An Arabidopsis mutant showing reduced feedback inhibition of photosynthesis.

Many plant genes are responsive to sugars but the mechanisms used by plants to sense sugars are unknown. A genetic approach has been used in Arabidopsis to identify genes involved in perception and transduction of sugar signals. For this purpose, an in vivo reporter system was established consisting of the light- and sugar-regulated plastocyanin promoter, fused to the luciferase coding sequence (PC-LUC construct). At the seedling stage, expression of the PC-LUC gene is repressed by sucrose, and a number of sucrose-uncoupled (sun) mutants were selected in which sucrose is unable to repress the activity of the PC promoter. Three mutants have been characterized in more detail. The sugar analog 2-deoxy-D-glucose (2DG) was used to repress whole plant photosynthesis, PC-LUC gene expression and total ribulose-1,5-bisphosphate activity. It was found that the sun6 mutation makes plants unresponsive to these 2DG-induced effects. Moreover, unlike wild-type plants, sun6 mutants are insensitive to elevated levels of glucose in the growth medium. These findings suggest that the SUN6 gene is active in a hexose-activated signal transduction pathway.

A Short-Term Decrease in Nitrogenase Activity (C2H2 Reduction) Is Induced by Exposure of Soybean Shoots to Their CO2 Compensation Point.

Photosynthesis and nitrogenase acetylene-reducing activity (ARA) were measured in soybeans (Glycine max [L.] Merr.) in which the shoots were exposed for 48 h to 60 [mu]L L-1 CO2, a value corresponding to their CO2 compensation point. Six hours after the beginning of the light period at low CO2, the ARA started to decrease, reaching a rate of 50% of the control rate in 14 to 24 h and 20% of the control rate in 34 to 38 h after the beginning of the CO2 treatment. At these times, there was no net photosynthesis, and the transpiration rate was 20% lower than that in the control plants. An increase in the partial pressure of O2 around the nodules alleviated this inhibition of ARA. The maximal ARA achieved at 40 kPaO2 was 3 times higher than that at 20 kPa O2 and similar to the maximal ARA of the control plants. It was argued that the decrease in ARA of soybean exposed to the CO2 compensation point was due to a decrease in the nodule's permeability to O2 diffusion.

Effect of acetylene on root respiration and acetylene reducing activity in nodulated soya bean.

Acetylene decreased root and nodule respiration, as measured by CO(2) evolution of nodulated or non-nodulated Glycine max. An inhibition of 25 to 35% in 15 to 30 minutes occurred when 13% C(2)H(2) was introduced in the gas flux which aerated the root nutrient solution. When the light intensity was doubled to 800 microeinsteins per square meter per second, the inhibition increased to 50% and nodule acetylene reduction activity was inhibited 50%.

Differing substomatal and chloroplastic CO2 concentrations in water-stressed wheat.

Gas exchanges of wheat (Triticum aestivum L. cv. Courtot) shoots were measured before and during a water stress. While photosynthesis, transpiration and dark respiration decreased because of the stress, photorespiration increased initially, up to a maximum of 50% above its initial value. The CO2 concentration in the intercellular space was calculated from gas-diffusion resistances, and remained approximately constant before and during the stress. On the other hand, the CO2 concentration in the chloroplast, in the vicinity of Ribulose-1,5-biphosphate carboxylase/oxygenase (Rubisco), was evaluated from the ratio of CO2 to O2 uptake, using the known kinetic constants of the oxygenation and carboxylation reactions which compete for Rubisco. In the well-watered plants, the calculated chloroplastic concentration was slightly smaller than the substomatal concentration. During water stress, this concentration decreased while the substomatal CO2 concentration remained constant. Hypotheses to explain this difference between substomatal and chloroplastic CO2 concentrations are discussed.

Effect of CO2 and O2 on development and fructification of wheat in closed systems.

The cultivation of wheat (Triticum aestivum L.) was performed in controlled environment chambers with the continuous monitoring of photosynthesis, dark respiration, transpiration and main nutrient uptakes. A protocol in twin chambers was developed to compare the specific effects of low O2 and high CO2. Each parameter is able to influence photosynthesis but different effects are obtained In the development, fructification and seed production, because of the different effects of each parameter on the ratio of reductive to oxidative cycle of carbon. The first main conclusion is that low level of O2, at the same rate of biomass production, strongly acts on the rate of ear appearance and on seed production. Ear appearance was delayed and seed production reduced with a low O2 treatment (approximately 4%). The O2 effect was not mainly due to the repression of the oxidative cycle. The high CO2 treatment (700 to 900 microl l-1) delayed ear appearance by 4 days but did not reduce seed production. High CO2 treatment also reduced transpiration by 20%. Two hypothesis were proposed to explain the similarities and the difference in the O2 and CO2 effects on the growth of wheat.

