Enhanced isoprene emission capacity and altered light responsiveness in aspen grown under elevated atmospheric CO2 concentration

Controversial evidence of CO₂‐responsiveness of isoprene emission has been reported in the literature with the response ranging from inhibition to enhancement, but the reasons for such differences are not understood. We studied isoprene emission characteristics of hybrid aspen (Populus tremula x P. tremuloides) grown under ambient (380 μmol mol^?1) and elevated (780 μmol mol^?1) [CO₂] to test the hypothesis that growth [CO₂] effects on isoprene emission are driven by modifications in substrate pool size, reflecting altered light use efficiency for isoprene synthesis. A novel in vivo method for estimation of the pool size of the immediate isoprene precursor, dimethylallyldiphosphate (DMADP) and the activity of isoprene synthase was used. Growth at elevated [CO₂] resulted in greater leaf thickness, more advanced development of mesophyll and moderately increased photosynthetic capacity due to morphological “upregulation”, but isoprene emission rate under growth light and temperature was not significantly different among ambient‐ and elevated‐[CO₂]‐grown plants independent of whether measured at 380 μmol mol^?1 or 780 μmol mol^?1 CO₂. However, DMADP pool size was significantly less in elevated‐[CO₂]‐grown plants, but this was compensated by increased isoprene synthase activity. Analysis of CO₂ and light response curves of isoprene emission demonstrated that the [CO₂] for maximum isoprene emission was shifted to lower [CO₂] in elevated‐[CO₂]‐grown plants. The light‐saturated isoprene emission rate (I_max,Q) was greater, but the quantum efficiency at given I_max,Q was less in elevated‐[CO₂]‐grown plants, especially at higher CO₂ measurement concentration, reflecting stronger DMADP limitation at lower light and higher [CO₂]. These results collectively demonstrate important shifts in light and CO₂‐responsiveness of isoprene emission in elevated‐[CO₂]‐acclimated plants that need consideration in modeling isoprene emissions in future climates.     

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.     

A cycad's non‐saturating response to carbon dioxide enrichment indicates Cenozoic carbon limitation in pre‐historic plants

Cycads were a dominant plant functional type during the Mesozoic Era when atmospheric carbon dioxide [CO₂] greatly exceeded current conditions. Cycads, now rare and endangered, are slow‐growing perennial gymnosperms that develop carbon‐rich structural biomass, such as sclerophyllous leaves, dense stems and massive reproductive cones. Is cycad carbon partitioning to specific organs a constraint of their high [CO₂] evolutionary history (CO₂ legacy hypothesis, CLH)? To explore changes in cycad growth, carbon partitioning and assimilation responses that could be expected during the CO₂ depletion of the Cenozoic Era, individuals of the cycad species Encephalartos villosus plants were grown at four CO₂ levels: 400, 550, 750 and 1000 μmol mol^?1. The CLH predicts that cycad biomass and growth rates would increase in elevated [CO₂] due to increased net assimilation rates, and that carbon‐dense structures would provide sufficient carbohydrate sinks to prevent photosynthetic down‐regulation even under super‐ambient [CO₂] of 1000 μmol mol^?1. Both hypotheses were confirmed, though the latter less strongly. Plant relative growth rates increased 23% and biomass accumulation increased 65% in 1000 μmol mol^?1relative to 400 μmol mol^?1 treatment groups. Mean net assimilation rates increased 130% at 1000 μmol mol^?1 relative to 400 μmol mol^?1 CO₂, though there was some down‐regulation of maximum rate of carboxylation (Vc_max). Assimilation rates, relative growth rates, biomass and mean leaf sugar content were linearly related to [CO₂] over the entire experimental range. Photosynthesis appears to be regulated by stomata at low CO₂ levels and by non‐stomatal (i.e. biochemical limitations) at greater concentrations. In general, our results suggest that growth and physiological performance of cycads have been severely compromised by declining [CO₂] during the Cenozoic Era, possibly contributing to the current rare and endangered status of this functional type.     

