Seasonal CO2 exchange patterns of developing peach (Prunus persica) fruits in response to temperature, light and CO2 concentration

CO₂ exchange rates per unit dry weight, measured in the field on attached fruits of the late-maturing Cal Red peach cultivar, at 1200 μmol photons m^?2S^?1 and in dark, and photosynthetic rates, calculated by the difference between the rates of CO₂ evolution in light and dark, declined over the growing season. Calculated photosynthetic rates per fruit increased over the season with increasing fruit dry matter, but declined in maturing fruits apparently coinciding with the loss of chlorophyll. Slight net fruit photosynthetic rates ranging from 0. 087 ± 0. 06 to 0. 003 ± 0. 05 nmol CO₂ (g dry weight)^?1 S^?1 were measured in midseason under optimal temperature (15 and 20°C) and light (1200 μmol photons m^?2 S^?1) conditions. Calculated fruit photosynthetic rates per unit dry weight increased with increasing temperatures and photon flux densities during fruit development. Dark respiration rates per unit dry weight doubled within a temperature interval of 10°C; the mean seasonal O₁₀ value was 2. 03 between 20 and 30°C. The highest photosynthetic rates were measured at 35°C throughout the growing season. Since dark respiration rates increased at high temperatures to a greater extent than CO₂ exchange rates in light, fruit photosynthesis was apparently stimulated by high internal CO₂ concentrations via CO₂ refixation. At 15°C, fruit photosynthetic rates tended to be saturated at about 600 μmol photons m^?2 S^?1. Young peach fruits responded to increasing ambient CO₂ concentrations with decreasing net CO₂ exchange rates in light, but more mature fruits did not respond to increases in ambient CO₂. Fruit CO₂ exchange rates in the dark remained fairly constant, apparently uninfluenced by ambient CO₂ concentrations during the entire growing season. Calculated fruit photosynthetic rates clearly revealed the difference in CO₂ response of young and mature peach fruits. Photosynthetic rates of younger peach fruits apparently approached saturation at 370 μl CO₂1^?2. In CO₂ free air, fruit photosynthesis was dependent on CO₂ refixation since CO₂ uptake by the fruits from the external atmosphere was not possible. The difference in photosynthetic rates between fruits in CO₂-free air and 370 μl CO₂ 1^?1 indicated that young peach fruits were apparently able to take up CO₂ from the external atmosphere. CO₂ uptake by peach fruits contributed between 28 and 16% to the fruit photosynthetic rate early in the season, whereas photosynthesis in maturing fruits was supplied entirely by CO₂ refixation.     

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.     

Responses of Photosynthetic and Defence Systems to High Temperature Stress in Quercus suber L Seedlings Grown under Elevated CO2

Abstract: Growth in elevated CO₂ led to an increase in biomass production per plant as a result of enhanced carbon uptake and lower rates of respiration, compared to ambient CO₂-grown plants. No down-regulation of photosynthesis was found after six months of growth under elevated CO₂. Photosynthetic rates at 15°C or 35 °C were also higher in elevated than in ambient CO₂-grown plants, when measured at their respective CO₂ growth condition. Stomata of elevated CO₂-grown plants were less responsive to temperature as compared to ambient CO₂ plants. The after effect of a heat-shock treatment (4 h at 45 °C in a chamber with 80% of relative humidity and 800–1000 tmol m^-2 s^-1 photon flux density) on A_max was less in elevated than in ambient CO₂-grown plants. At the photochemical level, the negative effect of the heat-shock treatment was slightly more pronounced in ambient than in elevated CO₂-grown plants. A greater tolerance to oxidative stress caused by high temperatures in elevated CO₂-grown plants, in comparison to ambient CO₂ plants, is suggested by the increase in superoxide dismutase activity, after 1 h at 45 °C, as well as its relatively high activity after 2 and 4 h of the heat shock in the elevated CO₂-grown plants in contrast with the decrease to residual levels of superoxide dismutase activity in ambient CO₂-grown plants immediately after 1 h at 45 °C. The observed increase in catalase after 1 h at 45 °C in both ambient and elevated CO₂-grown plants, can be ascribed to the higher rates of photorespiration and respiration under this high temperature.     

