External CO2 Concentration Research Articles

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 μL L(-1) near the leaf base to below atmospheric (<350 μL 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 μmol 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 (14)CO2 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 μL 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.

A Fiber-Optic Carbon Dioxide Sensor for Fermentation Monitoring

We have developed a fiber-optic chemical sensor for determining dissolved carbon dioxide and assessed its performance for the on-line monitoring of fermentation. The sensor operates on the Severinghaus pCO2 electrode principle; it consists of a pH sensitive dye (hydroxypyrenetrisulfonic acid, HPTS) in an HCO3− buffer solution entrapped in an expanded PTFE support held at the distal end of an optical fiber by a gas permeable membrane. CO2 crossing the membrane produces a pH change in the indicator solution. This change is related to the external CO2 concentration by the Henderson-Hasselbach equation. The sensor has a reversible working dissolved CO2 dynamic range of 0–0.25 atm. The sensor can be auto-claved without affecting its calibration. Results are presented for the on-line determination of CO2 production in beer fermentation.

Induction of CO2 and Bicarbonate Transport in the Green Alga Chlorella ellipsoidea (II. Evidence for Induction in Response to External CO2 Concentration).

The critical species and concentrations of dissolved inorganic carbon (DIC) required for the induction of DIC transport during adaptation to low CO2 were determined for the green alga Chlorella ellipsoidea. The concentration of dissolved CO2 needed for the induction of both CO2 and HCO3- transport was independent of pH during adaptation, whereas the total DIC concentration required increased at alkaline pH. At pH 7.5, the minimum equilibrium DIC concentration at which high CO2 characteristics were maintained, i.e. transport was repressed, was 2100 [mu]M, whereas the maximum equilibrium DIC concentration below which DIC transport was fully induced (DICIND) was 500 [mu]M. Intracellular DIC concentration during adaptation to DICIND decreased temporarily after 2 h to 60% of the maximum level but recovered after 3 h of adaptation. After 3 h of adaptation to DICIND, cells exhibited maximum O2 evolution rate at DICIND. When cells partially adapted to DICIND were returned to high CO2, there was an immediate halt to the induction of transport and a gradual decrease in transport capacity over 23 h. The capacity for the induction of transport was unaffected by the absence of light. These results indicate that changes in the internal DIC pool during adaptation to low CO2 do not trigger the induction of DIC transport and that the induction is not light dependent. Induction of DIC transport in C. ellipsoidea appears to occur in response to the continuous exposure of cells to a critical CO2 concentration in the external medium.

Chloroplast pH values and buffer capacities in darkened leaves as revealed by CO2 solubilization in vivo

Leaves of the C3 plants Brassica oleracea L., Datura suaveolens Humb. & Bonpl. ex Willd., Helianthus annuus L. and Nicotiana tabacum L. with open stomata were exposed in a leaf chamber in the dark to CO2 concentrations varying from 1 to 20% in air. When they were transferred back into CO2-free air, CO2 was rapidly released. It originated from dissolved CO2 and from the bicarbonate in the chloroplast stroma, since vacuoles are acidic and chloroplasts contain carbonic anhydrase which rapidly liberates CO2 from bicarbonate. The data were fitted to a model which accounts for the CO2/bicarbonate equilibrium in buffers with different CO2 concentrations and initial pH values. From this, pH values and CO2-dependent pH changes in the chloroplast stroma were calculated. The full range of external CO2 concentration caused acidic shifts up to 1 pH unit. The best fits of the data points were obtained with stromal buffer concentrations ranging from 45 to 65 mM and stromal pH values at low CO2 between 7.5 and 7.9. Calculated buffer capacities ranged from 23 to 31 mM H+ per pH unit. The work shows that measurements of solubilized CO2 are useful to investigate proton buffering and pH regulation in the chloroplast stroma of intact leaves.

