Respiratory CO2 Fluxes in Photosynthesizing Leaves of C3 Species Varying in Rates of Starch Synthesis1

The rates of CO₂ fixation and respiratory CO₂ fluxes in six C₃ species, namely Solanum tuberosum, Nicotiana tabacum, Arabidopsis thaliana, Hordeum vulgare, Triticum aestivum, and Secale cereale, were determined under steady-state photosynthesis. The plants may be divided into two groups: (a) cereals with a low rate of starch synthesis (7–5% of true photosynthesis); (b) plants with a high rate of starch synthesis (45–35% of true photosynthesis). In the light, primary and stored photosynthates are consumed as substrates for both respiratory and photorespiratory pathways. In leaves of cereals, the total rate of respiratory and photorespiratory decarboxylations of stored photosynthates was higher in the light than in the dark, while, in starch-synthesizing species, stored photosynthates were consumed at a higher rate in the dark. Under normal environmental conditions, respiratory decarboxylation of stored photosynthates was suppressed by light in all species studied. The total rate of respiration as the sum of decarboxylation of stored and primary photosynthates was not affected by light in cereals, but suppressed in starch-accumulating plants. This suppression was not compensated for by the additional supply of respiratory substrates from primary photosynthates in the light.     

Carbon dioxide concentration at night affects translocation from soybean leaves

Studies have indicated that the concentration of carbon dioxide [CO2] during the dark period may influence plant dry matter accumulation. It is often suggested that these effects on growth result from effects of [CO2] on rates of respiration, but responses of respiration to [CO2] remain controversial, and connections between changes in respiration rate and altered growth rate have not always been clear. The present experiments tested whether translocation, a major consumer of energy from respiration in exporting leaves, was sensitive to [CO2]. Nineteen-day-old soybean plants grown initially at a constant [CO2] of 350 micromol mol(-1) were exposed to three consecutive nights with a [CO2] of 220-1400 micromol mol(-1), with a daytime [CO2] of 350 micromol mol(-1). Change in dry mass of the individual second, third and fourth trifoliate leaves over the 3-d period was determined, along with rates of respiration and photosynthesis of second leaves, measured by net CO2 exchange. Translocation was determined from mass balance for second leaves. Additional experiments were conducted where the [CO2] around individual leaves was controlled separately from that of the rest of the plant. Results indicated that low [CO2] at night increased both respiration and translocation and elevated [CO2] decreased both processes, to similar relative extents. The effect of [CO2] during the dark on the change in leaf mass over 3 d was largest in second leaves, where the change in mass was about 50% greater at 1400 micromol mol(-1) CO2 than at 220 micromol mol(-1) CO2. The response of translocation to [CO2] was localized in individual leaves. Results indicated that effects of [CO2] on net carbon dioxide exchange rate in the dark either caused or reflected a change in a physiologically important process which is known to depend on energy supplied by respiration. Thus, it is unlikely that the observed effects of [CO2] on respiration were artefacts of the measurement process in this case.     

Stem CO2 release under illumination: corticular photosynthesis,photorespiration or inhibition of mitochondrial respiration?

In illuminated stems and branches, CO2 release is often reduced. Many light-triggered processes are thought to contribute to this reduction, namely photorespiration, corticular photosynthesis or even an inhibition of mitochondrial respiration. In this study, we investigated these processes with the objective to discriminate their influence to the overall reduction of branch CO2 release in the light. CO2 gas-exchange measurements of young birch (Betula pendula Roth.) branches (< 1.5 cm) performed under photorespiratory (20% O2) and non-photorespiratory (< 2%) conditions revealed that photorespiration does not play a pre-dominant role in carbon exchange. This suppression of photorespiration was attributed to the high CO2 concentrations (C(i)) within the bark tissues (1544 +/- 227 and 618 +/- 43 micromol CO2 mol(-1) in the dark and in the light, respectively). Changes in xylem CO2 were not likely to explain the observed decrease in stem CO2 release as gas-exchange measurements before and after cutting of the branches did not effect CO2 efflux to the atmosphere. Combined fluorescence and gas-exchange measurements provided evidence that the light-dependent reduction in CO2 release can pre-dominantly be attributed to corticular refixation, whereas an inhibition of mitochondrial respiration in the light is unlikely to occur. Corticular photosynthesis was able to refix up to 97% of the CO2 produced by branch respiration, although it rarely led to a positive net photosynthetic rate.     