Combined effects of elevated CO2 and air temperature on carbon assimilation of Pinus taeda trees

Branches of 22-year-old loblolly pine (Pinus taeda, L.) trees growing in a plantation were exposed to ambient CO₂, ambient + 165 μmol mol^?1 CO₂ or ambient + 330 μmol mol^?1 CO₂ concentrations in combination with ambient or ambient + 2°C air temperatures for 3 years. Field measurements in the third year indicated that net carbon assimilation was enhanced in the elevated CO₂ treatments in all seasons. On the basis of A/C_i, curves, there was no indication of photosynthetic down-regulation. Branch growth and leaf area also increased significantly in the elevated CO₂ treatments. The imposed 2°C increase in air temperature only had slight effects on net assimilation and growth. Compared with the ambient CO₂ treatment, rates of net assimilation were ～1·6 times greater in the ambient + 165 μmol mol^?1 CO₂ treatment and 2·2 times greater in the ambient + 330 μmol mol^?1 CO₂ treatment. These ratios did not change appreciably in measurements made in all four seasons even though mean ambient air temperatures during the measurement periods ranged from 12·6 to 28·2°C. This indicated that the effect of elevated CO₂ concentrations on net assimilation under field conditions was primarily additive. The results also indicated that the effect of elevated CO₂ (+ 165 or + 330 μmol mol^?1) was much greater than the effect of a 2°C increase in air temperature on net assimilation and growth in this species.     

Some characteristics of photosynthetic inorganic carbon uptake of a marine macrophytic red alga

Abstract Some characteristics of photosynthetic inorganic carbon uptake by Palmaria palmata, a marine red macroalga, have been measured under physiological conditions in artificial seawater. The apparent affinity of thallus for CO₂ [K_1/2(CO₂)] at pH 8.0 and 15°C was 21.4±3.0mmol m^?3 CO₂ under air, and 25.7±70mmol m^?3 CO₂ under N₂. The corresponding values of V_max were 2.98 ± 0.42 and 3.65±0.87 mmol O₂ evolved g Chr^?1 s^?l. The apparent K_m(CO₂) of isolated ribulose bisphosphate carboxylase was determined at pH 8.0 and 30 °C to be 30.2 mmol m^?3 CO₂, and the corresponding value of V_max was 19.67 μniol CO₂ g protein^?1 s^?1. The CO₂ compensation points of the thallus were measured in artificial seawater at pH 8.0 under air and N₂, using a gas-chromatographic method. The values were relatively low, rising from 10 cm³ m^?3 at 15°C, to 35 cm³ m^?3 at 25°C, but were not affected by the O₂ concentration. The lack of an effect of O₂ on photosynthesis and on compensation point indicates that there is little photorespiratory CO₂ loss in this macroalga. The high affinity of the thallus for CO₂, and the low CO₂ compensation concentrations, are consistent with the occurrence of bicarbonate uptake in this alga.     

Measurement of the photorespiratory activity of the submerged aquatic plant Myriophyllum spicatum L.

Abstract The CO₂ compensation concentrations (points) of leaves of the submerged vascular aquatic plant Myriophyllum spicatum L. were determined in a closed aqueous system at pH 7.0 by a gas chromatographic technique and over the range 10–30deg;C were found to range from 36 to 46 cm³m^?3 in medium equilibrated with 21% O₂ (0.03 kgm^?3), and 25 to 35 cm³m^?3 in medium equilibrated with 2% O₂ (0.03 kgm^?3). The rates of true (TPS) and apparent (APS) photosynthesis of leaves were measured in medium equilibrated with 21% O₂ and buffered at pH 7.0, at subsaturating concentrations (12.8–18.8 mmol m^?3) of dissolved inorganic carbor. (DIC) containing H¹⁴CO₃, by determining the initial rates of uptake by the leaves of DIC and ¹⁴C-activity from the medium. The rate of photorespiration, the difference between TPS and APS, was 7.0–13.3% of TPS over the range of 10–25°C and rose to 29% of TPS at 35°C. The magnitude of the compensation point of this plant is therefore similar to, but is much less O₂-sensitive than, those of C₃ plants, and the photorespiratory rate, at DIC concentrations near the CO₂ compensation point, is very low compared to that of C₃ plants.     