Photobiont-related differences in carbon acquisition among green-algal lichens

The photosynthetic properties of a range of lichens (eight species) containing green algal primary photobionts of either the genus Coccomyxa, Dictyochloropsis or Trebouxia were examined with the aim of obtaining a better understanding for the different CO2 acquisition strategies of lichenized green algae. Fast transients of light/dark-dependent CO2 uptake and release were measured in order to screen for the presence or absence of a photosynthetic CO2-concentrating mechanism (CCM) within the photobiont. It was found that lichens with Trebouxia photobionts (four species) were able to accumulate a small pool of inorganic carbon (DIC; 70–140 nmol per mg chlorophyll (Chl)), in the light, which theoretically may result in, at least, a two to threefold increase in the stromal CO2 concentration, as compared to that in equilibrium with ambient air. The other lichens (four species), which were tripartite associations between a fungus, a cyanobacterium (Nostoc) and a green alga (Coccomyxa or Dictyochloropsis) accumulated a much smaller pool of DIC (10–30 nmol·(mg Chl)−1). This pool is most probably associated with the previously documented CCM of Nostoc, inferred from the finding that free-living cells of Coccomyxa did not show any signs of DIC accumulation. In addition, the kinetics of fast CO2 exchange for free-living Nostoc were similar to those of intact tripartite lichens, especially in their responses to the CCM and the carbonic anhydrase (CA) inhibitor ethoxyzolamide. Trebouxia lichens had a higher photosynthetic capacity at low and limiting external CO2 concentrations, with an initial slope of the CO2-response curve of 2.6–3.9 μmol·(mg Chl)−1·h−1·Pa−1, compared to the tripartite lichens which had an initial slope of 0.5–1.1 μmol-(mg Chl)−1·h−1·-Pa−1, suggesting that the presence of a CCM in the photobiont affects the photosynthetic performance of the whole lichen. Regardless of these indications for the presence or absence of a CCM, ethoxyzolamide inhibited the steady-state rate of photosynthesis at low CO2 in all lichens, indicating a role of CA in the photosynthetic process within all of the photobionts. Measurements of CA activity in photobiont-enriched homogenates of the lichens showed that Coccomyxa had by far the highest activity, while the other photobionts displayed only traces or no activity at all. As the CCM is apparently absent in Coccomyxa, it is speculated that this alga compensates for this absence with high internal CA activity, which may function to reduce the CO2-diffusion resistance through the cell.

Regulation of Periplasmic Carbonic Anhydrase Expression in Chlamydomonas reinhardtii by Acetate and pH.

The effects of mixotrophic growth with acetate and growth medium pH on expression of extracellular carbonic anhydrase (CA) in Chlamydomonas reinhardtii were evaluated. Addition of 10 mM acetate to the culture medium resulted in reduction of CA activity that was parallel to the reduction generated by growth of the algae in high external CO2 concentrations. This reduction in activity is a consequence of lower level of the CA protein as determined by western analysis. Transcript abundance of cah-1, the gene encoding the low CO2-induced CA, is also reduced by the addition of acetate as verified by northern analysis. Measurements of photosynthesis and respiration suggest that the acetate-induced reduction of CA expression is not a function of lowered photosynthetic capacity, but may be the result of increased internal CO2 concentration generated by high, acetate-stimulated respiratory rates. Growth medium pH can also influence extracellular CA expression. The induction of CA activity, protein abundance, and transcript levels by exposure to limiting inorganic carbon (Ci) concentrations is much more pronounced at higher than at lower pH values. The relationship between pH regulation of CA expression and its role in the Ci-concentrating mechanism are discussed.

