Water relations and gas exchange in poplar and willow under water stress and elevated atmospheric CO2

Predictions of shifts in rainfall patterns as atmospheric [CO2] increases could impact the growth of fast growing trees such as Populus spp. and Salix spp. and the interaction between elevated CO2 and water stress in these species is unknown. The objectives of this study were to characterize the responses to elevated CO2 and water stress in these two species, and to determine if elevated CO2 mitigated drought stress effects. Gas exchange, water potential components, whole plant transpiration and growth response to soil drying and recovery were assessed in hybrid poplar (clone 53-246) and willow (Salix sagitta) rooted cuttings growing in either ambient (350 &mgr;mol mol-1) or elevated (700 &mgr;mol mol-1) atmospheric CO2 concentration ([CO2]). Predawn water potential decreased with increasing water stress while midday water potentials remained unchanged (isohydric response). Turgor potentials at both predawn and midday increased in elevated [CO2], indicative of osmotic adjustment. Gas exchange was reduced by water stress while elevated [CO2] increased photosynthetic rates, reduced leaf conductance and nearly doubled instantaneous transpiration efficiency in both species. Dark respiration decreased in elevated [CO2] and water stress reduced Rd in the trees growing in ambient [CO2]. Willow had 56% lower whole plant hydraulic conductivity than poplar, and showed a 14% increase in elevated [CO2] while poplar was unresponsive. The physiological responses exhibited by poplar and willow to elevated [CO2] and water stress, singly, suggest that these species respond like other tree species. The interaction of [CO2] and water stress suggests that elevated [CO2] did mitigate the effects of water stress in willow, but not in poplar.     

Increased C availability at elevated carbon dioxide concentration improves N assimilation in a legume

Plant growth is typically stimulated at elevated carbon dioxide concentration ([CO2]), but a sustained and maximal stimulation of growth requires acquisition of additional N in proportion to the additional C fixed at elevated [CO2]. We hypothesized that legumes would be able to avoid N limitation at elevated [CO2]. Soybean was grown without N fertilizer from germination to final senescence at elevated [CO2] over two growing seasons under fully open-air conditions, providing a model legume system. Measurements of photosynthesis and foliar carbohydrate content showed that plants growing at elevated [CO2] had a c. 25% increase in the daily integral of photosynthesis and c. 58% increase in foliar carbohydrate content, suggesting that plants at elevated [CO2] had a surplus of photosynthate. Soybeans had a low leaf N content at the beginning of the season, which was a further c. 17% lower at elevated [CO2]. In the middle of the season, ureide, total amino acid and N content increased markedly, and the effect of elevated [CO2] on leaf N content disappeared. Analysis of individual amino acid levels supported the conclusion that plants at elevated [CO2] overcame an early-season N limitation. These soybean plants showed a c. 16% increase in dry mass at final harvest and showed no significant effect of elevated [CO2] on leaf N, protein or total amino acid content in the latter part of the season. One possible explanation for these findings is that N fixation had increased, and that these plants had acclimated to the increased N demand at elevated [CO2].     

Soil and plant water relations determine photosynthetic responses of C3 and C4 grasses in a semi-arid ecosystem under elevated CO2

To model the effect of increasing atmospheric CO2 on semi-arid grasslands, the gas exchange responses of leaves to seasonal changes in soil water, and how they are modified by CO2, must be understood for C3 and C4 species that grow in the same area. In this study, open-top chambers were used to investigate the photosynthetic and stomatal responses of Pascopyrum smithii (C3) and Bouteloua gracilis (C4) grown at 360 (ambient CO2) and 720 micro mol mol-1 CO2 (elevated CO2) in a semi-arid shortgrass steppe. Assimilation rate (A) and stomatal conductance (gs) at the treatment CO2 concentrations and at a range of intercellular CO2 concentrations and leaf water potentials (psileaf) were measured over 4 years with variable soil water content caused by season and CO2 treatment. Carboxylation efficiency of ribulose bisphosphate carboxylase/oxygenase (Vc,max), and ribulose bisphosphate regeneration capacity (Jmax) were reduced in P. smithii grown in elevated CO2, to the degree that A was similar in elevated and ambient CO2 (when soil moisture was adequate). Photosynthetic capacity was not reduced in B. gracilis under elevated CO2, but A was nearly saturated at ambient CO2. There were no stomatal adaptations independent of photosynthetic acclimation. Although photosynthetic capacity was reduced in P. smithii growing in elevated CO2, reduced gs and transpiration improved soil water content and psileaf in the elevated CO2 chambers, thereby improving A of both species during dry periods. These results suggest that photosynthetic responses of C3 and C4 grasses in this semi-arid ecosystem will be driven primarily by the effect of elevated CO2 on plant and soil water relations.     