Net ecosystem CO2 exchange in Mojave Desert shrublands during the eighth year of exposure to elevated CO2

Arid ecosystems, which occupy about 35% of the Earth's terrestrial surface area, are believed to be among the most responsive to elevated [CO₂]. Net ecosystem CO₂ exchange (NEE) was measured in the eighth year of CO₂ enrichment at the Nevada Desert Free‐Air CO₂ Enrichment (FACE) Facility between the months of December 2003–December 2004. On most dates mean daily NEE (24 h) (μmol CO₂ m^?2 s^?1) of ecosystems exposed to elevated atmospheric CO₂ were similar to those maintained at current ambient CO₂ levels. However, on sampling dates following rains, mean daily NEEs of ecosystems exposed to elevated [CO₂] averaged 23 to 56% lower than mean daily NEEs of ecosystems maintained at ambient [CO₂]. Mean daily NEE varied seasonally across both CO₂ treatments, increasing from about 0.1 μmol CO₂ m^?2 s^?1 in December to a maximum of 0.5–0.6 μmol CO₂ m^?2 s^?1 in early spring. Maximum NEE in ecosystems exposed to elevated CO₂ occurred 1 month earlier than it did in ecosystems exposed to ambient CO₂, with declines in both treatments to lowest seasonal levels by early October (0.09±0.03 μmol CO₂ m^?2 s^?1), but then increasing to near peak levels in late October (0.36±0.08 μmol CO₂ m^?2 s^?1), November (0.28±0.03 μmol CO₂ m^?2 s^?1), and December (0.54±0.06 μmol CO₂ m^?2 s^?1). Seasonal patterns of mean daily NEE primarily resulted from larger seasonal fluctuations in rates of daytime net ecosystem CO₂ uptake which were closely tied to plant community phenology and precipitation. Photosynthesis in the autotrophic crust community (lichens, mosses, and free‐living cyanobacteria) following rains were probably responsible for the high NEEs observed in January, February, and late October 2004 when vascular plant photosynthesis was low. Both CO₂ treatments were net CO₂ sinks in 2004, but exposure to elevated CO₂ reduced CO₂ sink strength by 30% (positive net ecosystem productivity=127±17 g C m^?2 yr^?1 ambient CO₂ and 90±11 g C m^?2 yr^?1 elevated CO₂, P=0.011). This level of net C uptake rivals or exceeds levels observed in some forested and grassland ecosystems. Thus, the decrease in C sequestration seen in our study under elevated CO₂– along with the extensive coverage of arid and semi‐arid ecosystems globally – points to a significant drop in global C sequestration potential in the next several decades because of responses of heretofore overlooked dryland ecosystems.     

Interactive effects of elevated CO2 and soil fertility on isoprene emissions from Quercus robur

The effects of global change on the emission rates of isoprene from plants are not clear. A factor that can influence the response of isoprene emission to elevated CO₂ concentrations is the availability of nutrients. Isoprene emission rate under standard conditions (leaf temperature: 30°C, photosynthetically active radiation (PAR): 1000 μmol photons m^?2 s^?1), photosynthesis, photosynthetic capacity, and leaf nitrogen (N) content were measured in Quercus robur grown in well‐ventilated greenhouses at ambient and elevated CO₂ (ambient plus 300 ppm) and two different soil fertilities. The results show that elevated CO₂ enhanced photosynthesis but leaf respiration rates were not affected by either the CO₂ or nutrient treatments. Isoprene emission rates and photosynthetic capacity were found to decrease with elevated CO₂, but an increase in nutrient availability had the converse effect. Leaf N content was significantly greater with increased nutrient availability, but unaffected by CO₂. Isoprene emission rates measured under these conditions were strongly correlated with photosynthetic capacity across the range of different treatments. This suggests that the effects of CO₂ and nutrient levels on allocation of carbon to isoprene production and emission under near‐saturating light largely depend on the effects on photosynthetic electron transport capacity.     

The effect of temperature and oxygen on the CO2 compensation point of the marine alga Ulva lactuca

Abstract The CO₂ compensation point of Ulva lactuca frond sections has been measured in artificial seawater using a sensitive gas-chromatographic method. Under nitrogen the compensation point remained relatively constant at 3–6 cm³ m⁻³ at temperatures from 10 to 30°C while in air-saturated medium (0.3 kg m⁻³ O₂) the compensation point rose from 5 cm³ m⁻³ at 10°C to 11 cm³ m⁻³ at 30°C. These responses of the compensation point to temperature and oxygen concentration indicate that there is little photorespiratory CO₂ loss in this marine macroalga, and the low values of these compensation points indicate that inorganic carbon is actively accumulated by the plant.     

