Respiration rhythms and heartbeats of diapausing Colorado potato beetles, Leptinotarsa decemlineata, at low temperatures

Discontinuous gas exchange cycles (DGCs), active muscular ventilation, microcycles of repetitive openings, and heartbeats of diapausing adult Colorado potato beetle, Leptinotarsa decemlineata Say (Coleoptera: Chrysomelidae), were studied at low temperatures (0, 5, and 10 °C) using an electrolytic respirometer combined with an infrared actograph. The DGC of the adult constriction-flutter-open type was the main respiration mode in fully quiescent beetles at temperatures from 5 to 10 °C. The CO₂ bursts were actively ventilated at temperatures above 5 °C. During the flutter period, a series of microcycles appeared, but no muscular contractions associated with the microcycles were detected. We identified this respiration mode as discontinuous suction ventilation.
The hydration condition of the beetles did not influence the frequency of the gas exchange cycles, but dehydrated beetles showed significantly longer flutter periods and shorter ventilation periods than hydrated beetles. The heartbeat frequencies were influenced by both temperature and hydration status.
We conclude from the results that DGCs are used at rest in adult L. decemlineata under various environmental conditions and also at low temperatures. Our results showed that DGCs are the main respiration mode of resting adult Colorado potato beetle irrespective of its hydration state and temperature. Our method resolves O₂ uptake and subsequent CO₂ release in flutter and ventilation periods and shows that diffusion is replaced by convection to reduce water loss in adult beetles.     

Gas exchange studies in two Portuguese grapevine cultivars

Gas exchange characteristics of leaves of Vitis vinifera L. cvs Tinta Amarela and Periquita, two grapevine cultivars grown in distinct climatic regions of Portugal, were studied under natural and controlled conditions. Daily time courses of gas exchange were measured on both a hot, sunny day and a cooler, partly cloudy day. Responses of net photosynthesis to irradiance and internal partial pressure of CO₂, were also obtained. A strong correlation between net photosynthesis (P_N) and leaf conductance (g_s) was found during the diurnal time courses of gas exchange, as well as a relatively constant internal partial pressure of CO₂ (P_i), even under non-steady-state conditions. On the cloudless day, both P_N and g_s were lower in the afternoon than in the morning, despite similar conditions of leaf temperature, air to leaf water vapor deficit and irradiance. The response curves of net photosynthesis to internal CO₂ showed linearity up to p_i values of 50 Pa, possibly indicating a substantial excess of photosynthetic capacity. When measured at low partial pressures of O₂ (1 kPa), P_N became inhibited at high CO₂ levels. Inhibition of P_N at high CO₂ was absent under normal levels of O₂ (21 kPa). Significant differences in gas exchange characteristics were found between the two cultivars, with T. Amarela having higher rates under similar measurement conditions. In particular, the superior performance of T. Amarela at high temperatures may represent adaptation to the warmer conditions at its place of origin.     

Effects of graded hypoxia on Atlantic salmon infected with amoebic gill disease

Atlantic salmon Salmo salar with amoebic gill disease (AGD) were exposed to a graded hypoxia (135–40 mmHg water P O₂) and blood samples analysed for respiratory gases and pH at 119, 79·5 and 40 mmHg water P O₂. There were no differences in the rate of oxygen uptake between infected and control fish. However, arterial P O₂, and pH were significantly lower in the infected fish whereas P CO₂ was significantly higher in infected fish compared with controls prior to hypoxia and at 119 mmHg water P O₂. At 79·5 and 40 mmHg water P O₂ saturation, there were no significant differences in blood P O₂ or pH although blood P CO₂ was elevated in AGD affected fish at 50% hypoxia (79·5 mmHg water P O₂). The elevated levels of P CO₂ in fish affected by AGD resulted in a persistent respiratory acidosis even during hypoxic challenge. These data suggest that even though the fish were severely affected by AGD, the presence of AGD while impairing gas transfer under normoxic conditions, did not contribute to respiratory failure during hypoxia.     

