Elevated CO2 enhances stomatal responses to osmotic stress and abscisic acid in Arabidopsis thaliana

A , carbon assimilation rate
ABA, abscisic acid
Ci , intercellular space CO₂ concentration
g , leaf conductance
WUE, water use efficiency

Carbon dioxide and abscisic acid (ABA) are two major signals triggering stomatal closure. Their putative interaction in stomatal regulation was investigated in well-watered air-grown or double CO₂-grown Arabidopsis thaliana plants, using gas exchange and epidermal strip experiments. With plants grown in normal air, a doubling of the CO₂ concentration resulted in a rapid and transient drop in leaf conductance followed by recovery to the pre-treatment level after about two photoperiods. Despite the fact that plants placed in air or in double CO₂ for 2 d exhibited similar levels of leaf conductance, their stomatal responses to an osmotic stress (0·16–0·24 MPa) were different. The decrease in leaf conductance in response to the osmotic stress was strongly enhanced at elevated CO₂. Similarly, the drop in leaf conductance triggered by 1 μ M ABA applied at the root level was stronger at double CO₂. Identical experiments were performed with plants fully grown at double CO₂. Levels of leaf conductance and carbon assimilation rate measured at double CO₂ were similar for air-grown and elevated CO₂-grown plants. An enhanced response to ABA was still observed at high CO₂ in pre-conditioned plants. It is concluded that: (i) in the absence of stress, elevated CO₂ slightly affects leaf conductance in A. thaliana ; (ii) there is a strong interaction in stomatal responses to CO₂ and ABA which is not modified by growth at elevated CO₂.     

The effect of different growth conditions on dark and light carbon assimilation in Littorella uniflora

The effect of long-term exposure to different inorganic carbon, nutrient and light regimes on CAM activity and photosynthetic performance in the submerged aquatic plant, Littorella uniflora (L.) Aschers was investigated. The potential CAM activity of Littorella was highly plastic and was reduced upon exposure to low light intensities (43 μmol m⁻² s⁻¹), high CO₂ concentrations (5.5 mM, pH 6.0) or low levels of inorganic nutrients, which caused a 25–80% decline in the potential maximum CAM activity relative to the activity in the control experiments (light: 450 μmol m⁻² s⁻¹; free CO₂: 1.5 mM). The CAM activity was regulated more by light than by CO₂, while nutrient levels only affected the activity to a minor extent. The minor effect of low nutrient regimes may be due to a general adaptation of isoetid species to low nutrient levels.
The photosynthetic capacity and CO₂ affinity was unaffected or increased by exposure to low CO₂, irrespective of nutrient levels. High CO₂, low nutrient and low light, however, reduced the capacity by 22–40% and the CO₂ affinity by 35-45%, relative to control.
The parallel effect of growth conditions on CAM activity and photosynthetic performance of Littorella suggest that light and dark carbon assimilation are interrelated and constitute an integrated part of the carbon assimilation physiology of the plant. The results are consistent with the hypothesis that CAM is a carbon-conserving mechanism in certain aquatic plants. The investment in the CAM enzyme system is beneficial to the plants during growth at high light and low CO₂ conditions.     

Effect of elevated CO2 on the stomatal distribution and leaf physiology of Alnus glutinosa

Variation in stomatal development and physiology of mature leaves from Alnus glutinosa plants grown under reference (current ambient, 360 μmol mol⁻¹ CO₂) and double ambient (720 μmol mol⁻¹ CO₂) carbon dioxide (CO₂) mole fractions is assessed in terms of relative plant growth, stomatal characters (i.e. stomatal index and density) and leaf photosynthetic characters. This is the first study to consider the effects of elevated CO₂ concentration on the distribution of stomata and epidermal cells across the whole leaf and to try to ascertain the cause of intraleaf variation. In general, a doubling of the atmospheric CO₂ concentration enhanced plant growth and significantly increased stomatal index. However, there was no significant change in relative stomatal density. Under elevated CO₂ concentration there was a significant decrease in stomatal conductance and an increase in assimilation rate. However, no significant differences were found for the maximum rate of carboxylation ( V _cmax) and the light saturated rate of electron transport ( J _max) between the control and elevated CO₂ treatment.     

