Lower photorespiration in elevated CO2 reduces leaf N concentrations in mature Eucalyptus trees in the field

Rising atmospheric CO₂ concentrations is expected to stimulate photosynthesis and carbohydrate production, while inhibiting photorespiration. By contrast, nitrogen (N) concentrations in leaves generally tend to decline under elevated CO₂ (eCO₂), which may reduce the magnitude of photosynthetic enhancement. We tested two hypotheses as to why leaf N is reduced under eCO₂: (a) A “dilution effect” caused by increased concentration of leaf carbohydrates; and (b) inhibited nitrate assimilation caused by reduced supply of reductant from photorespiration under eCO₂. This second hypothesis is fully tested in the field for the first time here, using tall trees of a mature Eucalyptus forest exposed to Free‐Air CO₂ Enrichment (EucFACE) for five years. Fully expanded young and mature leaves were both measured for net photosynthesis, photorespiration, total leaf N, nitrate () concentrations, carbohydrates and reductase activity to test these hypotheses. Foliar N concentrations declined by 8% under eCO₂ in new leaves, while the fraction and total carbohydrate concentrations remained unchanged by CO₂ treatment for either new or mature leaves. Photorespiration decreased 31% under eCO₂ supplying less reductant, and in situ reductase activity was concurrently reduced (?34%) in eCO₂, especially in new leaves during summer periods. Hence, assimilation was inhibited in leaves of E. tereticornis and the evidence did not support a significant dilution effect as a contributor to the observed reductions in leaf N concentration. This finding suggests that the reduction of reductase activity due to lower photorespiration in eCO₂ can contribute to understanding how eCO₂‐induced photosynthetic enhancement may be lower than previously expected. We suggest that large‐scale vegetation models simulating effects of eCO₂ on N biogeochemistry include both mechanisms, especially where is major N source to the dominant vegetation and where leaf flushing and emergence occur in temperatures that promote high photorespiration rates.     

Yield response of field‐grown soybean exposed to heat waves under current and elevated [CO2]

Elevated atmospheric CO₂ concentration ([CO₂]) generally enhances C₃ plant productivity, whereas acute heat stress, which occurs during heat waves, generally elicits the opposite response. However, little is known about the interaction of these two variables, especially during key reproductive phases in important temperate food crops, such as soybean (Glycine max). Here, we grew soybean under elevated [CO₂] and imposed high‐ (+9°C) and low‐ (+5°C) intensity heat waves during key temperature‐sensitive reproductive stages (R1, flowering; R5, pod‐filling) to determine how elevated [CO₂] will interact with heat waves to influence soybean yield. High‐intensity heat waves, which resulted in canopy temperatures that exceeded optimal growth temperatures for soybean, reduced yield compared to ambient conditions even under elevated [CO₂]. This was largely due to heat stress on reproductive processes, especially during R5. Low‐intensity heat waves did not affect yields when applied during R1 but increased yields when applied during R5 likely due to relatively lower canopy temperatures and higher soil moisture, which uncoupled the negative effects of heating on cellular‐ and leaf‐level processes from plant‐level carbon assimilation. Modeling soybean yields based on carbon assimilation alone underestimated yield loss with high‐intensity heat waves and overestimated yield loss with low‐intensity heat waves, thus supporting the influence of direct heat stress on reproductive processes in determining yield. These results have implications for rain‐fed cropping systems and point toward a climatic tipping point for soybean yield when future heat waves exceed optimum temperature.     