Measurements of photosynthesis and respiration in plants

Methods for measuring the rates of photosynthesis and respiration in plants are reviewed. Closed systems that involve manometric techniques, ¹⁴CO₂ fixation, O₂ electrodes and other methods for measuring dissolved and gas phase O₂ are described. These methods typically provide time-integrated rate measurements, and limitations to their use are discussed. Open gas exchange systems that use infra-red CO₂ gas analysers and differential O₂ analysers for measuring instantaneous rates of CO₂ and O₂ exchange are described. Important features of the analysers, design features of gas exchange systems, and sources of potential error are considered. The analysis of chlorophyll fluorescence parameters for estimating the quantum yield for O₂ evolution and CO₂ fixation is described in relation to new fluorescence imaging systems for large scale screening of photosynthetic phenotypes, and the microimaging of individual chloroplasts.     

An integrated portable apparatus for the simultaneous field measurement of photosynthetic CO2 and water vapour exchange, light absorption and chlorophyll fluorescence emission of attached leaves

Abstract. A portable apparatus has been constructed to measure simultaneously the quantum yield of CO₂ assimilation, light absorption, chlorophyll fluorescence emission and water vapour exchange of attached intact leaves in the field. The core of the instrument is a light-integrating spherical leaf chamber which includes ports for a light source, photosynthetically active radiation sensor, fluorescence probes and gas inlet and outlet manifolds. Measurement of the quantum flux inside the empty chamber and with a leaf present allows determination of leaf absorptance. An open gas exchange system is employed using an infra-red analyser to measure leaf CO₂ exchange. Using a DC white light source the quantum yield of CO₂ assimilation based on absorbed light (φ_abs) may be determined rapidly in either ambient air or artificial gas mixtures. Inclusion of capacitance humidity probes into the gas inlet and outlet ports allows simultaneous determination of water vapour exchange and subsequent estimation of stomatal conductance to CO₂ and intercellular CO₂ concentration. Measurement of fluorescence emission by the sample leaf exposed to white light is achieved by a modulated fluorescence detection system. In addition to determination of the minimal, maximal and variable fluorescence levels, a further analysis allows the photochemical and non-photochemical components of fluorescence quenching, to be estimated. The theory and design of this apparatus is described in detail. The use of the apparatus in the field is demonstrated through a study of the photosynthetic performance of a maize and bean crop during the growing season and by analysis of the photosynthetic performance of crops subjected to nitrogen-stress and a herbicide treatment.     

Gas exchange and carbon isotope composition of Ananas comosus in response to elevated CO2 and temperature

Ananas comosus L. (Merr.) (pineapple) was grown at three day/night temperatures and 350 (ambient) and 700 (elevated) μ mol mol^–1 CO₂ to examine the interactive effects of these factors on leaf gas exchange and stable carbon isotope discrimination ( Δ ,‰). All data were collected on the youngest mature leaf for 24 h every 6 weeks. CO₂ uptake (mmol m^–2 d^–1) at ambient and elevated CO₂, respectively, were 306 and 352 at 30/20 °C, 175 and 346 at 30/25 °C and 187 and 343 at 35/25 °C. CO₂ enrichment enhanced CO₂ uptake substantially in the day in all environments. Uptake at night at elevated CO₂, relative to that at ambient CO₂, was unchanged at 30/20 °C, but was 80% higher at 30/25 °C and 44% higher at 35/25 °C suggesting that phosphoenolpyruvate carboxylase was not CO₂-saturated at ambient CO₂ levels and a 25 °C night temperature. Photosynthetic water use efficiency (WUE) was higher at elevated than at ambient CO₂. Leaf Δ -values were higher at elevated than at ambient CO₂ due to relatively higher assimilation in the light. Leaf Δ was significantly and linearly related to the fraction of total CO₂ assimilated at night. The data suggest that a simultaneous increase in CO₂ level and temperature associated with global warming would enhance carbon assimilation, increase WUE, and reduce the temperature dependence of CO₂ uptake by A. comosus .     

Implications of non-uniform stomatal closure on gas exchange calculations

Abstract. This paper discusses the consequences of non-uniform (= patchwise) stomatal closure on the estimation of gas exchange parameters. The estimation of the partial pressure of internal CO₂ (c_i) appears to be little sensitive to complete non-uniform stomatal closure. During the process of closure of these patches, however, a lower c_i will be calculated. For gas exchange measurements done at low wind speeds, it can be shown that an error is made in the partitioning of the total vapour transfer resistance into boundary layer and stomatal resistance. This error influences the calculated total transfer resistance of gases other than water vapour (e.g. CO₂). The apparent negative internal gas concentrations that have sometimes been found in fumigation experiments with SO₂ can possibly be explained by this error.     

