol O₂ (mg Chl h)^-1, An enhancement of photosystem I electron transfer, which is produced by incubation of thylakoid membranes at temperatures above 30 °C, was detected in native submembrane fractions, but not in the immobilized preparations. It is suggested that the increased activity at high temperature results from conformational modifications not allowed in the immobilization matrix. The insensitivity of immobilized photosystem I particles to prolonged storage at 4°C and to strong light exposure, as well as their high electron-transfer rates, demonstrates that the immobilization procedure used can be successfully applied to submembrane fractions.     

OXYGEN MEDIATED PHOTOSYSTEM I ACTIVITY IN THYLAKOID MEMBRANES MONITORED BY A PHOTOELECTROCHEMICAL CELL

Abstract— A photoelectrochemical cell has been used to monitor the effects of three enzymes on the photocurrent produced by isolated spinach thylakoids. The enzymes were glucose oxidase, superoxide dismutase and catalase. It is shown that all three inhibit the photocurrent to varying degrees. The results demonstrate that electron transport to the working electrode is mediated by oxygen. Further, the activity monitored originated from photosystem I with oxygen as the acceptor and photosystem II/plastoquinone as the donor. Thus, the photoelectrochemical cell constitutes a potential new approach for the monitoring of pseudocyclic electron transport.     

Calcium-manganese oxides as structural and functional models for active site in oxygen evolving complex in photosystem II: lessons from simple models

The oxygen evolving complex in photosystem II which induces the oxidation of water to dioxygen in plants, algae and certain bacteria contains a cluster of one calcium and four manganese ions. It serves as a model to split water by sunlight. Reports on the mechanism and structure of photosystem II provide a more detailed architecture of the oxygen evolving complex and the surrounding amino acids. One challenge in this field is the development of artificial model compounds to study oxygen evolution reaction outside the complicated environment of the enzyme. Calcium-manganese oxides as structural and functional models for the active site of photosystem II are explained and reviewed in this paper. Because of related structures of these calcium-manganese oxides and the catalytic centers of active site of the oxygen evolving complex of photosystem II, the study may help to understand more about mechanism of oxygen evolution by the oxygen evolving complex of photosystem II.     

Relationship between the structural and functional changes of the photosynthetic apparatus during chloroplast-chromoplast transition in flower bud of Lilium longiflorum

The relationship between the structural and functional changes of the photosynthetic apparatus in the flower bud of Lilium longiflorum during chloroplast-chromoplast transition was examined. Compared with green petals, there was a five-fold increase of the carotenoid content in yellow petals, whereas the chlorophyll content decreased five-fold. Absorption and emission fluorescence spectra of chromoplasts indicated that newly synthesized carotenoids were not associated with photosystem II (PSII) photochemistry. The maximum quantum yield in the remaining PSII reaction centers remained constant during the chromoplast formation, whereas the photosynthetic electron transport beyond PSII became inhibited, as indicated by a marked decrease of the O2 evolution capacity, of the photochemical quenching of chlorophyll-alpha fluorescence and of the operational quantum yield of photosynthetic electron transport. Deconvoluted fluorescence emission spectra indicated a more rapid degradation of photosystem I (PSI) complexes than of PSII during chromoplast formation. Compared with green petals, the spillover between PSII and PSI decreased by approximately 40% in yellow petals. Our results indicate that during chloroplast-chromoplast transition in the flower bud of L. longiflorum, PSII integrity was preserved longer than the rest of the photosynthetic apparatus.     

Evidence for PSII donor-side damage and photoinhibition induced by cadmium treatment on rice (Oryza sativa L.)

The effects of cadmium (from 7.5 to 75 microM) on chloroplasts of rice were studied at the structural and biochemical level. Loss of pigments, reduction of thylakoids and decrease in oxygen evolution and Fv/Fm ratio occur in leaves following cadmium treatment. However, the amount of photosystem II reaction center proteins and that of its light harvesting complex is not affected, indicating that cadmium does not adversely influence the structural organization of this photosystem. In thylakoids isolated from cadmium-treated plants a loss in the capability to reduce 2,6-dichlorophenolindophenol is observed, which is partially restored if diphenylcarbazide is used as an electron donor, indicating that cadmium affects water splitting activity. In thylakoids isolated from control plants and treated with cadmium, diphenylcarbazide preserves most of the photosystem II activity lost after incubation with cadmium; most of the S(2) multiline electron paramagnetic resonance signal from the manganese cluster is lost, whereas the TyrD(+) and other signals are retained. Light-induced photosystem II damage, in vitro, is promoted by Cd-treatment as deduced from the mobility shift of the D1 protein observed by immunoblot.     

