Photorespiratory CO2 Evolution Research Articles

Is the recovery of (photo) respiratory CO2 and intermediates minimal?

Is the recovery of (photo) respiratory <scp>CO</scp>₂ and intermediates minimal?

A New Approach to Measure Gross CO2 Fluxes in Leaves. Gross CO2 Assimilation, Photorespiration, and Mitochondrial Respiration in the Light in Tomato under Drought Stress

We developed a new method using 13CO2 and mass spectrometry to elucidate the role of photorespiration as an alternative electron dissipating pathway under drought stress. This was achieved by experimentally distinguishing between the CO2 fluxes into and out of the leaf. The method allows us to determine the rates of gross CO2 assimilation and gross CO2 evolution in addition to net CO2 uptake by attached leaves during steady-state photosynthesis. Furthermore, a comparison between measurements under photorespiratory and non-photorespiratory conditions may give information about the contribution of photorespiration and mitochondrial respiration to the rate of gross CO2 evolution at photosynthetic steady state. In tomato (Lycopersicon esculentum Mill. cv Moneymaker) leaves, drought stress decreases the rates of net and gross CO2 uptake as well as CO2 release from photorespiration and mitochondrial respiration in the light. However, the ratio of photorespiratory CO2 evolution to gross CO2 assimilation rises with water deficit. Also the contribution of re-assimilation of (photo) respiratory CO2 to gross CO2 assimilation increases under drought.

Distribution of Inorganic Carbon Among its Component Species in Cyanobacteria: Do Cyanobacteria in fact Actively Accumulate Inorganic Carbon?

A mathematical model is presented, which describes the distribution of inorganic carbon (Ci) between the species CaCO3, CaHCO3+, CO32−, HCO3−and CO2in the cytosol of a high CO2- requiring mutant of the cyanobacteriumSynechocystisPCC 6803, which lacks carboxysomes. The model assumes that entry and efflux of Ci occurs via a CO32−transport and diffusion of CO2. HCO3−is considered as impermeable. Intracellular Ci is distributed among the species according to the cytoplasmic pH and the calcium concentration. Former models considered entry of HCO3−, and not CO32−, and the intracellular interactions of calcium with CO32−and their implications for the Ci accumulating mechanism were not considered.The model predicts that in the presence of 10−3–10−2M calcium, at low and moderate external inorganic carbon concentrations, CaCO3is the main cytoplasmic Ci species, whereas in its absence HCO3−is the main Ci species. Due to sequestration of CO32−by calcium and low permeability of the cells to CO2, the CO2, concentration is low and no net photosynthesis is observed. Photosynthesis is predicted to occur only at external Ci concentrations higher than 1 mM, where the CO32−transport is saturated and the main source of Ci is diffusion of CO2. This converts the mutant to a CO2user with a high photosynthetic Km(CO2), unlike the wild type which utilizes external CO32−as a substrate for photosynthesis at low external concentration.The sequestration of CO32−by Ca2+efficiently reduced the Co32−and the CO2accumulation ratio in comparison with that of total Ci resulting in an inward CO2gradient and reduced CO32−gradient over a wide range of external Ci concentrations. This raises doubts as to whether the transport of Ci as such is an active transport.The simulation predicts that in cases where the affinity of the uptake mechanism for CO32−is higher than that measured for this mutant and the accumulation ratio for Ci is consequently higher, the predicted accumulation ratio for the transported species CO32−remains low. Taking the permeability of the cells to gases to be as low as previously measured, the simulation predicts that O2may be accumulated in the cytoplasm up to saturation at CO2saturated photosynthesis rates. This results in a high rate of photorespiration and competitive inhibition of CO2fixation.Entry of CO32−, accumulation of charged Ci species, Ci utilization and efflux of carbon stimulates ions movements required to compensate the charges of CO32−, HCO3−and CaHCO3+.The model predicts accumulation of cations up to 50 mEq × 1−1or exclusion of the same amount of anions required to electroneutralize the accumulation of charged Ci species in the absence of calcium and up to 28 mEq × liter−1in the presence of 20 mM calcium. The steady-state CO32−dependent flux of electroneutralizing ions was found to be equal to the steady-state rate of Ci utilization and efflux (photosynthesis, photorespiratory CO2evolution and CO2leakage) multiplied by the valency of CO32−. The interconversion of Ci species resulting from Ci utilization and dissipation of Ci involves H+liberation in the hydration reaction and the dissociation of HCO3−to CO32−or H+consumption in the conversion of CO32−to HCO3−and dehydration of the latter. The H+production and the CO32−dependent transport of positive ions are interrelated and are non-monotonously altered in anti-parallel fashion as a function of the external Ci concentration. The presence of calcium reduces the magnitude of both these fluxes.

