Equilibrium of CO2 reactions in the pulmonary capillary

Steady-state CO2 excretion was measured in isolated blood-free rabbit lungs perfused with bicarbonate solutions. CO2 in the expired ventilation was either present initially in the perfusate as dissolved CO2 or produced from bicarbonate during pulmonary capillary transit. The two components were separated by measurement of simultaneous acetylene excretion. Bovine carbonic anhydrase and acetazolamide were sequentially added to the perfusate to determine the effects of maximal enzyme catalysis and inhibition of native lung carbonic anhydrase on CO2 production. Control CO2 production was significantly greater than that observed during inhibition of native lung carbonic anhydrase, confirming previous observations that bicarbonate has access to the tissue enzyme. Addition of excess carbonic anhydrase increased CO2 production by a statistically, but not physiologically, significant amount. These data demonstrate that CO2 reactions outside the erythrocyte attain 97% completion during pulmonary capillary transit. Under control and catalyzed conditions, alveolar and venous CO2 tens ions and pH were essentially identical to equilibrium values determined by in vitro tonometry.     

Transient phenomena in capillary exchange

The case of a vessel, which supplies a region through which it passes with some substance, is considered for the situation in which the permeability is the limiting factor. Diffusion parallel to the vessel is neglected. The substance may, however, be consumed proportional to its concentration in the inner or outer region. A solution is given for the case in which the input is an arbitrary function of time. It is suggested that the results may be applied in some cases to data on the injection of substances into blood vessels, or they may be applied to the transient effects in the case of vapors or gases passing through the respiratory passages.     

Moisture content and CO2 exchange of lichens

Summary Net photosynthesis (10 klx light intensity, 150 E m^-2 s^-1 PAR) and dark respiration of the lichen Ramalina maciformis at different temperatures are measured in relation to thallus water content. Both first increase with increasing hydration. Dark respiration then remains constant with increased water content until thallus saturation. In contrast, a further increase in water content leads to a depression of net photosynthesis, as shown in previous studies, after a maximum of CO₂ uptake has been attained. However, the extent of this depression depends strongly on temperature. In saturated thalli (160% water content in relation to lichen dry weight) the depression amounts to about 15% and 63% of the maximum unsaturated rate at 5°C and 25°C thallus temperature, respectively. The moisture compensation-point of net photosynthesis is also decisively determined by temperature (for 0°C at 20% water content; for 25°C at 15%), and the water content that allows maximum rates of CO₂ uptake (for 0°C at 80%; for 25°C at less than 40% water content). An electrical analogue of CO₂ exchange in a lichen thallus is presented, and it is suggested that the experimental results may be interpreted in terms of temperature-dependent CO₂ diffusion resistances in imbibed lichen thalli.     

CO2 and bicarbonate exchange in the rat liver

Several forms of carbonic anhydrase (CA) have been detected in hepatocytes. The distribution of these enzymes appears to be heterogeneous in the hepatic lobule, and the specific isoenzyme that predominates is influenced by sex steroid levels in the animal. In the present study, experiments were conducted in isolated male rat livers perfused with erythrocyte-free solutions, which were devoid of CA to see if there were sufficient tissue CA activity accessible to the plasma to ensure equilibration between labeled HCO3- and CO2 during a single passage from the portal vein to the hepatic vein. After injection of H14CO3- into the portal vein, emergence of the 14C label from the hepatic vein was slightly more rapid than after injections of 14CO2. After infusion of 5-250 microM of acetazolamide, an inhibitor of CA, H14CO3- was virtually confined to the extracellular space during a single transit through the organ, whereas the outflow of 14CO2 was very prolonged, suggesting that some of the 14C had been "trapped" within the hepatic cells as H14CO3-. Inhibition of CA activity in the intact organ with low doses of acetazolamide suggests the presence of a readily inhibitable isoenzyme of CA on the surface of the hepatocytes, which is directly accessible to both HCO3- and acetazolamide. The outflow patterns of 14CO2 and H14CO3- became the same after infusion of erythrocyte CA into the portal vein. On the basis of the pH of the perfusate and the cellular distribution of 14CO2 and H14CO3- in the presence of CA, an intracellular pH value of 7.26 was calculated.(ABSTRACT TRUNCATED AT 250 WORDS)     