Leaf temperature of soybean grown under elevated CO2 increases Aphis glycines (Hemiptera: Aphididae) population growth

Abstract Plants grown under elevated carbon dioxide (CO₂) experience physiological changes that influence their suitability as food for insects. To determine the effects of living on soybean (Glycine max Linnaeus) grown under elevated CO₂, population growth of the soybean aphid (Aphis glycines Matsumura) was determined at the SoyFACE research site at the University of Illinois, Urbana‐Champaign, Illinois, USA, grown under elevated (550 μL/L) and ambient (370 μL/L) levels of CO₂. Growth of aphid populations under elevated CO₂ was significantly greater after 1 week, with populations attaining twice the size of those on plants grown under ambient levels of CO₂. Soybean leaves grown under elevated levels of CO₂ were previously demonstrated at SoyFACE to have increased leaf temperature caused by reduced stomatal conductance. To separate the increased leaf temperature from other effects of elevated CO₂, air temperature was lowered while the CO₂ level was increased, which lowered overall leaf temperatures to those measured for leaves grown under ambient levels of CO₂. Aphid population growth on plants grown under elevated CO₂ and reduced air temperature was not significantly greater than on plants grown under ambient levels of CO₂. By increasing Glycine max leaf temperature, elevated CO₂ may increase populations of Aphis glycines and their impact on crop productivity.     

Light inhibition of leaf respiration in field-grown Eucalyptus saligna in whole-tree chambers under elevated atmospheric CO2 and summer drought

We investigated whether the degree of light inhibition of leaf respiration (R) differs among large Eucalyptus saligna grown in whole‐tree chambers and exposed to present and future atmospheric [CO₂] and summer drought. Associated with month‐to‐month changes in temperature were concomitant changes in R in the light (R_light) and darkness (R_dark), with both processes being more temperature dependent in well‐watered trees than under drought. Overall rates of R_light and R_dark were not significantly affected by [CO₂]. By contrast, overall rates of R_dark (averaged across both [CO₂]) were ca. 25% lower under drought than in well‐watered trees. During summer, the degree of light inhibition of leaf R was greater in droughted (ca. 80% inhibition) than well‐watered trees (ca. 50% inhibition). Notwithstanding these treatment differences, an overall positive relationship was observed between R_light and R_dark when data from all months/treatments were combined (R² = 0.8). Variations in R_light were also positively correlated with rates of Rubisco activity and nitrogen concentration. Light inhibition resulted in a marked decrease in the proportion of light‐saturated photosynthesis respired (i.e. reduced R/A_sat). Collectively, these results highlight the need to account for light inhibition when assessing impacts of global change drivers on the carbon economy of tree canopies.     

Modelling assimilation and intercellular CO2 from measured conductance: a synthesis of approaches

A spectrum of models that estimate assimilation rate A from intercellular carbon dioxide concentration (C_i) and measured stomatal conductance to CO₂ (g_c) were investigated using leaf‐level gas exchange measurements. The gas exchange measurements were performed in a uniform loblolly pine stand (Pinus taeda L.) using the Free Air CO₂ Enrichment (FACE) facility under ambient and elevated atmospheric CO₂ for 3 years. These measurements were also used to test a newly proposed framework that combines basic properties of the A–C_i curve with a Fickian diffusion transport model to predict the relationship between C_i/C_a and g_c, where C_a is atmospheric carbon dioxide concentration. The widely used Ball–Berry model and five other models as well as the biochemical model proposed by Farquhar et al. (1980) were also reformulated to express variations in C_i/C_a as a function of their corresponding driving mechanisms. To assess the predictive capabilities of these approaches, their respective parameters were estimated from independent measurements of long‐term stable carbon isotope determinations (δ¹³C), meteorological variables, and ensemble A–C_i curves. All eight approaches reproduced the measured A reasonably well, in an ensemble sense, from measured water vapour conductance and modeled C_i/C_a. However, the scatter in the instantaneous A estimates was sufficiently large for both ambient and elevated C_a to suggest that other transient processes were not explicitly resolved by all eight parameterizations. An important finding from our analysis is that added physiological complexity in modeling C_i/C_a (when g_c is known) need not always translate to increased accuracy in predicting A. Finally, the broader utility of these approaches to estimate assimilation and net ecosystem exchange is discussed in relation to elevated atmospheric CO₂.     