Partitioning of the Leaf CO2 Exchange into Components Using CO2 Exchange and Fluorescence Measurements.

Photorespiration was calculated from chlorophyll fluorescence and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) kinetics and compared with CO2 evolution rate in the light, measured by three gas-exchange methods in mature sunflower (Helianthus annuus L.) leaves. The gas-exchange methods were (a) postillumination CO2 burst at unchanged CO2 concentration, (b) postillumination CO2 burst with simultaneous transfer into CO2-free air, and (c) extrapolation of the CO2 uptake to zero CO2 concentration at Rubisco active sites. The steady-state CO2 compensation point was proportional to O2 concentration, revealing the Rubisco specificity coefficient (Ksp) of 86. Electron transport rate (ETR) was calculated from fluorescence, and photorespiration rate was calculated from ETR using CO2 and O2 concentrations, Ksp, and diffusion resistances. The values of the best-fit mesophyll diffusion resistance for CO2 ranged between 0.3 and 0.8 s cm-1. Comparison of the gas-exchange and fluorescence data showed that only ribulose-1,5-bisphosphate (RuBP) carboxylation and photorespiratory CO2 evolution were present at limiting CO2 concentrations. Carboxylation of a substrate other than RuBP, in addition to RuBP carboxylation, was detected at high CO2 concentrations. A simultaneous decarboxylation process not related to RuBP oxygenation was also detected at high CO2 concentrations in the light. We propose that these processes reflect carboxylation of phosphoenolpyruvate, formed from phosphoglyceric acid and the subsequent decarboxylation of malate.

Comparison of Methods to Estimate Dark Respiration in the Light in Leaves of Two Woody Species.

Dark respiration in the light was estimated in leaves of two woody species (Heteromeles arbutifolia Ait. and Lepechinia fragans Greene) using two different approaches based on gas-exchange techniques: the Kok method and the Laisk method. In all cases, dark respiration in the light was lower (P < 0.05) than respiration in darkness, indicating that dark respiration was inhibited in the light. Rates of dark respiration in the light estimated by the Laisk method were 52% higher (P < 0.05) than those estimated by the Kok method. Differences between the methods could be explained by the low ambient CO2 concentrations required by the Laisk approach. The mean value of the inhibition of respiration by light for the two species, corrected for the ambient CO2 concentration effect, was 55%. Despite the differences in leaf characteristics between the species, values of the CO2 photocompensation point, at which the rate of photosynthetic CO2 uptake equaled that of photorespiratory CO2 evolution, were very constant, suggesting an excellent consistency in the results obtained with the Laisk approach.

Effect of temperature on the CO2/O2 specificity of ribulose-1,5-bisphosphate carboxylase/oxygenase and the rate of respiration in the light

Responses of the rate of net CO2 assimilation (A) to the intercellular partial pressure of CO2 (p i ) were measured on intact spinach (Spinacia oleracea L.) leaves at different irradiances. These responses were analysed to find the value of p i at which the rate of photosynthetic CO2 uptake equalled that of photorespiratory CO2 evolution. At this CO2 partial pressure (denoted Г), net rate of CO2 assimilation was negative, indicating that there was non-photorespiratory CO2 evolution in the light. Hence Г was lower than the CO2 compensation point, Γ. Estimates of Г were obtained at leaf temperatures from 15 to 30°C, and the CO2/O2 specificity of ribulose 1,5-bisphosphate (RuBP) carboxylase/oxygenase (E.C. 4.1.1.39) was calculated from these data, taking into account changes in CO2 and O2 solubilities with temperature. The CO2/O2 specificity decreased with increasing temperature. Therefore we concluded that temperature effects on the ratio of photorespiration to photosynthesis were not solely the consequence of differential effects of temperature on the solubilities of CO2 and O2. Our estimates of the CO2/O2 specificity of RuBP carboxylase/oxygenase are compared with in-vitro measurements by other authors. The rate of nonphotorespiratory CO2 evolution in the light (R d ) was obtained from the value of A at Г. At this low CO2 partial pressure, R d was always less than the rate of CO2 evolution in darkness and appeared to decrease with increasing irradiance. The decline was most marked up to about 100 μmol quanta m(-2) s(-1) and less marked at higher irradiances. At one particular irradiance, however, R d as a proportion of the rate of CO2 evolution in darkness was similar in different leaves and this proportion was unaffected by leaf temperature or by [O2] (ambient and greater). After conditions of high [CO2] and high irradiance for several hours, the rate of CO2 evolution in darkness increased and R d also increased.

