Carbon Dioxide-Induced Oscillations in Fluorescence and Photosynthesis: Role of Thylakoid Membrane Energization in Regulation of Photosystem II Activity

The response of CO₂ fixation to a sudden increase in ambient CO₂ concentration has been investigated in intact leaf tissue from spinach (Spinacia oleracea) using a dual channel infrared gas analyzer. Simultaneous with these measurements, changes in fluorescence emission associated with a weak, modulated measuring beam were recorded. Application of brief (2-3 seconds) dark intervals enabled estimation of the dark fluorescence level (F_o) under both steady state and transient conditions. The degree of suppression of F_o level fluorescence in the light was strongly correlated with nonphotochemical quenching under all conditions. During CO₂-induced oscillations in photosynthesis under 2% O₂ the changes in nonphotochemical quenching anticipate changes in the rate of uptake of CO₂. At such low levels of O₂ and constant illumination, changes in the relative quantum efficiency of open photosystem II units were estimated as the ratio of the rate of CO₂ uptake and the photochemical quenching coefficient. Under the same conditions the relative quantum efficiency of photosystem II was found to vary inversely with the degree of nonphotochemical quenching. The relationship between changes in the rate of CO₂ uptake: photochemical quenching coefficient and nonphotochemical quenching was altered somewhat when the same experiment was conducted under 20% O₂. The results suggest that electron transport coupled to reduction of O₂ occurs to varying degrees with time during oscillations, especially when ambient O₂ concentrations are high.     

Relationship between Steady-State Fluorescence Yield and Photosynthetic Efficiency in Spinach Leaf Tissue

The relationship between steady-state photosynthetic efficiency, as moles CO₂ per mole of incident visible photons under 2% O₂, and chlorophyll fluorescence quenching has been investigated in intact leaf tissue of Spinacia oleracia. Fluorescence yield was measured using a pulse amplitude modulation technique that permitted rapid and sensitive resolution and quantitation of photochemical and nonphotochemical quenching coefficients. A highly linear relationship was observed between photosynthetic efficiency and the ratio of photochemical:nonphotochemical quenching coefficients for values of the latter less than 1.6. This relationship applied whether irradiance or CO₂ concentration was varied. The observed relationships between photochemical yield and fluorescence yield were compatible with the photosystem II model proposed by Butler and Kitajima (1975 Biochim Biophys Acta 376: 116-125). The results are discussed with respect to the proposed role of nonphotochemical quenching in regulating radiant energy utilization and also the applicability of fluorescence measurements as a means of estimation of the rate of photosynthetic electron transport.     

Effects of Irradiance on the in Vivo CO(2):O(2) Specificity Factor in Tobacco Using Simultaneous Gas Exchange and Fluorescence Techniques

The effects of gas phase O₂ concentration (1%, 20.5%, and 42.0%, v/v) on the quantum yield of net CO₂ fixation and fluorescence yield of chlorophyll a are examined in leaf tissue from Nicotiana tabacum at normal levels of CO₂ and 25 to 30°C. Detectable decreases in nonphotochemical quenching of absorbed excitation occurred at the higher O₂ levels relative to 1% O₂ when irradiance was nearly or fully saturating for photosynthesis. Photochemical quenching was increased by high O₂ levels only at saturating irradiance. Simultaneous measurements of CO₂ and H₂O exchange and fluorescence yield permit estimation of partitioning of linear photosynthetic electron transport between net CO₂ fixation and O₂-dependent, dissipative processes such as photorespiration as a function of leaf internal CO₂ concentration. Changes in the in vivo CO₂:O₂ `specificity factor' (K_sp) with increasing irradiance are examined. The magnitude K_sp was found to decline from a value of 85 at moderate irradiance to 68 at very low light, and to 72 at saturating photon flux rates. The results are discussed in terms of the applicability of the ribulose bisphosphate carboxylase/oxygenase enzyme model to photosynthesis in vivo.     

