Interactive Effects of Elevated CO2 and Ozone on Leaf Thermotolerance in Field-grown Glycine max

Humans are increasing atmospheric CO2, ground-level ozone (O3), and mean and acute high temperatures. Laboratory studies show that elevated CO2 can increase thermotolerance of photosynthesis in C3 plants. O3-related oxidative stress may offset benefits of elevated CO2 during heat-waves. We determined effects of elevated CO2 and O3 on leaf thermotolerance of field-grown Glycine max (soybean, C3). Photosynthetic electron transport (φet) was measured in attached leaves heated in situ and detached leaves heated under ambient CO2 and O3. Heating decreased φet, which O3 exacerbated. Elevated CO2 prevented O3-related decreases during heating, but only increased φet under ambient O3 in the field. Heating decreased chlorophyll and carotenoids, especially under elevated CO2. Neither CO2 nor O3 affected heat-shock proteins. Heating increased catalase (except in high O3) and CulZn-superoxide dismutase (SOD), but not MnSOD; CO2 and O3 decreased catalase but neither SOD. Soluble carbohydrates were unaffected by heating, but increased in elevated CO2. Thus, protection of photosynthesis during heat stress by elevated CO2 occurs in field-grown soybean under ambient O3, as in the lab, and high CO2 limits heat damage under elevated O3, but this protection is likely from decreased photorespiration and stomatal conductance rather than production of heat-stress adaptations.     

Mechanisms underlying the amelioration of O3-induced damage by elevated atmospheric concentrations of CO2

There is growing evidence that rising atmospheric CO2 concentrations will reduce or prevent reductions in the growth and productivity of C3 crops attributable to ozone (O3) pollution. In this study, the role of pollutant exclusion in mediating this response was investigated through growth chamber-based investigations on leaves 4 and 7 of spring wheat (Triticum aestivum cv. Hanno). In the core experiments, plants were raised at two atmospheric CO2 concentrations (ambient [350 micro l l(-1)] or elevated CO2 [700 micro l l(-1)] under two O3 regimes (charcoal/Purafil-filtered air [<5 nl l(-1) O3] or ozone-enriched air [75 nl l(-1) 7 h d(-1)]). A subsequent experiment used an additional O3 treatment where the goal was to achieve equivalent daily O3 uptake over the life-span of leaves 4 and 7 under ambient and CO2-enriched conditions, through daily adjustment of exposures based on measured shifts in stomatal conductance. Plant growth and net CO2 assimilation were stimulated by CO2-enrichment and reduced by exposure to O3. However, the impacts of O3 decreased with plant age (i.e. leaf 7 was more resistant to O3 injury than leaf 4); a finding consistent with ontogenic shifts in the tolerance of plant tissue and/or acclimation to O3-induced oxidative stress. In the combined treatment, elevated CO2 protected against the adverse effects of O3 and reduced cumulative O3 uptake (calculated from measurements of stomatal conductance) by c. 10% and 35% over the life-span of leaves 4 and 7, respectively. Analysis of the relationship between O3 uptake and the decline in the maximum in vivo rate of Rubisco carboxylation (Vcmax) revealed the protection afforded by CO2-enrichment to be due, to a large extent, to the exclusion of the pollutant from the leaf interior (as a consequence of the decline in stomatal conductance triggered by CO2-enrichment), but there was evidence (especially from flux-response relationships constructed for leaf 4) that CO2-enrichment resulted in additional effects that alleviated the impacts of ozone-induced oxidative stress on photosynthesis.     

臭氧胁迫对大豆叶片抗坏血酸-谷胱甘肽循环的影响

由于城市化的加剧导致近地面臭氧（O3）浓度日益增加,对植物生长和生态系统的功能产生了显著影响,因此准确评估近地层O3浓度升高对植物的影响具有重要意义。本文利用开顶式气室（OTCs）,系统探讨了模拟O3胁迫下大豆抗氧化系统抗坏血酸（AsA）-谷胱甘肽（GSH）循环清除活性氧（ROS）的机制及其对植株生长发育的影响。结果表明,在整个生育期内,与对照相比, 80?10 nL?L-1和110?10 nL?L-1 O3可以使大豆叶片丙二醛（MDA）含量、相对电导率增大,超氧阴离子（O2 ）产生速率、过氧化氢（H2O2）含量升高,超氧化物歧化酶（SOD）活性减弱; AsA-GSH循环中的AsA、GSH含量减少,脱氢抗坏血酸（DHA）、氧化型谷胱甘肽（GSSG）含量增加,过氧化物酶（APX）、单脱氢抗坏血酸还原酶（MDHAR）、谷胱甘肽还原酶（GR）活性呈现出前期增强后期减弱趋势,而脱氢抗坏血酸还原酶（DHAR）活性呈现出增强-减弱-增强的趋势。以上结果说明,O3浓度升高促进了大豆叶片ROS的代谢速率,降低了AsA-GSH循环效率,表明抗氧化系统不能长时间忍受高浓度O3带来的氧化伤害,从而使膜脂过氧化程度加重,对大豆表现为伤害效应。     

