Inhibition of glycine decarboxylation and serine formation in tobacco by glycine hydroxamate and its effect on photorespiratory carbon flow

Glycine hydroxamate is a competitive inhibitor of glycine decarboxylation and serine formation (referred to as glycine decarboxylase activity) in particulate preparations obtained from both callus and leaf tissue of tobacco. In preparations from tobacco callus tissues, the K_i for glycine hydroxamate was 0.24 ± 0.03 millimolar and the K_m for glycine was 5.0 ± 0.5 millimolar. The inhibitor was chemically stable during assays of glycine decarboxylase activity, but reacted strongly when incubated with glyoxylate. Glycine hydroxamate blocked the conversion of glycine to serine and CO₂in vivo when callus tissue incorporated and metabolized [1-¹⁴C]glycine, [1-¹⁴C]glycolate, or [1-¹⁴C]glyoxylate. The hydroxamate had no effect on glyoxylate aminotransferase activities in vivo, and the nonenzymic reaction between glycine hydroxamate and glyoxylate did not affect the flow of carbon in the glycolate pathway in vivo. Glycine hydroxamate is the first known reversible inhibitor of the photorespiratory conversion of glycine to serine and CO₂.     

Mitochondrial Complex I Impairment and Nitrate Reductase Activity in Leaves of Nicotiana sylvestris

Loss of major mitochondrial complex I subunit Nad 7 in the leaves of CMS II mutant of Nicotiana sylvestris caused increase in both in vivo and in vitro nitrate reductase activities and the per cent activation state of NR was also higher in CMS II as against wild type. The differences suggest a possibility for export of mitochondrial NADH due to impairment of complex I, a major and high affinity sink for NADH. Loss of complex I subunit also resulted in constitutive expression of alternate oxidase (Aox) thereby providing a mechanism for continuous supply of carbon skeletons required for nitrate assimilation. The possibility of close coordination between mitochondrial redox perturbation and nitrate reduction and its further assimilation is discussed.     

Evaluation of photosynthetic electron flow using simultaneous measurements of gas exchange and chlorophyll fluorescence under photorespiratory conditions

Simultaneous measurements of leaf gas exchange and chlorophyll fluorescence for Koelreuteria paniculata Laxm. at 380 ± 5.6 and 600 ± 8.5 ??mol mol^?1 were conducted, and the photosynthetic electron flow via photosystem II (PSII) to photosynthesis, photorespiration, and other electron-consuming processes were calculated. The results showed that the photosynthetic electron flow associated with carboxylation (J _c), oxygenation (J _o), and other electron-consuming processes (J _r) were 72.7, 45.7, and 29.4 ??mol(e^?) m^?2 s^?1 at 380 ??mol mol^?1, respectively; and 86.1, 35.3, and 48.2 ??mol(e^?) m^?2 s^?1 at 600 ??mol mol^?1, respectively. Our results revealed that other aspects associated with electronconsuming processes, except for photosynthesis and respiration, were neither negligible nor constant under photorespiratory conditions. Using maximum net photosynthetic rate (P _max), day respiration (R), photorespiration rate (R _l), and maximum electron flow via PSII (J _max), the use efficiency of electrons via PSII at saturation irradiance to fix CO₂ was calculated. The calculated results showed that the use efficiency of electrons via PSII to fix CO₂ at 600 ??mo^l mol^?1 was almost as effective as that at 380 ??mol mol^?1, even though more electrons passed through PSII at 600 ??mol mol^?1 than at 380 ??mol mol^?1.     

Combined effects of elevated CO2 and air temperature on carbon assimilation of Pinus taeda trees

Branches of 22-year-old loblolly pine (Pinus taeda, L.) trees growing in a plantation were exposed to ambient CO₂, ambient + 165 μmol mol^?1 CO₂ or ambient + 330 μmol mol^?1 CO₂ concentrations in combination with ambient or ambient + 2°C air temperatures for 3 years. Field measurements in the third year indicated that net carbon assimilation was enhanced in the elevated CO₂ treatments in all seasons. On the basis of A/C_i, curves, there was no indication of photosynthetic down-regulation. Branch growth and leaf area also increased significantly in the elevated CO₂ treatments. The imposed 2°C increase in air temperature only had slight effects on net assimilation and growth. Compared with the ambient CO₂ treatment, rates of net assimilation were ～1·6 times greater in the ambient + 165 μmol mol^?1 CO₂ treatment and 2·2 times greater in the ambient + 330 μmol mol^?1 CO₂ treatment. These ratios did not change appreciably in measurements made in all four seasons even though mean ambient air temperatures during the measurement periods ranged from 12·6 to 28·2°C. This indicated that the effect of elevated CO₂ concentrations on net assimilation under field conditions was primarily additive. The results also indicated that the effect of elevated CO₂ (+ 165 or + 330 μmol mol^?1) was much greater than the effect of a 2°C increase in air temperature on net assimilation and growth in this species.     

