In vitro multiplication and plantlet establishment of avocado

Summary A procedure is described to regenerate shoots and plants from the embryonic axis of avocado seedlings with benzyladenine or thidiazuron. Explants were grown in the dark for 10 d and then transferred to an 18-h photoperiod to induce multiple shoots. Increased concentrations of benzyladenine or thidiazuron resulted in increased shoot production. Shoots were transferred to shoot promoting media where additional shoots were formed. Under the best conditions, six rooted shoots were produced per original explant. About 80% of the shoots with roots survived in the glasshouse and produced normal phenotypic plants.     

Regulation of C4-phosphoenolpyruvate carboxylase activity by ambient CO2

Light activation of phosphoenolpyruvate carboxylase from the leaves of the C₄ plant Setaria verticillata (L.) is more pronounced at low CO₂ levels. The 2-fold activation observed at physiological ambient CO₂ becomes 3.64-fold at 5 L/L and completely abolished above 700 L/L. When the stomata close under the influence of abscisic acid at 330 L/L CO₂, the extent of light activation is high (3.59-fold), probably because the increased diffusive resistance keeps the internal CO₂ at much lower levels. Under darkness. CO₂ and absicisic acid do not affect the extractable phosphoenolpyruvate carboxylase activity. Internal CO₂ levels may determine phosphoenolpyruvate concentratio in the cytoplasm through the control of its utilization by phosphoenolpyruvate carboxylase. We have recently proposed (Samaras et al. 1988) that photosynthetically produced phosphoenolpyruvate could be an activator of the enzyme. It is therefore suggested that CO₂ indirectly affects the activation state of phosphoenolpyruvate carboxylase by controlling the levels of phosphoenolpyruvate which may act as an activator.Abbreviations PEPCase phosphoenolpyruvate carboxylase - PEP phosphoenolpyruvate - PAR photosynthetically active radiation - G6P glucose-6-phosphate - ABA abscisic acid - MDH malate dehydrogenase - PPDK pyruvate, Pi, dikinase - CAM Crassulacean Acid Metabolism     

Chloroflexus aurantiacus secretes 3-hydroxypropionate,a possible intermediate in the assimilation of CO2 and acetate

Chloroflexus aurantiacus OK-70 fl secreted 3-hydroxypropionate (3HP) during phototrophic growth. The greatest amounts were secreted by cells grown on propionate (0.35 mM 3HP) while the lowest levels were found in autotrophically grown cultures (1.5 M). Large amounts of 2-fluoro,3-hydroxypropionate were formed by autotrophically grown cells exposed to fluoroacetate (FAc). Increased levels of 3HP were observed in these cultures when incubated with acctate. The secretion of 3HP was further stimulated by 0.2 mM KCN, an inhibitor of CO₂ fixation, but only in the presence of acetate. The pathway of 3HP formation was studied by using ¹³C-labelled substrates and NMR. The 3HP formed in the presence of C1-labelled acetate and FAc was labelled at C3 and somewhat less at C2 while with C2-labelled acetate as the tracer 3HP was labelled predominantly at C2. The carboxyl group was derived from CO₂. The 3HP formed by cells grown on propionate and ¹³CO₂ was labelled at all carbon atoms, the label content of C2 and C3 was about 25 and 65% of that of C1 respectively. It is suggested that 3HP is an intermediate in a pathway for acetate assimilation and in a new reductive carboxylic acid cycle for autotrophic CO₂ fixation.Abbreviations 3HP 3-hydroxypropionate - 2F3HP 2,fluoro,3-hydroxypropionate - FAc fluoroacetate - GC gas chromatography - MS mass spectrometry - NMR nuclear magnetic resonance     

Effect of Gamma Irradiation on Embryogenic Avocado Cultures and Somatic Embryo Development

Embryogenic avocado cultures were exposed to ionizing irradiation in order to determine its effect on proliferation and subsequent somatic embryo development. The approximate PD₅₀ as determined by linear regression is 35 Gy 2 weeks after irradiation for Fuerte 2.11.1 and 4 weeks after irradiation for T362 2.11.1. Irradiation of embryogenic cultures did not significantly affect the number of early stage Fuerte 2.11.1 somatic embryos that developed directly from irradiated cultures; however, 10–50 Gy inhibited somatic embryo development. Irradiation of T362 2.11.1 embryogenic cultures at 25–50 Gy inhibited the number of intermediate and mature stages of somatic embryos that developed directly from irradiated cultures, and 50 Gy inhibited somatic embryo maturation. Inhibition of somatic embryo development could be partially offset by proliferation of irradiated embryogenic cultures as suspensions. Irradiation up to 10 Gy significantly increased the number of mature Fuerte 2.11.1 somatic embryos that developed from suspension cultures. Irradiation with doses up to 25 Gy stimulated development of heart stage T362 2.11.1 somatic embryos; however, mature somatic embryo development was suppressed at dosages of 10 Gy and greater.     