Carbon balance during CAM: an assessment of respiratory CO2 recycling in the epiphytic bromeliads Aechmea nudicaulis and Aechmea fendleri

Abstract The regulation of crassulacean acid metabolism (CAM) under controlled environmental conditions has been investigated for two tropical epiphytes, relating plant water and carbon balance to growth form and habitat preference under natural conditions. Aechmea fendleri is restricted to wet, upper montane regions of Trinidad, while A. nudicaulis has a wider distribution extending into more arid regions of the island. Morphological characteristics of these plants are related to habitat preference in terms of leaf succulence (0.44 and 0.94 kg m^?2 for the two species respectively) and a distinct layer of water storage parenchyma in A. nudicaulis In contrast, the thinner leaves of A. fendleri contain little water-storage parenchyma and less chlorenchyma per unit area, but the plants have a more open leaf rosette. The two species differ in expression of CAM, since the proportion of respiratory CO₂ recycled as part of CAM had been found to be much lower in A. fendleri This study compared the efficiency of water use and role of respiratory CO₂ recycling under two PAR regimes (300 and 120 μnol m^?2 s^?1) and three night temperatures (12, 18 and 25 °C). Dark CO₂ uptake rates for both species were comparable to plants in the field (maximum of 2.3 ± 0.2 μmol m^?2s^?1± SD, n= 3). Total net CO₂ uptake at night increased on leaf area basis with temperature for both species under higher PAR, although under the low PAR regime CO₂ uptake was maximal at 18 °C. Water-use efficiency (WUE) increased at 18 °C and 25 °C during dark CO₂ uptake (Phase I) and also during late afternoon photosynthesis (Phase IV) in both species. For A. fendleri, dawn to dusk changes in titrable acidity (ΔH ⁺) were similar under high and low PAR, although ΔH⁺ was correlated to night temperature and PAR in A. nudicaulis. The proportion of ΔH⁺ derived from respiratory CO₂ also varied with experimental conditions. Thus percentage recycling was lower in A. fendleri under high PAR (0–10%), but was only reduced at 18 °C under low PAR. Recycling by A. nudicaulis ranged from 32–42% under high PAR, but was also reduced to 6% under low PAR at 18 °C; at 12 °C and 25 °C, recycling was 37% and 52% respectively. Previous studies have suggested a relationship between the proportion of recycling and degree of water stress. This study indicated that CAM as a CO₂ concentrating mechanism regulates both water-use efficiency and plant carbon balance in these epiphytes, in response to PAR and night temperature. However, the precise relationship between respiratory processes and the balance between external and internal sources of CO₂ is as yet unresolved.     

Growth and physiological responses of cotton (Gossypium hirsutum L.) to elevated carbon dioxide and ultraviolet-B radiation under controlled environmental conditions

Better understanding of crop responses to projected changes in climate is an important requirement. An experiment was conducted in sunlit, controlled environment chambers known as soil–plant–atmosphere–research units to determine the interactive effects of atmospheric carbon dioxide concentration [CO₂] and ultraviolet‐B (UV‐B) radiation on cotton (Gossypium hirsutum L.) growth, development and leaf photosynthetic characteristics. Six treatments were used, comprising two levels of [CO₂] (360 and 720 µmol mol^?1) and three levels of 0 (control), 7.7 and 15.1 kJ m^?2 d^?1 biologically effective UV‐B radiations within each CO₂ level. Treatments were imposed for 66 d from emergence until 3 weeks after the first flower stage. Plants grown in elevated [CO₂] had greater leaf area and higher leaf photosynthesis, non‐structural carbohydrates, and total biomass than plants in ambient [CO₂]. Neither dry matter partitioning among plant organs nor pigment concentrations was affected by elevated [CO₂]. On the other hand, high UV‐B (15.1 kJ m^?2 d^?1) radiation treatment altered growth resulting in shorter stem and branch lengths and smaller leaf area. Shorter plants at high UV‐B radiation were related to internode lengths rather than the number of mainstem nodes. Fruit dry matter accumulation was most sensitive to UV‐B radiation due to fruit abscission. Even under 7.7 kJ m^?2 d^?1 of UV‐B radiation, fruit dry weight was significantly lower than the control although total biomass and leaf photosynthesis did not differ from the control. The UV‐B radiation of 15.1 kJ m^?2 d^?1 reduced both total (43%) and fruit (88%) dry weights due to smaller leaf area and lower leaf net photosynthesis. Elevated [CO₂] did not ameliorate the adverse effects of UV‐B radiation on cotton growth and physiology, particularly the boll retention under UV‐B stress.     