Glycine supports in vivo reduction of nitrate in barley leaves

Glycine, a photorespiratory intermediate, enhanced the in vivo reduction of nitrate in barley (Hordeum vulgare L.) leaf slices, when included in the assay medium. Isonicotinyl hydrazide, an inhibitor of glycine oxidation, partially reduced NO₂⁻ production. The enhancement caused by glycine treatment was reversed by isonicotinyl hydrazide when both were present together in the medium. Similar effects were observed when the excised leaves were preincubated with the metabolite and the inhibitor. Glycine also partially relieved the inhibition of nitrate reduction caused by malonate, an inhibitor of the tricarboxylic acid cycle. The results support the hypothesis that glycine decarboxylation activity is a source of NADH for nitrate reductase activity.     

A field study of the effects of elevated CO2 on carbon assimilation, stomatal conductance and leaf and branch growth of Pinus taeda trees

A study was conducted in 21-year-old loblolly pine (Pinus taeda L.) trees growing in plantation in north central Georgia, USA. The experiment used branch chambers to impose treatments of ambient, ambient +165 and ambient + 330 μmol mol^?1 CO₂. After one growing season there was no indication of acclimation to elevated CO₂. In August and September, carbon assimilation, measured by two different methods, was twice as high at ambient +330 μmol mol^?1 CO₂ than at ambient. Dark respiration was suppressed by 6% at ambient +165 and by 14% at ambient + 330 μmol mol^?1 CO₂. This suppression was immediate, and not an effect of exposure to elevated CO₂ during growth, since respiration was reduced by the same amount in all treatments when measured at a high CO₂ concentration. Elevated CO₂ increased the growth of foliage and woody tissue. It also increased instantaneous transpiration efficiency, but it had no effect on stomatal conductance. Since the soil at the study site had low to moderate fertility, these results suggest that the growth potential of forests on many sites may be enhanced by global increases in atmospheric CO₂, concentration.     

CO2 uptake and fluorescence responses for a shade-tolerant cactus Hylocereus undatus under current and doubled CO2 concentrations

Hylocereus undatus (Haworth) Britton and Rose growing in controlled environment chambers at 370 and 740 μmol CO₂ mol^?1 air showed a Crassulacean acid metabolism (CAM) pattern of CO₂ uptake, with 34% more total daily CO₂ uptake under the doubled CO₂ concentration and most of the increase occurring in the late afternoon. For both CO₂ concentrations, 90% of the maximal daily CO₂ uptake occurred at a total daily photosynthetic photon flux density (PPFD) of only 10 mol m^?2 day^?1 and the best day/night air temperatures were 25/15°C. Enhancement of the daily net CO₂ uptake by doubling the CO₂ concentration was greater under the highest PPFD (30 mol m^?2 day^?1) and extreme day/night air temperatures (15/5 and 45/35°C). After 24 days of drought, daily CO₂ uptake under 370 μmol CO₂ mol^?1 was 25% of that under 740 μmol CO₂ mol^?1. The ratio of variable to maximal chlorophyll fluorescence (F_y/F_m) decreased as the PPFD was raised above 5 mol m^?2 day^?1, at extreme day/night temperatures and during drought, suggesting that stress occurred under these conditions. F_v/F_m was higher under the doubled CO₂ concentration, indicating that the current CO₂ concentration was apparently limiting for photosynthesis. Thus net CO₂ uptake by the shade-tolerant H. undatus, the photosynthetic efficiency of which was greatest at low PPFDs. showed a positive response to doubling the CO₂ concentration, especially under stressful environmental conditions.     