二氧化碳和臭氧浓度升高对春小麦生长及次生代谢的影响

通过开顶式气室(OTCs)研究了OTC对照（自然CO₂浓度约342 μmol·mol^-1,O₃浓度约30 nmol·mol^-1）、高浓度CO₂（550 μmol·mol^-1）、高浓度O₃（浓度为80 nmol·mol^-1）及其交互作用（CO₂ 550 μmol ·mol^-1,O₃ 80 nmol·mol^-1）对春小麦不同发育时期生物量、总酚量、黄酮含量及成熟期产量性状的影响.结果表明：CO₂浓度增加条件下,春小麦生物量和产量性状都显著高于OTC对照（P<0.05）;而O₃浓度升高条件下,小麦生物量降低,株高、穗长、穗粒质量及千粒重也显著低于对照;CO₂和O₃交互作用下各项指标处于二者之间.说明CO₂可以缓解O₃对小麦的负效应,而O₃对CO₂的正效应具有削弱作用,但二者的作用并非简单的叠加.CO₂、O₃浓度增加及其交互作用显著增加了春小麦叶片中的总酚含量,其中两者交互作用的效应更大,但在小麦生长后期,总酚含量增加量比对照有所降低.在小麦生长前期,各处理总黄酮含量均低于对照;而在成熟期,各处理都显著高于对照.     

Photosynthetic acclimation to elevated CO2 in a sunflower canopy

Sunflower canopies were grown in mesocosom gas exchange chambers at ambient and elevated CO2 concentrations (360 and 700 ppm) and leaf photosynthetic capacities measured at several depths within each canopy. Elevated [CO2] had little effect on whole-canopy photosynthetic capacity and total leaf area, but had marked effects on the distribution of photosynthetic capacity and leaf area within the canopy. Elevated [CO2] did not significantly reduce the photosynthetic capacities per unit leaf area of young leaves at the top of the canopy, but it did reduce the photosynthetic capacities of older leaves by as much as 40%. This effect was not dependent on the canopy light environment since elevated [CO2] also reduced the photosynthetic capacities of older leaves exposed to full sun on the south edge of the canopy. In addition to the effects on leaf photosynthetic capacity, elevated [CO2] shifted the distribution of leaf area within the canopy so that more leaf area was concentrated near the top of the canopy. This change resulted in as much as a 50% reduction in photon flux density in the upper portions of the elevated [CO2] canopy relative to the ambient [CO2] canopy, even though there was no significant difference in the total canopy leaf area. This reduction in PFD appeared to account for leaf carbohydrate contents that were actually lower for many of the shaded leaves in the elevated as opposed to the ambient [CO2] canopy. Photosynthetic capacities were not significantly correlated with any of the individual leaf carbohydrate contents. However, there was a strong negative correlation between photosynthetic capacity and the ratio of hexose sugars to sucrose, consistent with the hypothesis that sucrose cycling is a component of the biochemical signalling pathway controlling photosynthetic acclimation to elevated [CO2].     