Lack of discontinuous gas exchange in a tracheate arthropod, Leiobunum townsendi (Arachnida, Opiliones)

Abstract The discontinuous gas exchange cycle, characterized by stringent spiracular control and periods of near-zero external CO₂ emission separated by 'bursts' of CO₂ emission, has evolved independently in several taxa of tracheate arthropods. These include the hexapoda, diplopoda, and several arachnid taxa; ticks, pseudoscorpions and solphugids. This paper presents the first data on gas exchange kinetics in a harvestman (Arachnida; Opiliones). The experimental animal, Leiobunum townsendi Weed, from an arid area of the south-western United States, displayed a metabolic rate similar to those of other arthropods at 25 °C (129 ± 22 µW). Their CO₂ emission kinetics showed, when the animals were motionless, only minor variations about a mean value of 0.0217 ± 0.0037 mL/h ( n  = 6, mean body mass 86 mg). Expressed on an intra-recording basis, the coefficient of variation of CO₂ emission (= SD/MEAN), which is an index of short-term gas emission fluctuations and thus of spiracular control, had a mean value of only 0.082. In contrast, the coefficient of variation of animals employing a discontinuous gas exchange cycle is > 1.5. Gas exchange in opilionids, unlike the case with most other tracheate arthropods, may therefore be dominated by simple diffusion without a prominent role for wide modulations of spiracular conductance. Contributory to this conservative spiracular control strategy may be the weak degree of tracheation in opilionids, combined with circulating haemocyanin, which acts as both a transport medium and a buffering reservoir for respiratory gas exchange.     

Carbon Dioxide Exchange Between the Atmosphere and an Alpine Shrubland Meadow During the Growing Season on the Qinghai-Tibetan Plateau

Abstract: In the present study, we used the eddy covariance method to measure CO₂ exchange between the atmosphere and an alpine shrubland meadow ecosystem (37°36'N, 101°18'E; 3 250 m a.s.l.) on the Qinghai-Tibetan Plateau, China, during the growing season in 2003, from 20 April to 30 September. This meadow is dominated by formations of Potentilla fruticosa L. The soil is Mol-Cryic Cambisols. During the study period, the meadow was not grazed. The maximum rates of CO₂ uptake and release derived from the diurnal course of CO₂ flux were -9.38 and 5.02 μmol·m^-2·s^-1, respectively. The largest daily CO₂ uptake was 1.7 g C·m^-2·d^-1 on 14 July, which is less than half that of an alpine Kobresia meadow ecosystem at similar latitudes. Daily CO₂ uptake during the measurement period indicated that the alpine shrubland meadow ecosystem may behave as a sink of atmospheric CO₂ during the growing season. The daytime CO₂ uptake was correlated exponentially or linearly with the daily photo synthetic photon flux density each month. The daytime average water use efficiency of the ecosystem was 6.47 mg CO₂/g H₂O. The efficiency of the ecosystem increased with a decrease in vapor pressure deficit.
(Managing editor: Ya-Qin HAN)     

Elevated CO2 enhances stomatal responses to osmotic stress and abscisic acid in Arabidopsis thaliana

A , carbon assimilation rate
ABA, abscisic acid
Ci , intercellular space CO₂ concentration
g , leaf conductance
WUE, water use efficiency

Carbon dioxide and abscisic acid (ABA) are two major signals triggering stomatal closure. Their putative interaction in stomatal regulation was investigated in well-watered air-grown or double CO₂-grown Arabidopsis thaliana plants, using gas exchange and epidermal strip experiments. With plants grown in normal air, a doubling of the CO₂ concentration resulted in a rapid and transient drop in leaf conductance followed by recovery to the pre-treatment level after about two photoperiods. Despite the fact that plants placed in air or in double CO₂ for 2 d exhibited similar levels of leaf conductance, their stomatal responses to an osmotic stress (0·16–0·24 MPa) were different. The decrease in leaf conductance in response to the osmotic stress was strongly enhanced at elevated CO₂. Similarly, the drop in leaf conductance triggered by 1 μ M ABA applied at the root level was stronger at double CO₂. Identical experiments were performed with plants fully grown at double CO₂. Levels of leaf conductance and carbon assimilation rate measured at double CO₂ were similar for air-grown and elevated CO₂-grown plants. An enhanced response to ABA was still observed at high CO₂ in pre-conditioned plants. It is concluded that: (i) in the absence of stress, elevated CO₂ slightly affects leaf conductance in A. thaliana ; (ii) there is a strong interaction in stomatal responses to CO₂ and ABA which is not modified by growth at elevated CO₂.     