Elevated CO2 increases water use efficiency by sustaining photosynthesis of water-limited maize and sorghum

Maize and grain sorghum seeds were sown in pots and grown for 39 days in sunlit controlled-environment chambers at 360 (ambient) and 720 (double-ambient, elevated) μmol mol⁻¹ carbon dioxide concentrations [CO₂]. Canopy net photosynthesis (PS) and evapotranspiration (TR) was measured throughout and summarized daily from 08:00 to 17:00 h Eastern Standard Time. Irrigation was withheld from matched pairs of treatments starting on 26 days after sowing (DAS). By 35 DAS, cumulative PS of drought-stress maize, compared to well-watered plants, was 41% lower under ambient [CO₂] but only 13% lower under elevated [CO₂]. In contrast, by 35 DAS, cumulative PS of drought-stress grain sorghum, compared to well-watered plants, was only 9% lower under ambient [CO₂] and 7% lower under elevated [CO₂]. During the 27-35 DAS drought period, water use efficiency (WUE, mol CO₂ Kmol⁻¹ H₂O), was 3.99, 3.88, 5.50, and 8.65 for maize and 3.75, 4.43, 5.26, and 9.94 for grain sorghum, for ambient-[CO₂] well-watered, ambient-[CO₂] stressed, elevated-[CO₂] well-watered and elevated-[CO₂] stressed plants, respectively. Young plants of maize and sorghum used water more efficiently at elevated [CO₂] than at ambient [CO₂], especially under drought. Reductions in biomass by drought for young maize and grain sorghum plants were 42 and 36% at ambient [CO₂], compared to 18 and 14% at elevated [CO₂], respectively. Results of our water stress experiment demonstrated that maintenance of relatively high canopy photosynthetic rates in the face of decreased transpiration rates enhanced WUE in plants grown at elevated [CO₂]. This confirms experimental evidence and conceptual models that suggest that an increase of intercellular [CO₂] (or a sustained intercellular [CO₂]) in the face of decreased stomatal conductance results in relative increases of growth of C₄ plants. In short, drought stress in C₄ crop plants can be ameliorated at elevated [CO₂] as a result of lower stomatal conductance and sustaining intercellular [CO₂]. Furthermore, less water might be required for C₄ crops in future higher CO₂ atmospheres, assuming weather and climate similar to present conditions.     

Interactive direct and plant‐mediated effects of elevated atmospheric [CO2] and temperature on a eucalypt‐feeding insect herbivore

Understanding the direct and indirect effects of elevated [CO₂] and temperature on insect herbivores and how these factors interact are essential to predict ecosystem‐level responses to climate change scenarios. In three concurrent glasshouse experiments, we measured both the individual and interactive effects of elevated [CO₂] and temperature on foliar quality. We also assessed the interactions between their direct and plant‐mediated effects on the development of an insect herbivore of eucalypts. Eucalyptus tereticornis saplings were grown at ambient or elevated [CO₂] (400 and 650 μmol mol^?1 respectively) and ambient or elevated ( + 4 °C) temperature for 10 months. Doratifera quadriguttata (Lepidoptera: Limacodidae) larvae were feeding directly on these trees, on their excised leaves in a separate glasshouse, or on excised field‐grown leaves within the temperature and [CO₂] controlled glasshouse. To allow insect gender to be determined and to ensure that any sex‐specific developmental differences could be distinguished from treatment effects, insect development time and consumption were measured from egg hatch to pupation. No direct [CO₂] effects on insects were observed. Elevated temperature accelerated larval development, but did not affect leaf consumption. Elevated [CO₂] and temperature independently reduced foliar quality, slowing larval development and increasing consumption. Simultaneously increasing both [CO₂] and temperature reduced these shifts in foliar quality, and negative effects on larval performance were subsequently ameliorated. Negative nutritional effects of elevated [CO₂] and temperature were also independently outweighed by the direct positive effect of elevated temperature on larvae. Rising [CO₂] and temperature are thus predicted to have interactive effects on foliar quality that affect eucalypt‐feeding insects. However, the ecological consequences of these interactions will depend on the magnitude of concurrent temperature rise and its direct effects on insect physiology and feeding behaviour.     

