Effect of CO2 enrichment on non-structural carbohydrates in leaves, stems and ears of spring wheat

Field-grown spring wheat (Triticum aestivum L. cv. Dragon) was exposed to ambient and elevated CO₂ concentrations (1.5 and 2 times ambient) in open-top chambers. Contents of non-structural carbohydrates were analysed enzymatically in leaves, stems and ears six times during the growing season. The impact of elevated CO₂ on wheat carbohydrates was non-significant in most harvests. However, differences in the carbohydrate contents due to elevated CO₂ were found in all plant compartments. Before anthesis, at growth stage (GS) 30 (the stem is 1 cm to the shoot apex), the plants grown in elevated CO₂ contained significantly more water soluble carbohydrates (WSC), fructans, starch and total non-structural carbohydrates (TNC) in the leaves in comparison with the plants grown in ambient CO₂. It is hypothesised that the plants from the treatments with elevated CO₂ were sink-limited at GS30. After anthesis, the leaf WSC and TNC contents of the plants from elevated CO₂ started to decline earlier than those of the plants from ambient CO₂. This may indicate that the leaves of plants grown in the chambers with elevated CO₂ senesced earlier. Elevated CO₂ accelerated grain development: 2 weeks after anthesis, the plants grown in elevated CO₂ contained significantly more starch and significantly less fructans in the ears compared to the plants grown in ambient CO₂. Elevated CO₂ had no effect on ear starch and TNC contents at the final harvest. Increasing the CO₂ concentration from 360 to 520 μmol mol^?1 had a larger effect on wheat non-structural carbohydrates than the further increase from 520 to 680 μmol mol^?1. The results are discussed in relation to the effects of elevated CO₂ on yield and yield components.     

Effects of elevated CO2 on the protein concentration of food crops: a meta-analysis

Meta‐analysis techniques were used to examine the effect of elevated atmospheric carbon dioxide [CO₂] on the protein concentrations of major food crops, incorporating 228 experimental observations on barley, rice, wheat, soybean and potato. Each crop had lower protein concentrations when grown at elevated (540–958 μmol mol^?1) compared with ambient (315–400 μmol mol^?1) CO₂. For wheat, barley and rice, the reduction in grain protein concentration was ～10–15% of the value at ambient CO₂. For potato, the reduction in tuber protein concentration was 14%. For soybean, there was a much smaller, although statistically significant reduction of protein concentration of 1.4%. The magnitude of the CO₂ effect on wheat grains was smaller under high soil N conditions than under low soil N. Protein concentrations in potato tubers were reduced more for plants grown at high than at low concentrations of ozone. For soybean, the ozone effect was the reverse, as elevated CO₂ increased the protein concentration of soybean grown at high ozone concentrations. The magnitude of the CO₂ effect also varied depending on experimental methodology. For both wheat and soybean, studies performed in open‐top chambers produced a larger CO₂ effect than those performed using other types of experimental facilities. There was also indication of a possible pot artifact as, for both wheat and soybean, studies performed in open‐top chambers showed a significantly greater CO₂ effect when plants were rooted in pots rather than in the ground. Studies on wheat also showed a greater CO₂ effect when protein concentration was measured in whole grains rather than flour. While the magnitude of the effect of elevated CO₂ varied depending on the experimental procedures, a reduction in protein concentration was consistently found for most crops. These findings suggest that the increasing CO₂ concentrations of the 21st century are likely to decrease the protein concentration of many human plant foods.     

Interacting effects of CO2 concentration, temperature and nitrogen supply on the photosynthesis and composition of winter wheat leaves

Winter wheat (Triticum aestivum L., cv. Mercia) was grown at two different atmospheric CO₂ concentrations (350 and 700 μmol mol⁻¹), two temperatures [ambient temperature (i.e. tracking the open air) and ambient +4°C] and two rates of nitrogen supply (equivalent to 489 kg ha⁻¹ and 87 kg ha⁻¹). Leaves grown at 700 μmol mol⁻¹ CO₂ had slightly greater photosynthetic capacity (10% mean increase over the experiment) than those grown at ambient CO₂ concentration, but there were no differences in carboxylation efficiency or apparent quantum yield. The amounts of chlorophyll, soluble protein and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) per unit leaf area did not change with long-term exposure to elevated CO₂ concentration. Thus winter wheat, grown under simulated field conditions, for which total biomass was large compared to normal field production, did not experience loss of components of the photosynthetic system or loss of photosynthetic competence with elevated CO₂ concentration. However, nitrogen supply and temperature had large effects on photosynthetic characteristics but did not interact with elevated CO₂ concentration. Nitrogen deficiency resulted in decreases in the contents of protein, including Rubisco, and chlorophyll, and decreased photosynthetic capacity and carboxylation efficiency. An increase in temperature also reduced these components and shortened the effective life of the leaves, reducing the duration of high photosynthetic capacity.     