Impact of rising CO2 on emissions of volatile organic compounds: isoprene emission from Phragmites australis growing at elevated CO2 in a natural carbon dioxide spring†

Isoprene basal emission (the emission of isoprene from leaves exposed to a light intensity of 1000 µmol m^?2 s^?1 and maintained at a temperature of 30 °C) was measured in Phragmites australis plants growing under elevated CO₂ in the Bossoleto CO₂ spring at Rapolano Terme, Italy, and under ambient CO₂ at a nearby control site. Gas exchange and biochemical measurements were concurrently taken. Isoprene emission was lower in the plants growing at elevated CO₂ than in those growing at ambient CO₂. Isoprene emission and isoprene synthase activity (IsoS) were very low in plants growing at the bottom of the spring under very rich CO₂ and increased at increasing distance from the spring (and decreasing CO₂ concentration). Distance from the spring did not significantly affect photosynthesis making it therefore unlikely that there is carbon limitation to isoprene formation. The isoprene emission rate was very quickly reduced after rapid switches from elevated to ambient CO₂ in the gas‐exchange cuvette, whereas it increased when switching from ambient to elevated CO₂. The rapidity of the response may be consistent with post‐translational modifications of enzymes in the biosynthetic pathway of isoprene formation. Reduction of IsoS activity is interpreted as a long‐term response. Basal emission of isoprene was not constant over the day but showed a diurnal course opposite to photosynthesis, with a peak during the hottest hours of the day, independent of stomatal conductance and probably dependent on external air temperature or temporary reduction of CO₂ concentration. The present experiments show that basal emission rate of isoprene is likely to be reduced under future elevated CO₂ levels and allow improvement in the modelling of future isoprene emission rates.     

The influence of elevated CO2 and temperature on biomass production of continuously defoliated white clover

Clonal plants of white clover (Trifolium repens L.), grown singly in pots of Perlite and solely dependent for nitrogen on root nodule N₂ fixation, were maintained in controlled environments which provided four environments: 18/13 °C day/night temperature at 340 and 680 μmol mol^?1 CO₂ and 20·5/15·5°C day/night temperature at 340 and 680 μmol mol^?1 CO₂. The daylength was 12 h and the photon flux density 500±25 μmol m^?2 s^?1 (PFD). All plants were defoliated for about 80d, nominally every alternate day, to leave the youngest expanded leaf intact on 50% of stolons, plus expanding leaves (simulated grazing). Elevated CO₂ increased the yield of biomass removed at defoliation by a constant 45% during the second 40d of the experiment and by a varying amount in the first half of the experiment. Elevated temperature had little effect on biomass yield. Nitrogen, as a proportion of the harvested biomass, was only fractionally affected by elevated CO₂ or temperature. In contrast, N₂ fixation increased in concert with the promoting effect of elevated CO₂ on biomass production. The increased yield of biomass harvested in 680 μmol mol^?1 CO₂ was primarily due to the early development and continued maintenance of more stolons. However, the stolons of plants grown in elevated CO₂ also developed leaves which were heavier and slightly larger in area than their counterparts in ambient CO₂. The conclusion is that, when white clover plants are maintained at constant mass by simulated grazing, they continue to respond to elevated CO₂ in terms of a sustained increase in biomass production.     

Carbon dioxide balance of a fen ecosystem in northern Finland under elevated UV-B radiation