CO2 exchange in the Empetrum nigrum-Sphagnum fuscum community

Summary The CO₂ exchange of the Empetrum nigrum-Sphagnum fuscum community of a raised bog was studied in the laboratory at different temperature (from 5 to 30° C) and irradiance (up to 128 W m^-2) combinations during one growing season. The total CO₂ exchange was divided into three components, namely those due to Empetrum nigrum, Sphagnum fuscum, and peat, respectively. At the optimum temperature (10 to 15° C) the maximum net CO₂ exchange of Empetrum nigrum was c. 200 and that of Sphagnum fuscum c. 250 mg CO₂ m^-2h^-1. The total respiration in peat increased exponentially from 50 to 350 mg CO₂ m^-2h^-1 with increasing temperature from 5 to 30° C. About 40% of the CO₂ fixed by the community in optimal temperature and irradiation conditions was released immediately.     

Leaf cavity CO2 concentrations and CO2 exchange in onion,Allium cepa L.

Onion (Allium cepa L.) plants were examined to determine the photosynthetic role of CO₂ that accumulates within their leaf cavities. Leaf cavity CO₂ concentrations ranged from 2250 L L^–1 near the leaf base to below atmospheric (<350 L L^–1) near the leaf tip at midday. There was a daily fluctuation in the leaf cavity CO₂ concentrations with minimum values near midday and maximum values at night. Conductance to CO₂ from the leaf cavity ranged from 24 to 202 mol m^–2 s^–1 and was even lower for membranes of bulb scales. The capacity for onion leaves to recycle leaf cavity CO₂ was poor, only 0.2 to 2.2% of leaf photosynthesis based either on measured CO₂ concentrations and conductance values or as measured directly by ¹⁴CO₂ labeling experiments. The photosynthetic responses to CO₂ and O₂ were measured to determine whether onion leaves exhibited a typical C₃-type response. A linear increase in CO₂ uptake was observed in intact leaves up to 315 L L^–1 of external CO₂ and, at this external CO₂ concentration, uptake was inhibited 35.4±0.9% by 210 mL L^–1 O₂ compared to 20 mL L^–1 O₂. Scanning electron micrographs of the leaf cavity wall revealed degenerated tissue covered by a membrane. Onion leaf cavity membranes apparently are highly impermeable to CO₂ and greatly restrict the refixation of leaf cavity CO₂ by photosynthetic tissue.Abbreviations C_a external CO₂ concentration - C_i intercellular CO₂ concentration - CO₂ compensation concentration - PPFR photosynthetic photon fluence rate     

Short-term CO2 exchange response to temperature,irradiance, and CO2 concentration in strawberry

Relative importance of short-term environmental interaction and preconditioning to CO₂ exchange response was examined in Fragaria ananasa (strawberry, cv. Quinault). Tests included an orthogonal comparison of 15 to 60-min and 6 to 7-h exposures to different levels of temperature (16 to 32°C), photosynthetically active radiation (PAR, 200 to 800 E m² s^-1), and CO₂ (300 to 600 l/l) on successive days of study. Plants were otherwise maintained at 21°C, 300 E m² s^-1 PAR and 300–360 l/l CO₂ as standard conditions. Treatment was restricted to the mean interval of 14 h daily illumination and the first 3–4 days of each test week over a 12-week cultivation period. CO₂ exchange rates were followed with each step-change in environmental level including ascending/descending temperature/PAR within a test period, initial response at standard conditions on successive days of testing, and measurement at reduced O₂. Response generally supported prior concepts of leaf biochemical modeling in identifying CO₂ fixation as the major site of environmental influence, while overall patterns of whole plant CO₂ exchange suggested additional effects for combined environmental factors and preconditioning. These included a positive interaction between temperature and CO₂ concentration on photosynthesis at high irradiance and a greater contribution by dark respiration at lower PAR than previously indicated. The further importance of estimating whole plant CO₂ exchange from repetitive tests and measurements was evidenced by a high correlation of response to prior treatment both during the daily test period and on consecutive days of testing.Abbreviations C₃ plant a plant in which the product of CO₂ fixation is a 3-carbon acid (3-phosphoglyceric acid) - IRGA intra-red gas analyzer - PAR photosynthetically active radiation - RH relative humidity - RuBisCO ribulose-1,5-bisphosphate carboxylase/oxygenase Reference to a company and/or product named by the Department is only for purposes of information and does not imply approval or recommendation of the product to the exclusion of others which may also be suitable.     