Daily measurement of CO2assimilation rate on a kiwifruit vine

Abstract CO2 assimilation rate of a ‘Hayward’ kiwifruit (Actinidia deliciosa (A. Chev.) C.F. Liang et A.R. Ferguson) adult vine was measured during the summer of 1989 by two methods. One, an open‐system chamber enclosing a whole vine canopy, was designed for continuous investigation of photosynthesis via the monitoring of external light, humidity, air flow, and air CO2 concentration. The other was the ADC‐LCA1 system, which sampled only a part of a single leaf. The results indicate that photosynthesis rate value increases with light intensity in both instances, but, with respect to the single‐leaf CO2 measurements, the whole canopy CO2 exchange measurement shows lower variability. Also absolute values for single leaf photosynthesis are higher than those observed for the whole plant.

Leaf and canopy photosynthetic CO2 uptake of a stand of Echinochloa polystachya on the Central Amazon floodplain : Are the high potential rates associated with the C4 syndrome realized under the near-optimal conditions provided by this exceptional natural habitat?

The C4 grass Echinochloa polystachya, which forms dense and extensive monotypic stands on the Varzea floodplains of the Amazon region, provides the most productive natural higher plant communities known. The seasonal cycle of growth of this plant is closely linked to the annual rise and fall of water level over the floodplain surface. Diurnal cycles of leaf photosynthesis and transpiration were measured at monthly intervals, in parallel with measurements of leaf area index, canopy light interception and biomass. By artificial manipulation of the light flux incident on leaves in the field light-response curves of photosynthesis at the top and near to the base of the canopy were generated. Fitted light-response curves of CO2 uptake were combined with information of leaf area index, incident light and light penetration of the canopy to estimate canopy rates of photosynthesis. Throughout the period in which the floodplains were submerged photosynthetic rates of CO2 uptake (A) for the emergent leaves were high with a mean of c. 30 μmol m-2 s-1 at mid-day and occasional values of 40 μmol m-2 s-1. During the brief dry phase, when the floodplain surface is uncovered, there was a significant depression of A, with mid-day mean values of c. 17 μmol m-2 s-1. This corresponded with a c. 50% decrease in stomatal conductance, and a c. 35% depression in the ratio of the leaf inter-cellular to external CO2 concentration (c i/c a). During the dry phase, a midday depression of rates of CO2 assimilation was observed. The lowest leaf area index (F) was c. 2 in November-December, when the flood plain was dry, and again in May, when the rising floodwaters were submerging leaves faster than they were replaced. The maximum F of c. 5 was in August when the floodwaters were receding rapidly. Canopy light interception efficiency varied from 0.90 to 0.98. Calculated rates of canopy photosynthesis exceeded 18 mol C m-2 mo-1 throughout the period of flooding, with a peak of 37 mol C m-2 mo-1 in August, but declined to 13 mol C m-2 mo-1 in November during the dry phase. Estimated uptake of carbon by the canopy from the atmosphere, over 12 months, was 3.57 kg C m-2. This was insufficient to account for the 3.99 kg C m-2 of net primary production, measured simultaneously by destructive harvesting. It is postulated that this discrepancy might be accounted for by internal diffusion of CO2 from the CO2-rich waters and sediments via the roots and stems to the sites of assimilation in the leaves.

CDNA cloning and gene expression of carbonic anhydrase in Chlamydomonas reinhardtii

cDNA and genes encoding periplasmic carbonic anhydrase (CA) polypeptides of Chlamydomonas reinhardtii have been isolated and characterized. Nucleotide sequence analysis of cDNA clones revealed that the large subunit (35 kDa or 36.5 kDa) and the small subunit (4 kDa) are cotranslated as a precursor polypeptide (41 626 Da) with a NH2-terminal hydrophobic signal peptide of 20 amino acids. The amino acid sequence of Chlamydomonas CA showed 20–22% identity with animal CA isozymes (CAI, CAII, CAIII, and CAVII). Three zinc-liganded histidine residues and those forming the hydrogen-bond network to zinc-bound solvent molecules were highly conserved. No significant sequence similarity was observed between Chlamydomonas CA and chloroplast CAs of spinach and pea. Two copies of structurally related CA genes (CAH1 and CAH2) were tandemly clustered in Chlamydomonas nuclear genome and regulated by external CO2 concentration in a reverse manner. The 5′ upstream gene CAH1 encodes the major periplasmic CA whose mRNA level is induced under low CO2 condition in light. Photosynthesis is absolutely required for the accumulation of the CAH1 mRNA. The 3′ downstream gene CAH2 is possibly a gene for another periplasmic CA isozyme, which is induced under high CO2 conditions. Light has an inhibitory effect on the accumulation of the CAH2 mRNA. Key words: photosynthesis, light regulation, zinc, CO2-concentrating mechanism, intracellular processing.