Effects of Methionine Sulfoximine and Glycine on Nitrogen Metabolism of Maize Leaves in the Light

Detached maize leaves with their cut bases in water or in solutions containing 15 mM [14C, 15N]glycine, 15 mM [14C]glutamate, 5 mM methionine sulfoximine (an inhibitor of glutamine synthetase) or the appropriate amino acid plus inhibitor, were incubated for up to 135 min in the light. The concentrations and the 15N content of ammonia and of amino acids involved in photorespiratory nitrogen cycling were determined. Incubation with methionine sulfoximine or glycine increased the ammonia content significantly, whereas glutamate showed no effect. The nitrogen of glycine was metabolized into serine and ammonia. Ammonia was first recycled into glutamine, and then into glutamate. The glycine carbon skeleton served as a precursor for serine. Based on the data for ammonia accumulation the minimum rate of photorespiratory CO2 evolution in maize leaves was estimated to be about 1% of the rate of CO2 assimilation.

Effect of carbon dioxide and temperature on photosynthetic CO2 uptake and photorespiratory CO2 evolution in sunflower leaves

Using an open gas-exchange system, apparent photosynthesis, true photosynthesis (TPS), photorespiration (PR) and dark respiration of sunflower (Helianthus annuus L.) leaves were determined at three temperatures and between 50 and 400 μl/l external CO2. The ratio of PR/TPS and the solubility ratio of O2/CO2 in the intercellular spaces both decreased with increasing CO2. The rate of PR was not affected by the CO2 concentration in the leaves and was independent of the solubility ratio of oxygen and CO2 in the leaf cell. At photosynthesis-limiting concentrations of CO2, the ratio of PR/TPS significantly increased from 18 to 30°C and the rate of PR increased from 4.3 mg CO2 dm(-2) h(-1) at 18°C to 8.6 mg CO2 dm(-2) h(-1) at 30°C. The specific activity of photorespired CO2 was CO2-dependent but temperature-independent, and the carbon traversing the glycolate pathway appeared to be derived both from recently fixed assimilate and from older reserve materials. It is concluded that PR as a percentage of TPS is affected by the concentrations of O2 and CO2 around the photosynthesizing cells, but the rate of PR may also be controlled by other factors.

Metabolic regulation of carbon flux during C4 photosynthesis

The potential for glycolate and glycine metabolism and the mechanism of refixation of photorespiratory CO2 in leaves of C4 plants were studied by parallel inhibitor experiments with thin leaf slices, different leaf cell types and isolated mitochondria of C3 and C4 Panicum species. CO2 evolution by leaf slices of P. bisulcatum, a C3 species, fed glycolate or glycine was light-independent and O2-sensitive. The C4 P. maximum and P. miliaceum leaf slices fed glycolate or glycine evolved CO2 in the dark but not in the light. In C4 species, dark CO2 evolution was abolished by the addition of phosphoenolpyruvate (PEP)(4). The addition of maleate, a PEP carboxylase inhibitor, resulted in photorespiratory CO2 efflux by C4 leaf slices in the light also. However, PEP and maleate had no effect on either glycolate-dependent O2 uptake by the C4 leaf slices or on glycolate and glycine metabolism in C3 leaf slices. The rate of photorespiratory CO2 evolution in the C3 Panicum species was 3 times higher than that observed with the C4 species. The ratio of glycolate-dependent CO2 evolution to O2 uptake in both groups was 1:2. Isolated C4 mesophyll protoplasts or their mitochondria did not metabolize glycolate or glycine. However, both C3 mesophyll protoplasts and C4 bundle sheath strands readily metabolized glycolate and glycine in a light-independent, O2-sensitive manner, and the addition of PEP or maleate had no effect. C4 bundle sheath- and C3-mitochondria were capable of oxidizing glycine. This oxidation was linked to the mitochondrial electron transport chain, was coupled to three phosphorylation sites and was sensitive to electron transport inhibitors. C4 bundle sheath- and C3-mitochondrial glycine decarboxylation was stimulated by oxaloacetate and NAD had no effect. In marked contrast, mitochondria isolated from C4 mesophyll cells were incapable of oxidizing or decarboxylating added glycine. The results suggest that in leaves of C4 plants bundle sheath cells are the primary site of O2-sensitive photorespiratory CO2 evolution and the PEP carboxylase present in the mesophyll cells has the Potential for efficiently refixing CO2 before it escapes out of the leaf. The relative role of the PEP carboxylase mediated CO2 pump and reassimilation of photorespiratory CO2 are discussed in relation to the apparent lack of photorespiration in leaves of C4 species.