Polymer-Modified Single-Walled Carbon Nanotubes Affect Photosystem II Photochemistry,Intersystem Electron Transport Carriers and Photosystem I End Acceptors in Pea Plants

Single-walled carbon nanotubes (SWCNT) have recently been attracting the attention of plant biologists as a prospective tool for modulation of photosynthesis in higher plants. However, the exact mode of action of SWCNT on the photosynthetic electron transport chain remains unknown. In this work, we examined the effect of foliar application of polymer-grafted SWCNT on the donor side of photosystem II, the intersystem electron transfer chain and the acceptor side of photosystem I. Analysis of the induction curves of chlorophyll fluorescence via JIP test and construction of differential curves revealed that SWCNT concentrations up to 100 mg/L did not affect the photosynthetic electron transport chain. SWCNT concentration of 300 mg/L had no effect on the photosystem II donor side but provoked inactivation of photosystem II reaction centres and slowed down the reduction of the plastoquinone pool and the photosystem I end acceptors. Changes in the modulated reflection at 820 nm, too, indicated slower re-reduction of photosystem I reaction centres in SWCNT-treated leaves. We conclude that SWCNT are likely to be able to divert electrons from the photosynthetic electron transport chain at the level of photosystem I end acceptors and plastoquinone pool in vivo. Further research is needed to unequivocally prove if the observed effects are due to specific interaction between SWCNT and the photosynthetic apparatus.     

SHORT-TERM ADAPTATION OF HIGHER PLANTS TO CHANGING LIGHT INTENSITIES AND EVIDENCE FOR THE INVOLVEMENT OF PHOSPHORYLATION OF THE LIGHT HARVESTING CHLOROPHYLL alb PROTEIN COMPLEX OF PHOTOSYSTEM II

Abstract The short-term adaptation of intact leaves to an increase in light intensity was studied by an analysis of chlorophyll fluorescence and oxygen evolution monitored by photoacoustics. An increase in light intensity led to an oxygen “gush”. This “gush” was followed by a large (up to 120%) biphasic increase in the yield of oxygen evolution characterized by a fast phase (T = 0.5–2 min) and a slow phase (T = 4–20 min). The fast phase of the increase in oxygen yield was coupled to a decrease of fluorescence, whereas the slow phase was accompanied by a parallel fluorescence increase. A comparison of fluorescence parameters with oxygen yield indicates that the slow phase of the increase in oxygen yield was coupled to an increase in the antenna size of photosystem II. The slow phase was not inhibited by the uncoupler Nigericin but it was absent in chlorophyll-b-less barley mutants dencient in the light harvesting chlorophyll a/b protein complex of photosystem II (LHC II). These experiments indicate that changes in the LHC II mediated energy distribution, which occur in the time-range of several minutes, are involved in the adaptation to changing light intensities. Moreover, electrophoretic analysis of ³²P orthophosphate labeled leaf discs adapted to low and high light intensities suggests that the slow phase of the increase in oxygen evolution involves dephosphorylation of the 25 kDa polypeptide of LHC II, by a small extent of 12%. The trigger for the slow phase of the increase in oxygen yield does not involve the oxidation of the plastoquinone pool. It was found that in response to the increased light intensity, the plastoquinone pool became more reduced as judged by model calculations. Experiments with the uncoupler Nigericin suggest that the control of the slow phase of adaptation to increased light intensity was also not exerted by the pH gradient across the thylakoid membrane. The similarities between the adaptation to increased light intensity and the state II to state I transition suggest that both adaptation phenomena involve LHC II dephosphorylation possibly triggered by the cytochrome b₆/f complex.     

SHORT-TERM ADAPTATION OF HIGHER PLANTS TO CHANGING LIGHT INTENSITIES AND EVIDENCE FOR THE INVOLVEMENT OF PHOSPHORYLATION OF THE LIGHT HARVESTING CHLOROPHYLL alb PROTEIN COMPLEX OF PHOTOSYSTEM II

Abstract— The short-term adaptation of intact leaves to an increase in light intensity was studied by an analysis of chlorophyll fluorescence and oxygen evolution monitored by photoacoustics. An increase in light intensity led to an oxygen “gush”. This “gush” was followed by a large (up to 120%) biphasic increase in the yield of oxygen evolution characterized by a fast phase (T = 0.5–2 min) and a slow phase (T = 4–20 min). The fast phase of the increase in oxygen yield was coupled to a decrease of fluorescence, whereas the slow phase was accompanied by a parallel fluorescence increase. A comparison of fluorescence parameters with oxygen yield indicates that the slow phase of the increase in oxygen yield was coupled to an increase in the antenna size of photosystem II. The slow phase was not inhibited by the uncoupler Nigericin but it was absent in chlorophyll-b-less barley mutants deñcient in the light harvesting chlorophyll a/b protein complex of photosystem II (LHC II). These experiments indicate that changes in the LHC II mediated energy distribution, which occur in the time-range of several minutes, are involved in the adaptation to changing light intensities. Moreover, electrophoretic analysis of ³²P orthophosphate labeled leaf discs adapted to low and high light intensities suggests that the slow phase of the increase in oxygen evolution involves dephosphorylation of the 25 kDa polypeptide of LHC II, by a small extent of 12%. The trigger for the slow phase of the increase in oxygen yield does not involve the oxidation of the plastoquinone pool. It was found that in response to the increased light intensity, the plastoquinone pool became more reduced as judged by model calculations. Experiments with the uncoupler Nigericin suggest that the control of the slow phase of adaptation to increased light intensity was also not exerted by the pH gradient across the thylakoid membrane. The similarities between the adaptation to increased light intensity and the state II to state I transition suggest that both adaptation phenomena involve LHC II dephosphorylation possibly triggered by the cytochrome b₆/f complex.     