The effect of elevated UV‐B radiation on CO₂ exchange of a natural flark fen was studied in open‐field conditions during 2003–2005. The experimental site was located in Sodankylä in northern Finland (67°22′N, 26°38′E, 179 m a.s.l.). Altogether 30 study plots, each 120 cm × 120 cm in size, were randomly distributed between three treatments (n=10): ambient control, UV‐A control and UV‐B treatment. The UV‐B‐treated plots were exposed to elevated UV‐B radiation level for three growing seasons. The instantaneous net ecosystem CO₂ exchange (NEE) and dark respiration (R_TOT) were measured during the growing season using a closed chamber method. The wintertime CO₂ emissions were estimated using a gradient technique by analyzing the CO₂ concentration in the snow pack. In addition to the instantaneous CO₂ exchange, the seasonal CO₂ balances during the growing seasons were modeled using environmental data measured at the site. In general, the instantaneous NEE at light saturation was slightly higher in the UV‐B treatment compared with the ambient control, but the gross photosynthesis was unaffected by the exposure. The R_TOT was significantly lower under elevated UV‐B in the third study year. The modeled seasonal (June–September) CO₂ balance varied between the years depending on the ground water level and temperature conditions. During the driest year, the seasonal CO₂ balance was negative (net release of CO₂) in the ambient control and the UV‐B treatment was CO₂ neutral. During the third year, the seasonal CO₂ uptake was 43±36 g CO₂‐C m⁻² in the ambient control and 79±45 g CO₂‐C m⁻² in the UV‐B treatment. The results suggest that the long‐term exposure to high UV‐B radiation levels may slightly increase the CO₂ accumulation to fens resulting from a decrease in microbial activity in peat. However, it is unlikely that the predicted development of the level of UV‐B radiation would significantly affect the CO₂ balance of fen ecosystems in future.     

CO2 uptake and fluorescence responses for a shade-tolerant cactus Hylocereus undatus under current and doubled CO2 concentrations

Hylocereus undatus (Haworth) Britton and Rose growing in controlled environment chambers at 370 and 740 μmol CO₂ mol^?1 air showed a Crassulacean acid metabolism (CAM) pattern of CO₂ uptake, with 34% more total daily CO₂ uptake under the doubled CO₂ concentration and most of the increase occurring in the late afternoon. For both CO₂ concentrations, 90% of the maximal daily CO₂ uptake occurred at a total daily photosynthetic photon flux density (PPFD) of only 10 mol m^?2 day^?1 and the best day/night air temperatures were 25/15°C. Enhancement of the daily net CO₂ uptake by doubling the CO₂ concentration was greater under the highest PPFD (30 mol m^?2 day^?1) and extreme day/night air temperatures (15/5 and 45/35°C). After 24 days of drought, daily CO₂ uptake under 370 μmol CO₂ mol^?1 was 25% of that under 740 μmol CO₂ mol^?1. The ratio of variable to maximal chlorophyll fluorescence (F_y/F_m) decreased as the PPFD was raised above 5 mol m^?2 day^?1, at extreme day/night temperatures and during drought, suggesting that stress occurred under these conditions. F_v/F_m was higher under the doubled CO₂ concentration, indicating that the current CO₂ concentration was apparently limiting for photosynthesis. Thus net CO₂ uptake by the shade-tolerant H. undatus, the photosynthetic efficiency of which was greatest at low PPFDs. showed a positive response to doubling the CO₂ concentration, especially under stressful environmental conditions.     

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.     

NMR imaging of root water distribution in intact Vicia faba L plants in elevated atmospheric CO2

The effect of elevated atmospheric CO₂ on water distribution in the intact roots of Vicia faba L. bean seedlings grown in natural soil was studied noninvasively with proton (¹H) nuclear magnetic resonance (NMR) imaging. Exposure of 24-d-old plants to atmospheric CO₂-enriched air at 650 cm³ m^?3 produced significant increases in water imaged in upper roots, hypogeal cotyledons and lower stems in response to a short-term drying-stress cycle. Above ground, drying produced negligible stem shrinkage and stomatal resistance was unchanged. In contrast, the same drying cycle caused significant depletion of water imaged in the same upper root structures in control plants subject to ambient CO₂ (350 m³ m^?3), and stem shrinkage and increased stomatal resistance. The results suggest that inhibition of transpiration caused by elevated CO₂ does not necessarily result in attenuation of water transport from lower root structures. Inhibition of water loss from upper roots and lower stem in elevated CO₂ environments may be a mitigating factor in assessing deleterious effects of greenhouse changes on crops during periods of dry climate.     