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     

CO2 environment,microclimate and photosynthetic characteristics of the moss Hylocomium splendens in a subarctic habitat

Summary In order to document the natural CO₂ environment of the moss Hylocomium splendens, and ascertain whether or not the moss was adapted to this, and its interactions with other microenvironmental factors, two studies were carried out. Firstly, the seasonal variations of CO₂ concentration, photosynthetically active radiation (PAR), tissue water content and temperature were measured in the natural microenvironment of H. splendens in a subarctic forest during the summer period (July–September). Secondly, the photosynthetic responses of the species to controlled CO₂ concentrations, PAR, temperature, and hydration were measured in the laboratory. CO₂ concentrations around the upper parts of the plant, when PAR was above the compensation point (30 mol m^–2 s^–1), were mostly between 400 and 450 ppm. They occasionally increased up to 1143 ppm for short periods. PAR flux densities below saturating light levels for photosynthesis (100 mol m^–2 s^–1), occurred during 65% (July), 76% (August) and 96% (September) of the hours of the summer period. The temperature optimum of photosynthesis was 20° C: this temperature coincided with PAR above the compensation point during 5%, 6% and 0% of the time in July, August and September, respectively. Optimal hydration of tissues was infrequent. Hence PAR, temperature and water limit CO₂ uptake for most of the growing season. Our data suggest that the higher than normal ambient CO₂ concentration in the immediate environment of the plant counteracts some of the limitations in PAR supply that it experiences in its habitat. This species already experiences concentrations of atmospheric CO₂ predicted to occur over the next 50 years.     

Effect of O2 and CO2 on net CO2 exchange in a high-CO2-requiring mutant of Chlamydomonas reinhardtii during dark-light-dark transitions

Net CO₂ exchange was monitored through a dark-light-dark transition, under 2% and 21% O₂ in the presence and absence of CO₂, in Chlamydomonas reinhardtii wild type and the high-CO₂-requiring mutant ca-1-12-1C. Upon illumination at 350 l/l CO₂, ca-1-12-1C cell exhibited a large decrease in net CO₂ uptake following an initial surge of CO₂ uptake. Net CO₂ uptake subsequently attained a steady-state rate substantially lower than the maximum. A large, O₂-enchanced post-illumination burst of CO₂ efflux was observed after a 10-min illumination period, corresponding to a minimum in the net CO₂ uptake rate. A smaller, but O₂-insensitive post-illumination burst was observed following a 30-min illumination period, when net CO₂ uptake was at a steady-state rate. These post-illumination bursts appeared to reflect the release of an intracellular pool of inorganic carbon, which was much larger following the initial surge of net CO₂ uptake than during the subsequent steady-state CO₂ uptake period.With the mutant in CO₂-free gas, O₂-stimulated, net CO₂ efflux was observed in the light, and a small, O₂-dependent post-illumination burst was observed. With wild-type cells no CO₂ efflux was observed in the light in CO₂-free gas under either 2% or 21% O₂, but a small, O₂-dependent post-illumination burst was observed. These results were interpreted as indicating that photorespiratory rates were similar in the mutant and wild-type cells in the absence of CO₂, but that the wild-type cells were better able to scavenge the photorespiratory CO₂.     

Rhizodeposition under ambient and elevated CO2 levels

As global CO₂ levels rise, can soils store more carbon and so buffer atmospheric CO₂ levels? Answering this question requires a knowledge of the rates of C inputs to soil and of CO₂ outputs via decomposition. Below-ground inputs from roots are a major component of the C flow into soils but are still poorly understood. In this article, new techniques for measuring rhizodeposition are reviewed and discussed and the need for cross-comparisons between methods is identified. One component of rhizodeposition, root exudation, is examined in more detail and evidence is presented which suggests that current estimates of exudate flow into soils are incorrect. A mechanistic mathematical model is used to explore how exudate flows might change under elevated CO₂.     