Effects of NAD kinase 2 overexpression on primary metabolite profiles in rice leaves under elevated carbon dioxide

The concentration of carbon dioxide (CO₂) in the atmosphere is projected to double by the end of the 21st century. In C₃ plants, elevated CO₂ concentrations promote photosynthesis but inhibit the assimilation of nitrate into organic nitrogen compounds. Several steps of nitrate assimilation depend on the availability of ATP and sources of reducing power, such as nicotinamide adenine dinucleotide phosphate (NADPH). Plastid‐localised NAD kinase 2 (NADK2) plays key roles in increasing the ATP/ADP and NADP(H)/NAD(H) ratios. Here we examined the effects of NADK2 overexpression on primary metabolism in rice (Oryza sativa) leaves in response to elevated CO₂. By using capillary electrophoresis mass spectrometry, we showed that the primary metabolite profile of NADK2‐overexpressing plants clearly differed from that of wild‐type plants under ambient and elevated CO₂. In NADK2‐overexpressing leaves, expression of the genes encoding glutamine synthetase and glutamate synthase was up‐regulated, and the levels of Asn, Gln, Arg, and Lys increased in response to elevated CO₂. The present study suggests that overexpression of NADK2 promotes the biosynthesis of nitrogen‐rich amino acids under elevated CO₂.     

Reduction of transpiration and altered nutrient allocation contribute to nutrient decline of crops grown in elevated CO2 concentrations

Plants grown in elevated [CO₂] have lower protein and mineral concentrations compared with plants grown in ambient [CO₂]. Dilution by enhanced production of carbohydrates is a likely cause, but it cannot explain all of the reductions. Two proposed, but untested, hypotheses are that (1) reduced canopy transpiration reduces mass flow of nutrients to the roots thus reducing nutrient uptake and (2) changes in metabolite or enzyme concentrations caused by physiological changes alter requirements for minerals as protein cofactors or in other organic complexes, shifting allocation between tissues and possibly altering uptake. Here, we use the meta‐analysis of previous studies in crops to test these hypotheses. Nutrients acquired mostly by mass flow were decreased significantly more by elevated [CO₂] than nutrients acquired by diffusion to the roots through the soil, supporting the first hypothesis. Similarly, Mg showed large concentration declines in leaves and wheat stems, but smaller decreases in other tissues. Because chlorophyll requires a large fraction of total plant Mg, and chlorophyll concentration is reduced by growth in elevated [CO₂], this supports the second hypothesis. Understanding these mechanisms may guide efforts to improve nutrient content, and allow modeling of nutrient changes and health impacts under future climate change scenarios.     

Reduced early growing season freezing resistance in alpine treeline plants under elevated atmospheric CO2

The frequency of freezing events during the early growing season and the vulnerability to freezing of plants in European high‐altitude environments could increase under future atmospheric and climate change. We tested early growing season freezing sensitivity in 10 species, from four plant functional types (PFTs) spanning three plant growth forms (PGFs), from a long‐term in situ CO₂ enrichment (566 vs. 370 ppm) and 2‐year soil warming (+4 K) experiment at treeline in the Swiss Alps (Stillberg, Davos). By additionally tracking plant phenology, we distinguished indirect phenology‐driven CO₂ and warming effects from direct physiology‐related effects on freezing sensitivity. The freezing damage threshold (lethal temperature 50) under ambient conditions of the 10 treeline species spanned from ?6.7±0.3 °C (Larix decidua) to ?9.9±0.6 °C (Vaccinium gaultherioides). PFT, but not PGF, explained a significant amount of this interspecific variation. Long‐term exposure to elevated CO₂ led to greater freezing sensitivity in multiple species but did not influence phenology, implying that physiological changes caused by CO₂ enrichment were responsible for the effect. The elevated CO₂ effect on freezing resistance was significant in leaves of Larix, Vaccinium myrtillus, and Gentiana punctata and marginally significant in leaves of Homogyne alpina and Avenella flexuosa. No significant CO₂ effect was found in new shoots of Empetrum hermaphroditum or in leaves of Pinus uncinata, Leontodon helveticus, Melampyrum pratense, and V. gaultherioides. Soil warming led to advanced leaf expansion and reduced freezing resistance in V. myrtillus only, whereas Avenella showed greater freezing resistance when exposed to warming. No effect of soil warming was found in any of the other species. Effects of elevated CO₂ and soil warming on freezing sensitivity were not consistent within PFTs or PGFs, suggesting that any future shifts in plant community composition due to increased damage from freezing events will likely occur at the individual species level.     