自由空气中臭氧浓度升高对大豆的影响

人类活动导致的大气和气候变化将极大地改变作物未来的生长环境,其中一个显著的变化就是近地层空气污染物臭氧浓度的迅速上升:从工业革命前低于10nL/L上升到现在的50nL/L(夏季每天8h平均),最新预测这一浓度将在2015-2050年增加20%-25%,本世纪末将增加40%-60%。目前大气背景臭氧浓度已经超过敏感植物的伤害阀值(即40nL/L),广泛地造成农作物减产,而未来臭氧浓度增加将使这种影响变得更为严重。与封闭式和开顶式试验相比,FACE(free-air gas concentration enrichment)研究使用标准的作物管理技术,在完全开放的农田条件下运行,代表了人类对未来大气环境的最好模拟。作为人类食物蛋白的重要来源,大豆是世界上种植面积最大的双子叶植物,也是1年生C3作物的模式作物,同时也被认为对臭氧污染最为敏感的作物之一。美国伊利诺伊大学的大豆FACE(SoyFACE)是世界上第1个利用FACE技术开展农作物对高浓度臭氧(模拟本世纪中叶近地层臭氧浓度)响应和适应的多学科合作研究。在阐述气室研究的局限性和介绍SoyFACE运行特点的基础上,首次综述了FACE情形下高浓度臭氧对大豆光合特性、冠层结构、物质生产与分配、产量及其构成因素以及虫害等方面的影响,并比较了FACE与气室研究结果的异同点。SoyFACE研究清楚地表明臭氧对未来粮食安全的影响必须作为一个重要的全球变化因子来加以考虑。利用FACE技术深入开展臭氧及其与其它全球变化因子的互作对世界主要粮食作物的影响、机制和调控的系统研究,是该领域未来优先考虑的方向。     

Chronic ozone exposure affects leaf senescence of adult beech trees: a chlorophyll fluorescence approach

Accelerated leaf senescence is one of the harmful effects of elevated tropospheric ozone concentrations ([O(3)]) on plants. The number of studies dealing with mature forest trees is scarce however. Therefore, five 66-year-old beech trees (Fagus sylvatica L.) have been exposed to twice-ambient (2xambient) [O(3)] levels by means of a free-air canopy O(3) exposure system. During the sixth year of exposure, the hypothesis of accelerated leaf senescence in 2xambient [O(3)] compared with ambient [O(3)] trees was tested for both sun and shade leaves. Chlorophyll (chl) fluorescence was used to assess the photosynthetic quantum yield, and chl fluorescence images were processed to compare functional leaf homogeneity and the proportion of O(3)-injured leaf area (stipples) under ambient and 2xambient [O(3)] regimes. Based on the analysis of chl fluorescence images, sun leaves of both ambient and 2xambient [O(3)] trees had apparently developed typical necrotic O(3) stipples during high O(3) episodes in summer, while accelerated senescence was only observed with sun leaves of 2xambient [O(3)] trees. This latter effect was indicated along with a faster decrease of photosynthetic quantum yield, but without evidence of changes in non-photochemical quenching. Overall, treatment effects were small and varied among trees. Therefore, compared with ambient [O(3)], the consequence of the observed O(3)-induced accelerated leaf senescence for the carbon budget is likely limited.     

大豆对臭氧、二氧化碳及其复合效应的响应

以大豆‘中黄14'为试验材料,首次模拟研究大气中O₃、CO₂浓度增加,及其逐渐和持续增加O₃、CO₂浓度复合效应对大豆的影响.结果表明,CO₂浓度增加可缓解O₃对叶片的伤害程度,受害时间推迟,受害症状无实质性变化.熏气20 d测定各处理叶片生理参数发现,在本底大气环境下,叶片气孔阻力和蒸腾速率与对照差异较小,熏气时O₃、CO₂浓度增加诱导叶片气孔关闭,气孔阻力明显增加,蒸腾速率显著降低.与对照相比,O₃浓度增加,大豆干物质积累、产量和收获指数明显降低,籽粒粗脂肪含量明显减少,粗蛋白含量显著增加;CO₂浓度增加,干物质积累和产量显著提高,收获指数无明显差异,籽粒粗脂肪和粗蛋白含量均明显减少;逐渐和持续增加O₃和CO₂浓度复合效应处理下,大豆干物质积累、产量和收获指数差异不明显,籽粒粗蛋白含量不同程度地减少,粗脂肪含量显著增加.     