Growth and physiological responses of canola (Brassica napus) to UV-B and CO2 under controlled environment conditions

Few studies have investigated the interaction of ultraviolet (UV)-B radiation and CO₂ concentration on plants. We studied the combined effects of UV-B radiation and CO₂ concentration on canola ( Brassica napus cv. 46A65) under four growth conditions – ambient CO₂ with UV-B (control), elevated CO₂ with UV-B, ambient CO₂ without UV-B, and elevated CO₂ without UV-B – to determine whether the adverse effects of UV-B are mitigated by elevated CO₂. Elevated CO₂ significantly increased plant height and seed yield, whereas UV-B decreased them. Elevated CO₂ ameliorated the adverse effects of UV-B in plant height. UV-B did not affect the physical characteristics of leaf but CO₂ did. Certain flower and fruit characteristics were affected negatively by UV-B and positively by CO₂. UV-B did not affect net photosynthesis, transpiration and stomatal conductance but decreased water use efficiency (WUE). Elevated CO₂ significantly increased net photosynthesis and WUE. Neither UV-B nor CO₂ affected chlorophyll (Chl) fluorescence. UV-B significantly decreased Chl b and increased the ratio of Chl a / b . Elevated CO₂ decreased only the ratio of Chl a / b . UV-B significantly increased UV-absorbing compounds while CO₂ had no effect on them. Both UV-B and CO₂ significantly increased epicuticular wax content. Many significant relationships were found between morphological, physiological, and chemical parameters. This study showed that elevated CO₂ can partially ameliorate some of the adverse effects of UV-B radiation in B . napus .     

Oxygen-dependent stomatal opening in Zea mays leaves. Effect of Light and carbon dioxide

The oxygen requirement for stomatal opening in maize plants ( Zea mays L. hybrid INRA 508) was studied at different CO₂ concentrations and light intensities. In the absence of CO₂, stomatal opening always required O₂, but this requirement decreased with increasing light intensity. In darkness, the lowest O₂ partial pressure needed to obtain a weak stomatal movement was about 50 Pa. This value was lowered to ca 10 Pa in light (320 μmol m⁻² s⁻¹).
On the other hand. in the absence of O₂, CO₂enabled stomatal opening to occur in the light, presumably due to the evolved photosynthetic O₂. Thus, CO₂, which generally reduced stomatal aperture, could induce stomatal movement in anoxia and light. The effect of CO₂ on stomatal opening was closely dependent on O₂ concentration and light intensity. Stomatal aperture appeared CO₂-independent at an O₂ partial pressure which was dependent on light intensity and was about 25 Pa at 320 umol m⁻² s⁻¹.
The presence of a plasmalemma oxidase, in addition to mitochondrial oxidase, might explain the differences in the O² requirement at various light intensities. The possible involvement of such a system in relation to the effect of CO₂ is discussed.     

Responses of individual stomata in Ipomoea pes-caprae to various CO2 concentrations

The responses of individual stomata to CO₂ concentrations ranging from 0 to 900 μmol mol⁻¹ air were analysed in Ipomoea pes-caprae L. Sweet (Convolvulaceae). The stomata were directly observed using a measurement system that permitted continuous observation of stomatal movement under controlled light and CO₂ conditions. A CO₂ concentration of 350 μmol mol⁻¹ or higher induced stomatal closure, whereas concentrations below 350 μmol mol⁻¹ did not. The time lag before stomatal closure decreased with increasing CO₂ concentration, as did the steady-state aperture of the stomata after a change in CO₂ concentration. However, the rate of stomatal closure increased with increasing CO₂ concentration. Therefore, not only the stomatal closure rate but also the time from the CO₂ concentration change to the beginning of stomatal closure changed with increasing CO₂ concentration. These results suggest that atmospheric CO₂ may be the stimulus for the closure of guard cells. No significant differences were observed between adaxial and abaxial stomata in terms of their responses to CO₂. However, heterogeneous responses were detected between neighbouring stomata on each leaf surface.     