Effect of photon flux density on inorganic carbon accumulation and net CO2 exchange in a high-CO2-requiring mutant of Chlamydomonas reinhardtii

The effect of photon flux density on inorganic carbon accumulation and photosynthetic CO2 assimilation was determined by CO2 exchange studies at three, limiting CO2 concentrations with a ca-1 mutant of Chlamydomonas reinhardiii. This mutant accumulates a large internal inorganic carbon pool in the light which apparently is unavailable for photosynthetic assimilation. Although steady-state photosynthetic CO2 assimilation did not respond to the varying photon flux densities because of CO2 limitation, components of inorganic-carbon accumulation were not clearly light saturated even at 1100 μmol photons m(-2) s(-1), indicating a substantial energy requirement for inorganic carbon transport and accumulation. Steady-state photosynthetic CO2 assimilation responded to external CO2 concentrations but not to changing internal inorganic carbon concentrations, confirming that diffusion of CO2 into the cells supplies most of the CO2 for photosynthetic assimilation and that the internal inorganic carbon pool is essentially unavailable for photosynthetic assimilation. The estimated concentration of the internal inorganic carbon pool was found to be relatively insensitive to the external CO2 concentration over the small range tested, as would be expected if the concentration of this pool is limited by the internal to external inorganic carbon gradient. An attempt to use this CO2 exchange method to determine whether inorganic carbon accumulation and photosynthetic CO2 assimilation compete for energy at low photon flux densities proved inconclusive.

Short-term water stress leads to a stimulation of sucrose synthesis by activating sucrose-phosphate synthase

The aim of this work was to identify which aspects of photosynthetic metabolism respond most sensitively to leaf water deficit. Spinach (Spinacia oleracea L.) leaf discs were floated on sorbitol concentrations of increasing molarity and changes of the protoplast volume were estimated using [(14)C]sorbitol and (3)H2O penetration. Detached leaves were also wilted until 10% of their fresh weight was lost. Photosynthesis was studied at very high external CO2 concentrations, to eliminate the effect of closing stomata. There was no large inhibition of CO2 fixation after wilting leaves, or until the external water deficit was greater than-1.2 MPa. However, partitioning changed markedly at these moderate water deficits: more sucrose and less starch was made. When an inhibition of CO2-saturated photosynthesis did appear at a water deficit of-2.0 MPa and above, measurements of chlorophyll-fluorescence quenching and metabolite levels showed the thylakoid reactions were not especially susceptible to short-term water stress. The inhibition was accompanied by a small increase of the triose phosphate: ribulose-1,5-bisphosphate ratio, showing regeneration of ribulose-1,5-bisphosphate was affected. However, there was also a general increase of the estimated concentrations of most metabolites, indicating that there is no specific site for the inhibition of photosynthesis. Increasing water deficit led to a large increase of fructose-2,6-bisphosphate. This is explained in terms of a simultaneous increase of fructose-6-phosphate and inorganic phosphate as the cell shrinks. The high fructose-2,6-bisphosphate led to the accumulation of triose phosphates, and the potential significance of this for protection against photoinhibition is discussed. There was an increase in the extractable activity of sucrose-phosphate synthase. This was only detected when the enzyme was assayed in conditions which distinguish between different kinetic forms which have previously been identified in spinach leaves. It is proposed that activation of sucrose-phosphate synthase is one of the first sites at which spinach leaves respond to a rising water deficit. This could be of importance for osmoregulation.