CO2浓度升高对沉水克隆植物生长速率及营养元素积累的影响

 研究在不同CO2浓度下水生克隆植物刺苦草（Vallisneria spinulosa）整个生活周期中生长的动态变化及对营养元素积累情况。在不损伤植物体的前提下,采用刺苦草形态学指标组合史估计了植株生物量的动态变化。结果表明：刺苦草鳞茎的萌发不受CO2浓度变化的影响。在高浓度CO2即(1000±50) μmol·mol-1下,刺苦草源株地上部分生长速率在整个生长前期和中期都远远高于低浓度CO2即(400±20) μmol·mol-1,而在后期则出现相反的现象,其中一个原因是因为高浓度CO2下后期光合物质向地下大量转移形成鳞茎引起地上部分生长减慢。但高浓度CO2下克隆株中的初级和次级分株生长速率均高于低浓度CO2。在两种CO2浓度下相同克隆植株构件中的总碳含量没有明显差异;除鳞茎外,根、叶、匍匐茎中的总磷含量随CO2浓度升高显著增加;由于各构件生物量增加有明显差异,导致叶和鳞茎因为生物稀释作用而使其含氮量降低了12%～14%,但根和茎中含量基本保持不变。在高浓度CO2中植株总生物量显著升高,所以总碳、总氮和总磷吸收量均显著大于在低浓度CO2中的吸收量。研究结果揭示,大气CO2浓度升高对沉水克隆植物生长的促进,有利于提高水生克隆植物在群落中的竞争能力;水生植物克隆生长将增加水生生态系统中碳的沉积;水环境中N、P含量将直接影响到水生克隆植物生长。     

The relationship between leaf composition and morphology at elevated CO2 concentrations

The composition and morphology of leaves exposed to elevated [CO₂] usually change so that the leaf nitrogen (N) per unit dry mass decreases and the leaf dry mass per unit area increases. However, at ambient [CO₂], leaves with a high leaf dry mass per unit area usually have low leaf N per unit dry mass. Whether the changes in leaf properties induced by elevated [CO₂] follow the same overall pattern as that at ambient [CO₂] has not previously been addressed. Here we address this issue by using leaf measurements made at ambient [CO₂] to develop an empirical model of the composition and morphology of leaves. Predictions from that model are then compared with a global database of leaf measurements made at ambient [CO₂]. Those predictions are also compared with measurements showing the impact of elevated [CO₂]. In the empirical model both the leaf dry mass and liquid mass per unit area are positively correlated with leaf thickness, whereas the mass of C per unit dry mass and the mass of N per unit liquid mass are constant. Consequently, both the N:C ratio and the surface area:volume ratio of leaves are positively correlated with the liquid content. Predictions from that model were consistent with measurements of leaf properties made at ambient [CO₂] from around the world. The changes induced by elevated [CO₂] follow the same overall trajectory. It is concluded that elevated [CO₂] enhances the rate at which dry matter is accumulated but the overall trajectory of leaf development is conserved.     