An evaluation of the recycling in measurements of photorespiration.

All measurements of photorespiration and gross photosynthesis in leaves, whether using isotopes or not, are underestimated because of the recycling of O(2) or CO(2). On the basis of a simple diffusion model, we propose a method for the calculation of the recycling and the corresponding underestimation of the measurements. This procedure can be applied when the stomatal resistance is known, and allows for a correction of certain results in the literature. It is found that measurements of the photorespiratory CO(2) release are usually underestimated by 20 to 100%, which sets the estimated rate of CO(2) photorespired at 30 to 50% of the net photosynthesis in C3 plants under normal conditions. In water stress studies, the correction of the photorespiration is still more important (1.5-3.3) because the stomata are closed more. Analysis of the diffusion of O(2) shows that its recycling is low and that the underestimation of photorespiration with (18)O(2) is negligible.

Preferential Photosynthetic Uptake of Exogenous HCO(3) in the Marine Macroalga Chondrus crispus.

The rate of HCO(3) (-) uptake by the red macroalga Chondrus crispus has been investigated. Unbalanced concentrations of free CO(2) and HCO(3) (-), generated by the photosynthetic activity, were detected in steady state conditions by using an exchange column apparatus linked to an assimilation chamber. Observing the variations of this gradient as influenced by the time of seawater transit from the assimilation chamber towards the column allowed an experimental determination of: (a) the actual gradient created by the photosynthetic activity, (b) the rate constant of the chemical conversion of free CO(2) to HCO(3) (-). With a value of 0.115 per second at pH 8.92, this rate constant was in good agreement with a previous estimation. By using a simple model, we show that the photosynthetic rate of HCO(3) (-) consumption can be estimated by the product of the actual gradient and the rate constant. In the conditions of the experiments reported here, this rate represented more than 90% of the whole photosynthetic flux.

CO(2) and O(2) Exchange in Two Mosses, Hypnum cupressiforme and Dicranum scoparium.

Photosynthetic CO(2) and O(2) exchange was studied in two moss species, Hypnum cupressiforme Hedw. and Dicranum scoparium Hedw. Most experiments were made during steady state of photosynthesis, using (18)O(2) to trace O(2) uptake. In standard experimental conditions (photoperiod 12 h, 135 micromoles photons per square meter per second, 18 degrees C, 330 microliters per liter CO(2), 21% O(2)) the net photosynthetic rate was around 40 micromoles CO(2) per gram dry weight per hour in H. cupressiforme and 50 micromoles CO(2) per gram dry weight per hour in D. scoparium. The CO(2) compensation point lay between 45 and 55 microliters per liter CO(2) and the enhancement of net photosynthesis by 3% O(2)versus 21% O(2) was 40 to 45%. The ratio of O(2) uptake to net photosynthesis was 0.8 to 0.9 irrespective of the light intensity. The response of net photosynthesis to CO(2) showed a high apparent K(m) (CO(2)) even in nonsaturating light. On the other hand, O(2) uptake in standard conditions was not far from saturation. It could be enhanced by only 25% by increasing the O(2) concentration (saturating level as low as 30% O(2)), and by 65% by decreasing the CO(2) concentration to the compensation point. Although O(2) is a competitive inhibitor of CO(2) uptake it could not replace CO(2) completely as an electron acceptor, and electron flow, expressed as gross O(2) production, was inhibited by both high O(2) and low CO(2) levels. At high CO(2), O(2) uptake was 70% lower than the maximum at the CO(2) compensation point. The remaining activity (30%) can be attributed to dark respiration and the Mehler reaction.

Evidence for Endogenous Cyclic Photophosphorylation in Intact Chloroplasts: CO(2) Fixation with Dihydroxyacetone Phosphate.