Elevated atmospheric [CO2] can dramatically increase wheat yields in semi‐arid environments and buffer against heat waves

Wheat production will be impacted by increasing concentration of atmospheric CO₂ [CO₂], which is expected to rise from about 400 μmol mol^?1 in 2015 to 550 μmol mol^?1 by 2050. Changes to plant physiology and crop responses from elevated [CO₂] (e[CO₂]) are well documented for some environments, but field‐level responses in dryland Mediterranean environments with terminal drought and heat waves are scarce. The Australian Grains Free Air CO₂ Enrichment facility was established to compare wheat (Triticum aestivum) growth and yield under ambient (~370 μmol^?1 in 2007) and e[CO₂] (550 μmol^?1) in semi‐arid environments. Experiments were undertaken at two dryland sites (Horsham and Walpeup) across three years with two cultivars, two sowing times and two irrigation treatments. Mean yield stimulation due to e[CO₂] was 24% at Horsham and 53% at Walpeup, with some treatment responses greater than 70%, depending on environment. Under supplemental irrigation, e[CO₂] stimulated yields at Horsham by 37% compared to 13% under rainfed conditions, showing that water limited growth and yield response to e[CO₂]. Heat wave effects were ameliorated under e[CO₂] as shown by reductions of 31% and 54% in screenings and 10% and 12% larger kernels (Horsham and Walpeup). Greatest yield stimulations occurred in the e[CO₂] late sowing and heat stressed treatments, when supplied with more water. There were no clear differences in cultivar response due to e[CO₂]. Multiple regression showed that yield response to e[CO₂] depended on temperatures and water availability before and after anthesis. Thus, timing of temperature and water and the crop's ability to translocate carbohydrates to the grain postanthesis were all important in determining the e[CO₂] response. The large responses to e[CO₂] under dryland conditions have not been previously reported and underscore the need for field level research to provide mechanistic understanding for adapting crops to a changing climate.     

Temporal dynamics and spatial variability in the enhancement of canopy leaf area under elevated atmospheric CO2

Increased canopy leaf area (L) may lead to higher forest productivity and alter processes such as species dynamics and ecosystem mass and energy fluxes. Few CO₂ enrichment studies have been conducted in closed canopy forests and none have shown a sustained enhancement of L. We reconstructed 8 years (1996–2003) of L at Duke's Free Air CO₂ Enrichment experiment to determine the effects of elevated atmospheric CO₂ concentration ([CO₂]) on L before and after canopy closure in a pine forest with a hardwood component, focusing on interactions with temporal variation in water availability and spatial variation in nitrogen (N) supply. The dynamics of L were reconstructed using data on leaf litterfall mass and specific leaf area for hardwoods, and needle litterfall mass and specific leaf area combined with needle elongation rates, and fascicle and shoot counts for pines. The dynamics of pine L production and senescence were unaffected by elevated [CO₂], although L senescence for hardwoods was slowed. Elevated [CO₂] enhanced pine L and the total canopy L (combined pine and hardwood species; P<0.050); on average, enhancement following canopy closure was ～16% and 14% respectively. However, variation in pine L and its response to elevated [CO₂] was not random. Each year pine L under ambient and elevated [CO₂] was spatially correlated to the variability in site nitrogen availability (e.g. r²=0.94 and 0.87 in 2001, when L was highest before declining due to droughts and storms), with the [CO₂]‐induced enhancement increasing with N (P=0.061). Incorporating data on N beyond the range of native fertility, achieved through N fertilization, indicated that pine L had reached the site maximum under elevated [CO₂] where native N was highest. Thus closed canopy pine forests may be able to increase leaf area under elevated [CO₂] in moderate fertility sites, but are unable to respond to [CO₂] in both infertile sites (insufficient resources) and sites having high levels of fertility (maximum utilization of resources). The total canopy L, representing the combined L of pine and hardwood species, was constant across the N gradient under both ambient and elevated [CO₂], generating a constant enhancement of canopy L. Thus, in mixed species stands, L of canopy hardwoods which developed on lower fertility sites (～3 g N inputs m^?2 yr^?1) may be sufficiently enhanced under elevated [CO₂] to compensate for the lack of response in pine L, and generate an appreciable response of total canopy L (～14%).     