Changes in leaf nutrient traits and photosynthesis of four tree species: effects of elevated [CO2], N fertilization and canopy positions

Aims Leaf traits of trees exposed to elevated [CO₂] in association with other environmental factors are poorly understood in tropical and subtropical regions. Our goal was to investigate the impacts of elevated [CO₂] and N fertilization on leaf traits in southern China.Methods Four tree species, Schima superba Gardn. et Champ. (S. superba), Ormosia pinnata (Lour.) Merr (O. pinnata), Castanopsis hystrix AC. DC. (C. hystrix) and Acmena acuminatissima (Blume) Merr. et Perry (A. acuminatissima) were studied in a factorial combination of atmospheric [CO₂] (ambient at ~390 μmol mol ? 1 and elevated [CO₂] at ~700 μmol mol^-1) and N fertilization (ambient and ambient + 100 kg N ha^-1 yr^-1) in open-top chambers in southern China for 5 years. Leaf mass per unit leaf area (LMA), leaf nutrient concentration and photosynthesis (A sat) were measured.Important findings Results indicated that leaf traits and photosynthesis were affected differently by elevated [CO₂] and N fertilization among species. Elevated [CO₂] decreased LMA in all species, while N fertilization did not affect LMA. Leaf mass-based N concentration (N M) was significantly greater in O. pinnata and C. hystrix grown in elevated [CO₂] but was lower in S. superba. Leaf mass-based P concentration (P M) was significantly greater in C. hystrix and A. acuminatissima exposed to elevated [CO₂] but was lower in S. superba. N fertilization significantly increased P M in O. pinnata but decreased P M in S. superba. Photosynthetic stimulation in O. pinnata, C. hystrix and A. acuminatissima was sustained after 5 years of CO₂ fumigation. N fertilization did not modify the effects of elevated [CO₂] on photosynthesis. Leaf traits (N M, N A, P M, P A) and light-saturated photosynthesis were decreased from the upper to lower canopy. Canopy position did not alter the responses of leaf traits and photosynthesis to elevated [CO₂]. Results suggest that photosynthetic stimulation by elevated [CO₂] in native species in subtropical regions may be sustained in the long term.     

Nitrification,denitrification, and related functional genes under elevated CO2: A meta-analysis in terrestrial ecosystems

Global change may have profound effects on soil nitrogen (N) cycling that can induce positive feedback to climate change through increased nitrous oxide (N₂O) emissions mediated by nitrification and denitrification. We conducted a meta-analysis of the effects of elevated CO₂ on nitrification and denitrification based on 879 observations from 58 publications and 46 independent elevated CO₂ experiments in terrestrial ecosystems. We investigated the effects of elevated CO₂ alone or combined with elevated temperature, increased precipitation, drought, and N addition. We assessed the response to elevated CO₂ of gross and potential nitrification, potential denitrification, and abundances of related functional genes (archaeal amoA, bacterial amoA, nirK, nirS, and nosZ). Elevated CO₂ increased potential nitrification (+28%) and the abundance of bacterial amoA functional gene (+62%) in cropland ecosystems. Elevated CO₂ increased potential denitrification when combined with N addition and higher precipitation (+116%). Elevated CO₂ also increased the abundance of nirK (+25%) and nirS (+27%) functional genes in terrestrial ecosystems and of nosZ (+32%) functional gene in cropland ecosystems. The increase in the abundance of nosZ under elevated CO₂ was larger at elevated temperature and high N (+62%). Four out of 14 two-way interactions tested between elevated CO₂ and elevated temperature, elevated CO₂ and increased precipitation, and elevated CO₂ and N addition were marginally significant and mostly synergistic. The effects of elevated CO₂ on potential nitrification and abundances of bacterial amoA and nirS functional genes increased with mean annual temperature and mean annual precipitation. Our meta-analysis thus suggests that warming and increased precipitation in large areas of the world could reinforce positive responses of nitrification and denitrification to elevated CO₂ and urges the need for more investigations in the tropical zone and on interactive effects among multiple global change factors, as we may largely underestimate the effects of global change on soil N₂O emissions.     