Non-stomatal limitation of CO2 assimilation in three tree species during natural drought conditions

The effect of drought on CO₂ assimilation and leaf conductance was studied in three northern hardwood species: Quercus rubra L., Acer rubrum L. and Populus grandidentata Michx. Leaf gas exchange characteristics at two CO₂ levels (320 and 620 μl I⁻¹) and temperatures from 20 to 35°C were measured at the end of a dry period and shortly after 10 cm of rainfall. The effects of drought varied with species, temperature and CO₂ level. Calculated values of internal CO₂ concentration showed little or no decline during drought. Differences in assimilation, before vs after the rains, were most apparent at the higher CO₂ level. These latter two observations indicate nonstomatal disruption of CO₂ assimilation during the dry period. In P. grandidentata there was a substantial interaction between drought and temperature, with a resultant shift in the temperature for maximum assimilation to lower temperatures during drought. During drought, internal CO₂ concentrations increased sharply in all three species under the combined conditions of high temperatures and the higher CO₂ level.     

Photosynthesis, carbohydrate levels and chlorophyll fluorescence-estimated intercellular CO2 in water-stressed Casuarina equisetifolia Forst. & Forst.

The effect of long-term water stress on photosynthetic carbon metabolism in Casuarina equisetifolia Forst. & Forst. was analysed by measuring CO₂ assimilation, stomatal conductance, the quantum yield of photosystem II ( Φ _PSII), enzyme activities, and the levels of photosynthetic intermediates and carbohydrates. CO₂ assimilation decreased under water stress while the intercellular CO₂ concentration ( C _i) as estimated by gas exchange measurements remained high. However, the estimates of C _i from measurements of Φ _PSII suggest that the decrease in photosynthesis can be explained in terms of stomatal closure. Water stress decreased total stromal fructose-1,6-bisphosphatase activity and did not alter the activities and activation states of ribulose bisphosphate carboxylase oxygenase and NADP-dependent malate dehydrogenase (NADP-MDH). The concentration of photosynthetic metabolites, glucose, fructose and sucrose decreased, whereas starch concentrations increased under drought conditions.     

Soil drying and nitrogen availability modulate carbon and water exchange over a range of annual precipitation totals and grassland vegetation types

Increased intensity in precipitation events and longer periods of water deficit are predicted as a general trend under future climate scenarios with potentially large effects on terrestrial ecosystem function. The primary objective of this study was to understand how variation in the intensity of precipitation inputs followed by intermittent soil drying events influence leaf and ecosystem carbon dioxide (CO₂) and water exchange in a California annual grassland mesocosm experiment. We further examined how nitrogen (N) availability, and differences in plant community composition (grass-forb combinations) affected gas exchange responses to the precipitation treatments. Net ecosystem CO₂ exchange (NEE) and evapotranspiration (ET) increased significantly with greater precipitation and were positively correlated with soil moisture. A repeated 10-day soil drying period, following 11 days of watering, strongly depressed NEE over a range of annual precipitation totals (297, 657 and 987 mm), and plant community types. Ecosystem dark respiration ( R _e) and leaf level photosynthesis ( A _max) showed greater sensitivity to periods of soil drying in the low precipitation plots (297 mm). N additions significantly increased NEE and R _e, particularly as water availability was increased. Across the range of precipitation totals and plant community types, intermittent periods of soil moisture deficit and native soil N availability constrained leaf and ecosystem level CO₂ exchange, while the influence on water vapor exchange was less pronounced.     

Rhythms of passive and active ventilation, and circulation recorded in diapausing pupae of Mamestra brassicae using constant volume respirometry