Photosynthetic stimulation under long-term CO2 enrichment and fertilization is sustained across a closed Populus canopy profile (EUROFACE)

The long-term response of leaf photosynthesis to rising CO2 concentrations [CO2] depends on biochemical and morphological feedbacks. Additionally, responses to elevated [CO2] might depend on the nutrient availability and the light environment, affecting the net carbon uptake of a forest stand. After 6 yr of exposure to free-air CO2 enrichment (EUROFACE) during two rotation cycles (with fertilization during the second cycle), profiles of light, leaf characteristics and photosynthetic parameters were measured in the closed canopy of a poplar (Populus) short-rotation coppice. Net photosynthetic rate (A(growth)) was 49% higher in poplars grown in elevated [CO2], independently of the canopy position. Jmax significantly increased (15%), whereas leaf carboxylation capacity (Vcmax), leaf nitrogen (N(a)) and chlorophyll (Chl(a)) were unaffected in elevated [CO2]. Leaf mass per unit area (LMA) increased in the upper canopy. Fertilization created more leaves in the top of the crown. These results suggest that the photosynthetic stimulation by elevated [CO2] in a closed-canopy poplar coppice might be sustained in the long term. The absence of any down-regulation, given a sufficient sink capacity and nutrient availability, provides more carbon for growth and storage in this bioenergy plantation.     

Phosphorus supply and arbuscular mycorrhizas increase growth and net gas exchange responses of two Citrus spp. grown at elevated [CO2]

We hypothesized that greater photosynthate supply at elevated [CO₂] could compensate for increased below-ground C demands of arbuscular mycorrhizas. Therefore, we investigated plant growth, mineral nutrition, starch, and net gas exchange responses of two Citrus spp. to phosphorus (P) nutrition and mycorrhizas at elevated atmospheric [CO₂]. Half of the seedlings of sour orange (C. aurantium L.) and ‘Ridge Pineapple’ sweet orange (C. sinensis L. Osbeck) were inoculated with the arbuscular mycorrhizal (AM) fungus, Glomus intraradices Schenck and Smith and half were non-mycorrhizal (NM). Plants were grown at ambient or 2X ambient [CO₂] in unshaded greenhouses for 11 weeks and fertilized daily with nutrient solution either without added P or with 2 mM P in a low-P soil. High P supply reduced AM colonization whereas elevated [CO₂] counteracted the depressive effect of P on intraradical colonization and vesicle development. Seedlings grown at either elevated [CO₂], high P or with G. intraradices had greater growth, net assimilation of CO₂ (A _CO2) in leaves, leaf water-use efficiency, leaf dry wt/area, leaf starch and carbon/nitrogen (C/N) ratio. Root/whole plant dry wt ratio was decreased by elevated [CO₂], P, and AM colonization. Mycorrhizal seedlings had higher leaf-P status but lower leaf N and K concentrations than nonmycorrhizal seedlings which was due to growth dilution effects. Starch in fibrous roots was increased by elevated [CO₂] but reduced by G. intraradices, especially at low-P supply. In fibrous roots, elevated [CO₂] had no effect on C/N, but AM colonization decreased C/N in both Citrus spp. grown at low-P supply. Overall, there were no species differences in growth or A _CO2. Mycorrhizas did not increase plant growth at ambient [CO₂]. At elevated [CO₂], however, mycorrhizas stimulated growth at both P levels in sour orange, the more mycorrhiza-dependent species, but only at low-P in sweet orange, the less dependent species. At low-P and elevated [CO₂], colonization by the AM fungus increased A _CO2 in both species but more so in sour orange than in sweet orange. Leaf P and root N concentrations were increased more and root starch level was decreased less by AM in sour orange than in sweet orange. Thus, the additional [CO₂] availability to mycorrhizal plants increased CO₂ assimilation, growth and nutrient uptake over that of NM plants especially in sour orange under P limitation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Atmospheric carbon dioxide and fertilizer nitrogen effects on radiation interception by rice 1