Acclimation of Respiratory O2 Uptake in Green Tissues of Field-Grown Native Species after Long-Term Exposure to Elevated Atmospheric CO2

C3 and C4 plants were grown in open-top chambers in the field at two CO2 concentrations, normal ambient (ambient) and normal ambient + 340 [mu]LL-1 (elevated). Dark oxygen uptake was measured in leaves and stems using a liquid-phase Clark-type oxygen electrode. High CO2 treatment decreased dark oxygen uptake in stems of Scirpus olneyi (C3) and leaves of Lindera benzoin (C3) expressed on either a dry weight or area basis. Respiration of Spartina patens (C4) leaves was unaffected by CO2 treatment. Leaf dry weight per unit area was unchanged by CO2, but respiration per unit of carbon or per unit of nitrogen was decreased in the C3 species grown at high CO2. The component of respiration in stems of S. olneyi and leaves of L. benzoin primarily affected by long-term exposure to the elevated CO2 treatment was the activity of the cytochrome pathway. Elevated CO2 had no effect on activity and capacity of the alternative pathway in S. olneyi. The cytochrome c oxidase activity, assayed in a cell-free extract, was strongly decreased by growth at high CO2 in stems of S. olneyi but it was unaffected in S. patens leaves. The activity of cytochrome c oxidase and complex III extracted from mature leaves of L. benzoin was also decreased after one growing season of plant exposure to elevated CO2 concentration. These results show that in some C3 species respiration will be reduced when plants are grown in elevated atmospheric CO2. The possible physiological causes and implications of these effects are discussed.     

Decreases in stomatal conductance of soybean under open-air elevation of [CO2] are closely coupled with decreases in ecosystem evapotranspiration

Stomatal responses to atmospheric change have been well documented through a range of laboratory- and field-based experiments. Increases in atmospheric concentration of CO(2) ([CO(2)]) have been shown to decrease stomatal conductance (g(s)) for a wide range of species under numerous conditions. Less well understood, however, is the extent to which leaf-level responses translate to changes in ecosystem evapotranspiration (ET). Since many changes at the soil, plant, and canopy microclimate levels may feed back on ET, it is not certain that a decrease in g(s) will decrease ET in rain-fed crops. To examine the scaling of the effect of elevated [CO(2)] on g(s) at the leaf to ecosystem ET, soybean (Glycine max) was grown in field conditions under control (approximately 375 micromol CO(2) mol(-1) air) and elevated [CO(2)] (approximately 550 micromol mol(-1)) using free air CO(2) enrichment. ET was determined from the time of canopy closure to crop senescence using a residual energy balance approach over four growing seasons. Elevated [CO(2)] caused ET to decrease between 9% and 16% depending on year and despite large increases in photosynthesis and seed yield. Ecosystem ET was linked with g(s) of the upper canopy leaves when averaged across the growing seasons, such that a 10% decrease in g(s) results in a 8.6% decrease in ET; this relationship was not altered by growth at elevated [CO(2)]. The findings are consistent with model and historical analyses that suggest that, despite system feedbacks, decreased g(s) of upper canopy leaves at elevated [CO(2)] results in decreased transfer of water vapor to the atmosphere.     

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

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

Flux-based response of sucrose and starch in leaves of adult beech trees (Fagus sylvatica L.) under chronic free-air O3 fumigation

Investigations on sucrose and starch contents in leaves of 60-year-old beech trees ( FAGUS SYLVATICA L.) are the focus of the present study. Five trees were exposed to a twice ambient ozone regime (2 x O(3)) with a free-air canopy exposure system throughout the seasons and five trees under the prevailing ambient ozone regime served as controls (1 x O(3)). In order to examine chronic ozone (O(3)) effects, leaf samples from the sun and shade crowns of the trees were analyzed five times throughout the growing seasons in 2003 and 2004. Sucrose concentrations of leaves collected in 2004 were consistently lower than those taken in 2003, regardless of the O(3) treatment and crown position. However, the opposite was found for starch. O(3) caused a reduction of sucrose and starch contents of sun leaves in both years. Due to the fact that O(3)-responsiveness depends on the O(3) uptake through stomata during the season, all carbohydrate data were related to the cumulative O(3) uptake (COU). Little differences were found comparing sucrose and starch contents in leaves of trees grown under ambient or elevated O (3) regimes, possibly indicating the high capacity of leaves of adult beech to cope with rising O(3) exposure. Even under 2 x O(3), leaves were still able to regulate the O(3) intake by narrowing their stomata at the cost of CO(2)-uptake and sugar synthesis. In order to clarify whole-tree response patterns carbohydrate data were compared with photosynthesis, stomatal conductance and electron transport rates. In 2004 all parameters revealed a significant common response pattern to COU that indicated a reduction for all parameters under 2 x O(3).     