Water stress, carbon dioxide, and light effects on sucrosephosphate synthase activity in Phaseolus vulgaris

The characteristics of sucrose-phosphate synthase (SPS; EC 2.4.1.14) activity in leaves of Phaseolus vulgaris L. cv. Linden was studied in plants subjected to water stress and various CO₂ and light treatments. When water was withheld for 3 days causing mild water stress (–0.9 MPa), the activity of SPS measured in crude extracts was reduced ca 50%. The effect of water stress was most evident when the enzyme was assayed with saturating amounts of its substrates fructose 6-phosphate and UDP glucose. Placing a water-stressed plant in an atmosphere containing 1% CO₂ reversed the effect of water stress on SPS activity over 5 h even though the water stress was not relieved. Holding unstressed leaves in low CO₂ partial pressure reduced the extractable activity of SPS. After 1 h of low CO₂ treatment the effect of low CO₂ could be reversed by 20 min of 5% CO₂. However, after 24 h of low CO₂ treatment, less SPS activity was recovered by the 20 min treatment. The cytosolic protein synthesis inhibitor cycloheximide prevented the slow recovery of SPS activity, but did not affect the rapid recovery of SPS. We conclude that the effect of water stress on SPS activity was a consequence of the inhibition of photosynthesis caused by stomatal closure. Responses of Phaseolus vulgaris SPS to light were similar to the response to low CO₂ in that the effects were most pronounced under V_max assay conditions. This is the first report of this type of light response of SPS in a dicotyledonous species.     

The stomatal response to CO2 is linked to changes in guard cell zeaxanthin*

The mechanisms mediating CO₂ sensing and light–CO₂ interactions in guard cells are unknown. In growth chamber-grown Vicia faba leaves kept under constant light (500 μ mol m^–2 s^–1) and temperature, guard cell zeaxanthin content tracked ambient [CO₂] and stomatal apertures. Increases in [CO₂] from 400 to 1200 cm³ m^–3 decreased zeaxanthin content from 180 to 80 mmol mol^–1 Chl and decreased stomatal apertures by 7·0 μ m. Changes in zeaxanthin and aperture were reversed when [CO₂] was lowered. Guard cell zeaxanthin content was linearly correlated with stomatal apertures. In the dark, the CO₂-induced changes in stomatal aperture were much smaller, and guard cell zeaxanthin content did not change with chamber [CO₂]. Guard cell zeaxanthin also tracked [CO₂] and stomatal aperture in illuminated stomata from epidermal peels. Dithiothreitol (DTT), an inhibitor of zeaxanthin formation, eliminated CO₂-induced zeaxanthin changes in guard cells from illuminated epidermal peels and reduced the stomatal CO₂ response to the level observed in the dark. These data suggest that CO₂-dependent changes in the zeaxanthin content of guard cells could modulate CO₂-dependent changes of stomatal apertures in the light while a zeaxanthin-independent CO₂ sensing mechanism would modulate the CO₂ response in the dark.     

CO2 enrichment, drought stress and growth of Alaska pea plants (Pisum sativum)

The interaction of CO₂ enrichment and drought on water status and growth of pea plants was investigated. Pisum sativum L. (cv. Alaska) plants were grown from seeds in growth chambers using 350 and 675 μl I1 CO₂, a photon flux density of 600 μmol M-2 S-1, a 16 h photoperiod and a temperature regime of 20/14°C. The drought treatment was started at the beginning of branch initiation and lasted for 9 or 11 days. The water status of the plants was monitored daily by measuring total leaf water potential and stomatal conductance. The total leaf water potential of well-watered plants was not affected by the CO₂ level. Under draughting conditions total leaf water potential decreased, with a slower decrease under the high CO₂ regime, due, at least in part, to reduced stomatal conductance. Upon rewatering, total leaf water potential and stomatal conductance recovered within one day. High CO₂ counteracted the reduction in height and, to some extent, leaf area that developed in low CO₂ unwatered plants. Additional CO₂ had no effect on branch number and did not prevent the complete inhibition of branch development that resulted from drought stress. Removing the drought conditions resulted in a rapid recovery of the internal water status and also a rapid recovery of most, but not all, plant growth parameters.     

Modifications of the leaf surface structures of Quercus ilex L. in open, naturally CO2-enriched environments

Two Italian CO₂ springs allowed us to study the long-term effect of a 350–2600 μ mol mol^–1 increase in CO₂ concentrations on the surface structures of leaves of Quercus ilex L. Carbon dioxide increased the quantity of cuticular waxes, above an apparent threshold of 750 μ mol mol^–1 CO₂. Leaf wettability was not modified by CO₂ concentrations. Reduction in stomatal frequency was observable up to 750 μ mol mol^–1 CO₂, the slope being almost the same as that estimated for the increase in CO₂ concentration from pre-industrial times to the present. At higher concentrations, CO₂ seemed to exert no more impact on stomatal frequency.     