Consequence of Absence of Nitrate Reductase Activity on Photosynthesis in Nicotiana plumbaginifolia Plants

Chlorate-resistant Nicotiana plumbaginifolia (cv Viviani) mutants were found to be deficient in the nitrate reductase apoprotein (NR⁻nia). Because they could not grow with nitrate as sole nitrogen source, they were cultivated as graftings on wild-type Nicotiana tabacum plants. The grafts of mutant plants were chlorotic compared to the grafts of wild type. Mutant leaves did not accumulate nitrogen and nitrate but contained less malate and more glutamine than wild leaves. They exhibited a slight increase of the proportion of the light-harvesting chlorophyll a/b protein complexes and a lowering of the efficiency of energy transfer between these complexes and the active centers. After a 3 second ¹⁴CO₂ pulse, the total ¹⁴C incorporation of the mutant leaves was approximately 20% of that of the control. The ¹⁴C was essentially recovered in ribulose bisphosphate in these plants. It was consistent with a decline of ribulose bisphosphate carboxylase activity observed in the mutant. After a 3 second ¹⁴CO₂ pulse followed by a 60 second chase with normal CO₂, ¹⁴C was mainly accumulated in starch which was labeled more in the mutant than in the wild type. These results confirm the observation that in the nitrate reductase deficient leaves, chloroplasts were loaded with large starch inclusions preceding disorganization of the photosynthetic apparatus.     

The influence of elevated CO2 and temperature on biomass production of continuously defoliated white clover

Clonal plants of white clover (Trifolium repens L.), grown singly in pots of Perlite and solely dependent for nitrogen on root nodule N₂ fixation, were maintained in controlled environments which provided four environments: 18/13 °C day/night temperature at 340 and 680 μmol mol^?1 CO₂ and 20·5/15·5°C day/night temperature at 340 and 680 μmol mol^?1 CO₂. The daylength was 12 h and the photon flux density 500±25 μmol m^?2 s^?1 (PFD). All plants were defoliated for about 80d, nominally every alternate day, to leave the youngest expanded leaf intact on 50% of stolons, plus expanding leaves (simulated grazing). Elevated CO₂ increased the yield of biomass removed at defoliation by a constant 45% during the second 40d of the experiment and by a varying amount in the first half of the experiment. Elevated temperature had little effect on biomass yield. Nitrogen, as a proportion of the harvested biomass, was only fractionally affected by elevated CO₂ or temperature. In contrast, N₂ fixation increased in concert with the promoting effect of elevated CO₂ on biomass production. The increased yield of biomass harvested in 680 μmol mol^?1 CO₂ was primarily due to the early development and continued maintenance of more stolons. However, the stolons of plants grown in elevated CO₂ also developed leaves which were heavier and slightly larger in area than their counterparts in ambient CO₂. The conclusion is that, when white clover plants are maintained at constant mass by simulated grazing, they continue to respond to elevated CO₂ in terms of a sustained increase in biomass production.     

Direct and acclimatory responses of stomatal conductance to elevated carbon dioxide in four herbaceous crop species in the field

In order to separate the net effect of growth at elevated [CO₂] on stomatal conductance (g_s) into direct and acclimatory responses, mid‐day values of g_s were measured for plants grown in field plots in open‐topped chambers at the current ambient [CO₂], which averaged 350 μmol mol^?1 in the daytime, and at ambient + 350 μmol mol^?1[CO₂] for winter wheat, winter barley, potato and sorghum. The acclimatory response was determined by comparing g_s measured at 700 μmol mol^?1[CO₂] for plants grown at the two [CO₂]. The direct effect of increasing [CO₂] from 350 to 700 μmol mol^?1 was determined for plants grown at the lower concentration. Photosynthetic rates were measured concurrently with g_s. For all species, growth at the higher [CO₂] significantly reduced g_s measured at 700 μmol mol^?1[CO₂]. The reduction in g_s caused by growth at the higher [CO₂] was larger for all species on days with low leaf to air water vapour pressure difference for a given temperature, which coincided with highest conductances and also the smallest direct effects of increased [CO₂] on conductance. For barley, there was no other evidence for stomatal acclimation, despite consistent down‐regulation of photosynthetic rate in plants grown at the higher [CO₂]. In wheat and potato, in addition to the vapour pressure difference interaction, the magnitude of stomatal acclimation varied directly in proportion to the magnitude of down‐regulation of photosynthetic rate through the season. In sorghum, g_s consistently exhibited acclimation, but there was no down‐regulation of photosynthetic rate. In none of the species except barley was the direct effect the larger component of the net reduction in g_s when averaged over measurement dates. The net effect of growth at elevated [CO₂] on mid‐day g_s resulted from unique combinations of direct and acclimatory responses in the various species.     