Mass spectrometric determination of the inorganic carbon species assimilated by photoautotrophic cells of Euphorbia characias L.

The chemical forms of inorganic carbon, CO2 or HCO3-, incorporated during photosynthesis in photoautotrophic Euphorbia characias cell suspension cultures were determined in experiments using 13CO2 and a mass spectrometry technique. From the equations of the CO2 hydration reaction, a kinetic model was first developed, and the effect of photosynthesis on the external CO2 concentration was simulated. It was predicted from this model that CO2 and HCO3- uptakes could be differentiated by recording only the CO2 variation rate in the external medium, successively in absence then in presence of an exogenous carbonic anhydrase activity. The results obtained with either CO2-grown or air-grown photoautotrophic cells were in good agreement with the model and demonstrated that CO2 was the sole species taken up during photosynthesis. In addition no accumulation of inorganic carbon within the cells was observed in the light. Similarly, in dark, CO2 was the only species released by respiration in the external medium.

Carbon Dioxide and Light Responses of Photosynthesis in Cowpea and Pigeonpea during Water Deficit and Recovery

Greenhouse-grown pigeonpea (Cajanus cajan, [L.] Millsp.; cultivar UW-10) and cowpea (Vigna unguiculata, [L.] Walp.; cultivar California No. 5) were well-watered (control) or subjected to low water potential by withholding water to compare their modes of adaptation to water-limited conditions. Leaf CO(2) exchange rate (CER), leaf diffusive conductance to CO(2) (g(l)), and CO(2) concentration in the leaf intercellular air space (C(i)) were determined at various CO(2) concentrations and photon flux densities (PFD) of photosynthetically active radiation (400 to 700 nanometer). In cowpea, g(l) declined to less than 15% of controls and total water potential (psi(w)) at midafternoon declined to -0.8 megapascal after 5 days of withholding water, whereas g(l) in pigeonpea was about 40% of controls even though midafternoon psi(w) was -1.9 megapascal. After 8 days of withholding water, psi(w) at midafternoon declined to -0.9 and -2.4 megapascals in cowpea and pigeonpea, respectively. The solute component of water potential (psi(s)) decreased substantially less in cowpea than pigeonpea. Photosynthetic CER at saturation photon flux density (PFD) and ambient external CO(2) concentration (360 microliters per liter) on day 5 of withholding decreased by 83 and 55% in cowpea and pigeonpea, respectively. When measured at external, CO(2) concentration in bulk air of 360 microliters per liter, the CER of cowpea had fully recovered to control levels 3 days after rewatering; however, at 970 microliters per liter the PFD-saturated CERs of both species were substantially lower than in controls, indicating residual impairment. In stressed plants of both species the CER responses to C(i) from 250 to 600 microliters per liter indicated that a substantial nonstomatal inhibition of CER had occurred. Although the sensitivity of g(l) to water limitation in cowpea suggested a dehydration avoidance response, parallel measurements of CER at various C(i) and PFD indicated that photosynthetic activity of cowpea mesophyll was substantially inhibited by the water-limited treatment.

INTERACTIONS BETWEEN INTERNAL AND EXTERNAL CO2 POOLS IN THE PHOTOSYNTHESIS OF THE AQUATIC CAM PLANTS LITTORELLA UNIFLORA (L.) ASCHERS AND ISOETES LACUSTRIS L.