Photochemical efficiency of photosystem II,photon yield of O2 evolution,photosynthetic capacity,and carotenoid composition during the midday depression of net CO2 uptake in Arbutus unedo growing in Portugal

During the midday depression of net CO₂ exchange in the mediterranean sclerophyllous shrub Arbutus unedo, examined in the field in Portugal during August of 1987, several parameters indicative of photosynthetic competence were strongly and reversibly affected. These were the photochemical efficiency of photosystem (PS) II, measured as the ratio of variable to maximum chlorophyll fluorescence, as well as the photon yield and the capacity of photosynthetic O₂ evolution at 10% CO₂, of which the apparent photon yield of O₂ evolution was most depressed. Furthermore, there was a strong and reversible increase in the content of the carotenoid zeaxanthin in the leaves that occurred at the expense of both violaxanthin and -carotene. Diurnal changes in fluorescence characteristics were interpreted to indicate three concurrent effects on the photochemical system. First, an increase in the rate of radiationless energy dissipation in the antenna chlorophyll, reflected by changes in 77K fluorescence of PSII and PSI as well as in chlorophyll a fluorescence at ambient temperature. Second, a state shift characterized by an increase in the proportion of energy distributed to PSI as reflected by changes in PSI fluorescence. Third, an effect lowering the photon yield of O₂ evolution and PSII fluorescence at ambient temperature without affecting PSII fluorescence at 77K which would be expected from a decrease in the activity of the water splitting enzyme system, i.e. a donor side limitation.Abbreviations and symbols c_i concentration of CO₂ within the leaf - F_o instantaneous fluorescence emission - F_M maximum fluorescence emission - F_v variable fluorescence emission - PFD photon flux density (400–700 nm) - PSI, II photosystem I, II - T_L leaf temperature     

Changes in photosynthetic capacity,carboxylation efficiency,and CO2 compensation point associated with midday stomatal closure and midday depression of net CO2 exchange of leaves of Quercus suber

The carbon-dioxide response of photosynthesis of leaves of Quercus suber, a sclerophyllous species of the European Mediterranean region, was studied as a function of time of day at the end of the summer dry season in the natural habitat. To examine the response experimentally, a standard time course for temperature and humidity, which resembled natural conditions, was imposed on the leaves, and the CO₂ pressure external to the leaves on subsequent days was varied. The particular temperature and humidity conditions chosen were those which elicited a strong stomatal closure at midday and the simultaneous depression of net CO₂ uptake. Midday depression of CO₂ uptake is the result of i) a decrease in CO₂-saturated photosynthetic capacity after light saturation is reached in the early morning, ii) a decrease in the initial slope of the CO₂ response curve (carboxylation efficiency), and iii) a substantial increase in the CO₂ compensation point caused by an increase in leaf temperature and a decrease in humidity. As a consequence of the changes in photosynthesis, the internal leaf CO₂ pressure remained essentially constant despite stomatal closure. The effects on capacity, slope, and compensation point were reversed by lowering the temperature and increasing the humidity in the afternoon. Constant internal CO₂ may aid in minimizing photoinhibition during stomatal closure at midday. The results are discussed in terms of possible temperature, humidity, and hormonal effects on photosynthesis.Abbreviations and symbols CE carboxylation efficiency - NP net photosynthesis rate - PAR photosynthetically active radiation - P_i leaf internal CO₂ partial pressure - W water vapor mole fraction difference between leaf and air - T CO₂ compensation pressure Dedicated to Professor Dr. Hubert Ziegler on the occasion of his 60th birthday     

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.     

Above- and belowground response of Populus grandidentata to elevated atmospheric CO2 and soil N availability

Soil N availability may play an important role in regulating the long-term responses of plants to rising atmospheric CO₂ partial pressure. To further examine the linkage between above- and belowground C and N cycles at elevated CO₂, we grew clonally propagated cuttings of Populus grandidentata in the field at ambient and twice ambient CO₂ in open bottom root boxes filled with organic matter poor native soil. Nitrogen was added to all root boxes at a rate equivalent to net N mineralization in local dry oak forests. Nitrogen added during August was enriched with ¹⁵N to trace the flux of N within the plant-soil system. Above-and belowground growth, CO₂ assimilation, and leaf N content were measured non-destructively over 142 d. After final destructive harvest, roots, stems, and leaves were analyzed for total N and ¹⁵N. There was no CO₂ treatment effect on leaf area, root length, or net assimilation prior to the completion of N addition. Following the N addition, leaf N content increased in both CO₂ treatments, but net assimilation showed a sustained increase only in elevated CO₂ grown plants. Root relative extension rate was greater at elevated CO₂, both before and after the N addition. Although final root biomass was greater at elevated CO₂, there was no CO₂ effect on plant N uptake or allocation. While low soil N availability severely inhibited CO₂ responses, high CO₂ grown plants were more responsive to N. This differential behavior must be considered in light of the temporal and spatial heterogeneity of soil resources, particularly N which often limits plant growth in temperate forests.     