Leaf photosynthesis and carbohydrates of CO₂-enriched maize and grain sorghum exposed to a short period of soil water deficit during vegetative development

Among C₄ species, sorghum is known to be more drought tolerant than maize. The objective was to evaluate differences in leaf gas exchanges, carbohydrates, and two enzyme activities of these nicotinamide adenine dinucleotide phosphate-malic enzyme (NADP-ME) C₄ subtype monocots in response to water deficit and CO₂ concentration ([CO₂]). Maize and sorghum were grown in pots in sunlit environmental-controlled chambers. Treatments included well watered (WW) and water stressed (WS) (water withheld at 26 days) and daytime [CO₂] of 360 (ambient) and 720 (elevated) μmol mol⁻¹. Midday gas exchange rates, concentrations of nonstructural carbohydrates, and activities of sucrose-phosphate synthase (SPS) and adenosine 5′-diphosphoglucose pyrophosphorylase (ADGP) were determined for fully expanded leaf sections. There was no difference in leaf CO₂ exchange rates (CER) between ambient and elevated [CO₂] control plants for both maize and sorghum. After withholding water, leaf CER declined to zero after 8 days in maize and 10 days for sorghum. Sorghum had lower stomatal conductance and transpiration rates than maize, which resulted in a longer period of CER under drought. Nonstructural carbohydrates of both control maize and sorghum were hardly affected by elevated [CO₂]. Under drought, however, increases in soluble sugars and decreases in starch were generally observed for maize and sorghum at both [CO₂] levels. For stressed maize and sorghum, decreases in starch occurred earlier and were greater at ambient [CO₂] than at elevated [CO₂]. For maize, drought did not meaningfully affect SPS activity. However, a decline in SPS activity was observed for drought-stressed sorghum under both [CO₂] treatments. There was an increase in ADGP activity in maize under drought for both [CO₂] treatments. Such a response in ADGP to drought, however, did not occur for sorghum. The generally more rapid response of maize than sorghum to drought might be related to the more rapid growth of leaf area of maize.     

Response of archaeal communities in the rhizosphere of maize and soybean to elevated atmospheric CO2 concentrations

Background

Archaea are important to the carbon and nitrogen cycles, but it remains uncertain how rising atmospheric carbon dioxide concentrations ([CO₂]) will influence the structure and function of soil archaeal communities.

Methodology/Principal Findings

We measured abundances of archaeal and bacterial 16S rRNA and amoA genes, phylogenies of archaeal 16S rRNA and amoA genes, concentrations of KCl-extractable soil ammonium and nitrite, and potential ammonia oxidation rates in rhizosphere soil samples from maize and soybean exposed to ambient (∼385 ppm) and elevated (550 ppm) [CO₂] in a replicated and field-based study. There was no influence of elevated [CO₂] on copy numbers of archaeal or bacterial 16S rRNA or amoA genes, archaeal community composition, KCl-extractable soil ammonium or nitrite, or potential ammonia oxidation rates for samples from maize, a model C₄ plant. Phylogenetic evidence indicated decreased relative abundance of crenarchaeal sequences in the rhizosphere of soybean, a model leguminous-C₃ plant, at elevated [CO₂], whereas quantitative PCR data indicated no changes in the absolute abundance of archaea. There were no changes in potential ammonia oxidation rates at elevated [CO₂] for soybean. Ammonia oxidation rates were lower in the rhizosphere of maize than soybean, likely because of lower soil pH and/or abundance of archaea. KCl-extractable ammonium and nitrite concentrations were lower at elevated than ambient [CO₂] for soybean.

Conclusion

Plant-driven shifts in soil biogeochemical processes in response to elevated [CO₂] affected archaeal community composition, but not copy numbers of archaeal genes, in the rhizosphere of soybean. The lack of a treatment effect for maize is consistent with the fact that the photosynthesis and productivity of maize are not stimulated by elevated [CO₂] in the absence of drought.     

Canopy radiation‐ and water‐use efficiencies as affected by elevated [CO2]