Nitrogen balance for wheat canopies (Triticum aestivum cv. Veery 10) grown under elevated and ambient CO2 concentrations

We examined the hypothesis that elevated CO₂ concentration would increase NO₃^– absorption and assimilation using intact wheat canopies (Triticum aestivum cv. Veery 10). Nitrate consumption, the sum of plant absorption and nitrogen loss, was continuously monitored for 23 d following germination under two CO₂ concentrations (360 and 1000 μmol mol^–1 CO₂) and two root zone NO₃^– concentrations (100 and 1000 mmol m³ NO₃^–). The plants were grown at high density (1780 m^–2) in a 28 m³ controlled environment chamber using solution culture techniques. Wheat responded to 1000 μmol mol^–1 CO₂ by increasing carbon allocation to root biomass production. Elevated CO₂ also increased root zone NO₃^– consumption, but most of this increase did not result in higher biomass nitrogen. Rather, nitrogen loss accounted for the greatest part of the difference in NO₃^– consumption between the elevated and ambient [CO₂] treatments. The total amount of NO₃^–-N absorbed by roots or the amount of NO₃^–-N assimilated per unit area did not significantly differ between elevated and ambient [CO₂] treatments. Instead, specific leaf organic nitrogen content declined, and NO₃^– accumulated in canopies growing under 1000 μmol mol^–1 CO₂. Our results indicated that 1000 μmol mol^–1 CO₂ diminished NO₃^– assimilation. If NO₃^– assimilation were impaired by high [CO₂], then this offers an explanation for why organic nitrogen contents are often observed to decline in elevated [CO₂] environments.     

Does deep soil N availability sustain long‐term ecosystem responses to elevated CO2?

A scrub‐oak woodland has maintained higher aboveground biomass accumulation after 11 years of atmospheric CO₂ enrichment (ambient +350 μmol CO₂ mol^?1), despite the expectation of strong nitrogen (N) limitation at the site. We hypothesized that changes in plant available N and exploitation of deep sources of inorganic N in soils have sustained greater growth at elevated CO₂. We employed a suite of assays performed in the sixth and 11th year of a CO₂ enrichment experiment designed to assess soil N dynamics and N availability in the entire soil profile. In the 11th year, we found no differences in gross N flux, but significantly greater microbial respiration (P≤0.01) at elevated CO₂. Elevated CO₂ lowered extractable inorganic N concentrations (P=0.096) considering the whole soil profile (0–190 cm). Conversely, potential net N mineralization, although not significant in considering the entire profile (P=0.460), tended to be greater at elevated CO₂. Ion‐exchange resins placed in the soil profile for approximately 1 year revealed that potential N availability at the water table was almost 3 × greater than found elsewhere in the profile, and we found direct evidence using a ¹⁵N tracer study that plants took up N from the water table. Increased microbial respiration and shorter mean residence times of inorganic N at shallower depths suggests that enhanced SOM decomposition may promote a sustained supply of inorganic N at elevated CO₂. Deep soil N availability at the water table is considerable, and provides a readily available source of N for plant uptake. Increased plant growth at elevated CO₂ in this ecosystem may be sustained through greater inorganic N supply from shallow soils and N uptake from deep soil.     