Effects of atmospheric CO2 enrichment and root restriction on leaf gas exchange and growth of banana (Musa)

The effects of atmospheric CO₂ enrichment and root restriction on photosynthetic characteristics and growth of banana (Musa sp. AAA cv. Gros Michel) plants were investigated. Plants were grown aeroponically in root chambers in controlled environment glasshouse rooms at CO₂ concentrations of 350 or 1 000 μmol CO₂ mol^-1. At each CO₂ concentration, plants were grown in large (2001) root chambers that did not restrict root growth or in small (20 1) root chambers that restricted root growth. Plants grown at 350 μmol CO₂ mol^-1 generally had a higher carboxylation efficiency than plants grown at 1 000 μmol CO₂ mol^-1 although actual net CO₂ assimilation (A) was higher at the higher ambient CO₂ concentration due to increased intercellular CO₂ concentrations (C_i resulting from CO₂ enrichment. Thus, plants grown at 1 000 μmol CO₂ mol^-1 accumulated more leaf area and dry weight than plants grown at 350 μmol CO₂ mol^-1. Plants grown in the large root chambers were more photosynthetically efficient than plants grown in the small root chambers. At 350 μmol CO₂ mol^-1, leaf area and dry weights of plant organs were generally greater for plants in the large root chambers compared to those in the small root chambers. Atmospheric CO₂ enrichment may have compensated for the effects of root restriction on plant growth since at 1 000 μmol CO₂ mol^-1 there was generally no effect of root chamber size on plant dry weight.     

Purification and properties of human erythrocyte membrane NADH-cytochrome b5 reductase

An NADH:(acceptor) oxidoreductase (EC 1.6.99.3) of human erythrocyte membrane was purified by DEAE-cellulose anion exchange, hydroxyapatite adsorption, and 5′-ADP-hexane-agarose affinity chromatographies after solubilization with Triton X-100. The purified reductase preparation was homogeneous and estimated to have an apparent molecular weight of 36,000 on SDS-polyacrylamide slab gel electrophoresis and of 144,000 on Sephadex G-200 gel filtration in the presence of 0.2% Triton X-100, whereas a soluble NADH-cytochrome b₅ reductase of human erythrocyte had a molecular weight of 32,000 by both methods, indicating the existence of a distinct membrane reductase. Digestion of the membrane reductase with cathepsin D yielded a new polypeptide chain which gave the same relative mobility as the soluble reductase on SDS-polyacrylamide slab gel electrophoresis. The membrane enzyme, the cathepsin-digested enzyme, and the soluble enzyme all cross-reacted with the antibody to rat liver microsomal NADH-cytochrome b₅ reductase. The enzyme had one mole FAD per 36,000 as a prosthetic group and could reduce K₃Fe(CN)₆, 2,6-dichlorophenolindophenol, cytochrome c, methemoglobin-ferrocyanide complex, cytochrome b₅ and methemoglobin via cytochrome b₅ when NADH was used as an electron donor. NADPH was less effective as an electron donor than NADH. The specific activity of the purified enzyme was 790 μmol ferricyanide reduced min^?1 mg^?1 and the turnover number was 40,600 mol ferricyanide reduced min^?1 mol^?1 FAD at 25 °C. The apparent K_m values for NADH and cytochrome b₅ were 0.6 and 20 μm, respectively, and the apparent V value was 270 μmol cytochrome b₅ reduced min^?1 mg^?1. These kinetic properties were similar to those of the soluble NADH-cytochrome b₅ reductase. The results indicate that the NADH:(acceptor) oxidoreductase of human erythrocyte membrane could be characterized as a membrane NADH-cytochrome b₅ reductase.     

Effects of chronic ozone and elevated atmospheric CO2 concentrations on ribulose-1,5-bisphosphate in soybean (Glycine max)