This study used an environmentally controlled plant growth facility, EcoCELLs, to measure canopy gas exchanges directly and to examine the effects of elevated [CO₂] on canopy radiation‐ and water‐use efficiencies. Sunflowers (Helianthus annus var. Mammoth) were grown at ambient (399 μmol mol^?1) and elevated [CO₂] (746 μmol mol^?1) for 53 days in EcoCELLs. Whole canopy carbon‐ and water‐fluxes were measured continuously during the period of the experiment. The results indicated that elevated [CO₂] enhanced daily total canopy carbon‐ and water‐fluxes by 53% and 11%, respectively, on a ground‐area basis, resulting in a 54% increase in radiation‐use efficiency (RUE) based on intercepted photosynthetic active radiation and a 26% increase in water‐use efficiency (WUE) by the end of the experiment. Canopy carbon‐ and water‐fluxes at both CO₂ treatments varied with canopy development. They were small at 22 days after planting (DAP) and gradually increased to the maxima at 46 DAP. When canopy carbon‐ and water‐fluxes were expressed on a leaf‐area basis, no effect of CO₂ was found for canopy water‐flux while elevated [CO₂] still enhanced canopy carbon‐flux by 29%, on average. Night‐time canopy carbon‐flux was 32% higher at elevated than at ambient [CO₂]. In addition, RUE and WUE displayed strong diurnal variations, high at noon and low in the morning or afternoon for WUE but opposite for RUE. This study provided direct evidence that plant canopy may consume more, instead of less, water but utilize both water and radiation more efficiently at elevated than at ambient [CO₂], at least during the exponential growth period as illustrated in this experiment.     

Stomatal conductance and not stomatal density determines the long-term reduction in leaf transpiration of poplar in elevated CO<Subscript>2</Subscript>

Using a free-air CO₂ enrichment (FACE) experiment, poplar trees (Populus × euramericana clone I214) were exposed to either ambient or elevated [CO₂] from planting, for a 5-year period during canopy development, closure, coppice and re-growth. In each year, measurements were taken of stomatal density (SD, number mm⁻²) and stomatal index (SI, the proportion of epidermal cells forming stomata). In year 5, measurements were also taken of leaf stomatal conductance (g _s, μmol m⁻² s⁻¹), photosynthetic CO₂ fixation (A, mmol m⁻² s⁻¹), instantaneous water-use efficiency (A/E) and the ratio of intercellular to atmospheric CO₂ (C_i:C_a). Elevated [CO₂] caused reductions in SI in the first year, and in SD in the first 2 years, when the canopy was largely open. In following years, when the canopy had closed, elevated [CO₂] had no detectable effects on stomatal numbers or index. In contrast, even after 5 years of exposure to elevated [CO₂], g _s was reduced, A/E was stimulated, and C_i:C_a was reduced relative to ambient [CO₂]. These outcomes from the long-term realistic field conditions of this forest FACE experiment suggest that stomatal numbers (SD and SI) had no role in determining the improved instantaneous leaf-level efficiency of water use under elevated [CO₂]. We propose that altered cuticular development during canopy closure may partially explain the changing response of stomata to elevated [CO₂], although the mechanism for this remains obscure.     

Sustained effects of atmospheric [CO2] and nitrogen availability on forest soil CO2 efflux

Soil CO₂ efflux (F_soil) is the largest source of carbon from forests and reflects primary productivity as well as how carbon is allocated within forest ecosystems. Through early stages of stand development, both elevated [CO₂] and availability of soil nitrogen (N; sum of mineralization, deposition, and fixation) have been shown to increase gross primary productivity, but the long‐term effects of these factors on F_soil are less clear. Expanding on previous studies at the Duke Free‐Air CO₂ Enrichment (FACE) site, we quantified the effects of elevated [CO₂] and N fertilization on F_soil using daily measurements from automated chambers over 10 years. Consistent with previous results, compared to ambient unfertilized plots, annual F_soil increased under elevated [CO₂] (ca. 17%) and decreased with N (ca. 21%). N fertilization under elevated [CO₂] reduced F_soil to values similar to untreated plots. Over the study period, base respiration rates increased with leaf productivity, but declined after productivity saturated. Despite treatment‐induced differences in aboveground biomass, soil temperature and water content were similar among treatments. Interannually, low soil water content decreased annual F_soil from potential values – estimated based on temperature alone assuming nonlimiting soil water content – by ca. 0.7% per 1.0% reduction in relative extractable water. This effect was only slightly ameliorated by elevated [CO₂]. Variability in soil N availability among plots accounted for the spatial variability in F_soil, showing a decrease of ca. 114 g C m^?2 yr^?1 per 1 g m^?2 increase in soil N availability, with consistently higher F_soil in elevated [CO₂] plots ca. 127 g C per 100 ppm [CO₂] over the +200 ppm enrichment. Altogether, reflecting increased belowground carbon partitioning in response to greater plant nutritional needs, the effects of elevated [CO₂] and N fertilization on F_soil in this stand are sustained beyond the early stages of stand development and through stabilization of annual foliage production.     