Divergent terrestrial responses of soil N2O emissions to different levels of elevated CO2 and temperature

Understanding the responses of soil nitrous oxide (N₂O) emissions from terrestrial ecosystems to future CO₂ enrichment and warming is critical for the development of mitigation and adaptation policies. The effects of continuous increase in elevated CO₂ (EC) and elevated temperature (ET) on N₂O emissions are not fully known. We synthesized 209 measurements from 70 published studies and carried out a meta-analysis to examine individual and interactive effects of EC and ET on N₂O emissions from grasslands, croplands and forests. On average, a significant increase of 23% in N₂O emissions was observed under EC across all case studies. EC did not affect N₂O emissions from grasslands or forests, but significantly increased N₂O emissions in croplands by 38%. The extent of ET effects on N₂O emissions was nonsignificant and there was no significant difference in N₂O emission responses among these three terrestrial systems. ET only promoted N₂O emissions in forest by about 32% when ET was less than 2°C. The interactive effect of EC and ET on N₂O emissions was significantly synergistic, showing a greater increase than the sum of the effects caused by EC and ET alone. Our findings indicated that the combination of EC and ET substantially promoted soil N₂O and highlighted the urgent need to explore its mechanisms to better understand N₂O responses under future climate change.     

Effects of mycorrhizal colonization on biomass production and nitrogen fixation of black locust (Robinia pseudoacacia) seedlings grown under elevated atmospheric carbon dioxide

Interactive effects of elevated atmospheric CO₂ and arbuscular mycorrhizal (AM) fungi on biomass production and N₂ fixation were investigated using black locust ( Robinia pseudoacacia ). Seedlings were grown in growth chambers maintained at either 350 μmol mol⁻¹ or 710 μmol mol⁻¹ CO₂. Seedlings were inoculated with Rhizobium spp. and were grown with or without AM fungi. The ¹⁵N isotope dilution method was used to determine N source partitioning between N₂ fixation and inorganic fertilizer uptake. Elevated atmospheric CO₂ significantly increased the percentage of fine roots that were colonized by AM fungi. Mycorrhizal seedlings grown under elevated CO₂ had the greatest overall plant biomass production, nodulation, N and P content, and root N absorption. Additionally, elevated CO₂ levels enhanced nodule and root mass production, as well as N₂ fixation rates, of non- mycorrhizal seedlings. However, the relative response of biomass production to CO₂ enrichment was greater in non-mycorrhizal seedlings than in mycorrhizal seedlings. This study provides strong evidence that arbuscular mycorrhizal fungi play an important role in the extent to which plant nutrition of symbiotic N₂-fixing tree species is affected by enriched atmospheric CO₂.     

CO_2浓度升高对水体硝化、反硝化作用的影响研究进展

大气CO_2浓度升高潜移默化地影响着水体生态系统的碳循环过程.然而,该过程如何影响与其耦合的氮循环过程仍不明确.水体硝化、反硝化过程作为水体氮循环的重要环节,必然会对大气CO_2浓度升高产生一系列的响应.本文总结了国内外关于大气CO_2浓度升高对水体理化性质、硝化作用、反硝化作用及N形态转化影响方面的研究工作,发现大气CO_2浓度升高会降低水体的p H,增加水中CO_2和HCO_3^-含量,但对富营养化与寡营养化水体中硝化、反硝化作用的影响具有明显差异.大气CO_2浓度升高抑制寡营养化水体的硝化作用和反硝化作用,降低N2_O的释放通量,抑制富营养化水体的硝化作用,但当水体pH在7～9时,可能促进反硝化作用,增加N2_O的释放通量,最终可能导致水体中NH_4^+的积累及NO_3^-浓度的降低,影响水体中微生物的多样性.在此基础上提出目前相关研究存在的瓶颈问题及值得深入探讨的科学问题,为进一步深入理解温室效应背景下全球CO_2浓度升高对水体生态系统N循环的影响提供参考.     