Water vapour fluxes and their impact under elevated CO2 in a C4-tallgrass prairie

We measured leaf-level stomatal conductance, xylem pressure potential, and stomate number and size as well as whole plant sap flow and canopy-level water vapour fluxes in a C4-tallgrass prairie in Kansas exposed to ambient and elevated CO₂. Stomatal conductance was reduced by as much as 50% under elevated CO₂ compared to ambient. In addition, there was a reduction in stomate number of the C4 grass, Andropogon gerardii Vitman, and the C3 dicot herb, Salvia pitcheri Torr., under elevated CO₂ compared to ambient. The result was an improved water status for plants exposed to elevated CO₂ which was reflected by a less negative xylem pressure potential compared to plants exposed to ambient CO₂. Sap flow rates were 20 to 30% lower for plants exposed to elevated CO₂ than for those exposed to ambient CO₂. At the canopy level, evapotranspiration was reduced by 22% under elevated CO₂. The reduced water use by the plant canopy under elevated CO₂ extended the photosynthetically-active period when water became limiting in the ecosystem. The result was an increased above- and belowground biomass production in years when water stress was frequent.     

Diurnal variations in the kinetics of delayed luminescence from Scenedesmus obtusiusculus after exposure to various light and CO2 regimes

Delayed luminescence was measured from samples of a synchronously growing cell culture of the unicellular green alga, Scenedesmus obtusiusculus Chod., every second hour during the 24 h cell cycle under a 15/9 h lighi/dark regime. Both high (air + 2.5% CO₂) and low (0.03% CO₂) CO₂ conditions were used. Under high CO₂ conditions, while light excitation induces formation of a late (maximum reached after 10–60 s) transient peak in delayed luminescence in cells sampled after 10–16 h in the cell cycle. During most of the cell cycle low CO₂ conditions stimulate a late transient peak formation. Excitation with 700 nm light, but not with 680 nm light, induces a late transient peak in delayed luminescence under high CO₂ conditions. The transient peak is more or less pronounced depending on the cell stage. The variations might be due to a changing capacity for light-induced state I/stale II transitions during the cell cycle. It is assumed that the formation of a late transient peak in delayed luminescence is due to ATP hydrolyzation and is thus favoured by a high ATP/NADPH ratio. Hydrolyzation of ATP affects the transthylakoidal ΔpH, which regulates the reverse electron flow to the plastoquinone-pool and Q_A/Q_B, thus affecting the decay kinetics of the delayed luminescence.     

Stomatal responses to CO2 during a diel Crassulacean acid metabolism cycle in Kalanchoe daigremontiana and Kalanchoe pinnata

To investigate the diurnal variation of stomatal sensitivity to CO₂, stomatal response to a 30 min pulse of low CO₂ was measured four times during a 24 h time-course in two Crassulacean acid metabolism (CAM) species Kalanchoe daigremontiana and Kalanchoe pinnata , which vary in the degree of succulence, and hence, expression and commitment to CAM. In both species, stomata opened in response to a reduction in p CO₂ in the dark and in the latter half of the light period, and thus in CAM species, chloroplast photosynthesis is not required for the stomatal response to low p CO₂. Stomata did not respond to a decreased p CO₂ in K. daigremontiana in the light when stomata were closed, even when the supply of internal CO₂ was experimentally reduced. We conclude that stomatal closure during phase III is not solely mediated by high internal p CO₂, and suggest that in CAM species the diurnal variability in the responsiveness of stomata to p CO₂ could be explained by hypothesizing the existence of a single CO₂ sensor which interacts with other signalling pathways. When not perturbed by low p CO₂, CO₂ assimilation rate and stomatal conductance were correlated both in the light and in the dark in both species.     