Combined effects of chronic ozone and elevated CO2 on Rubisco activity and leaf components in soybean Glycine max

Content and activity of Rubisco and concentrations of leaf nitrogen, chlorophyll and total non-structural carbohydrates (TNC) were determined at regular intervals during the 1993 and 1994 growing seasons to understand the effects and interactions of [O3] and elevated [CO2] on biochemical limitations to photosynthesis during ontogeny. Soybean (Glycine max var. Essex) was grown in open-top field chambers in either charcoal-filtered air (CF, 20 nmol mol^-1) or non-filtered air supplemented with 1.5 x ambient [O3] (c. 80 nmol mol^-1) at ambient (AA, 360 mol mol^-1) or elevated [CO2] (700 mol mol^-1). Sampling period significantly affected all the variables examined. Changes included a decrease in the activity and content of Rubisco during seed maturation, and increased nitrogen (N), leaf mass per unit area (LMA) and total non-structural carbohydrates (TNC, including starch and sucrose) through the reproductive phases. Ontogenetic changes were most rapid in O2-treated plants. At ambient [CO2], O3 decreased initial activity (14-64% per unit leaf area and 14-29% per unit Rubisco) and content of Rubisco (9-53%), and N content per unit leaf area. Ozone decreased LMA by 17-28% of plants in CF-AA at the end of the growing season because of a 24-41% decrease in starch and a 59-80% decrease in sucrose. In general, elevated CO2], in CF or O3-fumigated air, reduced the initial activity of Rubisco and activation state while having little effect on Rubisco content, N and the chlorophyll content, per unit leaf area. Elevated CO2 decreased Rubisco activity by 14-34% per unit leaf area and 15-25% per unit Rubisco content of plants in grown CF-AA, nd increases LMA by 27-74% of the leaf mass per unit area in CF-AA because of a 23-148% increase in starch. However, the data suggest that, at elevated [CO2], increases in starch and sucrose are not directly responsible for the deactivation of Rubisco. Also, there was little evidence of an adjustment of Rubisco activity in response to starch and sucrose metabolism. Significant interactions between elevated [CO2] and [O3] on all variables examined generally resulted in alleviation or amelioration of the O3 effects at elevated CO2. These data provide further support to the idea that elevated atmospheric CO2 will reduce or prevent damage from pollutant O3.     

Rising ozone concentrations decrease soybean evapotranspiration and water use efficiency whilst increasing canopy temperature

? Here, we investigated the effects of increasing concentrations of ozone ([O(3)]) on soybean canopy-scale fluxes of heat and water vapor, as well as water use efficiency (WUE), at the Soybean Free Air Concentration Enrichment (SoyFACE) facility. ? Micrometeorological measurements were made to determine the net radiation (R(n)), sensible heat flux (H), soil heat flux (G(0)) and latent heat flux (λET) of a commercial soybean (Glycine max) cultivar (Pioneer 93B15), exposed to a gradient of eight daytime average ozone concentrations ranging from approximately current (c. 40 ppb) to three times current (c. 120 ppb) levels. ? As [O(3)] increased, soybean canopy fluxes of λET decreased and H increased, whereas R(n) and G(0) were not altered significantly. Exposure to increased [O(3)] also resulted in warmer canopies, especially during the day. The lower λET decreased season total evapotranspiration (ET) by c. 26%. The [O(3)]-induced relative decline in ET was half that of the relative decline in seed yield, driving a 50% reduction in seasonal WUE. ? These results suggest that rising [O(3)] will alter the canopy energy fluxes that drive regional climate and hydrology, and have a negative impact on productivity and WUE, key ecosystem services.     

Longevity and fecundity of Japanese beetle (Popillia japonica) on foliage grown under elevated carbon dioxide