In order to predict the potential impacts of global change, it is important to understand the impact of increasing global atmospheric [CO₂] on the growth and yield of crop plants. The objectives of this study were to determine the interaction of N fertilization rates and atmospheric [CO₂] on radiation interception and radiation-use efficiency of rice (Oryza sativa L. cv. IR72) grown under tropical field conditions. Rice plants were grown inside open top chambers in a lowland rice field at the International Rice Research Institute in the Philippines at ambient (about 350 μmol mol^-1) or elevated (about 600 μmol mol^-1 during the 1993 wet season and 700 μmol mol^-1 during the 1994 dry season) in combination with three levels of applied N (0, 50 or 100 kg N ha^-1 in the wet season; 0, 90 or 200 kg N ha-1 in the dry season). Light interception was not directly affected by [CO₂], but elevated [CO₂] indirectly increased light interception through increasing total absorbed N. Plant N requirement for radiation interception was similar for rice grown under ambient [CO₂] or elevated [CO₂] treatments. The conversion efficiency of intercepted radiation to dry matter, radiation-use efficiency (RUE), was about 35% greater at elevated [CO₂] than at ambient [CO₂]. The relationship between leaf N and RUE was curvilinear. At ambient [CO₂], RUE was fairly stable across levels of leaf N, but leaf N less than about 2.5% resulted in lower RUE for plants grown with elevated [CO₂] than for plant grown at ambient [CO₂]. Decreased leaf N with increased [CO₂], therefore decreased RUE of rice plants grown at elevated [CO₂]. When predicting responses of rice to elevated [CO₂], RUE should be adjusted with a decrease in leaf N. This revised version was published online in June 2006 with corrections to the Cover Date.     

不同CO2浓度和N肥水平对粳稻茎鞘非结构性碳水化合物含量与积累的影响

为了解CO2浓度升高和N肥水平对水稻茎鞘内非结构性碳水化合物(NSC)含量和积累量的影响,利用开顶式气室(OTC),以常规粳稻"南粳9108"为试验材料,设置3个CO2浓度水平:对照T0(背景大气)、T0+120μmol·mol-1(T1)和T0+200μmol·mol-1(T2)。在OTC内采用盆栽方式,设置3个氮(N)肥水平:10 g N·m^-2(N1)、20 g N·m^-2(N2)和30g N·m^-2(N3)。分别于水稻抽穗期、灌浆期(抽穗后20 d)和成熟期对地上部分各器官生物量、茎鞘NSC含量以及顶部四张叶片的N含量进行分析。结果表明:CO2浓度升高对抽穗期叶N含量总体无显著影响,但显著降低灌浆期N2和N3水平的叶N含量;CO2浓度升高对抽穗期茎鞘NSC含量和积累量无显著影响,抽穗期置换到高CO2浓度环境使灌浆期茎鞘NSC积累显著增加,置换到低CO2浓度环境使NSC积累显著减少。同一CO2浓度条件下,NSC含量和积累量均为N1>N2>N3,且N1处理均显著高于N3处理,CO2浓度升高和N水平的交互作用对灌浆期茎鞘NSC含量影响显著。水稻产量在不同CO2浓度水平间无显著差异,但随施氮水平的提高而增加。抽穗期与灌浆期水稻茎鞘NSC含量和积累量与茎鞘干重呈极显著正相关,与叶N含量呈极显著负相关;叶N衰减越慢,灌浆期水稻茎鞘NSC残留比(RNSC)越低;结实率和产量与RNSC呈显著负相关,RNSC越大,茎鞘NSC转移的越少,结实率和产量越低。     

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.      

Olfactory preferences of Popillia japonica, Vanessa cardui, and Aphis glycines for Glycine max grown under elevated CO2