Adaptation of the Photosynthetic Apparatus in Maize Leaves as a Result of Nitrogen Limitation : Relationships between Electron Transport and Carbon Assimilation

In maize (Zea mays L., cv Contessa), nitrogen (NO₃⁻) limitation resulted in a reduction in shoot growth and photosynthetic capacity and in an increase in the leaf zeaxanthin contents. Nitrogen deficiency had only a small effect on the quantum yield of CO₂ assimilation but a large effect on the light-saturated rate of photosynthesis. Linear relationships persisted between the quantum yield of CO₂ assimilation and that of photosystem II photochemistry in all circumstances. At high irradiances, large differences in photochemical quenching and nonphotochemical quenching of Chl a fluorescence as well as the ratio of variable to maximal fluorescence (Fv/Fm) were apparent between nitrogen-deficient plants and nitrogen-replete controls, whereas at low irradiances these parameters were comparable in all plants. Light intensity-dependent increases in nonphotochemical quenching were greatest in nitrogen-deficient plants as were the decreases in Fv/Fm ratio. In nitrogen-deficient plants, photochemical quenching decreased with increasing irradiance but remained higher than in controls at high irradiances. Thermal dissipative processes were enhanced as a result of nitrogen deficiency (nonphotochemical quenching was elevated and Fv/Fm was lowered) allowing PSII to remain relatively oxidised even when carbon metabolism was limited via nitrogen limitation.     

Relative contributions of zeaxanthin-related and zeaxanthin-unrelated types of ;high-energy-state' quenching of chlorophyll fluorescence in spinach leaves exposed to various environmental conditions

We have identified two rapidly relaxing components of non-photochemical fluorescence quenching which suggests that dissipative processes occur in two different sites in the photochemical system of leaves. Under a variety of treatment conditions involving different leaf temperatures, photon flux densities (PFD), exposure times, and in the presence of 5% CO₂ or 2% O₂, no CO₂, the components of nonphotochemical fluorescence quenching were characterized with respect to their sensitivity to dithiothreitol (DTT, which completely inhibits zeaxanthin formation), the effect on instantaneous fluorescence, and the rapidity of relaxation upon darkening. Under most circumstances the DTT-sensitive component (associated with a quenching of instantaneous fluorescence and correlated with zeaxanthin) represented the majority of the rapidly relaxing portion of fluorescence quenching. A DTT-insensitive (zeaxanthin-independent) component, which also relaxed rapidly upon darkening but was not associated with a quenching of instantaneous fluorescence, became proportionally greater in an atmosphere of 2% O₂ and no CO₂, at elevated leaf temperatures, and to some degree during the induction of photosynthesis (1 minute after the onset of illumination). A third component which was also DTT-insensitive and was sustained upon darkening, was largely suppressed in 2% O₂, O% CO₂. We conclude that, under conditions favorable for photosynthesis, energy dissipation occurred mainly in the chlorophyll antennae whereas, under conditions less favorable for photosynthesis, a second dissipation process, probably in or around the reaction center of photosystem II, also developed. Furthermore, evidence is presented that the zeaxanthin-associated dissipation process prevents sustained inactivation of photochemistry by excessive light.     

Effect of action potential on photosynthesis and spatially distributed H+ fluxes in cells and Chloroplasts of Chara corallina

When exposed to light, the cells of characean algae produce intermittent regions of H⁺ extrusion and H⁺ absorption, featuring different photosynthetic activities. Methods for local measurements of outer pH, O₂ content, and photochemical activity of photosystem II (PSII) were applied to examine microscopic regions of Chara coralline Klein ex Willd. internodes. The results show that the functional spatial heterogeneity of these excitable cells is controlled not only by light but also by electric excitation of the plasma membrane. Generation of a single action potential (AP) induced a reversible transition to the state with homogenous pH distribution and had different effects on photosynthesis in cell regions producing alkaline and acid zones. The effective quantum yield of PSII primary processes and the maximal chlorophyll fluorescence decreased after AP in the alkaline cell regions but were almost unaffected in the acidic cell regions. The suppression of photosynthesis after AP was also evident in the decrease of photosynthetic O₂ evolution. The results provide evidence that electric signals arising at the plasmalemma are transmitted to the level of thylakoid membranes. The effects of electric excitation on fluorescence and the quantum yield of PSII photochemistry were best pronounced at low light intensities and low level of nonphotochemical quenching. The sensitivity of chlorophyll fluorescence in resting and excited cells to light intensity and protonophores indicates that the AP-induced fluorescence changes derive from the increase in pH gradient at the thylakoid membrane. The temporal elimination of alkaline zones and inhibition of photosynthesis apparently arise from parallel operational sequences that have a common initial stage. A possible role of cytosolic Ca²⁺ rise in the mechanism of photosynthesis suppression after electric excitation of the plasma membrane is discussed.     