This study examines the capacity of intact spinach (Spinacia oleracea L.) chloroplasts to fix (14)CO(2) when supplied with Benson-Calvin cycle intermediates in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). Under these conditions, substantial (14)CO(2) fixation occurred in the light but not in the dark when either dihydroxyacetone phosphate, ribulose 5-phosphate, fructose 6-phosphate, or fructose bisphosphate was added. The highest rate of (14)CO(2) fixation (20-40 micromoles per milligram chlorophyll per hour) was obtained with dihydroxyacetone phosphate. In contrast, no (14)CO(2) fixation occurred when 3-phosphoglycerate was used. (14)CO(2) fixation in the presence of dihydroxyacetone phosphate and DCMU was inhibited by carbonylcyanide m-chlorophenylhydrazone, dl-glyceraldehyde, and pyridoxal 5'-phosphate. Low concentrations of O(2) (25-50 micromolar) stimulated (14)CO(2) fixation, but the activity decreased with increasing O(2) concentrations. The fixation of (14)CO(2) in the presence of DCMU and dihydroxyacetone phosphate was also observed in maize bundle sheath cells. These results provide direct evidence for cyclic photophosphorylation in intact chloroplasts. The activity measured is adequate to support all the extra ATP requirements for maximum rates of photosynthesis in these intact chloroplasts.

[Giant-cell arteritis with initial resistance to corticosteroids (author's transl)]. / Maladie de Horton initialement résistante à la corticothérapie. A propos de trois observations.

Treatment of giant-cell arteritis rests on corticosteroids. Three observations of Horton arteritis with initial resistance to cortisone are reported. Diagnosis was substantiated by the examination of biopsy specimens. Resistance to therapy was overcome by changing to another corticosteroid. Observations such as those reported here are very uncommon.

Effect of CO(2), O(2), and Light on Photosynthesis and Photorespiration in Wheat.

Unidirectional O(2) fluxes were measured with (18)O(2) in a whole plant of wheat cultivated in a controlled environment. At 2 or 21% O(2), O(2) uptake was maximum at 60 microliters per liter CO(2). At lower CO(2) concentrations, it was strongly inhibited, as was photosynthetic O(2) evolution. At 2% O(2), there remained a substantial O(2) uptake, even at high CO(2) level; the O(2) evolution was inhibited at CO(2) concentrations under 330 microliters per liter. The O(2) uptake increased linearly with light intensity, starting from the level of dark respiration. No saturation was observed at high light intensities. No significant change in the gas-exchange patterns occurred during a long period of the plant life. An adaptation to low light intensities was observed after 3 hours illumination. These results are interpreted in relation to the functioning of the photosynthetic apparatus and point to a regulation by the electron acceptors and a specific action of CO(2). The behavior of the O(2) uptake and the study of the CO(2) compensation point seem to indicate the persistence of mitochondrial respiration during photosynthesis.

Photosynthesis and photorespiration in whole plants of wheat.

Wheat was cultivated in a small phytotronic chamber. (18)O(2) was used to measure the O(2) uptake by the plant, which was recorded simultaneously with the O(2) evolution, net CO(2) uptake, and transpiration. At normal atmospheric CO(2) concentration, photorespiration, measured as O(2) uptake, was as important as the net photosynthesis. The level of true O(2) evolution was independent of CO(2) concentration and stayed nearly equal to the sum of net CO(2) photosynthesis and O(2) uptake. We conclude that at a given light intensity, O(2) and CO(2) compete for the reducing power produced at constant rate by the light reactions of photosynthesis.

Oxygen and carbon dioxide exchanges in crassulacean-acid-metabolism-plants.

The 24 h O2 uptake and release together with the CO2 balance have been measured in two CAM plants, one a non-succulent Sempervivum grandifolium, the other a succulent Prenia sladeniana. The O2 uptake was estimated by the use of (18)O2. It was found that the mean hourly O2 uptake in the light was 7 times that in the dark for Sempervivum and 5 times that for Prenia, after correction for the lightdark temperature difference. It was estimated that oxygen uptake in the light was 2.4 times greater than oxygen release (=net photosynthesis) in Sempervivum and 1.4 times greater in Prenia. In both plants there was a positive carbon balance over the 24 h period under the experimental conditions. It was estimated that malate formed during the night could, if completely oxidized to CO2 and water, account for 74% of the light phase O2 uptake in Sempervivum. In Prenia the O2 uptake was more than sufficient to account for a full oxidation of malate.