Components of CO<Subscript>2</Subscript> exchange in leaves of C<Subscript>3</Subscript> species with different ability of starch accumulation

Using a radiogasometric method the rates of photorespiratory and respiratory decarboxylations of primary and stored photosynthates in the leaves of two groups of C₃ species, differing in the ability of starch accumulation, were determined. One group included starch-accumulating (SA) species with rates of starch synthesis on the average 38 % the rate of photosynthesis [Solanum tuberosum L., Arabidopsis thaliana (L.) Heynh, Helianthus annuus L., and Plantago lanceolata L.]. The second group represented starch-deficient (SD) species with rates of starch synthesis less than 8 % the rate of photosynthesis (Secale cereale L., Triticum aestivum L., Hordeum vulgare L., and Poa trivialis L.). In SA species the rate of respiration in the dark was significantly higher than in SD species. No differences were found in the rates of photosynthesis, photorespiration, and respiration under irradiation. Thus, the degree of inhibition of respiration by irradiation was in SA species higher than in SD species. It is concluded that starch does not provide substrates for respiratory and photorespiratory decarboxylations in irradiated photosynthesizing leaves.     

Effect of Cold Hardening on the Components of Respiratory Decarboxylation in the Light and in the Dark in Leaves of Winter Rye

In the dark, all decarboxylation reactions are associated with the oxidase reactions of mitochondrial electron transport. In the light, photorespiration is also active in photosynthetic cells. In winter rye (Secale cereale L.), cold hardening resulted in a 2-fold increase in the rate of dark respiratory CO2 release from leaves compared with nonhardened (NH) controls. However, in the light, NH and cold-hardened (CH) leaves had comparable rates of oxidase decarboxylation and total intracellular decarboxylation. Furthermore, whereas CH leaves showed similar rates of total oxidase decarboxylation in the dark and light, NH leaves showed a 2-fold increase in total oxidase activity in the light compared with the dark. Light suppressed oxidase decarboxylation of end products of photosynthesis 2-fold in NH leaves and 3-fold in CH leaves in air. However, in high-CO2, light did not suppress the oxidase decarboxylation of end products. Thus, the decrease in oxidase decarboxylation of end products observed in the light and in air reflected glycolate-cycle-related inhibition of tricarboxylic acid cycle activity. We also showed that the glycolate cycle was involved in the decarboxylation of the end products of photosynthesis in both NH and CH leaves, suggesting a flow of fixed carbon out of the starch pool in the light.     

Response of respiration of soybean leaves grown at ambient and elevated carbon dioxide concentrations to day-to-day variation in light and temperature under field conditions

BACKGROUND AND AIMS: Respiration is an important component of plant carbon balance, but it remains uncertain how respiration will respond to increases in atmospheric carbon dioxide concentration, and there are few measurements of respiration for crop plants grown at elevated [CO(2)] under field conditions. The hypothesis that respiration of leaves of soybeans grown at elevated [CO(2)] is increased is tested; and the effects of photosynthesis and acclimation to temperature examined. METHODS: Net rates of carbon dioxide exchange were recorded every 10 min, 24 h per day for mature upper canopy leaves of soybeans grown in field plots at the current ambient [CO(2)] and at ambient plus 350 micromol mol(-1) [CO(2)] in open top chambers. Measurements were made on pairs of leaves from both [CO(2)] treatments on a total of 16 d during the middle of the growing seasons of two years. KEY RESULTS: Elevated [CO(2)] increased daytime net carbon dioxide fixation rates per unit of leaf area by an average of 48 %, but had no effect on night-time respiration expressed per unit of area, which averaged 53 mmol m(-2) d(-1) (1.4 micromol m(-2) s(-1)) for both the ambient and elevated [CO(2)] treatments. Leaf dry mass per unit of area was increased on average by 23 % by elevated [CO(2)], and respiration per unit of mass was significantly lower at elevated [CO(2)]. Respiration increased by a factor of 2.5 between 18 and 26 degrees C average night temperature, for both [CO(2)] treatments. CONCLUSIONS: These results do not support predictions that elevated [CO(2)] would increase respiration per unit of area by increasing photosynthesis or by increasing leaf mass per unit of area, nor the idea that acclimation of respiration to temperature would be rapid enough to make dark respiration insensitive to variation in temperature between nights.     