Elevated atmospheric CO2 affects decomposition of Festuca vivipara (L.) Sm. litter and roots in experiments simulating environmental change in two contrasting arctic ecosystems

Mass loss, together with nitrogen and carbon loss, from above-ground material and roots of Festuca vivipara were followed for 13 months in a high Arctic polar semi-desert and a low Arctic tree-line dwarf shrub heath. Festuca vivipara for the study was obtained from plants cultivated at two different CO₂ concentrations (350 and 500 μL L^–1) in controlled environment chambers in the UK. Each of the four resource types (shoots or roots from plants grown in elevated or ambient CO₂ concentrations) was subsequently placed in an experiment simulating aspects of environmental change in each Arctic ecosystem. Air, litter and soil temperatures were increased using open-topped polythene tents at both sites, and a 58% increase in summer precipitation was simulated at the high Arctic site. Mass loss was greatest at the low Arctic site, and from the shoot material, rather than the roots. Shoots grown under an elevated CO₂ concentration decomposed more slowly at the high Arctic site, and more quickly at the low Arctic one, than shoots grown at ambient CO₂. After 13 months, greater amounts of C and N remained in above-ground litter from plants grown under elevated, rather than ambient, CO₂ at the polar semi-desert site, although lower amounts of C remained in elevated CO₂ litter at the low Arctic ecosystem. In the high Arctic, roots grown in the 500 μL L^–1 CO₂ concentration decomposed significantly more slowly than below-ground material derived from the ambient CO₂ chambers. Elevated CO₂ concentrations significantly increased the inital C:N ratio, % soluble carbohydrates and α-cellulose content, and significantly decreased the inital N content, of the above-ground material compared to that derived from the ambient treatment. Initially, the C:N ratio and percentage N were similar in both sets of roots derived from the two different CO₂ treatments, but soluble carbohydrate and α-cellulose concentrations were higher, and percentage lignin lower, in the elevated CO₂ treatments.The tent treatments significantly retarded shoot decomposition in both ecosystems, probably because of lower litter bag moisture contents, although the additional precipitation treatment had no effect on mass loss from the above-ground material. The results suggest that neither additional summer precipitation (up to 58%), nor soil temperature increase of 1 °C, which may occur by the end of the next century as an effect of a predicted 4 °C rise in air temperature, had an appreciable effect on root decomposition in the short term in a high Arctic soil. However, at the low Arctic site, greater root decomposition, and a lower pool of root N remaining, were observed where soil temperature was increased by 2 °C in response to a 4 °C rise in air temperature. These results suggest that decomposition below-ground in this ecosystem would increase as an effect of predicted climate change. These data also show that there is a difference in the initial results of decomposition processes between the two Arctic ecosystems in response to simulated environmental change.     

Does elevated atmospheric CO2 concentrations affect wood decomposition?

This study was conducted to test the hypothesis that wood tissues generated under elevated atmospheric [CO₂] have lower quality and subsequent reduced decomposition rates. Chemical composition and subsequent field decomposition rates were studied for beech (Fagus sylvatica L.) twigs grown under ambient and elevated [CO₂] in open top chambers. Elevated [CO₂] significantly affected the chemical composition of beech twigs, which had 38% lower N and 12% lower lignin concentrations than twigs grown under ambient [CO₂]. The strong decrease in N concentration resulted in a significant increase in the C/N and lignin/N ratios of the beech wood grown at elevated [CO₂]. However, the elevated [CO₂] treatment did not reduce the decomposition rates of twigs, neither were the dynamics of N and lignin in the decomposing beech wood affected by the [CO₂] treatment, despite initial changes in N and lignin concentrations between the ambient and elevated [CO₂] beech wood. This revised version was published online in June 2006 with corrections to the Cover Date.     

Leaf gas exchange and nitrogen dynamics of N2-fixing, field-grown Alnus glutinosa under elevated atmospheric CO2

Few studies have investigated the effects of elevated CO₂ on the physiology of symbiotic N₂-fixing trees. Tree species grown in low N soils at elevated CO₂ generally show a decline in photosynthetic capacity over time relative to ambient CO₂ controls. This negative adjustment may be due to a reallocation of leaf N away from the photosynthetic apparatus, allowing for more efficient use of limiting N. We investigated the effect of twice ambient CO₂ on net CO₂ assimilation (A), photosynthetic capacity, leaf dark respiration, and leaf N content of N2-fixing Alnus glutinosa (black alder) grown in field open top chambers in a low N soil for 160 d. At growth CO₂, A was always greater in elevated compared to ambient CO₂ plants. Late season A vs. internal leaf p(CO₂) response curves indicated no negative adjustment of photosynthesis in elevated CO₂ plants. Rather, elevated CO₂ plants had 16% greater maximum rate of CO₂ fixation by Rubisco. Leaf dark respiration was greater at elevated CO₂ on an area basis, but unaffected by CO₂ on a mass or N basis. In elevated CO₂ plants, leaf N content (μg N cm^?2) increased 50% between Julian Date 208 and 264. Leaf N content showed little seasonal change in ambient CO₂ plants. A single point acetylene reduction assay of detached, nodulated root segments indicated a 46% increase in specific nitrogenase activity in elevated compared to ambient CO₂ plants. Our results suggest that N₂-fixing trees will be able to maintain high A with minimal negative adjustment of photosynthetic capacity following prolonged exposure to elevated CO₂ on N-poor soils.     