Atmospheric levels of carbon dioxide (CO(2)) have been increasing steadily over the last century. Plants grown under elevated CO(2) experience physiological changes that influence their suitability as food. Previous studies have found increased insect herbivory on plants grown under elevated CO(2). To determine effects of consuming foliage of soybean (Glycine max) grown under elevated CO(2) on adult survivorship and fecundity, Japanese beetles (Popillia japonica Newman) were fed for the duration of their adult lives leaves grown under elevated CO(2) (550 mumol/mol), under ambient atmosphere (370 mumol/mol), or grown under ambient atmosphere but supplemented with a solution of sugars. To determine effects of a diet of foliage grown under elevated ozone (O(3)), another anthropogenic gaseous pollutant, beetles in the laboratory were fed soybean leaves grown under elevated CO(2), elevated O(3), or a combination of both elevated gases. Leaf tissue was also analyzed for longevity-enhancing antioxidants, because increases in dietary antioxidants can increase lifespan. Lifespan of Japanese beetles was prolonged by 8-25% when fed foliage developed under elevated CO(2), but consuming foliage that had taken up sugars to approximately the same level as foliage grown under elevated CO(2) had no effect on fecundity or longevity. Females consuming elevated CO(2) foliage laid approximately twice as many eggs as females fed foliage grown under ambient conditions. Consuming foliage grown under elevated O(3) had no effect on fecundity. No significant differences in total antioxidant content of foliage from ambient and elevated CO(2) conditions were detected. Although the precise mechanism is unclear, by altering components of leaf chemistry other than sugar content, elevated CO(2) may increase populations of Japanese beetles and their impact on crop productivity.     

Effects of elevated carbon dioxide and ozone on the phytochemistry of aspen and performance of an herbivore

The purpose of this study was to assess the independent and interactive effects of CO(2), O(3), and plant genotype on the foliar quality of a deciduous tree and the performance of a herbivorous insect. Two trembling aspen (Populus tremuloides Michaux) genotypes differing in response to CO(2) and O(3) were grown at the Aspen FACE (Free Air CO(2) Enrichment) site located in northern Wisconsin, USA. Trees were exposed to one of four atmospheric treatments: ambient air (control), elevated carbon dioxide (+CO(2); 560 microl/l), elevated ozone (+O(3); ambient x1.5), and elevated CO(2)+O(3). We measured the effects of CO(2) and O(3) on aspen phytochemistry and on performance of forest tent caterpillar (Malacosoma disstria Hübner) larvae. CO(2) and O(3) treatments influenced foliar quality for both genotypes, with the most notable effects being that elevated CO(2) reduced nitrogen and increased tremulacin levels, whereas elevated O(3) increased early season nitrogen and reduced tremulacin levels, relative to controls. With respect to insects, the +CO(2) treatment had little or no effect on larval performance. Larval performance improved in the +O(3) treatment, but this response was negated by the addition of elevated CO(2) (i.e., +CO(2)+O(3) treatment). We conclude that tent caterpillars will have the greatest impact on aspen under current CO(2) and high O(3) levels, due to increases in insect performance and decreases in tree growth, whereas tent caterpillars will have the least impact on aspen under high CO(2) and low O(3) levels, due to moderate changes in insect performance and increases in tree growth.     

Hourly and seasonal variation in photosynthesis and stomatal conductance of soybean grown at future CO(2) and ozone concentrations for 3 years under fully open-air field conditions

It is anticipated that enrichment of the atmosphere with CO(2) will increase photosynthetic carbon assimilation in C3 plants. Analysis of controlled environment studies conducted to date indicates that plant growth at concentrations of carbon dioxide ([CO(2)]) anticipated for 2050 ( approximately 550 micromol mol(-1)) will stimulate leaf photosynthetic carbon assimilation (A) by 20 to 40%. Simultaneously, concentrations of tropospheric ozone ([O(3)]) are expected to increase by 2050, and growth in controlled environments at elevated [O(3)] significantly reduces A. However, the simultaneous effects of both increases on a major crop under open-air conditions have never been tested. Over three consecutive growing seasons > 4700 individual measurements of A, photosynthetic electron transport (J(PSII)) and stomatal conductance (g(s)) were measured on Glycine max (L.) Merr. (soybean). Experimental treatments used free-air gas concentration enrichment (FACE) technology in a fully replicated, factorial complete block design. The mean A in the control plots was 14.5 micromol m(-2) s(-1). At elevated [CO(2)], mean A was 24% higher and the treatment effect was statistically significant on 80% of days. There was a strong positive correlation between daytime maximum temperatures and mean daily integrated A at elevated [CO(2)], which accounted for much of the variation in CO(2) effect among days. The effect of elevated [CO(2)] on photosynthesis also tended to be greater under water stress conditions. The elevated [O(3)] treatment had no statistically significant effect on mean A, g(s) or J(PSII) on newly expanded leaves. Combined elevation of [CO(2)] and [O(3)] resulted in a slightly smaller increase in average A than when [CO(2)] alone was elevated, and was significantly greater than the control on 67% of days. Thus, the change in atmospheric composition predicted for the middle of this century will, based on the results of a 3 year open-air field experiment, have smaller effects on photosynthesis, g(s) and whole chain electron transport through photosystem II than predicted by the substantial literature on relevant controlled environment studies on soybean and likely most other C3 plants.     