Theoretical consideration of the use of a Langmuir adsorption isotherm to describe the effect of light intensity on electron transfer in photosystem II

Electron transport through photosystem II (PSII), measured as oxygen evolution, was investigated in isolated PSII particles and thylakoid membranes irradiated with white light of intensities (I) of 20 to about 4000 micromol of photons/(m2.s). In steady-state conditions, the evolution of oxygen varies with I according to the hyperbolic expression OEth = OEth(max)I/(L1/2 + I) (eq i) where OEth is the theoretical oxygen evolution, OEth(max) is the maximum oxygen evolution, and L1/2 is the light intensity giving OEth(max)/2. In this work, the mathematical derivation of this relationship was performed by using the Langmuir adsorption isotherm and assuming that the photon interaction with the chlorophyll (Chl) in the PSII reaction center is a heterogeneous reaction in which the light is represented as a stream of particles instead of an electromagnetic wave (see discussion in Turro, N. J. Modern Molecular Photochemistry; University Science Books: Mill Valley, CA, 1991). In accordance with this approximation, the Chl molecules (P680) were taken as the adsorption surfaces (or heterogeneous catalysts), and the incident (or exciting) photons as the substrate, or the reagent. Using these notions, we demonstrated that eq i (Langmuir equation) is a reliable interpretation of the photon-P680 interaction and the subsequent electron transfer from the excited state P680, i.e., P680*, to the oxidized pheophytin (Phe), then from Phe- to the primary quinone QA. First, eq i contains specific functional and structural information that is apparent in the definition of OEth(max) as a measure of the maximal number of PSII reaction centers open for photochemistry, and L1/2 as the equilibrium between the electron transfer from Phe- to QA and the formation of reduced Phe in the PSII reaction center by electrons in provenance from P680*. Second, a physiological control mechanism in eq i is proved by the observation that the magnitudes of OEth(max) and L1/2 are affected differently by exogenous PSII stimulators of oxygen evolution (Fragata, M.; Dudekula, S. J. Phys. Chem. B 2005, 109, 14707). Finally, an unexpected new concept, implicit in eq i, is the consideration of the photon as the substrate in the photochemical reactions taking place in the PSII reaction center. We conclude that the Langmuir equation (eq i) is a novel mathematical formulation of energy and electron transfer in photosystem II.     

PHOTOSYNTHETIC NITRITE REDUCTION BY DITHIOERYTHRITOL AND THE EFFECT OF NITRITE ON ELECTRON TRANSPORT IN ISOLATED CHLOROPLASTS

Abstract. Photosynthetic reduction of nitrite to ammonia with type C chloroplasts from the heterocont alga Bumilleriopsis filiformis was investigated using 3,6-diaminodurene/ascorbate and 3,6-diaminodurene/dithioerythritol (DAD/DTE) as electron donor couple. Rates approach 6–10 μmol NO^-₂ reduced/mg chlorophyll/h and are steady for up to 30 min. The presence of oxygen or NADP⁺ only slightly diminished the rates of nitrite reduction obtained with DAD/DTE. Illuminated chloroplasts reduce oxygen in the presence of DAD/DTE at 135 μmol/mg chlorophyll/h without acceptor supplied. Photosynthetic oxygen uptake by this system in the presence of ferredoxin and NO^-₂, however, is inhibited to 42% by nitrite reductase with concurrent nitrite reduction. NO^-₃ and NO^-₂ have no effect on photosystem I-mediated NADP⁺ reduction, NO^-₂ (10 m M ) inhibits ferricyanide-mediated oxygen evolution to 72%. Also photosystem II reactions assayed e.g. with silicomolybdate are inhibited significantly by NO^-₂ (1 m M ), but only slightly by NO^-₃. Nitrite reductase is inhibited by p -chloromercuribenzoate ( p CMB), and this inhibition is prevented by DTE. Results suggest that photosynthetic nitrite reduction can cope with low concentrations of either compound, provided relevant thiol groups are protected.