Ribulose-1,5-bisphosphate (RuBP) pool size was determined at regular intervals during the growing season to understand the effects of tropospheric ozone concentrations, elevated atmospheric carbon dioxide concentrations and their interactions on the photosynthetic limitation by RuBP regeneration. Soybean (Glycine max [L.] Merr. cv. Essex) was grown from seed to maturity in open-top field chambers in charcoal-filtered air (CF) either without (22 nmol O₃ mol^?1) or with added O₃ (83 nmol mol^?1) at ambient (AA, 369 μmol CO₂ mol^?1) or elevated CO₂ (710 μmol mol^?1). The RuBP pool size generally declined with plant age in all treatments when expressed on a unit leaf area and in all treatments but CF-AA when expressed per unit ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) binding site. Although O₃ in ambient CO₂ generally reduced the RuBP pool per unit leaf area, it did not change the RuBP pool per unit Rubisco binding site. Elevated CO₂, in CF or O₃-fumigated air, generally had no significant effect on RuBP pool size, thus mitigating the negative O₃ effect. The RuBP pools were below 2 mol mol^?1 binding site in all treatments for most of the season, indicating limiting RuBP regeneration capacity. These low RuBP pools resulted in increased RuBP regeneration via faster RuBP turnover, but only in CF air and during vegetative and flowering stages at elevated CO₂. Also, the low RuBP pool sizes did not always reflect RuBP consumption rates or the RuBP regeneration limitation relative to potential carboxylation (%RuBP). Rather, %RuBP increased linearly with decrease in the RuBP pool turnover time. These data suggest that amelioration of damage from O₃ by elevated atmospheric CO₂ to the RuBP regeneration may be in response to changes in the Rubisco carboxylation.     

Effects of climate change on productivity of cereals and legumes; model evaluation of observed year-to-year variability of the CO2 response

The effect of elevated [CO₂] on the productivity of spring wheat, winter wheat and faba bean was studied in experiments in climatized crop enclosures in the Wageningen Rhizolab in 1991–93. Simulation models for crop growth were used to explore possible causes for the observed differences in the CO₂ response. Measurements of the canopy gas exchange (CO₂ and water vapour) were made continuously from emergence until harvest. At an external [CO₂] of 700 μmol mol^?1 Maximum Canopy CO₂ Exchange Rate (CCERmax) at canopy closure was stimulated by 51% for spring wheat and by 71% for faba bean. At the end of the growing season, above ground biomass increase at 700 μmol mol^?1 was 58% (faba bean), 35% (spring wheat) and 19% (winter wheat) and the harvest index did not change. For model exploration, weather data sets for the period 1975-88 and 1991–93 were used, assuming adequate water supply and [CO₂] at 350 and 700 μmol mol^?1. For spring wheat the simulated responses (35–50%) were at the upper end of the experimental results. In agreement with experiments, simulations showed smaller responses for winter wheat and larger responses for faba bean. Further model explorations showed that this differential effect in the CO₂ response may not be primarily due to fundamental physiological differences between the crops, but may be at least partly due to differences in the daily air temperatures during comparable stages of growth of these crops. Simulations also showed that variations between years in CO₂ response can be largely explained by differences in weather conditions (especially temperature) between growing seasons.     

Control of steady-state photosynthesis in sunflowers growing in enhanced CO2

Control coefficients were used to describe the degree to which ribulose bisphosphate carboxylase/oxygenase (Rubisco) limits the steady-state rate of CO₂ assimilation in sunflower leaves from plants grown at high (800 μmol mol⁻¹) and low (350 μmol mol⁻¹) CO₂. The magnitude of a control coefficient is approximately the percentage change in the flux that would result from a 1% rise in enzyme active site concentration. In plants grown at low CO₂, leaves of different ages varied considerably in their photosynthetic capacities. In a saturating light flux and an ambient CO₂ concentration of 350 μmol mol⁻¹, the Rubisco control coefficient was about 0.7 in all leaves, indicating that Rubisco activity largely limited the assimilation flux. The Rubisco control coefficient for leaves grown at 350 μmol mol⁻¹ CO₂ dropped to about zero when the ambient CO₂ concentration was raised to 800 μmol mol⁻¹. In relatively young, fully expanded leaves of plants grown at high CO₂, the Rubisco control coefficient was also about 0.7 at a saturating light flux and at the CO₂ concentration at which the plants were grown (800 μmol mol⁻¹). This apparently resulted from a decrease in the concentration of Rubisco active sites. In older leaves, however, the control coefficient was about 0.2. Because, on the whole, Rubisco activity still largely limits the assimilation flux in plants grown at high CO₂, the kinetics of this enzyme can still be used to model photosynthesis under these conditions. The relatively high Rubisco control coefficient under enhanced CO₂ indicates that the young sunflower leaves have the capacity to acclimate their photosynthetic biochemistry in a way consistent with an optimal use of protein resources.     