CO2 transported in xylem sap affects CO2 efflux from Liquidambar styraciflua and Platanus occidentalis stems, and contributes to observed wound respiration phenomena

The [CO₂] in the xylem of tree stems is typically two to three orders of magnitude greater than atmospheric [CO₂]. In this study, xylem [CO₂] was experimentally manipulated in saplings of sycamore (Platanus occidentalis L.) and sweetgum (Liquidambar styraciflua L.) by allowing shoots severed from their root systems to absorb water containing [CO₂] ranging from 0.04% to 14%. The effect of xylem [CO₂] on CO₂ efflux to the atmosphere from uninjured and mechanically injured, i.e., wounded, stems was examined. In both wounded and unwounded stems, and in both species, CO₂ efflux was directly proportional to xylem [CO₂], and increased 5-fold across the range of xylem [CO₂] produced by the [CO₂] treatment. Xylem [CO₂] explained 76–77% of the variation in pre-wound efflux. After wounding, CO₂ efflux increased substantially but remained directly proportional to internal stem [CO₂]. These experiments substantiated our previous finding that stem CO₂ efflux was directly related to internal xylem [CO₂] and expanded our observations to two new species. We conclude that CO₂ transported in the xylem may confound measurements of respiration based on CO₂ efflux to the atmosphere. This study also provided evidence that the rapid increase in CO₂ efflux observed after tissues are excised or injured is likely the result of the rapid diffusion of CO₂ from the xylem, rather than an actual increase in the rate of respiration of wounded tissues.     

Photosynthetic characteristics of Cymbidium plantlet in vitro

The photosynthetic characteristics of the Cymbidium plantlet in vitro cultured on Hyponex-agar medium with 2% sucrose were determined based on the measurements of CO₂ concentration inside and outside of the culture vessels. The CO₂ measurements were made with a gas chromatograph at a PPF (photosynthetic photon flux) of 35, 102 and 226 mol m^-2 s^-1, a chamber air temperature of 15, 25 and 35°C and a CO₂ concentration outside the vessel of approximately 350, 1100 and 3000 ppm. The net photosynthetic rates were determined on individual plantlets and were expressed on a dry weight basis. The steady-state CO₂ concentration during the photoperiod was lower inside the vessel than outside the vessel at any PPF greater than 35 mol m^-2s^-1 and at any chamber air temperature. The photosynthetic response curves relating the net photosynthetic rate, PPF, and CO₂ concentration in the vessel and chamber air temperature were similar to those for Cymbidium plants grown outside and other C₃ plants grown outside under shade. The results indicate that CO₂ enrichment for the plantlets in vitro at a relatively high PPF would promote photosynthesis and hence the growth of chlorophyllous shoots/plantlets in vitro and that the plantlets in vitro would make photoautotrophic growth under environmental conditions favorable for photosynthesis.Abbreviations C_in CO₂ concentration in the culture vessel - C_out CO₂ concentration outside the vessel (in the culture room) - PPF photosynthetic photon flux     

Photoautotrophic growth of soybean cells in suspension culture III. Characterization of carbon fixation products under high and low CO2 levels

A photoautotrophic soybean suspension culture (SB-P) was used to study CO₂ assimilation while exposed to elevated or ambient CO₂ levels. These studies showed that under elevated CO₂ (5% v/v) malate is the dominant fixation product, strongly suggesting that phosphoenolpyruvate carboxylase (PEPCase) is the primary enzyme involved in carbon fixation in these cells under their normal growth conditions. Citrate and [aspartate + glutamate] were also significant fixation products during fifteen minutes of exposure to ¹⁴CO₂. During the ten minute unlabeled CO₂ chase however, ¹⁴C-malate continued to increase while citrate and [aspartate + glutamate] declined. Fixation of ¹⁴CO₂ under ambient CO₂ levels (0.037%) showed a very different product pattern as 3-phosphoglycerate was very high in the first one to two minutes followed by increases in [serine + glycine] and [aspartate + glutamate]. Hexose phosphates were also quite high initially but then declined relatively rapidly. Thus, the carbon fixation pattern at ambient CO₂ levels resembles somewhat that seen in C3 leaf cells while that seen at elevated CO₂ levels more closely resembles that of a C₄ plant. The initial fixation product of C₃ plants, 3-PGA, was never detectable under high CO₂ conditions. These data suggest that an in vitro photoautotrophic system would be suitable for studying carbon fixation physiology during photosynthetic and non-photosynthetic growth.Abbreviations SB-P photoautotrophic soybean cells - PEPCase phosphoenol-pyruvate carboxylase - RuBPCase ribulose bisphosphate carboxylase/oxygenase - 3-PGA 3-phosphoglycerate     