SummaryThe significance of CAM as a carbon‐conserving mechanism in two submerged aquatics, Littorella uniflora (L.) Aschers and hoetes lacustris L., was evaluated by determining (1) the loss of previously fixed CO2, released through decarboxylation of malic acid and (2) the quantitative importance of CAM relative to external CO2 uptake in photosynthesis.Using a 14C‐labelling technique it was found that the loss of CO2 derived from decarboxylation of malic acid constituted less than 2 % of nocturnal carbon uptake, confirming that the diurnal rhythm of acidity provides a good measure of the incorporation of carbon via CAM.The exchange pattern of inorganic carbon and oxygen was measured for plants incubated in open flow‐through systems. The contribution of internal and external CO2 to photosynthesis was determined as the difference in CO, uptake and oxygen release, where excess oxygen release reflected the assimilation of CO2 released from deacidification of malic acid. Despite a rapid deacidification, uptake of external CO2 was stimulated by 15 to 30% at intermediate external CO2 concentrations. It is suggested that this effect was due to a reduced photorespiratory activity caused by an enhanced internal CO2 concentration generated from malic acid. The simultaneous uptake of inorganic carbon from high internal and low external CO2 concentrations can only be explained by assuming a non‐linear CO, gradient from the lacunal air to the bulk medium, with the CO2 concentration in the outermost cell layers being lower than both the bulk medium and the lacunal air.The relative contribution of CAM to the total uptake of CO2 in daytime declined from 95% (both species) at an external CO2 concentration of 30μ CO2 to 38% (Littorella) and 34% (Isoetes) at 200μ CO2. This resulted from increased uptake of external CO2 at high external CO2 concentrations and a parallel suppression of internal decarboxylation of malic acid.The observed suppression of decarboxylation was confirmed by following the time course in the content of titratable acidity of the leaves. A reversible inhibition of daytime deacidification was seen for external CO2 concentrations higher than 3.0 to 5.4 mM CO2 in both Littorella and Isoetes.The functional significance of CAM for aquatics rests in the enhanced capacity for obtaining inorganic carbon resulting from the extension of the diel period in which inorganic carbon can be accumulated and in the high reassimilation efficiency of nocturnal respiratory CO2.

Remote-monitoring of the physiological-ecological status of crops. IV. Quantitative relationship between photosynthetic rate and transpiration rate per vapor pressure deficit for corn and soybean under field conditions.

The objective of the present study was to obtain fundamental knowledge to monitor remotely the physiological and ecological status of crops in fields. In this paper the relationships between photosynthetic rate (Pn), transpiration rate (Tr), vapor pressure deficit (VPD), and other meteorological data such as photosynthetically active radiation (PAR) were investigated under field conditions. A model relating Pn and Tr was presented. The influence of chlorophyll concentration (Chl) of leaves on Pn was also examined. 1. A theoretical model to interrelate Pn and Tr was presented, which was based on the biophysical processes for gas and vapor transfer via stomata and boundary layer (Eq.1-6). Pn = a[Ca-Ci][Tr/VPD1] (6) where, a is a physical constant, Ca, Ci are ambient and substomatal CO2 concentration, respectively. The Pn is a product of the difference of those two CO2 concentrations and Tr/VPD as indicated in the eq. (6). 2. Close positive correlations between Pn and Tr/VPD were obtained for both corn and soybean as the results of experiments under field conditions. The correlation coefficients were around 0.9 and consistently higher than those coefficients between Pn and Tr in all cases. This relationship held under a wide range of environmental and crop conditions such as air temperature, PAR, VPD, soil water content, crop variety, chloropyll concentration of leaves, developmental stage, leaf position, rolling or wilting of leaves and time of measurements (Tables 1, 2). 3. Linear regression equations were obtained on the above relationships for both corn and soybean. According to those equations the difference of ambient and substomatal CO2 concentrations remains constant, because Pn is proportional to Tr/VPD. The difference of external and internal CO2 concentration was estimated as 159ppm for corn and 51ppm for soybean from the regression coefficients of those equations (Figs. 1∼2). Since the possibility of remote estimation of transpiration rate Tr had been already shown by INOUE10), the photosynthetic activity could also be estimated remotely by means of combination of remotely sensed data and meteorological data connecting above relationships. 4. As a result of the regression analysis the chlorophyll concentration (Chl) of a leaf had as large positive influence as PAR on Pn under water stress free conditions. The partial regression coefficients for Chl were around 0.7. On the other hand the effect of VPD on Pn increased negatively under water stress conditions. The partial regression coefficients were around -0.5 (Table 3).