C4 Photosynthesis (The CO2-Concentrating Mechanism and Photorespiration)

Despite previous reports of no apparent photorespiration in C4 plants based on measurements of gas exchange under 2 versus 21% O2 at varying [CO2], photosynthesis in maize (Zea mays) shows a dual response to varying [O2]. The maximum rate of photosynthesis in maize is dependent on O2 (approximately 10%). This O2 dependence is not related to stomatal conductance, because measurements were made at constant intercellular CO2 concentration (Ci); it may be linked to respiration or pseudocyclic electron flow. At a given Ci, increasing [O2] above 10% inhibits both the rate of photosynthesis, measured under high light, and the maximum quantum yield, measured under limiting light ([phi]CO2). The dual effect of O2 is masked if measurements are made under only 2 versus 21% O2. The inhibition of both photosynthesis and [phi]CO2 by O2 (measured above 10% O2) with decreasing Ci increases in a very similar manner, characteristically of O2 inhibition due to photorespiration. There is a sharp increase in O2 inhibition when the Ci decreases below 50 [mu]bar of CO2. Also, increasing temperature, which favors photorespiration, causes a decrease in [phi]CO2 under limiting CO2 and 40% O2. By comparing the degree of inhibition of photosynthesis in maize with that in the C3 species wheat (Triticum aestivum) at varying Ci, the effectiveness of C4 photosynthesis in concentrating CO2 in the leaf was evaluated. Under high light, 30[deg]C, and atmospheric levels of CO2 (340 [mu]bar), where there is little inhibition of photosynthesis in maize by O2, the estimated level of CO2 around ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the bundle sheath compartment was 900 [mu]bar, which is about 3 times higher than the value around Rubisco in mesophyll cells of wheat. A high [CO2] is maintained in the bundle sheath compartment in maize until Ci decreases below approximately 100 [mu]bar. The results from these gas exchange measurements indicate that photorespiration occurs in maize but that the rate is low unless the intercellular [CO2] is severely limited by stress.     

Temperature Dependence of the Linkage of Quantum Yield of Photosystem II to CO2 Fixation in C4 and C3 Plants

The temperature dependence of quantum yields of electron transport from photosystem II (PSII) ([phi]II, determined from chlorophyll a fluorescence) and CO2 assimilation ([phi]CO2, apparent quantum yield for CO2 assimilation) were determined simultaneously in vivo. With C4 species representing NADP-malic enzyme, NAD-malic enzyme, and phosphoenolpyruvate carboxykinase subgroups, the ratio of [phi]II/[phi]CO2 was constant over the temperature range from 15 to 40[deg]C at high light intensity (1100 [mu]mol quanta m-2 s-1). A similar response was obtained at low light intensity (300 [mu]mol quanta m-2 s-1), except the ratio of [phi]II/[phi]CO2 increased at high temperature. When the true quantum yield for CO2 fixation ([phi]CO2*) was calculated by correcting for respiration in the light (estimated from temperature dependence of dark respiration), the ratio of [phi]II/[phi]C02* remained constant with varying temperature and under both light intensities in all C4 species examined. Because the [phi]II/[phi]CO2* ratio was the same in C4 monocots representing the three subgroups, the ratio was not affected by differences in the bio-chemical mechanism of concentrating CO2 in the bundle sheath cells. The results suggest that PSII activity is closely linked to the true rate of CO2 fixation in C4 plants. The close relationship between [phi]II and [phi]CO2* in C4 species under varying temperature and light intensity conditions is apparently due to a common low level of photorespiration and a primary requirement for reductive power in the C3 pathway. In contrast, in a C3 plant the [phi] II/[phi]CO2* ratio is higher under normal atmospheric conditions than under nonphotorespiratory conditions and it increases with rising temperature. This decrease in efficiency in utilizing energy derived from PSII for CO2 fixation is due to an increase in photorespiration. In both the C3 and C4 species, photochemistry is limited under low temperature, and thus excess energy must be dissipated by nonphotochemical means.     