Pan-Arctic modelling of net ecosystem exchange of CO2

Net ecosystem exchange (NEE) of C varies greatly among Arctic ecosystems. Here, we show that approximately 75 per cent of this variation can be accounted for in a single regression model that predicts NEE as a function of leaf area index (LAI), air temperature and photosynthetically active radiation (PAR). The model was developed in concert with a survey of the light response of NEE in Arctic and subarctic tundras in Alaska, Greenland, Svalbard and Sweden. Model parametrizations based on data collected in one part of the Arctic can be used to predict NEE in other parts of the Arctic with accuracy similar to that of predictions based on data collected in the same site where NEE is predicted. The principal requirement for the dataset is that it should contain a sufficiently wide range of measurements of NEE at both high and low values of LAI, air temperature and PAR, to properly constrain the estimates of model parameters. Canopy N content can also be substituted for leaf area in predicting NEE, with equal or greater accuracy, but substitution of soil temperature for air temperature does not improve predictions. Overall, the results suggest a remarkable convergence in regulation of NEE in diverse ecosystem types throughout the Arctic.     

Novel technique for component monitoring of CO2 exchange in plants

A novel technique designed for component monitoring of CO₂ exchange in plants is described. The system is based on application of self-clamping leaf chambers connected to an open gas-exchange measuring system and on automatic recording of CO₂ concentration. This technique was implemented in a commercially available instrument, PTM-48A Photosynthesis Monitor, which provides for long-term measurements of gas exchange and for discrimination of its separate components. Furthermore, many other plant functions can be monitored during plant growth and development under laboratory, greenhouse, and field conditions.     

Effect of a change in photoperiod on subsequent CO2 exchange

The effect of a shift from a long to a short photoperiod on CO₂ exchange of Chenopodium polyspermum was studied. Equal quantities of photosynthetic energy were given daily to the plants, long photoperiods being produced by low intensity red light extension. A change in the photoperiod was shown to affect the pattern of CO₂ loss at the beginning of the night period and the onset of CO₂ intake at the beginning of day time. These events seem to be under phytochrome control.The photoperiod had an effect on the slope of the CO₂ curve of photosynthesis, efficiency of photosynthesis being increased after a short day. This effect was not due to a variation in the stomatal resistance.The action of O₂ concentration on photosynthesis (Warburg effect) was affected by the photoperiodic treatment, being less important after a long day than after a short day.Involvement of phytochrome in photosynthetic efficiency and photorespiration is discussed.

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Résumé On étudie l'effet d'une variation de photopériode sur les échanges de CO₂ de Chenopodium polyspermum. Les plantes reçoivent la même quantité d'énergie utilisable par la photosynthèse, l'allongement de la photopériode étant obtenu par addition au cours de la nuit de lumière rouge de faible intensité.Un changement de photopériode affecte à court terme le déroulement de la chute de respiration en début de nuit ainsi que la mise en route de la photosynthèse le jour suivant.Une variation de traitement photopériodique modifie l'efficience de la photosynthèse: la pente de la courbe de photosynthèse en fonction du CO₂ est plus élevée après un jour court. Cet effet n'est pas dú à une variation de résistance stomatique.L'action de la concentration en oxygène de l'air sur la photosynthèse (effet Warburg) est également affectée par le traitement photopériodique: elle est moins importante après un jour long qu'après un jour court.On discute l'influence éventuelle du phytochrome sur l'efficience de la photosynthèse et la photorespiration.