Influence of certain environmental factors on photosynthesis and photorespiration in Simmondsia chinensis

The response of net photosynthesis and apparent light respiration to changes in [O2], light intensity, and drought stress was determined by analysis of net photosynthetic CO2 response curves. Low [O2] treatment resulted in a large reduction in the rate of photorespiratory CO2 evolution. Lightintensity levels influenced the maximum net photosynthetic rate at saturating [CO2]. These results indicate that [CO2], [O2] and light intensity affect the levels of substrates involved in the enzymatic reactions of photosynthesis and photorespiration. Intracellular resistance to CO2 uptake decreased in low [O2] and increased at low leaf water potentials. This response reflects changes in the efficiency with which photosynthetic and photorespiratory substrates are formed and utilized. Water stress had no effect on the CO2 compensation point or the [CO2] at which net photosynthesis began to saturate at high light intensity. The relationship between these data and recently published in-vitro kinetic measurements with ribulose-diphosphate carboxylase is discussed.

Photosynthesis and photorespiratory CO2 evolution of water-stressed sunflower leaves

Rates of true photosynthesis (TPS), apparent photosynthesis (APS) and photorespiration (PR) of sunflower (Helianthus annuus L., Var. Mennonite) leaves were measured in air (21% O2, 300 vpm CO2) at 25° C and 400 μEinsteins m(-2) s(-1) radiant flux density. The plants were water stressed by application of osmoticum (polyethylene glycol 4000) to the root system. TPS and APS decreased linearly from maxima at-4 bar leaf-water potential (ψ) to become very small and zero respectively at about-18 bar ψ; at smaller potential CO2 was evolved from the leaf. Statistical analysis shows that TPS and APS were more closely correlated with ψ than stomatal conductance (r s (-1)), because r s (-1) changed only in the range-4 to-13 bar but ψ exerted an effect at smaller potential. Photorespiration decreased linearly with stress and at-18 bar was 30% of the control plant rate; ψ and TPS accounted for only part of the variance in PR, both independently and in combination, and r s (-1) accounted for little of the variance. Tricarboxylic acid cycle respiration of leaves placed for 20 min in darkness, remained almost constant with changing ψ and r s (-1). It was one-third of photorespiration in control plants but increased as a proportion in severely stressed plants. The relative specific activity (RSA) of the CO2 released by PR of wellwatered plants was 90% after 20 min photosynthesis in (14)CO2 but decreased to 18% at-18 bar ψ. Therefore, under stress mpre CO2 was derived by respiration from reserve materials and less from immediate photosynthate. Elimination of CO2 production by the glycollate pathway with small oxygen concentration (1.5%), showed that the contribution of TCA cycle respiration to photorespiration was small in unstressed plants but increased at small ψ to almost the same rate as photorespiration. It is concluded that desiccation decreased photosynthesis by decreasing the stomatal conductance to CO2 diffusion and by changing the balance between CO2 assimilation and production of the leaf. As a consequence carbon flux through the glycollate pathway decreased as did the rate of CO2 produced by it. However, TCA cycle respiration in the light increased with stress, so that total photorespiration remained large. The importance of maintaining carbon flux through the glycollate pathway and TCA cycle is discussed.