Levels of atmospheric CO(2) have been increasing steadily over the last century and are projected to increase even more dramatically in the future. Soybeans (Glycine max L.) grown under elevated levels of CO(2) have larger herbivore populations than soybeans grown under ambient levels of CO(2). Increased abundance could reflect the fact that these herbivores are drawn in by increased amounts of volatiles or changes in the composition of volatiles released by plants grown under elevated CO(2) conditions. To determine impacts of elevated CO(2) on olfactory preferences, Japanese beetles (Popillia japonica Newman) and soybean aphids (Aphis glycines Matsumura) were placed in Y-tube olfactometers with a choice between ambient levels of CO(2) gas versus elevated levels of CO(2) gas or damaged and undamaged leaves and plants grown under ambient levels of CO(2) versus damaged and undamaged plants grown under elevated levels of CO(2). All plants had been grown from seeds under ambient or elevated levels of CO(2). Painted lady butterflies (Vanessa cardui L.) were placed in an oviposition chamber with a choice between plants grown under ambient and elevated levels of CO(2). A. glycines and V. cardui showed no significant preference for plants in either treatment. P. japonica showed no significant preference between ambient levels and elevated levels of CO(2) gas. There was a significant P. japonica preference for damaged plants grown under ambient CO(2) versus undamaged plants but no preference for damaged plants grown under elevated CO(2) versus undamaged plants. P. japonica also preferred damaged plants grown under elevated levels of CO(2) versus damaged plants grown under ambient levels of CO(2). This lack of preference for damaged plants grown under elevated CO(2) versus undamaged plants could be the result of the identical elevated levels of a green leaf volatile (2-hexenal) present in all foliage grown under elevated CO(2) regardless of damage status. Green leaf volatiles are typically released from damaged leaves and are used as kairomones by many herbivorous insects for host plant location. An increase in production of volatiles in soybeans grown under elevated CO(2) conditions may lead to larger herbivore outbreaks in the future.     

Elevated CO2 reduces leaf damage by insect herbivores in a forest community

By altering foliage quality, exposure to elevated levels of atmospheric CO(2) potentially affects the amount of herbivore damage experienced by plants. Here, we quantified foliar carbon (C) and nitrogen (N) content, C : N ratio, phenolic levels, specific leaf area (SLA) and the amount of leaf tissue damaged by chewing insects for 12 hardwood tree species grown in plots exposed to elevated CO(2) (ambient plus 200 microl l(-1)) using free-air CO(2) enrichment (FACE) over 3 yr. The effects of elevated CO(2) varied considerably by year and across species. Elevated CO(2) decreased herbivore damage across 12 species in 1 yr but had no detectable effect in others. Decreased damage may have been related to lower average foliar N concentration and SLA and increased C : N ratio and phenolic content for some species under elevated compared with ambient CO(2). It remains unclear how these changes in leaf properties affect herbivory. Damage to the leaves of hardwood trees by herbivorous insects may be reduced in the future as the concentration of CO(2) continues to increase, perhaps altering the trophic structure of forest ecosystems.     

大气CO2浓度升高和N沉降对南亚热带主要乡土树种叶片元素含量的影响

大气CO₂浓度升高和N沉降以及二者之间的耦合作用对陆地森林生态系统的影响是当前国际生态学界关注的热点之一。该实验运用大型开顶箱(open-top chamber, OTC)研究: 1)高CO₂浓度(700 μmol×mol^-1) +高N沉降(100 kg N×hm^-2×a^-1) (CN); 2)高CO₂浓度(700 μmol×mol^-1)和背景N沉降(CC); 3)高N沉降(100 kg N×hm^-2×a^-1)和背景CO₂浓度(NN); 4)背景CO₂和背景N沉降(CK) 4种处理对南亚热带主要乡土树种木荷(Schima superba)、红锥(Castanopsis hystrix)、肖蒲桃(Acmena acuminatissima)、红鳞蒲桃(Syzygium hancei)、海南红豆(Ormosia pinnata)叶片元素含量的影响。研究结果表明, 大气CO₂浓度升高对5种乡土树种叶片元素含量有较大的影响, 除海南红豆叶片的Ca含量外, 其他树种的叶片元素含量在高CO₂浓度处理下都显著升高(p < 0.05); 而在N沉降处理下, 5个树种的叶片K和Ca含量都降低。大气CO₂浓度升高与N沉降处理对5种乡土树种植物叶片元素含量影响的交互作用不是很明显, 仅仅木荷和红鳞蒲桃的叶片Ca和Mn以及海南红豆的叶片Mn含量在大气CO₂浓度上升和N沉降交互处理下显著下降, 而肖蒲桃的叶片P含量在大气CO₂浓度上升和N沉降交互处理下显著上升。     