Short-term decomposition of litter produced by plants grown in ambient and elevated atmospheric CO2 concentrations

The effects of elevated atmospheric CO₂ (ambient + 340 μmol mol^–1) on above-ground litter decomposition were investigated over a 6-week period using a field-based mesocosm system. Soil respiratory activity in mesocosms incubated in ambient and elevated atmospheric CO₂ concentrations were not significantly different (t-test, P > 0.05) indicating that there were no direct effects of elevated atmospheric CO₂ on litter decomposition. A study of the indirect effects of CO₂ on soil respiration showed that soil mesocosms to which naturally senescent plant litter had been added (0.5% w/w) from the C₃ sedge Scirpus olneyi grown in elevated atmospheric CO₂ was reduced by an average of 17% throughout the study when compared to soil mesocosms to which litter from Scirpus olneyi grown in ambient conditions had been added. In contrast, similar experiments using senescent material from the C₄ grass Spartina patens showed no difference in soil respiration rates between mesocosms to which litter from plants grown in elevated or ambient CO₂ conditions had been added. Analysis of the C:N ratio and lignin content of the senescent material showed that, while the C:N ratio and lignin content of the Spartina patens litter did not vary with atmospheric CO₂ conditions, the C:N ratio (but not the lignin content) of the litter from Scirpus olneyi was significantly greater (t-test;P < 0.05) when derived from plants grown under elevated CO₂ (105:1 compared to 86:1 for litter derived from Scirpus olneyi grown under ambient conditions). The results suggest that the increased C:N ratio of the litter from the C₃ plant Scirpus olneyi grown under elevated CO₂ led to the lower rates of biodegradation observed as reduced soil respiration in the mesocosms. Further long-term experiments are now required to determine the effects of elevated CO₂ on C partitioning in terrestrial ecosystems.     

Decomposition and nitrogen release from leaves of three hardwood species grown under elevated O3 and/or CO2

Elevated concentrations of O₃ and CO₂ have both been shown to affect structure, nutrient status, and deposition of secondary metabolites in leaves of forest trees. While such studies have produced robust models of the effects of such air pollutants on tree ecophysiology and growth, few have considered the potential for broader, ecosystem-level effects after these chemically and structurally altered leaves fall as leaf litter and decay. To determine the effects of elevated O₃ and/or CO₂ on the subsequent decomposition and nutrient release from the leaves grown in such altered atmospheres, we grew seedlings of three widespread North American forest trees, black cherry (Prunus serotina) (BC), sugar maple (Acer saccharum) (SM), and yellow-poplar (Liriodendron tulipifera) (YP) for two growing seasons in charcoal-filtered air (CF-air=approximately 25% ambient O₃), ambient O₃ (1X) or twice-ambient O₃ (2X) in outdoor open-top chambers. We then assayed the loss of mass and N from the litter derived from those seedlings through one year litterbag incubations in the forest floor of a neighboring forest stand. Mass loss followed linear functions and was not affected by the O₃ regime in which the leaves were grown. Instantaneous decay rates (i.e. k values) averaged SM:–0.707 y^-1, BC:–0.613 y^-1, and YP:–0.859 y^-1. N loss from ambient (1X) O₃-grown SM leaves was significantly greater than from CF-air leaves: N loss from BC leaves did not differ among treatments. Significantly less N was released from CF-air-grown YP leaves than from 1X or 2X O₃-treated leaves. YP leaves from plants grown in pots at 2X O₃ and 350 ppm supplemental CO₂ in indoor pollutant fumigation chambers (CSTRs or Continuously Stirred Tank Reactors) loss 40% as much mass and 27% as much N over one year as did leaves from YP grown in CF-air or 2X O₃. Thus, for leaves from plants grown in pots in controlled environment fumigation chambers, the concentrations of both O₃ and CO₂ can affect N release from litter incubated in the field whereas mass loss rate was affected only by CO₂. Because both mass loss and N release from leaves grown at elevated CO₂ were reduced significantly (at least for yellow-poplar), forests exposed to elevated CO₂ may have significantly reduced N turnover rates, thereby resulting in increased N limitation of tree growth, especially in forests which are already N-limited.     