Abstract.  The periodically occurring convective inflow of air into the tracheal system, or passive suction ventilation, together with the cyclic bursts of release of CO₂ and active ventilation, is recorded in diapausing pupae of Mamestra brassicae . A constant volume respirometer combined with an opto-cardiograph-actograph is used. In all pupae with a metabolic rate of 0.025–0.054 mL g⁻¹ h⁻¹, the bouts of almost imperceptible abdominal contractions are recorded during the bursts of carbon dioxide release and this mode of active ventilation is qualified as extracardiac haemocoelic pulsations. The pupae whose metabolic rate is 0.052–0.075 mL⁻¹ g⁻¹ h⁻¹ show more vigorous abdominal contractions. The results demonstrate that, in diapausing pupae, characterized with low metabolic rates, both passive suction ventilation, referred to also as passive suction inspiration, and active ventilation occurs. In approximately 50% of the pupae, each gas exchange microcycle during the interburst periods begins with a miniature PSI followed by a microburst of CO₂ release; in approximately 30% of the individuals, passive suction inspirations occur separately from CO₂ microbursts; in the remaining pupae, miniature ones without microbursts of CO₂ are recorded. A typical event is heartbeat reversion: in longer periods, the heart peristalses are directed forward (anterograde of heartbeat) and, in shorter periods, the heart peristalses are directed backward (retrograde of heartbeat). At 0 °C, the cyclic release of CO₂ and miniature passive suction inspirations during the interburst periods are preserved at lower frequencies but active ventilation is lost.     

Modelling environmental controls on ecosystem photosynthesis and the carbon isotope composition of ecosystem-respired CO2 in a coastal Douglas-fir forest

We developed and applied an ecosystem-scale model that calculated leaf CO₂ assimilation, stomatal conductance, chloroplast CO₂ concentration and the carbon isotope composition of carbohydrate formed during photosynthesis separately for sunlit and shaded leaves within multiple canopy layers. The ecosystem photosynthesis model was validated by comparison to leaf-level gas exchange measurements and estimates of ecosystem-scale photosynthesis from eddy covariance measurements made in a coastal Douglas-fir forest on Vancouver Island. A good agreement was also observed between modelled and measured δ ¹³C values of ecosystem-respired CO₂ ( δ _R). The modelled δ _R values showed strong responses to variation in photosynthetic photon flux density (PPFD), air temperature, vapour pressure deficit (VPD) and available soil moisture in a manner consistent with leaf-level studies of photosynthetic ¹³C discrimination. Sensitivity tests were conducted to evaluate the effect of (1) changes in the lag between the time of CO₂ fixation and the conversion of organic matter back to CO₂; (2) shifts in the proportion of autotrophic and heterotrophic respiration; (3) isotope fractionation during respiration; and (4) environmentally induced changes in mesophyll conductance, on modelled δ _R values. Our results indicated that δ _R is a good proxy for canopy-level C _c/ C _a and ¹³C discrimination during photosynthetic gas exchange, and therefore has several applications in ecosystem physiology.     

Adaptation to high CO2 concentration in an optimal environment: radiation capture, canopy quantum yield and carbon use efficiency

The effect of elevated [CO₂] on wheat (Triticum aestivum L. Veery 10) productivity was examined by analysing radiation capture, canopy quantum yield, canopy carbon use efficiency, harvest index and daily C gain. Canopies were grown at either 330 or 1200 μ mol mol^–1[CO₂] in controlled environments, where root and shoot C fluxes were monitored continuously from emergence to harvest. A rapidly circulating hydroponic solution supplied nutrients, water and root zone oxygen. At harvest, dry mass predicted from gas exchange data was 102·8 ± 4·7% of the observed dry mass in six trials. Neither radiation capture efficiency nor carbon use efficiency were affected by elevated [CO₂], but yield increased by 13% due to a sustained increase in canopy quantum yield. CO₂ enrichment increased root mass, tiller number and seed mass. Harvest index and chlorophyll concentration were unchanged, but CO₂ enrichment increased average life cycle net photosynthesis (13%, P < 0·05) and root respiration (24%, P < 0·05). These data indicate that plant communities adapt to CO₂ enrichment through changes in C allocation. Elevated [CO₂] increases sink strength in optimal environments, resulting in sustained increases in photosynthetic capacity, canopy quantum yield and daily C gain throughout the life cycle.     

Direct mass spectrometric measurement of gases in peat cores

Abstract Dissolved gas concentrations (O₂, CH₄, CO₂) in peat cores were monitored simultaneously using a fine (1.56 mm diameter) membrane inlet probe connected to a quadrupole mass spectrometer. This technique allows direct measurements at specific locations within the sample with minimal disturbance. Detailed gas profiles in completely waterlogged peat samples (hollows) and samples in which the water table was several cm below the vegetation surface (hummocks) were compared. The depth of the water table played a central role in the distribution of gases. In a hollow, oxygen was present (90 μM) at the surface but was not detectable (<0.5 μM) at depths greater than 2 cm. Concentrations of CH₄ and CO₂ increased from 6 and 300 μM respectively at the surface to maxima of 450 and 3900 μM at 13 cm depth. At a hummock, O₂ and CO₂ were present above the water table but CH₄ was not detectable. CH₄ was measurable 2 cm below the water table. Both CH₄ and CO₂ concentrations increased with depth but maxima were not attained in the sampled cores.     