Growth and physiological responses of canola (Brassica napus) to three components of global climate change: temperature, carbon dioxide and drought

Elevated CO₂ appears to be a significant factor in global warming, which will likely lead to drought conditions in many areas. Few studies have considered the interactive effects of higher CO₂, temperature and drought on plant growth and physiology. We grew canola ( Brassica napus cv. 45H72) plants under lower (22/18°C) and higher (28/24°C) temperature regimes in controlled-environment chambers at ambient (370 μmol mol⁻¹) and elevated (740 μmol mol⁻¹) CO₂ levels. One half of the plants were watered to field capacity and the other half at wilting point. In three separate experiments, we determined growth, various physiological parameters and content of abscisic acid (ABA), indole-3-acetic acid and ethylene. Drought-stressed plants grown under higher temperature at ambient CO₂ had decreased stem height and diameter, leaf number and area, dry matter, leaf area ratio, shoot/root weight ratio, net CO₂ assimilation and chlorophyll fluorescence. However, these plants had increased specific leaf weight, leaf weight ratio and chlorophyll concentration. Elevated CO₂ generally had the opposite effect, and partially reversed the inhibitory effects of higher temperature and drought on leaf dry weight accumulation. This study showed that higher temperature and drought inhibit many processes but elevated CO₂ partially mitigate some adverse effects. As expected, drought stress increased ABA but higher temperature inhibited the ability of plants to produce ABA in response to drought.     

Diurnal regulation of photosynthesis in understory saplings

Photosynthetic rates of plants grown in natural systems exhibit diurnal patterns often characterized by an afternoon decline, even when measured under constant light and temperature conditions. Since we thought changes in the carbohydrate status could cause this pattern through feedback from starch and sucrose synthesis, we studied the natural fluctuations in photosynthesis rates of plants grown at 36 and 56 Pa CO₂ at a FACE (free-air-CO₂-enrichment) research site. Light-saturated photosynthesis varied by 40% during the day and was independent of the light-limited quantum yield of photosynthesis, which varied little through the day. Photosynthesis did not correspond with xylem water potential or leaf carbohydrate build-up, but rather with diurnal changes in air vapor-pressure deficit and light. The afternoon decline in photosynthesis also corresponded with decreased stomatal conductance and decreased Rubisco carboxylation efficiency which in turn allowed leaf-airspace CO₂ partial pressure to remain constant. Growth at elevated CO₂ did not affect the afternoon decline in photosynthesis, but did stimulate early-morning photosynthesis rates relative to the rest of the day. Plants grown at 56 Pa CO₂ had higher light-limited quantum yields than those at 36 Pa CO₂ but, there was no growth–CO₂ effect on quantum yield when measured at 2 kPa O₂. Therefore, understory plants have a high light-limited quantum yield that does not vary through the day. Thus, the major diurnal changes in photosynthesis occur under light-saturated conditions which may help understory saplings maximize their sunfleck-use-efficiency.     

The inhibition of photosynthesis and photorespiration in isolated mesophyll cells of Phaseolus and Lycopersicon by reduced osmotic potentials

Mesophyll cells isolated from Phaseolus vulgaris and Lycopersicon esculentum show decreasing photosynthetic rates when suspended in media containing increasing concentrations of osmoticum. The photosynthetic activity was sensitive to small changes in osmotic potential over a range of sorbitol concentrations from 0.44 M (−1.08 MPa) to 0.77 M (−1.88 MPa). Photorespiration assayed by ¹⁴CO₂ release in CO₂-free air and by ¹⁴CO₂ release from the oxidation of [1–¹⁴C] glycolate also decreased as the osmotic potential of the incubation medium was reduced. The CO₂ compensation points of the cells increased with increasing concentration of osmoticum from approximately 60 μ I⁻¹1 at −1.08 MPa to 130 μl 1⁻¹ for cells stressed at −1.88 MPa. Changes in photosynthetic and photorespiratory activities occurred at moderate osmotic potentials in these cells suggesting that in whole leaves during a reduction in water potential, non- stomatal inhibition of CO₂ assimilation and glycolate pathway metabolism occurs simultaneously with stomatal closure.     

Effect of High Concentrations of Carbon Dioxide on Growth Rate of Pseudomonas fragi, Bacillus cereus and Streptococcus cremoris

The maximum specific growth rates of Pseudomonas fragi, Bacillus cereus and Streptococcus cremoris were studied over a wide range of carbon dioxide concentrations. The growth rate compared with a control was reduced to 50% in Ps. fragi at 0–5 atm CO₂, in B. cereus at 1—3 atm and in Strep, cremoris at 8–6 atm. B. cereus and Strep, cremoris were completely inhibited at 3 and 11 atm CO₂, respectively. The growth rate of the aerobic Ps. fragi at 0–99 atm CO₂ (0–01 atm oxygen) was reduced to about 20% of that in air. The growth rate of Ps. fragi was decreased at oxygen concentrations lower than 0–01 atm.
When Ps. fragi was grown at oxygen limitation (0.0025 atm oxygen) and exposed to 0.99 atm CO₂, the inhibiting effect of the CO₂ was added to that of the oxygen limitation. No indications of a synergistic effect between CO₂ inhibition and oxygen limitation were noted.
B. cereus and Strep, cremoris were tested under anaerobic conditions.     