提高CO2浓度对两种亚热带树苗生物量及叶片特性的影响

 广东鼎湖山季风常绿阔叶林的主要优势乔木树种荷木和黧蒴幼苗生长于自然光照和人工调节CO2浓度为500±50μl·L-1或空气CO2(350μl·L-1)的气罩中3个月。高CO2浓度下生长的黧蒴和荷木植株总干物质量分别增加26.6％和16.6%,根部增加量最大,地上部分所占的比例降低,根冠比上升,基径增大而株高降低。高CO2浓度下生长的叶片密度及比叶重增加,叶肉细胞间隙体积减少。单位干重的黧蒴叶片可溶性糖含量、全碳、磷、钾含量在高CO2浓度下稍为下降,果糖、葡萄糖、蔗糖、全氮、镁含量及N/C比明显降低。而全钙含量无明显变化。     

Plant respiration and elevated atmospheric CO2 concentration: cellular responses and global significance

BACKGROUND: Elevated levels of atmospheric [CO2] are likely to enhance photosynthesis and plant growth, which, in turn, should result in increased specific and whole-plant respiration rates. However, a large body of literature has shown that specific respiration rates of plant tissues are often reduced when plants are exposed to, or grown at, high [CO2] due to direct effects on enzymes and indirect effects derived from changes in the plant's chemical composition. SCOPE: Although measurement artefacts may have affected some of the previously reported effects of CO2 on respiration rates, the direction and magnitude for the effects of elevated [CO2] on plant respiration may largely depend on the vertical scale (from enzymes to ecosystems) at which measurements are taken. In this review, the effects of elevated [CO2] from cells to ecosystems are presented within the context of the enzymatic and physiological controls of plant respiration, the role(s) of non-phosphorylating pathways, and possible effects associated with plant size. CONCLUSIONS: Contrary to what was previously thought, specific respiration rates are generally not reduced when plants are grown at elevated [CO2]. However, whole ecosystem studies show that canopy respiration does not increase proportionally to increases in biomass in response to elevated [CO2], although a larger proportion of respiration takes place in the root system. Fundamental information is still lacking on how respiration and the processes supported by it are physiologically controlled, thereby preventing sound interpretations of what seem to be species-specific responses of respiration to elevated [CO2]. Therefore the role of plant respiration in augmenting the sink capacity of terrestrial ecosystems is still uncertain.     

Leaf structure and chemical composition as affected by elevated CO2: genotypic responses of two perennial grasses

Genotypic variability was studied in two Mediterranean grass species, Bromus erectus and Dactylis glomerata , with regard to the response to CO₂ of leaf total non-structural carbohydrate concentration ([TNC]_lf), specific leaf area (SLA), and leaf carbon and nitrogen concentrations ([C]_lf and [N]_lf, respectively). Fourteen genotypes of each species were grown together on intact soil monoliths at ambient and elevated CO₂ concentrations (350 and 700 μmol mol⁻¹, respectively). In both species, the most consistent effect of elevated CO₂ was an increase in [TNC]_lf and a decrease in leaf nitrogen concentration when expressed either as total dry mass [N_m]_lf, structural dry mass [N_mst]_lf or leaf area [N_a]_lf. The SLA decreased only in D. glomerata , due to an accumulation of total non-structural carbohydrates and to an increase in leaf density. No genotypic variability was found for any variable in B. erectus , suggesting that genotypes responded in a similar way to elevated CO₂. In D. glomerata , a genotypic variability was found only for [Cst], [N_m]_lf, [N_mst]_lf and [N_a]_lf. Since [N_m]_lf is related to plant growth and is a strong determinant of plant–herbivore interactions, our results suggest evolutionary consequences of elevated CO₂ through competitive interactions or herbivory.     

Does elevated atmospheric [CO2] alter diurnal C uptake and the balance of C and N metabolites in growing and fully expanded soybean leaves?