Lower responsiveness of canopy evapotranspiration rate than of leaf stomatal conductance to open‐air CO2 elevation in rice

An elevated atmospheric CO₂ concentration ([CO₂]) can reduce stomatal conductance of leaves for most plant species, including rice (Oryza sativa L.). However, few studies have quantified seasonal changes in the effects of elevated [CO₂] on canopy evapotranspiration, which integrates the response of stomatal conductance of individual leaves with other responses, such as leaf area expansion, changes in leaf surface temperature, and changes in developmental stages, in field conditions. We conducted a field experiment to measure seasonal changes in stomatal conductance of the uppermost leaves and in the evapotranspiration, transpiration, and evaporation rates using a lysimeter method. The study was conducted for flooded rice under open‐air CO₂ elevation. Stomatal conductance decreased by 27% under elevated [CO₂], averaged throughout the growing season, and evapotranspiration decreased by an average of 5% during the same period. The decrease in daily evapotranspiration caused by elevated [CO₂] was more significantly correlated with air temperature and leaf area index (LAI) rather than with other parameters of solar radiation, days after transplanting, vapor‐pressure deficit and FAO reference evapotranspiration. This indicates that higher air temperatures, within the range from 16 to 27 °C, and a larger LAI, within the range from 0 to 4 m² m^?2, can increase the magnitude of the decrease in evapotranspiration rate caused by elevated [CO₂]. The crop coefficient (i.e. the evapotranspiration rate divided by the FAO reference evapotranspiration rate) was 1.24 at ambient [CO₂] and 1.17 at elevated [CO₂]. This study provides the first direct measurement of the effects of elevated [CO₂] on rice canopy evapotranspiration under open‐air conditions using the lysimeter method, and the results will improve future predictions of water use in rice fields.     

Will photosynthesis of maize (Zea mays) in the US Corn Belt increase in future [CO2] rich atmospheres? An analysis of diurnal courses of CO2 uptake under free‐air concentration enrichment (FACE)

The C₄ grass Zea mays (maize or corn) is the third most important food crop globally in terms of production and demand is predicted to increase 45% from 1997 to 2020. However, the effects of rising [CO₂] upon C₄ plants, and Z. mays specifically, are not sufficiently understood to allow accurate predictions of future crop production. A rainfed, field experiment utilizing free‐air concentration enrichment (FACE) technology in the primary area of global corn production (US Corn Belt) was undertaken to determine the effects of elevated [CO₂] on corn. FACE technology allows experimental treatments to be imposed upon a complete soil–plant–atmosphere continuum with none of the effects of experimental enclosures on plant microclimate. Crop performance was compared at ambient [CO₂] (354 μ mol mol^?1) and the elevated [CO₂] (549 μmol mol^?1) predicted for 2050. Previous laboratory studies suggest that under favorable growing conditions C₄ photosynthesis is not typically enhanced by elevated [CO₂]. However, stomatal conductance and transpiration are decreased, which can indirectly increase photosynthesis in dry climates. Given the deep soils and relatively high rainfall of the US Corn Belt, it was predicted that photosynthesis would not be enhanced by elevated [CO₂]. The diurnal course of gas exchange of upper canopy leaves was measured in situ across the growing season of 2002. Contrary to the prediction, growth at elevated [CO₂] significantly increased leaf photosynthetic CO₂ uptake rate (A) by up to 41%, and 10% on average. Greater A was associated with greater intercellular [CO₂], lower stomatal conductance and lower transpiration. Summer rainfall during 2002 was very close to the 50‐year average for this site, indicating that the year was not atypical or a drought year. The results call for a reassessment of the established view that C₄ photosynthesis is insensitive to elevated [CO₂] under favorable growing conditions and that the production potential of corn in the US Corn Belt will not be affected by the global rise in [CO₂].     