Leaf Respiration in Light and Darkness (A Comparison of Slow- and Fast-Growing Poa Species)

We investigated whether leaf dark respiration (nonphotorespiratory mitochondrial CO2 release) is inhibited by light in several Poa species, and whether differences in light inhibition between the species are related to differences in the rate of leaf net photosynthesis. Four lowland (Poa annua L., Poa compressa L., Poa pratensis L., and Poa trivialis L.), one subalpine (Poa alpina L.), and two alpine (Poa costiniana Vick. and Poa fawcettiae Vick.) Poa species differing in whole plant relative growth rates were grown under identical controlled conditions. Nonphotorespiratory mitochondrial CO2 release in the light (Rd) was estimated according to the Laisk method. Photosynthesis was measured at ambient CO2 partial pressure (35 Pa) and 500 [mu]mol photons m-2 s-1. The rate of photosynthesis per unit leaf mass was positively correlated with the relative growth rate, with the slow-growing alpine Poa species exhibiting the lowest photosynthetic rates. Rates of both Rd and respiration in darkness were also substantially lower in the alpine species. Nonphotorespiratory CO2 release in darkness was higher than Rd in all species. However, despite some variation between the species in the level of light inhibition of respiration, no relationship was observed between the level of inhibition and the rate of photosynthesis. Similarly, the level of inhibition was not correlated with the relative growth rate. Our results support the suggestion that rates of leaf respiration in the light are closely associated with rates in darkness.     

Advanced radiogasometric method for the determination of the rates of photorespiratory and respiratory decarboxylations of primary and stored photosynthates under steady-state photosynthesis

An advanced radiogasometric method for the study of plant leaf CO₂ exchange is presented. The method enables determination of the rates of CO₂ fixation, photorespiration and respiration in the light under steady‐state photosynthesis and discrimination between primary and stored photosynthates as substrates of photorespiratory and respiratory decarboxylations. The method is based on the analysis of the time curves of ¹⁴CO₂ evolution from labeled primary and stored photosynthates in leaves previously exposed to ¹⁴CO₂. The molar rates of different decarboxylation reactions are calculated from the initial slopes of the curves taking into account the specific radioactivity of CO₂ fed to leaves and/or evolved from leaves. To estimate the contribution of primary and stored photosynthates, the measurements of ¹⁴CO₂ evolution are performed after feeding plant leaves for different periods with ¹⁴CO₂. Photorespiration and respiration are distinguished on the basis of data obtained from measurements of ¹⁴CO₂ evolution under normal (210 ml l⁻¹) and low (15 ml l⁻¹) concentrations of oxygen. A principally new method for the determination of the rate of intracellular refixation of respiratory CO₂ has been developed. The method is based on the measurements of ¹⁴CO₂ evolution from leaves into the medium of very high concentrations (30 ml l⁻¹) of ¹²CO₂, where the probability of refixation of ¹⁴CO₂ evolved inside the cell is close to zero. The results obtained were comparable with the data derived from parallel refixation measurements by means of gasometric methods. As an example of application, the data on CO₂ exchange in leaves of two contrasting groups of C₃‐species, differing in the ability of starch accumulation, are presented.     