Physiological response of Nannochloropsis sp. to saline stress in laboratory batch cultures

In the present paper, we investigated the physiological response of the marine microalga Nannochloropsis sp. to salt stress (13, 27, 54, and 81 g L^?1 NaCl). Increasing the sodium chloride concentration caused up to a 70 % decrease in the chlorophyll a concentration, cell growth, and net photosynthesis rate. The chlorophyll a fluorescence measurements indicated a strong reduction in the effective quantum yield of photosystem II (?60 %) and an increase in nonphotochemical quenching when the cells were exposed to NaCl concentrations greater than 27 g L^?1 (control). In contrast, the specific lipid content increased up to 80 % when the sodium chloride concentration was increased from 27 to 54–81 g L^?1. These results are relevant for the outdoor cultivation of this microalga using open photobioreactors, in which microalgae are subjected to strong changes in salinity concentration caused by water evaporation.     

Photorespiration is Essential for the Protection of the Photosynthetic Apparatus of C3 Plants Against Photoinactivation Under Sunlight

CO₂ assimilation, transpiration and modulated chlorophyll fluorescence of leaves of Chenopodium bonus-henricus (L.) were measured in the laboratory and, at a high altitude location, in the field. Direct calibration of chlorophyll fluorescence parameters against carbon assimilation in the presence of 1 or 0.5% oxygen (plus CO₂) proved necessary to calculate electron transport under photorespiratory conditions in individual experiments. Even when stomata were open in the field, total electron transport was two to three times higher in sunlight than indicated by net carbon gain. It decreased when stomata were blocked by submerging leaves under water or by forcing them to close in air by cutting the petiole. Even under these conditions, electron transport behind closed stomata approached 10 nmol electrons m^?2 leaf area s^?1 at temperatures between 25 and 30 °C. No photoinactivation of photosystem II was indicated by fluorescence analysis after a day's exposure to full sunlight. Only when leaves were submerged in ice was appreciable photoinactivation noticeable after 4 h exposure to sunlight. Even then almost full recovery occurred overnight. Electron transport behind blocked stomata was much decreased when leaves were darkened for 70 min (in order to deactivate light-regulated enzymes of the Calvin cycle) before exposure to full sunlight. Brief exposure of leaves to HCN (to inhibit photoassimilation and photorespiration) also decreased electron transport drastically compared to electron transport in unpoisoned leaves with blocked stomata. Non-photochemical fluorescence quenching and reduction of Q_A, the primary electron acceptor of photosystem II was increased by HCN-poisoning. Very similar observations were made when glyceraldehyde was used instead of HCN to inhibit photosynthesis and photorespiration. In HCN-poisoned leaves, residual electron transport increased linearly with temperature and showed early light saturation revealing characteristics of the Mehler reaction. During short exposure of these leaves to photon flux densities equivalent to 25% of sunlight, no or only little photoinactivation of photosystem II was observed. However, prolonged exposure to sunlight caused inactivation even though non-photochemical quenching of chlorophyll fluorescence was extensive. Simultaneously, oxidation of cellular ascorbate and glutathione increased. Inactivation of photosystem II was reversible in dim light and in the dark only after short times of exposure to sunlight. Glyceraldehyde was very similar to HCN in increasing the sensitivity of photosystem II in leaves to sunlight. We conclude from the observations that the electron transport permitted by the interplay of photoassimilatory and photorespiratory electron transport is essential to prevent the photoinactivation of photosynthetic electron transport. The Mehler and Asada reactions, which give rise to strong nonphotochemical fluorescence quenching, are insufficient to protect the chloroplast electron transport chain against photoinactivation.     

Inhibition of zeaxanthin formation and of rapid changes in radiationless energy dissipation by dithiothreitol in spinach leaves and chloroplasts

Dithiothreitol, which completely inhibits the de-epoxidation of violaxanthin to zeaxanthin, was used to obtain evidence for a causal relationship between zeaxanthin and the dissipation of excess excitation energy in the photochemical apparatus in Spinicia oleracea L. In both leaves and chloroplasts, inhibition of zeaxanthin formation by dithiothreitol was accompanied by inhibition of a component of nonphotochemical fluorescence quenching. This component was characterized by a quenching of instantaneous fluorescence (F_o) and a linear relationship between the calculated rate constant for radiationless energy dissipation in the antenna chlorophyll and the zeaxanthin content. In leaves, this zeaxanthin-associated quenching, which relaxed within a few minutes upon darkening, was the major component of nonphotochemical fluorescence quenching determined in the light, i.e. it represented the `high-energy-state' quenching. In isolated chloroplasts, the zeaxanthin-associated quenching was a smaller component of total nonphotochemical quenching and there was a second, rapidly reversible high-energy-state component of fluorescence quenching which occurred in the absence of zeaxanthin and was not accompanied by F_o quenching. Leaves, but not chloroplasts, were capable of maintaining the electron acceptor, Q, of photosystem II in a low reduction state up to high degrees of excessive light and thus high degrees of nonphotochemical fluorescence quenching. When ascorbate, which serves as the reductant for violaxanthin de-epoxidation, was added to chloroplast suspensions, zeaxanthin formation at low photon flux densities was stimulated and the relationship between nonphotochemical fluorescence quenching and the reduction state in chloroplasts then became more similar to that found in leaves. We conclude that the inhibition of zeaxanthin-associated fluorescence quenching by dithiothreitol provides further evidence that there exists a close relationship between zeaxanthin and potentially photoprotective dissipation of excess excitation energy in the antenna chlorophyll.     