Responses of cotton and wheat photosynthesis and growth to cyclic variation in carbon dioxide concentration

The carbon dioxide concentration in free air carbon dioxide enrichment (FACE) systems typically has rapid fluctuations. In our FACE system, power spectral analysis of CO₂ concentration measured every second with an open path analyzer indicated peaks in variation with a period of about one minute. I used open-top chambers to expose cotton and wheat plants to either a constant elevated CO₂ concentration of 180 ??mol mol^?1 above that of outside ambient air, or to the same mean CO₂ concentration, but with the CO₂ enrichment cycling between about 30 and 330 ??mol mol^?1 above the concentration of outside ambient air, with a period of one minute. Three short-term replicate plantings of cotton were grown in Beltsville, Maryland with these CO₂ concentration treatments imposed for 27-day periods over two summers, and one winter wheat crop was grown from sowing to maturity. In cotton, leaf gas-exchange measurements of the continuously elevated treatment and the fluctuating treatment indicated that the fluctuating CO₂ concentration treatment consistently resulted in substantial down-regulation of net photosynthetic rate (P _N) and stomatal conductance (g _s). Total shoot biomass of the vegetative cotton plants in the fluctuating CO₂ concentration treatment averaged 30% less than in the constantly elevated CO₂ concentration treatment at 27 days after planting. In winter wheat, leaf gas-exchange measurements also indicated that down-regulation of P _N and g _s occurred in flag leaves in the fluctuating CO₂ concentration treatment, but the effect was not as consistent in other leaves, nor as severe as found in cotton. However, wheat grain yields were 12% less in the fluctuating CO₂ concentration treatment compared with the constant elevated CO₂ concentration treatment. Comparison with wheat yields in chambers without CO₂ addition indicated a nonsignificant increase of 5% for the fluctuating elevated CO₂ concentration treatment, and a significant increase of 19% for the constant elevated treatment. The results suggest that treatments with fluctuating elevated CO₂ concentrations could underestimate plant growth at projected future atmospheric CO₂ concentrations.     

Elevated CO2 enhances plant growth in droughted N2-fixing alfalfa without improving water status

The long-term interaction between elevated CO₂ and soil water deficit was analysed in N₂-fixing alfalfa plants in order to assess the possible drought tolerance effect of CO₂. Elevated CO₂ could delay the onset of drought stress by decreasing transpiration rates, but this effect was avoided by subjecting plants to the same soil water content. Nodulated alfalfa plants subjected to ambient (400 μmol mol^?1) or elevated (700 μmol mol^?1) CO₂ were either well watered or partially watered by restricting water to obtain 30% of the water content at field capacity (ampproximately 0.55 g water cm^?3). The negative effects of soil water deficit on plant growth were counterbalanced by elevated CO₂. In droughted plants, elevated CO₂ stimulated carbon fixation and, as a result, biomass production was even greater than in well-watered plants grown in ambient CO₂. Below-ground production was preferentially stimulated by elevated CO₂ in droughted plants, increasing nodule biomass production and the availability of photosynthates to the nodules. As a result, total nitrogen content in droughted plants was higher than in well-watered plants grown in ambient CO₂. The beneficial effect of elevated CO₂ was not correlated with a better plant water status. It is concluded that elevated CO₂ enhances growth of droughted plants by stimulating carbon fixation, preferentially increasing the availability of photosynthates to below-ground production (roots and nodules) without improving water status. This means that elevated CO₂ enhances the ability to produce more biomass in N₂-fixing alfalfa under given soil water stress, improving drought tolerance.     

Infection with the parasitic angiosperm Striga hermonthica influences the response of the C3 cereal Oryza sativa to elevated CO2