Sensing of atmospheric CO2 by plants

Abstract. Despite recent interest in the effects of high CO₂ on plant growth and physiology, very little is known about the mechanisms by which plants sense changes in the concentration of this gas. Because atmospheric CO₂ concentration is relatively constant and because the conductance of the cuticle to CO₂ is low, sensory mechanisms are likely to exist only for intercellular CO₂ concentration. Therefore, responses of plants to changes in atmospheric CO₂ will depend on the effect of these changes on intercellular CO₂ concentration. Although a variety of plant responses to atmospheric CO₂ concentration have been reported, most of these can be attributed to the effects of intercellular CO₂ on photosynthesis or stomatal conductance. Short-term and long-term effects of CO₂ on photosynthesis and stomatal conductance are discussed as sensory mechanisms for responses of plants to atmospheric CO₂. Available data suggest that plants do not fully realize the potential increases in productivity associated with increased atmospheric CO₂. This may be because of genetic and environmental limitations to productivity or because plant responses to CO₂ have evolved to cope with variations in intercellular CO₂ caused by factors other than changes in atmospheric CO₂.     

In situ responses to elevated CO2 in tropical forest understorey plants

1. Plants growing in deep shade and high temperature, such as in the understorey of humid tropical forests, have been predicted to be particularly sensitive to rising atmospheric CO₂. We tested this hypothesis in five species whose microhabitat quantum flux density (QFD) was documented as a covariable. After 7 (tree seedlings of Tachigalia versicolor and Beilschmiedia pendula ) and 18 months (shrubs Piper cordulatum and Psychotria limonensis, and grass Pharus latifolius ) of elevated CO₂ treatment ( c. 700 μl litre^–1) under mean QFD of less than 11 μmol m^–2 s^–1, all species produced more biomass (25–76%) under elevated CO₂.
2. Total plant biomass tended to increase with microhabitat QFD (daytime means varying from 5 to 11μmol m^–2 s^–1) but the relative stimulation by elevated CO₂ was higher at low QFD except in Pharus .
3. Non-structural carbohydrate concentrations in leaves increased significantly in Pharus (+ 27%) and Tachigalia (+ 40%).
4. The data support the hypothesis that tropical plants growing near the photosynthetic light compensation point are responsive to elevated CO₂. An improved plant carbon balance in deep shade is likely to influence understorey plant recruitment and competition as atmospheric CO₂ continues to rise.     

Effect of sink size on photosynthesis and carbohydrate content of leaves of three spring wheat varieties

Effects of CO₂ on stomatal movements of Commelina communis L. were studied with plants, epidermal strips and guard cell protoplasts. With plants, the stomatal response induced by a blue light pulse was studied for different ambient CO₂ concentration ranging from CO₂-deprived air to 100 Pa in darkness or under red light. It was observed that the blue light response could be obtained not only under a red light background but also in darkness and CO₂-free air, the two responses being quite similar.
With epidermal strips, the effect of CO₂ on ferricyanide reductase activity at the guard cell plasmalemma was studied by transmission electron microscopy. In the presence of ferric ions, reduced ferricyanide gives an electron dense precipitate of Prussian Blue. In darkness and air, no precipitate was observed. In darkness and CO₂-free air as well as under light and normal air, a precipitate was found along the plasmalemma of the guard cells, indicating a ferricyanide reductase activity. With guard cell protoplasts suspended in a medium either in equilibrium with air or in a CO₂-free medium the H⁺ extrusion induced by a blue light pulse added to a red light background was measured. A low CO₂ content was obtained by adding photosynthetic algae to the suspension of guard cell protoplasts. In a CO₂-free medium the rate of H⁺ extrusion was enhanced.
The results are discussed on the basis of a possible competition for reducing power between CO₂ fixation and a putative blue light dependent redox chain located on the plasma membrane.