Net grassland carbon flux over a subambient to superambient CO2 gradient

Increasing atmospheric CO₂ concentrations may have a profound effect on the structure and function of plant communities. A previously grazed, central Texas grassland was exposed to a 200‐µmol mol^?1 to 550 µmol mol^?1 CO₂ gradient from March to mid‐December in 1998 and 1999 using two, 60‐m long, polyethylene‐ covered chambers built directly onto the site. One chamber was operated at subambient CO₂ concentrations (200–360 µmol mol^?1 daytime) and the other was regulated at superambient concentrations (360–550 µmol mol^?1). Continuous CO₂ gradients were maintained in each chamber by photosynthesis during the day and respiration at night. Net ecosystem CO₂ flux and end‐of‐year biomass were measured in each of 10, 5‐m long sections in each chamber. Net CO₂ fluxes were maximal in late May (c. day 150) in 1998 and in late August in 1999 (c. day 240). In both years, fluxes were near zero and similar in both chambers at the beginning and end of the growing season. Average daily CO₂ flux in 1998 was 13 g CO₂ m^?2 day^?1 in the subambient chamber and 20 g CO₂ m^?2 day^?1 in the superambient chamber; comparable averages were 15 and 26 g CO₂ m^?2 day^?1 in 1999. Flux was positively and linearly correlated with end‐of‐year above‐ground biomass but flux was not linearly correlated with CO₂ concentration; a finding likely to be explained by inherent differences in vegetation. Because C₃ plants were the dominant functional group, we adjusted average daily flux in each section by dividing the flux by the average percentage C₃ cover. Adjusted fluxes were better correlated with CO₂ concentration, although scatter remained. Our results indicate that after accounting for vegetation differences, CO₂ flux increased linearly with CO₂ concentration. This trend was more evident at subambient than superambient CO₂ concentrations.     

Growth and water use of the mangroves Rhizophora apiculata and R. stylosa in response to salinity and humidity under ambient and elevated concentrations of atmospheric CO2

Two mangrove species, Rhizophora apiculata and R. stylosa, were grown for 14 weeks in a multifactorial combination of salinity (125 and 350 mol m^?3 NaCl), humidity (43 and 86% relative humidity at 30°C) and atmospheric CO₂ concentration (340 and 700 cm³ m^?3). Under ambient [CO₂], growth responses to different combinations of salinity and humidity were consistent with interspecific differences in distribution along natural gradients of salinity and aridity in northern Australia. Elevated [CO₂] had little effect on relative growth rate when it was limited by salinity but stimulated growth when limited by humidity. Both species benefited most from elevated [CO₂] under relatively low salinity conditions in which growth was vigorous, but relative growth rate was enhanced more in the less salt-tolerant and more rapidly growing species, R. apiculata. Changes in both net assimilation rate and leaf area ratio contributed to changes in relative growth rates under elevated [CO₂], with leaf area ratio increasing with decrease in humidity. Increase in water use efficiency under elevated [CO₂] occurred with increase, decrease or no change in evaporation rates; water use characteristics which depended on both the species and the growth conditions. In summary, elevated [CO₂] is unlikely to increase salt tolerance, but could alter competitive rankings of species along salinity × aridity gradients.     

Effects of increased CO2 concentration and temperature on growth and yield of winter wheat at two levels of nitrogen application

Winter wheat (Triticum aestivum L., cv. Mercia) was grown in chambers under light and temperature conditions similar to the UK field environment for the 1990/1991 growing season at two levels each of atmospheric CO₂ concentration (seasonal means: 361 and 692 μmol mol^?1), temperature (tracking ambient and ambient +4°C) and nitrogen application (equivalent to 87 and 489 kg ha^?1 total N applied). Total dry matter productivity through the season, the maximum number of shoots and final ear number were stimulated by CO₂ enrichment at both levels of the temperature and N treatments. At high N, there was a CO₂-induced stimulation of grain yield (+15%) similar to that for total crop dry mass (+12%), and there was no significant interaction with temperature. This contrasts with other studies, where positive interactions between the effects of increases in temperature and CO₂ have been found. Temperature had a direct, negative effect on yield at both levels of the N and CO₂ treatments. This could be explained by the temperature-dependent shortening of the phenological stages, and therefore, the time available for accumulating resources for grain formation. At high N, there was also a reduction in grain set at ambient +4°C temperature, but the overall negative effect of warmer temperature was greater on the number of grains (-37%) than on yield (-18%), due to a compensating increase in average grain mass. At low N, despite increasing total crop dry mass and the number of ears, elevated CO₂ did not increase grain yield and caused a significant decrease under ambient temperature conditions. This can be explained in terms of a stimulation of early vegetative growth by CO₂ enrichment leading to a reduction in the amount of N available later for the formation and filling of grain.