Upland rice (Oryza sativa L.) was grown at both ambient (350 μmol mol^?1) and elevated (700 μmol mol^?1) CO₂ in either the presence or absence of the root hemi‐parasitic angiosperm Striga hermonthica (Del) Benth. Elevated CO₂ alleviated the impact of the parasite on host growth: biomass of infected rice grown at ambient CO₂ was 35% that of uninfected, control plants, while at elevated CO₂, biomass of infected plants was 73% that of controls. This amelioration occurred despite the fact that O. sativa grown at elevated CO₂ supported both greater numbers and a higher biomass of parasites per host than plants grown at ambient CO₂. The impact of infection on host leaf area, leaf mass, root mass and reproductive tissue mass was significantly lower in plants grown at elevated as compared with ambient CO₂. There were significant CO₂ and Striga effects on photosynthetic metabolism and instantaneous water‐use efficiency of O. sativa. The response of photosynthesis to internal [CO₂] (A/C_i curves) indicated that, at 45 days after sowing (DAS), prior to emergence of the parasites, uninfected plants grown at elevated CO₂ had significantly lower CO₂ saturated rates of photosynthesis, carboxylation efficiencies and ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) contents than uninfected, ambient CO₂‐grown O. sativa. In contrast, infection with S. hermonthica prevented down‐regulation of photosynthesis in O. sativa grown at elevated CO₂, but had no impact on photosynthesis of hosts grown at ambient CO₂. At 76 DAS (after parasites had emerged), however, infected plants grown at both elevated and ambient CO₂ had lower carboxylation efficiencies and Rubisco contents than uninfected O. sativa grown at ambient CO₂. The reductions in carboxylation efficiency (and Rubisco content) were accompanied by similar reductions in nitrogen concentration of O. sativa leaves, both before and after parasite emergence. There were no significant CO₂ or infection effects on the concentrations of soluble sugars in leaves of O. sativa, but starch concentration was significantly lower in infected plants at both CO₂ concentrations. These results demonstrate that elevated CO₂ concentrations can alleviate the impact of infection with Striga on the growth of C₃ hosts such as rice and also that infection can delay the onset of photosynthetic down‐regulation in rice grown at elevated CO₂.     

Poa annua shows inter-generational differences in response to elevated CO2

Inter-generational effects on the growth of Poa annua (L.) in ambient and elevated atmospheric CO₂ conditions (350 and 550 μl l^–1, respectively) were studied in two different experiments. Both experiments showed similar results. In a greenhouse experiment growth, measured as the numbers of tillers produced per week, was compared for plants grown from first and second generation seeds. Second generation seeds were obtained from plants grown for one whole generation in either ambient or elevated atmospheric CO₂ (‘ambient’ and ‘elevated’ seeds, respectively). First generation plants and second generation ‘ambient’ plants did not respond to elevated CO₂. Second generation ‘elevated’ plants produced significantly more tillers in elevated CO₂. In the second experiment model terrestrial ecosystems growing in the Ecotron and which included Poa annua were used. Above-ground biomass after one and two generations of growth were compared. At the end of Generation 1 no difference was found in biomass production while at the end of Generation 2 biomass increased in elevated CO₂ by 50%. The implications for climate change research are discussed.     

Effects of elevated CO2 on the foraging behavior of cotton bollworm, Helicoverpa armigera

Effects of elevated CO2 on the foraging behavior of cotton bollworm Helicoverpa arrnigera Hübner reared on milky grains of spring wheat grown in ambient, 550μL/L and 750μL/L CO2 concentration atmospheres in open-top chambers （OTC） were studied. The results indicated that： （i） elevated CO2 significantly affected both the type and amount of food eaten by H.arrnigera reared on milky grains of ambient CO2-grown wheat were significant higher than those for bollworm larvae reared on wheat grains grown in 550 and 750μL/L CO2 atmospheres; （ii） when bollworm larvae were reared on mixed milky grains from different CO2-grown wheat （food-choice condition）, larval duration increased significantly-pupal weight, adult longevity, and fecundity decreased significantly, comparing with those reared on milky grains of ambient CO2-grown wheat, 550μL/L CO2-grown wheat and 750μL/L CO2-grown wheat respectively; （iii） significant decreases in the contents of fructose and gross protein （GP） and significant increases in the contents of glucose, amylose, total saccharides （TSC）, TSC： GP ratio, free amino acids and soluble protein in the wheat grains with CO2 rising; （iv） and selected-foraging amount/food-choice index of cotton bollworm H.armigera were significantly positive correlated with the contents of fructose and GP of wheat grains, but they had significantly negative relationships with the contents of glucose, amylose, TSC and TSC： GP ratio of wheat grains.     

Acclimation of photosynthesis in relation to Rubisco and non-structural carbohydrate contents and in situ carboxylase activity in Scirpus olneyi grown at elevated CO2 in the field