Zinc-inhibited Electron Transport of Photosynthesis in Isolated Barley Chloroplasts

In isolated barley chloroplasts, the presence of 2 millimolar ZnSO₄ inhibits the electron transport activity of photosystem II, as measured by photoreduction of dichlorophenolindophenol, O₂ evolution, and chlorophyll a fluorescence. The inhibition of photosystem II activity can be restored by the addition of the electron donor hydroxylamine or diphenylcarbazide, but not by benzidine and MnCl₂. These observations suggest that Zn inhibits electron flow at the oxidizing side of photosystem II at a site prior to the electron donating site(s) of hydroxylamine and diphenylcarbazide. No inhibition of photosystem I-dependent electron transport by 3 millimolar ZnSO₄ is observed. However, with concentrations of ZnSO₄ above 5 millimolar, photosystem I activity is partially inactivated. Washing Zn²⁺-treated chloroplasts partially restores the O₂-evolving activity.     

Active Transport of Inorganic Carbon Increases the Rate of O(2) Photoreduction by the Cyanobacterium Synechococcus UTEX 625

Chlorophyll a fluorescence of Synechococcus UTEX 625 was quenched during the transport of inorganic carbon, even when CO₂ fixation was inhibited by iodoacetamide. Measurements with a pulse modulation fluorometer showed that at least 75% of the quenching was due to oxidation of Q_a, the primary acceptor of photosystem II. Mass spectrometry revealed that transport of inorganic carbon increased the rate of O₂ photoreduction. Hence, O₂ could serve as an electron acceptor to allow oxidation of Q_a even in the absence of CO₂ fixation.     

Chlorophyll a Fluorescence Yield as a Monitor of Both Active CO(2) and HCO(3) Transport by the Cyanobacterium Synechococcus UTEX 625

Simultaneous measurements have been made of inorganic carbon accumulation (by mass spectrometry) and chlorophyll a fluorescence yield of the cyanobacterium Synechococcus UTEX 625. The accumulation of inorganic carbon by the cells was accompanied by a substantial quenching of chlorophyll a fluorescence. The quenching occurred even when CO₂ fixation was inhibited by iodoacetamide and whether the accumulation of inorganic carbon resulted from either active CO₂ or HCO₃⁻ transport. Measurement of chlorophyll a fluorescence yield of cyanobacteria may prove to be a rapid and convenient means of screening for mutants of inorganic carbon accumulation.     

Mehler-peroxidase reaction mediates zeaxanthin formation and zeaxanthin-related fluorescence quenching in intact chloroplasts

Induction of zeaxanthin formation and the associated nonphotochemical quenching in iodoacetamide-treated, non-CO₂-fixing intact chloroplasts of Lactuca sativa L. cv Romaine is reported. The electron transport needed to generate the required ΔpH for zeaxanthin formation and nonphotochemical quenching are ascribed to the Mehler-ascorbate peroxidase reaction. KCN, an inhibitor of ascorbate peroxidase, significantly affected these activities without affecting linear electron transport to methyl viologen or violaxanthin deepoxidase activity. At 1 millimolar KCN, zeaxanthin formation and ΔpH were inhibited 60 and 55%, respectively, whereas ascorbate peroxidase activity was inhibited almost totally. The KCN-resistant activity, which apparently was due to electron transport mediated by the Mehler reaction alone, however, was insufficient to support a high level of nonphotochemical quenching. We suggest that in vivo, as CO₂ fixation becomes limiting, the Mehler-peroxidase reaction protects photosystem II against the excess light by supporting the electron transport needed for zeaxanthin-dependent nonphotochemical quenching and concomitantly scavenging H₂O₂. Ascorbate is essential for this process to occur.     