Increases in growth at elevated [CO2] may be constrained by a plant's ability to assimilate the nutrients needed for new tissue in sufficient quantity to match the increase in carbon fixation and/or the ability to transport those nutrients and carbon in sufficient quantity to growing organs and tissues. Analysis of metabolites provides an indication of shifts in carbon and nitrogen partitioning due to rising atmospheric [CO2] and can help identify where bottlenecks in carbon utilization occur. In this study, the carbon and nitrogen balance was investigated in growing and fully expanded soybean leaves exposed to elevated [CO2] in a free air CO2 enrichment experiment. Diurnal photosynthesis and diurnal profiles of carbon and nitrogen metabolites were measured during two different crop growth stages. Diurnal carbon gain was increased by c. 20% in elevated [CO2] in fully expanded leaves, which led to significant increases in leaf hexose, sucrose, and starch contents. However, there was no detectable difference in nitrogen-rich amino acids and ureides in mature leaves. By contrast to mature leaves, developing leaves had high concentrations of ureides and amino acids relative to low concentrations of carbohydrates. Developing leaves at elevated [CO2] had smaller pools of ureides compared with developing leaves at ambient [CO2], which suggests N assimilation in young leaves was improved by elevated [CO2]. This work shows that elevated [CO2] alters the balance of carbon and nitrogen pools in both mature and growing soybean leaves, which could have down-stream impacts on growth and productivity.     

Variation in acclimation of photosynthesis in Trifolium repens after eight years of exposure to Free Air CO2 Enrichment (FACE)

The initial stimulation of photosynthesis observed on elevation of [CO2] in grasslands has been predicted to be a transient phenomenon constrained by the loss of photosynthetic capacity due to other limitations, notably nutrients and sinks for carbohydrates. Legumes might be expected partially to escape these feedbacks through symbiotic N2 fixation. The Free-Air Carbon dioxide Enrichment (FACE) experiment at Eschikon, Switzerland, has been the longest running investigation of the effects of open-air elevation of [CO2] on vegetation. The prediction of a long-term loss of photosynthetic capacity was tested by analysing photosynthesis in Trifolium repens L. (cv. Milkanova) in the spring and autumn of the eighth, ninth and tenth years of treatment. A high and low N treatment also allowed a test of the significance of exogenous N-supply in maintaining a stimulation of photosynthetic capacity in the long-term. Prior work in this Free Air CO2 Enrichment (FACE) experiment has revealed that elevated [CO2] increased both vegetative and reproductive growth of T. repens independent of N treatment. It is shown here that the photosynthetic response of T. repens was also independent of N fertilization under both current ambient and elevated (600 micro mol mol-1) [CO2]. There was a strong effect of season on photosynthesis, with light-saturated rates (Asat) 37% higher in spring than in autumn. Higher Asat in the spring was supported by higher maximum Rubisco carboxylation rates (Vc,max) and maximum rates of electron transport (Jmax) contributing to RuBP regeneration. Elevated [CO2] increased Asat by 37% when averaged across all measurement periods and both N fertilization levels, and decreased stomatal conductance by 25%. In spring, there was no effect of elevated [CO2] on photosynthetic capacity of leaves, but in autumn both Vc,max and Jmax were reduced by approximately 20% in elevated [CO2]. The results show that acclimation of photosynthetic capacity can occur in a nitrogen-fixing species, in the field where there are no artificial restrictions on sink capacity. However, even with acclimation there was a highly significant increase in photosynthesis at elevated [CO2].     

Growth and N Allocation in Rice Plants under CO2 Enrichment

The effects of CO2 enrichment on growth and N allocation of rice (Oryza sativa L.) were examined. The plants were grown hydroponically in growth chambers with a 14-h photoperiod (1000 [mu]mol quanta m-2 s-1) and a day/night temperature of 25/20[deg]C. From the 28th to 70th d after germination, the plants were exposed to two CO2 partial pressures, namely 36 and 100 Pa. The CO2 enrichment increased the final biomass, but this was caused by a stimulation of the growth rate during the first week of the exposure to elevated CO2 partial pressures. The disappearance of the initial stimulation of the growth rate was associated with a decreased leaf area ratio. Furthermore, CO2 enrichment decreased the investment of N in the leaf blades, whereas the N allocation into the leaf sheaths and roots increased. Thus, the decrease in leaf N content by CO2 enrichment was not due to dilution of N caused by a relative increase in the plant biomass but was due to the change in N allocation at the whole-plant level. We conclude that the growth responses of rice to CO2 enrichment are mainly controlled by leaf area expansion and N allocation into leaf blades at the whole-plant level.     

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.