Stands of Scirpus olneyi, a native saltmarsh sedge with C₃ photosynthesis, had been exposed to normal ambient and elevated atmospheric CO₂ concentrations (C_a) in their native habitat since 1987. The objective of this investigation was to characterize the acclimation of photosynthesis of Scirpus olneyi stems, the photosynthesizing organs of this species, to long-term elevated C_a treatment in relation to the concentrations of Rubisco and non-structural carbohydrates. Measurements were made on intact stems in the Held under existing natural conditions and in the laboratory under controlled conditions on stems excised in the field early in the morning. Plants grown at elevated C_a had a significantly higher (30–59%) net CO₂ assimilation rate (A) than those grown at ambient C_a when measurements were performed on excised stems at the respective growth C_a. However, when measurements were made at normal ambient C_a, A was smaller (45–53%) in plants grown at elevated C_a than in those grown at ambient C_a. The reductions in A at normal ambient C_a, carboxylation efficiency and in situ carboxylase activity were caused by a decreased Rubisco concentration (30–58%) in plants grown at elevated C_a; these plants also contained less soluble protein (39–52%). The Rubisco content was 43 to 58% of soluble protein, and this relationship was not significantly altered by the growth CO₂ concentrations. The Rubisco activation state increased slightly, but the in situ carboxylase activity decreased substantially in plants grown at elevated C_a. When measurements were made on intact stems in the field, the elevated C_a treatment caused a greater stimulation of,A (100%) and a smaller reduction in carboxylation efficiency (which was not statistically significant) than when measurements were made on excised stems in the laboratory. The possible reasons for this arc discussed. Plants grown at elevated C_a contained more non-structural carbohydrates (25–53%) than those grown at ambient C_a. Plants grown at elevated C_a appear to have sufficient sink capacity to utilize the additional carbohydrates formed during photosynthesis. Overall, our results are in agreement with the hypothesis that elevated C_a leads to an increased carbohydrate concentration and the ensuing acclimation of the photo-synthetic apparatus in C₃ plants results in a reduction in the protein complement, especially Rubisco, which reduces the photosynthetic capacity in plants grown at elevated C_a, relative to plants grown at normal ambient C_a. Nevertheless, when compared at their respective growth C_a, Scirpus olneyi plants grown at elevated C_a in their native habitat maintained a substantially higher rate of photosynthesis than those grown at normal ambient C_a even after 8 years of growth at elevated C_a.     

Elevated CO2 mitigates chilling-induced water stress and photosynthetic reduction during chilling

Bean, cucumber and corn plants were grown in controlled-environment chambers at 25/18 °C day/night temperature and either ambient (350 μmol mol^?1) or elevated (700 μmol mol^?1) CO₂ concentration, and at 20–30 d after emergence they were exposed to a 24 h chilling treatment (6.5 ± 1.5 °C) at their growth CO₂ concentration. Whole-plant transpiration rates (per unit leaf area basis) during the first 3 h of chilling were about 26,28 and 13% lower at elevated than at ambient CO₂ for bean, cucumber and corn, respectively. The decline in leaf water potential (ψ_L) and visible wilting of bean and cucumber during chilling were significantly less at elevated than at ambient CO₂. Corn ψ_L was not significantly affected by chilling, and corn did not exhibit any other symptoms of chilling-induced water stress. Leaf osmotic potentials (measured before chilling only) of bean and cucumber were more negative at elevated than at ambient CO₂, and the corresponding calculated leaf turgor potentials were significantly higher at elevated than at ambient CO₂. Leaf relative water content (RWC) during chilling at ambient CO₂fell to 62 and 48% for bean and cucumber, respectively. RWC during chilling at elevated CO₂ was never below 79% for bean or 63% for cucumber. Corn RWC was not measured. After 24 h of chilling at ambient CO₂, net photosynthetic rate (P_N) reductions were 83, 89 and 24% for bean, cucumber and corn, respectively. P_N reductions during chilling were less at elevated CO₂: 53, 40 and 4% for bean, cucumber and corn, respectively. At ambient CO₂, none of the species fully recovered to pre-chilling P_N, but at elevated CO₂ both bean and corn recovered fully. The average percentage leaf area with visible leaf damage due to chilling was 20.6 and 9.6% at ambient and elevated CO₂, respectively, for bean, and 32.4 and 23.6% at ambient and elevated CO₂, respectively, for cucumber. Corn showed no significant permanent leaf damage from chilling at either CO₂ concentration. These results indicate that cucumber was most sensitive to chilling as imposed in this study, followed by bean and corn. The results support the hypothesis that, at least in young plants under controlled-environment conditions, elevated CO₂ improves plant water relations during chilling and can mitigate photosynthetic depression and chilling damage. The implications for long-term growth and reproductive success in managed and natural ecosystems will require testing of this hypothesis under field conditions.     

Effects of free-air CO2 enrichment on the development of the photosynthetic apparatus in wheat, as indicated by changes in leaf proteins