Sites of inhibition by disulfiram in thylakoid membranes

Disulfiram (tetraethylthiuram disulfide), a metal chelator, inhibits photosynthetic electron transport in broken chloroplasts. A major site of inhibition is detected on the electron-acceptor side of photosystem II between Q_A, the first plastoquinone electron-acceptor, and the second plastoquinone electron-acceptor, Q_B. This site of inhibition is shown by a severalfold increase in the half-time of Q⁻_A oxidation, as monitored by the decay of the variable chlorophyll a flourescence after an actinic flash. Another site of inhibition is detected in the functioning of the reaction center of photosystem II; disulfiram is observed to quench the room temperature variable chlorophyll a fluorescence, as well as the intensity of the 695 nm peak, relative to the 685 nm peak, in the chlorophyll a fluorescence spectrum at 77 K. Electron transport from H₂O to the photosystem II electron-acceptor silicomolybdate is also inhibited. Disulfiram does not inhibit electron flow before the site(s) of donation by exogenous electron donors to photosystem II, and no inhibition is detected in the partial reactions associated with photosystem I.     

Cadmium exerts its toxic effects on photosynthesis via a cascade mechanism in the cyanobacterium,Synechocystis PCC 6803

Despite intense research, the mechanism of Cd²⁺ toxicity on photosynthesis is still elusive because of the multiplicity of the inhibitory effects and different barriers in plants. The quick Cd²⁺ uptake in Synechocystis PCC 6803 permits the direct interaction of cadmium with the photosynthetic machinery and allows the distinction between primary and secondary effects. We show that the CO₂‐dependent electron transport is rapidly inhibited upon exposing the cells to 40 µm Cd²⁺ (50% inhibition in ～15 min). However, during this time we observe only symptoms of photosystem I acceptor side limitation and a build of an excitation pressure on the reaction centres, as indicated by light‐induced P700 redox transients, O₂ polarography and changes in chlorophyll a fluorescence parameters. Inhibitory effects on photosystem II electron transport and the degradation of the reaction centre protein D1 can only be observed after several hours, and only in the light, as revealed by chlorophyll a fluorescence transients, thermoluminescence and immunoblotting. Despite the marked differences in the manifestations of these short‐ and long‐term effects, they exhibit virtually the same Cd²⁺ concentration dependence. These data strongly suggest a cascade mechanism of the toxic effect, with a primary effect in the dark reactions.     

Light Response of CO(2) Assimilation, Dissipation of Excess Excitation Energy, and Zeaxanthin Content of Sun and Shade Leaves

Intact attached sun leaves of Helianthus annuus and shade leaves of Monstera deliciosa and Hedera helix were used to obtain light response curves of CO₂ uptake, the content of the carotenoid zeaxanthin (formed by violaxanthin de-epoxidation), as well as nonphotochemical quenching (q_NP), and the rate constant of radiationless energy dissipation (k_D). The latter two parameters were calculated from the decrease of chlorophyll a fluorescence at closed photosystem II traps in saturating pulses in the light. Among the three species, the light-saturated capacity of CO₂ uptake differed widely and light saturation of CO₂ uptake occurred at very different photon flux densities. Fluorescence quenching and zeaxanthin content exhibited features which were common to all three species: below light-saturation of CO₂ uptake nonphotochemical quenching occurred in the absence of zeaxanthin and was not accompanied by a decrease in the yield of instantaneous fluorescence. Nonphotochemical quenching, q_NP, increased up to values which ranged between 0.35 and 0.5 when based on a control value of the yield of variable fluorescence determined after 12 hours of darkness. As light saturation of CO₂ uptake was approached, q_NP showed a secondary increase and the zeaxanthin content of the leaves began to rise. This was also the point from which the yield of instantaneous fluorescence began to decrease. The increase in zeaxanthin was paralleled by an increase in the rate constant for radiationless energy dissipation k_D, which opens the possibility that zeaxanthin is related to the rapidly relaxing “high-energy-state quenching” in leaves.     

Topography of photosynthetic activity of leaves obtained from video images of chlorophyll fluorescence

The distribution of photosynthetic activity over the area of a leaf and its change with time was determined (at low partial pressure of O₂) by recording images of chlorophyll fluorescence during saturating light flashes. Simultaneously, the gas exchange was being measured. Reductions of local fluorescence intensity quantitatively displayed the extent of nonphotochemical quenching; quench coefficients, q_N, were computed pixel by pixel. Because rates of photosynthetic electron transport are positively correlated with (1 − q_N), computed images of (1 − q_N) represented topographies of photosynthetic activity. Following application of abscisic acid to the heterobaric leaves of Xanthium strumarium L., clearly delineated regions varying in nonphotochemical quenching appeared that coincided with areoles formed by minor veins and indicated stomatal closure in groups.     