Kok effect and the quantum yield of photosynthesis : light partially inhibits dark respiration

The linear response of photosynthesis to light at low photon flux densities is known to change abruptly in the vicinity of the light compensation point so that the quantum yield seems to decrease as radiation increases. We studied this `Kok effect' in attached sunflower (Helianthus annuus L. cv IS894) leaves using gas exchange techniques. The effect was present even though respiration was constant in the dark. It was observed at a similar photon flux density (7 to 11 micromole photons per square meter per second absorbed photosynthetically active radiation) despite a wide range of light compensation points as well as rates of photosynthesis. The effect was not apparent when photorespiration was inhibited at low pO₂ (1 kilopascal), but this result was complicated because dark respiration was quite O₂-sensitive and was partially suppressed under these conditions. The Kok effect was observed at saturating pCO₂ and, therefore, could not be explained by a change in photorespiration. Instead, the magnitude of the effect varied as dark respiration varied in a single leaf, and was minimized when dark respiration was minimized, indicating that a partial suppression of dark respiration by light is responsible. Quantum yields measured at photon flux densities between 0 and 7 to 11 micromole photons per square meter per second, therefore, represent the combined yields of photosynthesis and of the suppression of a component of dark respiration by light. This leads to an overestimate of the quantum yield of photosynthesis. In view of these results, quantum yields of photosynthesis must be measured (a) when respiration is constant in the dark, and (b) when dark respiration has been inhibited either at low pO₂ to eliminate most of the light-induced suppression of dark respiration or at photon flux densities above that required to saturate the light-induced suppression of dark respiration. Significant errors in quantum yields of photosynthesis can result in leaves exhibiting this respiratory behavior if these principles are not followed.     

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.     

How does elevated CO2 or ozone affect the leaf-area index of soybean when applied independently?

Changes in leaf-area index (LAI) may alter ecosystem productivity in elevated [CO2] or [O3]. By increasing the apparent quantum yield of photosynthesis (phi(c,max)), elevated [CO2] may increase maximum LAI. However, [O3] when elevated independently accelerates senescence and may reduce LAI. Large plots (20 m diameter) of soybean (Glycine max) were exposed to ambient (approx. 370 micromol mol(-1)) or elevated (approx. 550 micromol mol(-1)) CO2 or 1.2 times ambient [O3] using soybean free-air concentration enrichment (SoyFACE). In 2001 elevated CO2 had no detectable effect on maximum LAI, but in 2002 maximum LAI increased by 10% relative to ambient air. Elevated [CO2] also increased the phi(c,max) of shade leaves in both years. Elevated [CO2] delayed LAI loss to senescence by approx. 54% and also increased leaf-area duration. Elevated [O3] accelerated senescence, reducing LAI by 40% near the end of the growing season. No effect of elevated [O3] on photosynthesis was detected. Elevated [CO2] or [O3] affected LAI primarily by altering the rate of senescence; knowledge of this may aid in optimizing future soybean productivity.     

NaCl胁迫对互花米草细胞膜和光响应曲线特征参数的影响

以大米草的互花米草为材料,研究了不同盐浓度对其细胞膜透性、丙二醛(MDA)含量和光响应曲线的特征参数的变化情况。结果表明:盐浓度低于300mmol·L-1时,互花米草细胞膜透性和MDA含量较对照组无显著差异;其较高的最大光合速率(>30μmol·m-2·s-1),表观量子效率(>0.05mol·mol-1Photons)以及较低的暗呼吸速率(<1.5μmolCO2·m-2·s-1)和光补偿点(<20μmol·m-2·s-1)为其有机物质积累、竞争、建立种群并扩散提供条件。盐浓度高于500mmol·L-1时,互花米草膜透性和MDA含量显著上升,最大光合速率(Amax)及表观量子效率(Q)显著下降,暗呼吸速率(Rday)和光补偿点(LCP)上升。表明细胞膜和光合作用有关酶受到迫害,抑制了其正常生长。盐胁迫下互花米草光合速率降低,但蒸腾速率的显著下降提高了单叶水分利用效率,从而部分缓解了渗透势变化对细胞的迫害,为其生存和生长提供条件。     

Seasonal changes in canopy photosynthesis and respiration, and partitioning of photosynthate, in rice (Oryza sativa L.) grown under free-air CO2 enrichment