A spring wheat crop was grown at ambient and elevated (550 μmol mol^?1) CO₂ concentrations under free-air CO₂ enrichment (FACE) in the field. Four experimental blocks, each comprising 21-m-diameter FACE and control experimental areas, were used. CO₂ elevation was maintained day and night from crop emergence to final grain harvest. This experiment provided a unique opportunity to examine the hypothesis that CO₂ elevation in the field would lead to acclimatory changes within the photosynthetic apparatus under open field conditions and lo assess whether acclimation was affected by crop developmental stage, leaf ontogeny and leaf age. Change in the photosynthetic apparatus was assessed by measuring changes in the composition of total leaf and thylakoid polypeptides separated by SDS-PAGE. For leaves at completion of emergence of the blade, growth at the elevated CO₂ concentration had no apparent effect on the amount of any of the major proteins of the photosynthetic apparatus regardless of the leaf examined. Leaf 5 on the main stem was in full sunlight at emergence, but then became shaded progressively as 3–4 further leaves formed above with continued development of the crop. By 35 d following completion of blade emergence, leaf 5 was in shade. At this point, the chlorophyll alb ratio had declined by 26% both in plants grown at the control CO₂ concentration and in those grown at the elevated CO₂ concentration, which is indicative of shade acclimation. The ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) content declined by 45% in the control leaves, but by 60% in the leaves grown at the elevated CO₂ concentration. The light- harvesting complex of photosystcm II (LHCII) and the chlorophyll content showed no decrease and no difference between treatments, indicating that the decrease in Rubisco was not an effect of earlier senescence in the leaves at the elevated CO₂ concentration. Following completion of the emergence of the flag-leaf blade, the elevated-CO₂ treatment inhibited the further accumulation of Rubisco which was apparent in control leaves over the subsequent 14 d. From this point onwards, the flag leaves from both treatments showed a loss of Rubisco, which was far more pronounced in the elevated-CO₂ treatment, so that by 36 d the Rubisco content of these leaves was just 70% of that of the controls and by 52 d it was only 20%. At 36 d, there was no decline in chlorophyll, LHCII or the chloroplast ATPase coupling factor (CFI) in the elevated CO₂ concentration treatment relative to the control. By 52 d, all of these proteins showed a significant decline relative to the control. This indicates that the decreased concentration of Rubisco at this final stage probably reflected earlier senescence in the elevated-CO₂ treatment, but that this was preceded by a CO₂-concentration-dependent decline in Rubisco.     

Effects of elevated CO2 and temperature on development and consumption rates of Octotoma championi and O. scabripennis feeding on Lantana camara

We carried out a factorial experiment to explore the effect of doubled CO₂ concentration and a 3 °C temperature increase on the development of a complete generation of the beetles Octotoma championi Baly and O. scabripennis Guérin‐Méneville (Coleoptera: Chrysomelidae). These species are biological control agents of Lantana camara L. (Verbenaceae), with a leaf‐mining larval phase and free‐living, leaf‐chewing adults. Plants grown at elevated CO₂ had enhanced above‐ground biomass, thicker leaves, reduced nitrogen concentration, and increased C:N ratios. Under the high temperature treatment, plants grown at ambient CO₂ suffered wilting and premature leaf loss, despite daily watering; this effect was ameliorated at elevated CO₂. The wilting of plants in the ambient CO₂/high temperature treatment reduced the emergence success of the beetles, particularly O. championi. Development time was accelerated by approximately 10–13 days at the higher temperature, but was not affected by CO₂. Neither CO₂ nor temperature affected adult beetle weight. Consumption rates of free‐living beetles were not affected by either CO₂ or temperature. By contrast, in the short‐term trials using excised foliage, beetles given no choice between ambient and elevated CO₂‐grown foliage, consumed more from ambient plants. When beetles were offered a choice between foliage grown at the two CO₂ levels, O. championi did not display a significant preference but O. scabripennis consumed more ambient CO₂‐grown foliage when feeding at the lower temperature. This study indicates that under future conditions of higher temperatures, amelioration of water stress in host plants growing in elevated CO₂ may benefit some endophagous insects by reducing premature leaf loss. Under some circumstances, this benefit may outweigh the deleterious effects of lower leaf nitrogen. Our results also indicate that foliage consumption under elevated CO₂ by mobile, adult insects on whole plants may not be significantly increased, as was previously indicated by short‐term experiments using excised foliage.     

Growth in elevated CO2 protects photosynthesis against high-temperature damage

We present evidence that plant growth at elevated atmospheric CO₂ increases the high‐temperature tolerance of photosynthesis in a wide variety of plant species under both greenhouse and field conditions. We grew plants at ambient CO₂ (~ 360 μ mol mol ^{? 1}) and elevated CO₂ (550–1000 μ mol mol ^{? 1}) in three separate growth facilities, including the Nevada Desert Free‐Air Carbon Dioxide Enrichment (FACE) facility. Excised leaves from both the ambient and elevated CO₂ treatments were exposed to temperatures ranging from 28 to 48 °C. In more than half the species examined (4 of 7, 3 of 5, and 3 of 5 species in the three facilities), leaves from elevated CO₂‐grown plants maintained PSII efficiency (F_v/F_m) to significantly higher temperatures than ambient‐grown leaves. This enhanced PSII thermotolerance was found in both woody and herbaceous species and in both monocots and dicots. Detailed experiments conducted with Cucumis sativus showed that the greater F_v/F_m in elevated versus ambient CO₂‐grown leaves following heat stress was due to both a higher F_m and a lower F_o, and that F_v/F_m differences between elevated and ambient CO₂‐grown leaves persisted for at least 20 h following heat shock. Cucumis sativus leaves from elevated CO₂‐grown plants had a critical temperature for the rapid rise in F_o that averaged 2·9 °C higher than leaves from ambient CO₂‐grown plants, and maintained a higher maximal rate of net CO₂ assimilation following heat shock. Given that photosynthesis is considered to be the physiological process most sensitive to high‐temperature damage and that rising atmospheric CO₂ content will drive temperature increases in many already stressful environments, this CO₂‐induced increase in plant high‐temperature tolerance may have a substantial impact on both the productivity and distribution of many plant species in the 21st century.     