Effect of low temperature on the photosynthetic metabolism of the C4 grass Echinochloa crus-galli.

CO2 curves of photosynthesis and activities of the four C4 enzymes and Ribulose bisphosphate carboxylase (RUBPc) were compared in two populations of the C4 grass Echinochloa crus-galli from contrasting thermal environments (Québec and Mississippi). Analyses were conducted both before and after 14 h of chilling at 7°C under high light conditions. This comparison provides the opportunity to assess which steps of the C4 pathway are more susceptible to become limiting at low temperatures. Both populations maintained, after chilling, a pattern of CO2 fixation typical of C4 plants with photosynthesis saturating at low external CO2 concentrations. However, the chilling treatment led to reductions in carbon uptake and in the activities of the C4 enzymes. RUBPc activity was not significantly affected by chilling. Reductions in photosynthesis and in C4 enzyme activities following the chilling treatment were significantly larger for plants of the Mississippi population. The enzyme data suggest that two steps of the C4 pathway, NADP+-malate dehydrogenase and pyruvate Pi dikinase, are likely to be associated with the reduction of CO2 uptake in C4 plants under cool conditions. When the experiment was replicated under enriched atmospheric CO2 (675 μl l-1 CO2), similar differences were observed between the two populations. CO2 enrichment resulted in an increase of activity for phospho-enol-pyruvate carboxylase and NADP+-malate dehydrogenase while activities of phospho-enol-pyruvate carboxylase and NADP+-malic enzyme were less reduced following chilling. Such an interaction was not observed for gas exchange parameters but net photosynthesis was lower when plants were grown under enriched CO2.

Damage components of powdery mildew in winter wheat

The photosynthesis and transpiration rates of winter wheat leaves were measured at different developmental stages and at various levels of mildew infection. Even at low levels of infection, circa 4% leaf area being covered, both the assimilation and transpiration rates at light satiation were considerably reduced. Light use efficiency and dark respiration were not significantly affected. The physiological background of these effects was analysed by photosynthesis measurements at different external CO2 concentrations. It was shown that the carboxylation resistance was increased due to mildew infection and that stomatal resistance, boundary layer resistance and transport resistance in the mesophyll were only indirectly affected. A simulation model was used to compute the consequences of these effects on daily growth rate of a winter wheat canopy at different development stages and with different levels of mildew severity. These computations show that a low infection level may result in a considerable reduction of the crop growth rate. This effect was more pronounced when the sky was clear than overcast. Bepalingen van fotosynthese en verdamping werden verricht bij verschillende ontwikkelingsstadia van wintertarwe, die in verschillende mate was aangetast door meeldauw. Zelfs bij vrij lage infectiepercentages (circa 4%) was er al een aanzienlijke reductie van assimilatie en verdampingssnelheid bij lichtverzadiging. Lichtbenuttingsefficiëntie en donkerademhaling werden niet significant beïnvloed. De fysiologische achtergrond van deze effecten werd onderzocht door fotosynthesemetingen bij verschillende externe CO2 concentraties. Er werd aangetoond dat de carboxylatieweerstand werd verhoogd door meeldauwinfectie en dat de huidmondjesweerstand, grenslaagweerstand en transportweerstand in het mesofyl indirect werden beïnvloed. De gevolgen van deze effecten op de dagelijkse groeisnelheid van een wintertarwegewas in verschillende ontwikkelingsfasen en met verschillende LAI werd nagegaan met een simulatiemodel. Deze berekeningen tonen aan dat een geringe meeldauwaantasting resulteert in een aanzienlijke reductie van de groeisnelheid van het gewas. Dit effect was duidelijker bij heldere dan bij bewolkte hemel.