An increase in atmospheric CO(2) concentration ( [CO(2)]) is generally expected to enhance photosynthesis and biomass. Rice plants (Oryza sativa L.) were grown in ambient CO(2) (AMB) or free-air CO(2)-enrichment (FACE), in which the target [CO(2)] was 200 micromol mol(-1) above AMB. (13)CO(2) was fed to the plants at different stages so we could examine the partitioning of photosynthates. Furthermore, canopy photosynthesis and respiration were measured at those stages. The ratio of (13)C content in the whole plant to the amount of fixed (13)C under FACE was similar to that under AMB at the vegetative stage. However, the ratio under FACE was greater than the ratio under AMB at the grain-filling stage. At the vegetative stage, plants grown under FACE had a larger biomass than those grown under AMB owing to enhancement of canopy photosynthesis by the increased [CO(2)]. On the other hand, at the grain-filling stage, CO(2) enrichment promoted the partitioning of photosynthate to ears, and plants grown under FACE had a greater weight of ears. However, enhancement of ear weight by CO(2) enrichment was not as great as that of biomass at the vegetative stage. Plants grown under FACE did not necessarily show higher canopy photosynthetic rates at the grain-filling stage. Therefore, we concluded that the ear weight did not increase as much as biomass at the vegetative stage owing to a loss of the advantage in CO(2) gain during the grain-filling period.     

Photosynthesis, productivity, and yield of maize are not affected by open-air elevation of CO2 concentration in the absence of drought

While increasing temperatures and altered soil moisture arising from climate change in the next 50 years are projected to decrease yield of food crops, elevated CO2 concentration ([CO2]) is predicted to enhance yield and offset these detrimental factors. However, C4 photosynthesis is usually saturated at current [CO2] and theoretically should not be stimulated under elevated [CO2]. Nevertheless, some controlled environment studies have reported direct stimulation of C4 photosynthesis and productivity, as well as physiological acclimation, under elevated [CO2]. To test if these effects occur in the open air and within the Corn Belt, maize (Zea mays) was grown in ambient [CO2] (376 micromol mol(-1)) and elevated [CO2] (550 micromol mol(-1)) using Free-Air Concentration Enrichment technology. The 2004 season had ideal growing conditions in which the crop did not experience water stress. In the absence of water stress, growth at elevated [CO2] did not stimulate photosynthesis, biomass, or yield. Nor was there any CO2 effect on the activity of key photosynthetic enzymes, or metabolic markers of carbon and nitrogen status. Stomatal conductance was lower (-34%) and soil moisture was higher (up to 31%), consistent with reduced crop water use. The results provide unique field evidence that photosynthesis and production of maize may be unaffected by rising [CO2] in the absence of drought. This suggests that rising [CO2] may not provide the full dividend to North American maize production anticipated in projections of future global food supply.     

Respiratory carbon metabolism following illumination in intact French bean leaves using (13)C/(12)C isotope labeling

The origin of the carbon atoms in the CO(2) respired by French bean (Phaseolus vulgaris) leaves in the dark has been studied using (13)C/(12)C isotopes as tracers. The stable isotope labeling was achieved through a technical device that uses an open gas-exchange system coupled online to an elemental analyzer and linked to an isotope ratio mass spectrometer. The isotopic analysis of the CO(2) respired in the dark after a light period revealed that the CO(2) was labeled, but the labeling level decreased progressively as the dark period increased. The pattern of disappearance depended on the amount of carbon fixed during the labeling and indicated that there were several pools of respiratory metabolites with distinct turnover rates. We demonstrate that the carbon recently assimilated during photosynthesis accounts for less than 50% of the carbon in the CO(2) lost by dark respiration and that the proportion is not influenced by leaf starvation in darkness before the labeling. Therefore, most of the carbon released by dark respiration after illumination does not come from new photosynthates.     

Interactions between Photosynthesis and Respiration in the Green Alga Chlamydomonas reinhardtii (Characterization of Light-Enhanced Dark Respiration)