In situ CO2 gas-exchange in fruits of a tropical tree,Durio zibethinus Murray

We examined the in situ CO₂ gas-exchange of fruits of a tropical tree, Durio zibethinus Murray, growing in an experimental field station of the Universiti Pertanian Malaysia. Day and night dark respiration rates were exponentially related to air temperature. The temperature dependent dark respiration rate showed a clockwise loop as time progressed from morning to night, and the rate was higher in the daytime than at night. The gross photosynthetic rate was estimated by summing the rates of daytime dark respiration and net photosynthesis. Photosynthetic CO₂ refixation, which is defined as the ratio of gross photosynthetic rate to dark respiration rate in the daytime, ranged between 15 and 45%. The photosynthetic CO₂ refixation increased rapidly as the temperature increased in the lower range of air temperature T _c (T _c <28.5 °C), while it decreased gradually as the temperature increased in the higher range (T _c 28.5 °C). Light dependence of photosynthetic CO₂ refixation was approximated by a hyperbolic formula, where light saturation was achieved at 100 mol m^–2 s^–1 and the asymptotic CO₂ refixation was determined to be 37.4%. The estimated gross photosynthesis and dark respiration per day were 1.15 and 4.90 g CO₂ fruit^–1, respectively. Thus the CO₂ refixation reduced the respiration loss per day by 23%. The effect of fruit size on night respiration rate satisfied a power function, where the exponent was larger than unity.     

Effect of CO2 enhancement in an outdoor algal production system usingTetraselmis

One of the objectives of microalgal culture is to provide reliable production technology for important live aquaculture feed organisms. Presented here are the results of experiments designed to provide a better understanding of the relationship between inorganic carbon availability and algal production.Our results suggest that through additions of CO₂ gas we were able to maintain sufficient dissolved carbon to stabilize outdoor algal cultures. Increases in the rate of addition of CO₂ increased levels of dissolved CO₂, total dissolved inorganic carbon (CO₂), and decreased pH in the growth medium. This translated into improved buffering capacity of the culture medium and higher growth rate. A minimum of 2.4 mM CO₂ was found necessary to maintain a maximal growth rate of 0.7 doublings/day. We also found that the increased productivity more than offsets the cost of adding the CO₂.     

Products of CO2 fixation and 14C labelling pattern of alanine in Methanobacterium thermoautotrophicum pulse-labelled with 14CO2

The incorporation of ¹⁴CO₂ by an exponentially growing culture of the autotrophic bacterium Methanobacterium thermoautotrophicum has been studied. The distribution of radioactivity during 2s–120s incubation periods has been analyzed by chromatography and radioautography. After a 2 s incubation most of the radioactivity of the ethanolsoluble fraction was present in the amino acids alanine, glutamate, glutamine and aspartate, whereas phosphorylated compounds were only weakly labelled. The percentage of the total radioactivity fixed, which was contained in the principal early labelled amino acid alanine, increased in the first 20 s and only then decreased, indicating that alanine is derived from primary products of CO₂ fixation.The labelling patterns of alanine produced during various incubation times have been determined by degradation. After a 2 s ¹⁴CO₂ pulse, 61% of the radioactivity was located in C-1, 23% in C-2, and 16% in C-3. The results are consistent with the operation of a previously proposed autotrophic CO₂ assimilation pathway which involves the formation of acetyl CoA from 2 CO₂ via one-carbon unit intermediates, followed by the reductive carboxylation of acetyl CoA to pyruvate.     

High CO2 partial pressure effects on dark and light CO2 fixation and metabolism in Vicia faba leaves

Stomatal opening on Vicia faba can be induced by high CO₂ partial pressures (10.2%) in dark as well as in light. Stomatal aperture was measured in both cases with a hydrogen porometer. The distribution of ¹⁴C among early products of photosynthesis was studied. Comparisons are made with carboxylations occurring when stomata were open in the dark with CO₂-free air and in light with 0.034% CO₂. Results showed that in high CO₂ partial pressure in light, less radioactivity was incorporated in Calvin cycle intermediates and more in sucrose. carboxylations and photorespiration seemed to be inhibited. In the dark in both CO₂ conditions, ¹⁴C incorporation was found in malate and aspartate but also in serine and glycerate in high CO₂ conditions. In light these changes in metabolic pathways may be related with the deleterious effects recorded on leaves after long-term expositions to high partial pressure of CO₂.Abbreviations DHAP dihydroxyacetone phosphate - PEP phosphonenolpyruvate - PEPCK phosphonenolpyruvatecarboxykinase - PGA 3-phosphoglyceric acid - RUBPc ribulose 1,5-bisphosphate carboxylase