Gas exchange and antioxidative compounds in young beech trees under free-air ozone exposure and comparisons to adult trees

Three-year-old beech (Fagus sylvatica) seedlings growing in containers were placed into the sun and shade crown of a mature beech stand exposed to ambient (1 x O(3)) and double ambient (2 x O(3)) ozone concentrations at a free-air exposure system ("Kranzberg Forst", Germany). Pigments, alpha-tocopherol, glutathione, ascorbate, and gas exchange were measured in leaves during 2003 (a drought year) and 2004 (an average year). Sun-exposed seedlings showed higher contents of antioxidants, xanthophylls, and beta-carotene and lower contents of chlorophyll, alpha-carotene, and neoxanthin than shade-exposed seedlings. In 2003 sun-exposed seedlings showed higher contents of carotenoids and total glutathione and lower net photosynthesis rates (A(max)) compared to 2004. O(3) exposure generally affected the content of chlorophyll, the xanthophyll cycle, and the intercellular CO(2) concentration (c(i)). Seedlings differed from the adjacent adult trees in most biochemical and physiological parameters investigated: Sun exposed seedlings showed higher contents of alpha-tocopherol and xanthophylls and lower contents of ascorbate, chlorophyll, neoxanthin, and alpha-carotene compared to adult trees. Shade exposed seedlings had lower contents of xanthophylls, alpha-carotene, and alpha-tocopherol than shade leaves of old-growth trees. In 2003, seedlings had higher A(max), stomatal conductance (g(s)), and c(i) under 2 x O(3) than adult trees. The results showed that shade acclimated beech seedlings are more sensitive to O(3), possibly due to a lower antioxidative capacity per O(3) uptake. We conclude that beech seedlings are uncertain surrogates for adult beech trees.     

Effects of elevated [CO2] and/or ozone on limitations to CO2 assimilation in soybean (Glycine max)

Soybean (Glycine max) was grown in open-top field chambers at ambient (360 mol mol^-1) or doubled [CO2] either in charcoal-filtered air (20 nmol mol^-1 [O3]) or in non-filtered air supplemented to 1,5 x ambient [O3] (70 nmol mol^-1) to determine the major limitations to assimilation under conditions of elevated [CO2] and/or [O3]. Through plant ontogeny, assimilation versus intercellular CO2 concentration (A/Ci) responses were measured to assess the limitations to assimilation imposed by the capacity for Rubisco carboxylation, RuBP regeneration, and stomatal diffusion.In the vegetative stages, no significant treatment effects of elevated [CO2] and/or [O3] were observed on Rubisco carboxylation efficiency (CE), light and CO2-saturated assimilation capacity (Amax), and chlorophyll content (Chl). However, for plants grown in elevated [CO2], the assimilation rate at growth [CO2] (A) was 60% higher than at ambient [CO2] up to the seed maturation stage, and the potential rate of assimilation by Rubisco capacity (Ap) was increased. Also in elevated [CO2]: A was 51% of Ap; the relative stomatal limitation (%Stomata) was 5%; and the relative RuBP regeneration limitation (%RuBP) was 44%. In ambient [CO2], O3 gradually decreased A per unit leaf area, but had little effect on Ap and the relative limitations to assimilation where A remained 51% of Ap, %Stomata was 27%, and %RuBP was 22%.During reproduction, CE declined for plants grown in elevated [CO2] and/or [O3]; Ap was unaffected by elevated [CO2], but was reduced by [O3] at ambient [CO2]; A increased to 72% of Ap in elevated [CO2] and/or [O3]-fumigated air; the %Stomata increased; and the %RuBP decreased, to become non significant in elevated [CO2] from the beginning of seed growth on, and in O3-fumigated air at ambient [CO2] at the seed maturation stage. The decrease in %RuBP occurred concomitantly with an increase in Amax and Chl. Significant [CO2] x [O3] interactions support the lack of an O3 effect on assimilation and its limitations at elevated [CO2] during seed maturation. These data suggest that elevated [CO2] alleviated some of the effects of O3 on photosynthesis.Keywords: CO2 by O3 interactions, elevated [CO2], O3 fumigation, Rubisco carboxylation efficiency, RuBP regeneration.