The rate of respiratory O2 consumption by Chlamydomonas reinhardtii cell suspensions was greater after a period of photosynthesis than in the preceding dark period. This "light-enhanced dark respiration" (LEDR) was a function of both the duration of illumination and the photon fluence rate. Mass spectrometric measurements of gas exchange indicated that the rate of gross respiratory O2 consumption increased during photosynthesis, whereas gross respiratory CO2 production decreased in a photon fluence rate-dependent manner. The rate of postillumination O2 consumption provided a good measure of the O2 consumption rate in the light. LEDR was substantially decreased by the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea or glycolaldehyde, suggesting that LEDR was photosynthesis-dependent. The onset of photosynthesis resulted in an increase in the cellular levels of phosphoglycerate, malate, and phosphoenolpyruvate, and a decrease in whole-cell ATP and citrate levels; all of these changes were rapidly reversed upon darkening. These results are consistent with a decrease in the rate of respiratory carbon flow during photosynthesis, whereas the increase in respiratory O2 consumption during photosynthesis may be mediated by the export of photogenerated reductant from the chloroplast. We suggest that photosynthesis interacts with respiration at more than one level, simultaneously decreasing the rate of respiratory carbon flow while increasing the rate of respiratory O2 consumption.     

Photosynthesis, Photorespiration and Respiration of Detached Spruce Twigs as Influenced by Oxygen Concentration and Light Intensity

The effects of oxygen concentration and light intensity on the rates of apparent photosynthesis, true photosynthesis, photorespiration and dark respiration of detached spruce twigs were determined by means of an infra-red carbon dioxide analyzer (IRCA). A closed circuit system IRCA was filled with either 1 per cent of oxygen in nitrogen, air (21 % O₂) or pure oxygen (100 % O₂). Two light intensities 30 × 10³ erg · cm ^?2· s^?1 and 120 × 10³ erg · cm^?2· s^?1 were applied. It has been found that the inhibitory effect of high concentration of oxygen on the apparent photosynthesis was mainly a result of a stimulation of the rate of CO₂ production in light (photorespiration). In the atmosphere of 100 % O₂, photorespiration accounts for 66–80 per cent of total CO₂ uptake (true photosynthesis). Owing to a strong acceleration of photorespiration by high oxygen concentrations, the rate of true photosynthesis calculated as the sum of apparent photosynthesis and photorespiration was by several times less inhibited by oxygen than the rate of apparent photosynthesis. The rates of dark respiration were essentially unaffected by the oxygen concentrations used in the experiments. An increase in the intensity of light from 30 × 10³ erg · cm^?3· s^?1 to 120 · 10³ erg · cm^?2· s^?1 enhanced the rate of photorespiration in the atmospheres of 21 and 100 % oxygen but not in 1 % O₂. The rate of apparent photosynthesis, however, was little affected by light intensity in an atmosphere of 1 % oxygen.     

The Effect of Leaf Temperature and Photorespiratory Conditions on Export of Sugars during Steady-State Photosynthesis in Salvia splendens

Export and photosynthesis in leaves of Salvia splendens were measured concurrently during steady-state 14CO2 labeling conditions. Under ambient CO2 and O2 conditions, photosynthesis and export rates were similar at 15 and 25[deg]C, but both declined as leaf temperature was raised from 25 to 40[deg]C. Suppressing photorespiration between 15 and 40[deg]C by manipulating CO2 and O2 levels resulted in higher rates of leaf photosynthesis, total sugar synthesis, and export. There was a linear relationship between the rate of photosynthesis and the rate of export between 15 and 35[deg]C. At these temperatures, 60 to 80% of the carbon fixed was readily exported with sucrose, raffinose, and stachyose, which together constituted over 90% of phloem mobile assimilates. Above 35[deg]C, however, export during photosynthesis was inhibited both in photorespiratory conditions, which inhibited photosynthesis, and in nonphotorespiratory conditions, which did not inhibit photosynthesis. Sucrose and raffinose but not stachyose accumulated in the leaf at 40[deg]C. When leaves were preincubated at 40[deg]C and then cooled to 35[deg]C, export recovered more slowly than photosynthesis. These data are consistent with the view that impairment of export processes, rather than photosynthetic processes associated with light trapping, carbon reduction, and sucrose synthesis, accounted for the marked reduction in export between 35 and 40[deg]C. Taken together, the data indicated that temperature changes between 15 and 40[deg]C had two effects on photosynthesis and concurrent export. At all temperatures, suppressing photorespiration increased both photosynthesis and export, but above 35[deg]C, export processes were more directly inhibited independent of changes in photorespiration and photosynthesis.