The effects of elevated CO2 (0.5%) on chloroplasts in the tetraploid black locust (Robinia pseudoacacia L.)

Some ploidy plants demonstrate environmental stress tolerance. Tetraploid (4×) black locust (Robinia pseudoacacia L.) exhibits less chlorosis in response to high CO₂ than do the corresponding diploid (2×) plants of this species. We investigated the plant growth, anatomy, photosynthetic ability, chlorophyll (chl) fluorescence, and antioxidase activities in 2× and 4× black locusts cultivated under high CO₂ (0.5%). Elevated CO₂ (0.5%) induced a global decrease in the contents of total chl, chl a, and chl b in 2× leaves, while few changes were found in the chl content of 4× leaves. Analyses of the chl fluorescence intensity, maximum quantum yield of photosystem II (PSII) photochemistry (Fv/Fm), K‐step (V_k), and J‐step (V_J) revealed that 0.5% CO₂ had a negative effect on the photosynthetic capacity and growth of the 2× plants, especially the performance of PSII. In contrast, there was no significant effect of high CO₂ on the growth of the 4× plants. These analyses indicate that the decreased inhibition of the growth of 4× plants by high CO₂ (0.5%) may be attributed to an improved photosynthetic capacity, pigment content, and ultrastructure of the chloroplast compared to 2× plants.     

大气一氧化碳浓度升高对植物生长的影响

大气ＣＯ２浓度同对植物生长有促进作用,对Ｃ３植物生长的促进作用最大。短期ＣＯ２浓度升高时,植物光和速率增加;在长期ＣＯ２浓度升高条件下,植物光鸽上降并发生光合适应现象。这可能是植物在长期ＣＯ２浓度升高条件下植物源库关系不平衡引起的反馈抑制作用以及营养吸收不能满足光合速率增加的需要所引起Ｒｕｂｉｓｅｏ活必和含量下降。在ＣＯ２浓度升高条件下植物的呼吸也会发生变化,根的分枝和数量增多,根系的分泌量和吸收     

Canopy warming caused photosynthetic acclimation and reduced seed yield in maize grown at ambient and elevated [CO2]

Rising atmospheric CO₂ concentration ([CO₂]) and attendant increases in growing season temperature are expected to be the most important global change factors impacting production agriculture. Although maize is the most highly produced crop worldwide, few studies have evaluated the interactive effects of elevated [CO₂] and temperature on its photosynthetic physiology, agronomic traits or biomass, and seed yield under open field conditions. This study investigates the effects of rising [CO₂] and warmer temperature, independently and in combination, on maize grown in the field throughout a full growing season. Free‐air CO₂ enrichment (FACE) technology was used to target atmospheric [CO₂] to 200 μmol mol^?1 above ambient [CO₂] and infrared heaters to target a plant canopy increase of 3.5 °C, with actual season mean heating of ~2.7 °C, mimicking conditions predicted by the second half of this century. Photosynthetic gas‐exchange parameters, leaf nitrogen and carbon content, leaf water potential components, and developmental measurements were collected throughout the season, and biomass and yield were measured at the end of the growing season. As predicted for a C₄ plant, elevated [CO₂] did not stimulate photosynthesis, biomass, or yield. Canopy warming caused a large shift in aboveground allocation by stimulating season‐long vegetative biomass and decreasing reproductive biomass accumulation at both CO₂ concentrations, resulting in decreased harvest index. Warming caused a reduction in photosynthesis due to down‐regulation of photosynthetic biochemical parameters and the decrease in the electron transport rate. The reduction in seed yield with warming was driven by reduced photosynthetic capacity and by a shift in aboveground carbon allocation away from reproduction. This field study portends that future warming will reduce yield in maize, and this will not be mitigated by higher atmospheric [CO₂] unless appropriate adaptation traits can be introduced into future cultivars.