     

Disentangling CO2 fluxes: direct measurements of mesocosm‐scale natural abundance 13CO2/12CO2 gas exchange, 13C discrimination,and labelling of CO2 exchange flux components in controlled environments

A ¹³C/¹²C mass spectrometer was interfaced with a open gas exchange system including four growth chambers to investigate CO₂ exchange components of perennial ryegrass (Lolium perenne L.) stands. Chambers were fed with air containing CO₂ with known δ¹³C (δ_CΟ2?2.6 or ?46.8‰). The system did not fractionate C isotopes and no extraneous CO₂ leaked into chambers. The on‐line ¹³C discrimination (Δ) of ryegrass stands in light was independent of δ_CΟ2 when δ_CΟ2 was constant. The δ of CO₂ exchanged by the stands in light (δ_Nd) and darkness (δ_Rn) differed by 0.7‰, suggesting some Δ in dark respiration at the stand‐level. However, Δ decreased by ～ 10‰ when δ_CΟ2 was switched from ?46.8 to ?2.5‰, and increased by ～ 10‰ following a shift from ?2.6 to ?46.7‰ due to isotopic disequilibria between photosynthetic and respiratory fluxes. Isotopic imbalances were used to assess (non‐photorespiratory) respiration in light and the replacement of the respiratory substrate pool(s) by new photosynthate. Respiration was partially inhibited by light, but increased during the light period and decreased in darkness, in association with temperature changes. The labelling kinetics of respiratory CO₂ indicated the existence of two major respiratory substrate pools: a fast pool which was exchanged within hours, and a slow pool accounting for ～ 60% of total respiration and having a mean residence time of 3.6 d.     

Blood osmolality during in vivo changes of CO2 pressure

In 16 experiments male subjects, age 22.4 +/- 0.5 (SE) yr, inspired CO2 for 15 min (8% end-tidal CO2) or hyperventilated for 30 min (2.5% end-tidal CO2). Osmolality (Osm) and acid-base status of arterialized venous blood were determined at short intervals until 30 min after hypo- and hypercapnia, respectively. During hypocapnia [CO2 partial pressure (PCO2) -2.31 +/- 0.32 kPa (-17.4 Torr), pH + 0.19 units], Osm decreased by 3.9 +/- 0.3 mosmol/kg H2O; during hypercapnia [PCO2 + 2.10 +/- 0.28 kPa (+15.8 Torr), pH -0.12 units], Osm increased by 5.8 +/- 0.7 mosmol/kg H2O. Presentation of the data in Osm-PCO2 or Osm-pH diagrams yields hysteresis loops probably caused by exchange between blood and tissues. The dependence of Osm on PCO2 must result mainly from CO2 buffering and therefore from the formation of bicarbonate. In spite of the different buffer capacities in various body compartments, water exchange allows rapid restoration of osmotic equilibrium throughout the organism. Thus delta Osm/delta pH during a PCO2 jump largely depends on the mean buffer capacity of the whole body. The high estimated buffer value during hypercapnia (38 mmol/kg H2O) compared with hypocapnia (19 mmol/kg H2O) seems to result from very strong muscle buffering during moderate acidosis.     

The time course of capillary exchange

The equations governing the time course of the exchange of substances between the blood in the capillaries and the extracellular space are solved for the case of substances which do not penetrate the cells. The equations given relate the time course of the exchange process to the various tissue and circulation parameters such as the specific capillary wall area, the pore area, the inter-capillary distance, the size of the extra-vascular, extra-cellular space, the diffusion coefficient in this space, and the velocity of blood in the capillaries. Some experimental work on capillary exchange is discussed in relation to the theory and estimates are made of the relative importance of the various tissue and circulation parameters in the exchange of substances in different tissues.     

Transcapillary exchange in relation to capillary circulation

Transcapillary exchange of diffusible solutes depends on capillary blood flow, Q; capillary permeability, P; and capillary surface area, S. In a single capillary, the extent of equilibration of a given solute depends on the ratio of Q, to the product of P and S. In a microvascular bed consisting of many capillaries, equilibration depends on the fraction of them which are open to blood flow at any time and on the distribution of Q/PS ratios in the open capillaries. Both these characteristics are subject to control by vascular smooth muscle, particularly by the precapillary sphincters. Vasomotor mechanisms have been shown experimentally to exert a wide range of effective control over blood-tissue transport. In skeletal muscle, effective PS measured with ⁴²K or ⁸⁶Rb may be increased 8-fold from maximum nervous vasoconstriction to optimum metabolic vasodilatation. Most probably, these changes are due to differences in functional capillary surface area and of blood flow distribution relative to permeability and surface area. The extent to which variations in permeability itself can contribute to control of transcapillary exchange is not known.     