Chlorophyll a fluorescence as a tool for a study of the Potato virus Y effects on photosynthesis of nontransgenic and transgenic Pssu-ipt tobacco

The effect of Potato virus Y ^NTN (PVY) infection upon photosynthesis was analysed in transgenic Pssu-ipt tobacco overproducing endogenous cytokinins in comparison with control, nontransgenic Nicotiana tabacum plants. The course of the infection from the early to the late stage was monitored by measuring of photosynthetic gas exchange and fast chlorophyll (Chl) a fluorescence induction kinetics. Leaf photosynthesis was also analysed using Chl fluorescence imaging (Chl-FI). From the different fluorescence parameters obtained using Chl-FI, the nonphotochemical quenching (NPQ) proved to be the most useful parameter to assess the effect of PVY infection. On the other hand, Chl-FI was found to be inapplicable for any presymptomatic detection of PVY infection in tobacco. The lower accumulation of the virus was found in transgenic plants and corresponded also with the presence of visible symptoms of PVY infection. The net photosynthetic rate (P _N), transpiration rate (E), and stomatal conductance (g _s) significantly decreased with the progress of the infection in both control plant types and transgenic rooted plants, while transgenic grafts were much less affected. The analysis of the Chl fluorescence transient revealed higher number of silent dissipative reaction centres, higher nonphotochemical dissipation, and significantly lower performance index, PI_(abs), in the healthy transgenic grafts. Chl-FI also confirmed significantly higher NPQ in transgenic grafts.     

Microscale Vertical Profiles of N2 Fixation, Photosynthesis, O2, Chlorophyll a, and Light in a Cyanobacterial Assemblage

Profiles of ¹⁵N₂ fixation, O₂ production (gross photosynthesis), O₂ concentration, chlorophyll a concentration, and photon fluence rates were measured with 50-μm resolution in colonies of the heterocyst-forming cyanobacterium Nostoc parmelioides. Microelectrode measurements were made after 20 h of incubation under ¹⁵N₂ gas. Colonies were frozen, and 50-μm sections were prepared by using a freezing microtome and analyzed for ¹⁵N enrichment and chlorophyll a concentration. Colonies exhibited steep spatial gradients in rates of gross photosynthesis, O₂ concentration, and irradiance, with the highest values generally occurring at the surface. O₂ concentration, photosynthesis, and irradiance all showed positive correlations, but chlorophyll a concentrations varied independently of photosynthesis and irradiance. Forty-four percent of the variation in ¹⁵N incorporation was explained by gross photosynthesis (a positive correlation) when incorporation of ¹⁵N was expressed per unit of biomass (chlorophyll a).     

Multiple effects of salinity on photosynthesis of the protist Euglena gracilis

The effect of NaCl in the culture medium on growth, photosynthesis and cell content of chlorophyll, K⁺, Na⁺, Ca²⁺ and Mg²⁺ in Euglena gracilis was studied. O₂ production, quantum yield of photosystem II (PSII), the non-photochemical quenching of chlorophyll fluorescence (q_N) and the chlorophyll alb ratio all diminished by 0.2 M NaCl. Respiration and chlorophyll a and b increased, whereas the photochemical quenching (q_p) of chlorophyll fluorescence was not affected by 0.2 M NaCl. Salt stress also induced an increase in cell volume and in K⁺ and Na⁺ concentrations, but decreased the concentrations of Ca²⁺ and Mg²⁺. Except for a protective effect on O₂ production, additional Ca²⁺ in the culture medium did not attenuate the salt effect on the parameters measured. The addition of HCO₃^? restored the PSII quantum yield of O₂ production in cells grown in high salt. Salt stress promoted a decrease in the apparent rate of quinone A (Q_A) reduction and an apparent obstruction of Q_B reduction, which were not prevented by excess HCO₃^?; the addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) did not increase chlorophyll fluorescence in salt-grown cells. These results indicate that photosynthesis in Euglena grown under salt stress exhibits: (1) diminution of the HCO₃^? dependent water-splitting activity of PSII; (2) inhibition of the electron transfer at the quinone pool level; (3) probable increase in thylakoid stacking (as indicated by the effect on the chlorophyll alb ratio); and (4) dissipation of the H⁺ gradient across the thylakoid membranes (as indicated by the decrease of q_N).