Photosynthesis of Ulva fasciata. IV. pH, carbonic anhydrase and inorganic carbon conversions in the unstirred layer

Abstract. The common marine macroalga Ulva was found to have a surface pH of about 10 during photosynthesis. Under such a condition, the equilibrium CO₂ concentration within the unstirred layer would be below reported CO₂ compensation points, and dehydration of HCO₃ could not occur. Even at a compensation point approaching zero, uncatalysed rates of HCO₃ to CO₂ conversion within the unstirred layer volume could not support photosynthetic rates as measured under stirred conditions in the presence of a carbonic anhydrase inhibitor. Based on this, it is concluded that Ulva takes up HCO₃. It is likely that HCO₃ uptake leads to high internal CO₂ levels which, in turn, suppress photorespiration and thus cause this plant's efficient gas exchange features. Carbonic anhydrase activity was measurable only in plant extracts. However, inhibitor studies suggest the presence of a surface enzyme. The possible functions of extracellular carbonic anhydrase in Ulva are assessed in terms of CO₂ hydration during emergence and a possible HCO₃, transport system.     

Large annual net ecosystem CO2 uptake of a Mojave Desert ecosystem

The net ecosystem CO₂ exchange (NEE) between a Mojave Desert ecosystem and the atmosphere was measured over the course of 2 years at the Mojave Global Change Facility (MGCF, Nevada, USA) using the eddy covariance method. The investigated desert ecosystem was a sink for CO₂, taking up 102±67 and 110±70 g C m⁻² during 2005 and 2006, respectively. A comprehensive uncertainty analysis showed that most of the uncertainty of the inferred sink strength was due to the need to account for the effects of air density fluctuations on CO₂ densities measured with an open-path infrared gas analyser. In order to keep this uncertainty within acceptable bounds, highest standards with regard to maintenance of instrumentation and flux measurement postprocessing have to be met. Most of the variability in half-hourly NEE was explained by the amount of incident photosynthetically active radiation (PAR). On a seasonal scale, PAR and soil water content were the most important determinants of NEE. Precipitation events resulted in an initial pulse of CO₂ to the atmosphere, temporarily reducing NEE or even causing it to switch sign. During summer, when soil moisture was low, a lag of 3–4 days was observed before the correlation between NEE and precipitation switched from positive to negative, as opposed to conditions of high soil water availability in spring, when this transition occurred within the same day the rain took place. Our results indicate that desert ecosystem CO₂ exchange may be playing a much larger role in global carbon cycling and in modulating atmospheric CO₂ levels than previously assumed – especially since arid and semiarid biomes make up >30% of Earth's land surface.     

Growth in elevated CO2 enhances temperature response of photosynthesis in wheat

The temperature dependence of C₃ photosynthesis may be altered by the growth environment. The effects of long-term growth in elevated CO₂ on photosynthesis temperature response have been investigated in wheat ( Triticum aestivum L.) grown in controlled chambers with 370 or 700 μmol mol⁻¹ CO₂ from sowing through to anthesis. Gas exchange was measured in flag leaves at ear emergence, and the parameters of a biochemical photosynthesis model were determined along with their temperature responses. Elevated CO₂ slightly decreased the CO₂ compensation point and increased the rate of respiration in the light and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) V_cmax, although the latter effect was reversed at 15°C. With elevated CO₂, J_max decreased in the 15–25°C temperature range and increased at 30 and 35°C. The temperature response (activation energy) of V_cmax and J_max increased with growth in elevated CO₂. CO₂ enrichment decreased the ribulose 1,5-bisphosphate (RuBP)-limited photosynthesis rates at lower temperatures and increased Rubisco- and RuBP-limited rates at higher temperatures. The results show that the photosynthesis temperature response is enhanced by growth in elevated CO₂. We conclude that if temperature acclimation and factors such as nutrients or water availability do not modify or negate this enhancement, the effects of future increases in air CO₂ on photosynthetic electron transport and Rubisco kinetics may improve the photosynthetic response of wheat to global warming.