Mass-spectrometric determination of O2 and CO2 gas exchange in illuminated higher-plant cells

In order to estimate photosynthetic and respiratory rates in illuminated photoautotrophic cells of carnation (Dianthus caryophyllus L.), simultaneous measurements of CO₂ and O₂ gas exchange were performed using ¹⁸O₂, ¹³CO₂ and a mass-spectrometry technique. This method allowed the determination, and thus the comparison, of unidirectional fluxes of O₂ and CO₂. In optimum photosynthetic conditions (i.e. in the presence of high light and a saturating level of CO₂), the rate of CO₂ influx represented 75±5% of the rate of gross O₂ evolution. After a dark-to-light transition, the rate of CO₂ efflux was inhibited by 50% whereas the O₂-uptake rate was little affected. The effect of a recycling of respiratory CO₂ through photosynthesis on the exchange of CO₂ gas was investigated using a mathematical model. The confliction of the experimental data with the simulated gas-exchange rates strongly supported the view that CO₂ recycling was a minor event in these cells and could not be responsible for the observed inhibition of CO₂ efflux. On the basis of this assumption it was concluded that illumination of carnation cells resulted in a decrease of substrate decarboxylations, and that CO₂ efflux and O₂ uptake were not as tightly coupled in the light as in the dark. Furthermore, it could be calculated from the rate of gross photosynthesis that the chloroplastic electron-transport chain produced enough ATP in the light to account for the measured CO₂-uptake rate without involving cyclic transfer of electrons around PS I or mitochondrial supplementation.Abbreviations Chl chlorophyll - K_d permeability coefficient The authors thank Drs A. Vermeglio and P. Thibault, Dépt. de Biologie, CEN-Cadarache, St. Paul Lez Durance, France, for helpful discussions.     

Numerical simulation of systemic O2 and CO2 exchange in a hyperbaric environment

The process of gas exchange in systemic capillaries and its surrounding tissue is simulated numerically in a hyperbaric environment, taking into account the molecular diffusion, convection, saturation of haemoglobin with O2 and CO2, and the metabolic activity in the tissue. Krogh tissue-cylinder is used as a geometrical representation of the capillary-tissue system. The resulting system of non-linear governing equations together with the physiologically relevant boundary conditions is solved numerically. It is found that the concentration of oxygen decreases from the axis of the capillary to the tissue periphery whereas the concentration of carbon dioxide increases. It is shown that very little CO2 is transported radially. The location of the vulnerable region from the point of view of CO2 accumulation is found to be the rim (r = R2, z = L) situated at the periphery of the tissue near the venous end of the capillary. It is also found that accumulation of O2 decreases whereas that of CO2 increases in a hyperbaric environment. Finally, it is surmised that one of the reasons in causing discomfort among divers could be excessive accumulation of CO2 in the tissue.     

Measurement of leaf and canopy photosynthetic CO2 exchange in the field

The principles and limitations of leaf gas exchange measurementsin portable gas exchange systems are described. Attention isgiven to the design and developments in infrared gas analysersused in portable systems, and the basic structure of singleand dual beam instruments is presented. The significance offlow measurement in these systems and the principles of thermalmass flow measurement are illustrated. Considerations of leafarea measurement, chamber design and choice of materials areoutlined. Two specific developments in field gas exchange systemsare described and their significance in field measurements isillustrated with examples. (1) An integrating sphere leaf chamberfor the determination of the quantum yield of photosynthesis,on the basis of absorbed light, is explained and equations forits use are developed. The significance of this approach isillustrated by a comparison of data for contrasting leaves plottedon an absorbed and incident light basis. This measurement oflight-limited photosynthesis is also critical in understandingthe contribution of shaded leaves to canopy photosynthesis.(2) A system for the measurement of canopy photosynthesis fromarable crops and low stature natural vegetation is described.Results from a season-long study of wheat CO₂ exchange are shownto illustrate its application. Key words: Leaf gas exchange, photosynthetic quantum efficiency, infrared gas analysis, canopy photosynthesis, integrating sphere