Growth, light interception and yield responses of spring wheat (Triticum aestivum L.) grown under elevated CO2 and O3 in open-top chambers

Spring wheat cv. Minaret was grown to maturity under three carbon dioxide (CO₂) and two ozone (O₃) concentrations in open-top chambers (OTC). Green leaf area index (LAI) was increased by elevated CO₂ under ambient O₃ conditions as a direct result of increases in tillering, rather than individual leaf areas. Yellow LAI was also greater in the 550 and 680 μmol mol^–1 CO₂ treatments than in the chambered ambient control; individual leaves on the main shoot senesced more rapidly under 550 μmol mol^–1 CO₂, but senescence was delayed at 680 μmol mol^–1 CO₂. Fractional light interception (f) during the vegetative period was up to 26% greater under 680 μmol mol^–1 CO₂ than in the control treatment, but seasonal accumulated intercepted radiation was only increased by 8%. As a result of greater carbon assimilation during canopy development, plants grown under elevated CO₂ were taller at anthesis and stem and ear biomass were 27 and 16% greater than in control plants. At maturity, yield was 30% greater in the 680 μmol mol^–1 CO₂ treatment, due to a combination of increases in the number of ears per m^–2, grain number per ear and individual grain weight (IGW). Exposure to a seasonal mean (7 h d^–1) of 84 nmol mol^–1 O₃ under ambient CO₂ decreased green LAI and increased yellow LAI, thereby reducing both f and accumulated intercepted radiation by ≈ 16%. Individual leaves senesced completely 7–28 days earlier than in control plants. At anthesis, the plants were shorter than controls and exhibited reductions in stem and ear biomass of 15 and 23%. Grain yield at maturity was decreased by 30% due to a combination of reductions in ear number m^–2, the numbers of grains per spikelet and per ear and IGW. The presence of elevated CO₂ reduced the rate of O₃-induced leaf senescence and resulted in the maintenance of a higher green LAI during vegetative growth under ambient CO₂ conditions. Grain yields at maturity were nevertheless lower than those obtained in the corresponding elevated CO₂ treatments in the absence of elevated O₃. Thus, although the presence of elevated CO₂ reduced the damaging impact of ozone on radiation interception and vegetative growth, substantial yield losses were nevertheless induced. These data suggest that spring wheat may be susceptible to O₃-induced injury during anthesis irrespective of the atmospheric CO₂ concentration. Possible deleterious mechanisms operating through effects on pollen viability, seed set and the duration of grain filling are discussed.     

Photosynthesis and conductance of spring-wheat leaves: field response to continuous free-air atmospheric CO2 enrichment

Spring wheat was grown from emergence to grain maturity in two partial pressures of CO₂ (pCO₂): ambient air of nominally 37 Pa and air enriched with CO₂ to 55 Pa using a free-air CO₂ enrichment (FACE) apparatus. This experiment was the first of its kind to be conducted within a cereal field without the modifications or disturbance of microclimate and rooting environment that accompanied previous studies. It provided a unique opportunity to examine the hypothesis that continuous exposure of wheat to elevated pCO₂ will lead to acclimatory loss of photosynthetic capacity. The diurnal courses of photosynthesis and conductance for upper canopy leaves were followed throughout the development of the crop and compared to model-predicted rates of photosynthesis. The seasonal average of midday photosynthesis rates was 28% greater in plants exposed to elevated pCO₂ than in contols and the seasonal average of the daily integrals of photosynthesis was 21% greater in elevated pCO₂ than in ambient air. The mean conductance at midday was reduced by 36%. The observed enhancement of photosynthesis in elevated pCO₂ agreed closely with that predicted from a mechanistic biochemical model that assumed no acclimation of photosynthetic capacity. Measured values fell below predicted only in the flag leaves in the mid afternoon before the onset of grain-filling and over the whole diurnal course at the end of grain-filling. The loss of enhancement at this final stage was attributed to the earlier senescence of flag leaves in elevated pCO₂. In contrast to some controlled-environment and field-enclosure studies, this field-scale study of wheat using free-air CO₂ enrichment found little evidence of acclimatory loss of photosynthetic capacity with growth in elevated pCO₂ and a significant and substantial increase in leaf photosynthesis throughout the life of the crop.