δ13C-variations of leaves in forests as an indication of reassimilated CO2 from the soil.

An attempt has been made to evaluate the contribution of soil respired CO2 to the total assimilation of a forest tree, by heeding the 13C-concentrations of CO2 from the free atmosphere and from mineralization processes within the soil respectively. An expression has been derived, according to which the assimilated fraction of CO2 from the soil at a particular height of a tree is given by the δ13C-value of the corresponding leaves, δ13C of atmospheric CO2, δ13C of soil respired CO2 and the physiological state of the leaves expressed as the ratio of total respiration over gross photosynthesis and internal over external CO2-concentration. In the particular case investigated, a δ13C-difference of 5‰ has been determined from bottom to top of a beech tree which results in a CO2 contribution from the soil of about 22% for the lower forest strata, while the total contribution of soil respired CO2 accounts for about 5% of the overall assimilation.

Responses of Soybean Canopy Photosynthesis and Transpiration to Whole‐Day Temperature Changes in Different CO2 Environments1

AbstractResponse of plants to increased temperatures, as well as increased CO2, is one of the concerns about the global consequences of continued burning of fossil fuel. In this study, soybeans [Glycine max (L.) Merr. cv. Bragg] were grown in controlled‐environment chambers located outdoors and acclimated to two environmental CO2 levels, 330 and 800 µmol mol−1. Canopies were exposed to different day‐long dry bulb temperature treatments that were maintained for a few days. Comparisons of CER‐light response curves and daytime total CO2 exchange from both low and high CO2 treatments indicated that canopy CER was not significantly affected by dry bulb temperature in the range 28°C to 35°C (nighttime temperature was 23°C). Canopy water use increased significantly in both CO2 treatments as dry bulb temperature was increased and the dewpoint temperature was kept at 21°C. Calculated total canopy resistances were constant throughout the temperature treatments in each of the CO2 treatments. This indicated that between dry bulb treatments, changes in canopy water use resulted strictly from temperature‐driven changes in the vapor pressure gradients within the canopy. Characteristic ratios of internal to external canopy CO2 concentrations were calculated and found to be similar among all temperature treatments and in both CO2 treatments. Across all temperature treatments, cumulative daytime transpiration was nearly equal between the two CO2 treatments, however, water‐use efficiency (WUE) decreased in both CO2 treatments as dry bulb temperature increased. Since CER response in both CO2 treatments was unaffected by air temperature across the range of 28 to 35°C, the decrease in WUE with increasing temperature was due solely to increasing transpiration.

Effects of temperature at constant air dew point on leaf carboxylation efficiency and CO2 compensation point of different leaf types

The effect of temperature on photosynthesis at constant water-vapor pressure in the air was investigated using two sclerophyll species, Arbutus unedo and Quercus suber, and one mesophytic species, Spinacia oleracea. Photosynthesis and transpiration were measured over a range of temperatures, 20-39° C. The external concentration of CO2 was varied from 340 μbar to near CO2 compensation. The initial slope (carboxylation efficiency, CE) of the photosynthetic response to intercellular CO2 concentration, the CO2 compensation point (Γ), and the extrapolated rate of CO2 released into CO2-free air (R i) were calculated. At an external CO2 concentration of 320-340 μbar CO2, photosynthesis decreased with temperature in all species. The effect of temperature on Γ was similar in all species. While CE in S. oleracea changed little with temperature, CE decreased by 50% in Q. suber as temperature increased from 25 to 34° C. Arbutus unedo also exhibited a decrease in CE at higher temperatures but not as marked as Q. suber. The absolut value of R i increased with temperature in S. oleracea, while changing little or decreasing in the sclerophylls. Variations in Γ and R i of the sclerophyll species are not consistent with greater increase of respiration with temperature in the light in these species compared with S. oleracea.