Interaction between atmospheric CO2 concentration and water deficit on gas exchange and crop growth: testing of ecosys with data from the Free Air CO2 Enrichment (FACE) experiment

Soil water deficits are likely to influence the response of crop growth and yield to changes in atmospheric CO₂ concentrations (C_a), but the extent of this influence is uncertain. To study the interaction of water deficits and C_a on crop growth, the ecosystem simulation model ecosys was tested with data for diurnal gas exchange and seasonal wheat growth measured during 1993 under high and low irrigation at C_a= 370 and 550 μmol mol^?1 in the Free Air CO₂ Enrichment (FACE) experiment near Phoenix, AZ. The model, supported by the data from canopy gas exchange enclosures, indicated that under high irrigation canopy conductance (g_c) at C_a= 550 μmol mol^?1 was reduced to about 0.75 that at C_a= 370 μmol mol^?1, but that under low irrigation, g_c was reduced less. Consequently when C_a was increased from 370 to 550 μmol mol^?1, canopy transpiration was reduced less, and net CO₂ fixation was increased more, under low irrigation than under high irrigation. The simulated effects of C_a and irrigation on diurnal gas exchange were also apparent on seasonal water use and grain yield. Simulated vs. measured seasonal water use by wheat under high irrigation was reduced by 6% vs. 4% at C_a= 550 vs. 370 μmol mol^?1 but that under low irrigation was increased by 3% vs. 5%. Simulated vs. measured grain yield of wheat under high irrigation was increased by 16% vs. 8%, but that under low irrigation was increased by 38% vs. 21%. In ecosys, the interaction between C_a and irrigation on diurnal gas exchange, and hence on seasonal crop growth and water use, was attributed to a convergence of simulated g_c towards common values under both C_a as canopy turgor declined. This convergence caused transpiration to decrease comparatively less, but CO₂ fixation to increase comparatively more, under high vs. low C_a. Convergence of g_c was in turn attributed to improved turgor maintenance under elevated C_a caused by greater storage C concentrations in the leaves, and by greater rooting density in the soil.     

Does elevated CO2 protect photosynthesis from damage by high temperature via modifying leaf water status in maize seedlings?

We hypothesized that decreased stomatal conductance (g _s) at elevated CO₂ might decrease transpiration (E), increase leaf water potential (Ψ_W), and thereby protect net photosynthesis rate (P _N) from heat damage in maize (Zea mays L) seedlings. To separate long-term effects of elevated CO₂, plants grew at either ambient CO₂ or elevated CO₂. During high-temperature treatment (HT) at 45°C for 15 min, leaves were exposed either to ambient CO₂ (380 μmol mol^?1) or to elevated CO₂ (560 μmol mol^?1). HT reduced P _N by 25 to 38% across four CO₂ combinations. However, the g _s and E did not differ among all CO₂ treatments during HT. After returning the leaf temperature to 35°C within 30 min, g _s and E were the same or higher than the initial values. Leaf water potential (Ψ_W) was slightly lower at ambient CO₂, but not at elevated CO₂. This study highlighted that elevated CO₂ failed in protecting P _N from 45°C via decreasing g _s and Ψ_W.     

The impact of recent increases in atmospheric CO2 on biomass production and vegetative retention of Cheatgrass (Bromus tectorum): implications for fire disturbance

Cheatgrass (Bromus tectorum) is a recognized, invasive annual weed of the western United States that reduces fire return times from decades to less than 5 years. To determine the interaction between rising carbon dioxide concentration ([CO₂]) and fuel load, we characterized potential changes in biomass accumulation, C : N ratio and digestibility of three cheatgrass populations from different elevations to recent and near‐term projections in atmospheric [CO₂]. The experimental CO₂ values (270, 320, 370, 420 μmol mol^?1) corresponded roughly to the CO₂ concentrations that existed at the beginning of the 19th century, that during the 1960s, the current [CO₂], and the near‐term [CO₂] projection for 2020, respectively. From 25 until 87 days after sowing (DAS), aboveground biomass for these different populations increased 1.5–2.7 g per plant for every 10 μmol mol^?1 increase above the 270 μmol mol^?1 preindustrial baseline. CO₂ sensitivity among populations varied with elevational origin with populations from the lowest elevation showing the greatest productivity. Among all populations, the undigestible portion of aboveground plant material (acid detergent fiber ADF, mostly cellulose and lignin) increased with increasing [CO₂]. In addition, the ratio of C : N increased with leaf age, with [CO₂] and was highest for the lower elevational population. These CO₂‐induced qualitative changes could, in turn, result in potential decreases in herbivory and decomposition with subsequent effects on the aboveground retention of cheatgrass biomass. Overall, these data suggest that increasing atmospheric [CO₂] above preambient levels may have contributed significantly to cheatgrass productivity and fuel load with subsequent effects on fire frequency and intensity.