Distinct seasonal dynamics of responses to elevated CO2 in two understorey grass species differing in shade‐tolerance

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Elevated ozone reduces photosynthetic carbon gain by accelerating leaf senescence of inbred and hybrid maize in a genotype‐specific manner

Exposure to elevated tropospheric ozone concentration ([O₃]) accelerates leaf senescence in many C₃ crops. However, the effects of elevated [O₃] on C₄ crops including maize (Zea mays L.) are poorly understood in terms of physiological mechanism and genetic variation in sensitivity. Using free air gas concentration enrichment, we investigated the photosynthetic response of 18 diverse maize inbred and hybrid lines to season‐long exposure to elevated [O₃] (~100 nl L^?1) in the field. Gas exchange was measured on the leaf subtending the ear throughout the grain filling period. On average over the lifetime of the leaf, elevated [O₃] led to reductions in photosynthetic CO₂ assimilation of both inbred (?22%) and hybrid (?33%) genotypes. There was significant variation among both inbred and hybrid lines in the sensitivity of photosynthesis to elevated [O₃], with some lines showing no change in photosynthesis at elevated [O₃]. Based on analysis of inbred line B73, the reduced CO₂ assimilation at elevated [O₃] was associated with accelerated senescence decreasing photosynthetic capacity and not altered stomatal limitation. These findings across diverse maize genotypes could advance the development of more O₃ tolerant maize and provide experimental data for parameterization and validation of studies modeling how O₃ impacts crop performance.     

How do various maize crop models vary in their responses to climate change factors?

Potential consequences of climate change on crop production can be studied using mechanistic crop simulation models. While a broad variety of maize simulation models exist, it is not known whether different models diverge on grain yield responses to changes in climatic factors, or whether they agree in their general trends related to phenology, growth, and yield. With the goal of analyzing the sensitivity of simulated yields to changes in temperature and atmospheric carbon dioxide concentrations [CO₂], we present the largest maize crop model intercomparison to date, including 23 different models. These models were evaluated for four locations representing a wide range of maize production conditions in the world: Lusignan (France), Ames (USA), Rio Verde (Brazil) and Morogoro (Tanzania). While individual models differed considerably in absolute yield simulation at the four sites, an ensemble of a minimum number of models was able to simulate absolute yields accurately at the four sites even with low data for calibration, thus suggesting that using an ensemble of models has merit. Temperature increase had strong negative influence on modeled yield response of roughly ?0.5 Mg ha^?1 per °C. Doubling [CO₂] from 360 to 720 μmol mol^?1 increased grain yield by 7.5% on average across models and the sites. That would therefore make temperature the main factor altering maize yields at the end of this century. Furthermore, there was a large uncertainty in the yield response to [CO₂] among models. Model responses to temperature and [CO₂] did not differ whether models were simulated with low calibration information or, simulated with high level of calibration information.     

Leaf and canopy scale drivers of genotypic variation in soybean response to elevated carbon dioxide concentration

The atmospheric [CO₂] in which crops grow today is greater than at any point in their domestication history and represents an opportunity for positive effects on seed yield that can counteract the negative effects of greater heat and drought this century. In order to maximize yields under future atmospheric [CO₂], we need to identify and study crop cultivars that respond most favorably to elevated [CO₂] and understand the mechanisms contributing to their responsiveness. Soybean (Glycine max Merr.) is a widely grown oilseed crop and shows genetic variation in response to elevated [CO₂]. However, few studies have studied the physiological basis for this variation. Here, we examined canopy light interception, photosynthesis, respiration and radiation use efficiency along with yield and yield parameters in two cultivars of soybean (Loda and HS93‐4118) previously reported to have similar seed yield at ambient [CO₂], but contrasting responses to elevated [CO₂]. Seed yield increased by 26% at elevated [CO₂] (600 μmol/mol) in the responsive cultivar Loda, but only by 11% in HS93‐4118. Canopy light interception and leaf area index were greater in HS93‐4118 in ambient [CO₂], but increased more in response to elevated [CO₂] in Loda. Radiation use efficiency and harvest index were also greater in Loda than HS93‐4118 at both ambient and elevated [CO₂]. Daily C assimilation was greater at elevated [CO₂] in both cultivars, while stomatal conductance was lower. Electron transport capacity was also greater in Loda than HS93‐4118, but there was no difference in the response of photosynthetic traits to elevated [CO₂] in the two cultivars. Overall, this greater understanding of leaf‐ and canopy‐level photosynthetic traits provides a strong conceptual basis for modeling genotypic variation in response to elevated [CO₂].