Effects of CO2 enrichment and water stress on gas exchange of Liquidambar styraciflua and Pinus taeda seedlings grown under different irradiance levels

Summary The effects of CO₂ enrichment and water stress on gas exchange of Liquidambar styraciflua L. (sweetgum) and Pinus taeda L. (loblolly pine) seedlings were examined for individuals grown from seed under high (1000 mol·m^-2·s^-1) and low (250 mol·m^-2·s^-1) photosynthetic photon flux density at 350, 675 and 1000 l·l^-1 CO₂. At 8 weeks of age, half the seedlings in each CO₂-irradiance treatment were subjected to a drying cycle which reduced plant water potential to about -2.5 MPa in the most stressed plants, while control plants remained well-watered (water potentials of -0.3 and -0.7 MPa for sweetgum and loblolly pine, respectively). During this stress cycle, whole seedling net photosynthesis, transpiration and stomatal conductance of plants from each CO₂-irradiance-water treatment were measured under respective growth conditions.For both species, water stress effects on gas exchange were greatest under high irradiance conditions. Waterstressed plants had significantly lower photosynthesis rates than well-watered controls throughout most of the drying cycle, with the most severe inhibition occurring for low CO₂, high irradiance-grown sweetgum seedlings. Carbon dioxide enrichment had little effect on gas exchange rates of either water-stressed or well-watered loblolly pine seedlings. In contrast, water stress effects were delayed for sweetgum seedlings grown at elevated CO₂, particularly in the 1000 l·l^-1 CO₂, high irradiance treatment where net photosynthesis, transpiration and conductance of stressed plants were 60, 36 and 33% of respective control values at the end of the drying cycle. Development of internal plant water deficits was slower for stressed sweetgum seedlings grown at elevated CO₂. As a result, these seedlings maintained higher photosynthetic rates over the drying cycle than stressed sweetgum seedlings grown at 350 l·l^-1 CO₂ and stressed loblolly pine seedlings grown at ambient and enriched CO₂ levels. In addition, water-stressed sweetgum seedlings grown at elevated CO₂ exhibited a substantial increase in water use efficiency.The results suggest that with the future increase in atmospheric CO₂ concentration, sweetgum seedlings should tolerate longer exposure to low soil moisture, resulting in greater first year survival of seedlings on drier sites of abandoned fields in the North Carolina piedmont.     

Combined Effects of CO2 and O3 on Antioxidative and Photoprotective Defense Systems in Needles of Ponderosa Pine

Ponderosa pine (Pinus ponderosa Dougl. ex Laws.) seedlings were exposed to near ambient or elevated CO₂ (average concentrations during the last growing season 446 versus 699 mol mol^–1), combined with low or elevated O₃ for three seasons. Ozone exposure during the last growing season (accumulated dose above threshold 0.06 mol mol^–1) was 0.05 versus 26.13 mol mol^–1 h. Needles of the youngest age class were harvested after the dormancy period. Ozone exposure decreased needle contents of chlorophyll a, chlorophyll b, and ascorbate, and resulted in a more oxidized total ascorbate and a more de-epoxidized xanthophyll cycle pool irrespective of the CO₂ level. Trees under elevated CO₂ had a more oxidized glutathione pool and lower chlorophyll a content. Contents of glutathione, tocopherol, and carotenoids were not affected by the CO₂ or O₃ treatments. There were no interactive effects between elevated CO₂ and elevated O₃ on any of the parameters measured. The results suggest that elevated atmospheric CO₂ concentration does not compensate for ozone stress by increasing antioxidative capacity in ponderosa pine.     

Effects of salt stress on the growth,ion content,stomatal behaviour and photosynthetic capacity of a salt-sensitive species,Phaseolus vulgaris L.

Phaseolus vulgaris (cv. Hawkesbury Wonder) was grown over a range of NaCl concentrations (0–150 mM), and the effects on growth, ion relations and photosynthetic performance were examined. Dry and fresh weight decreased with increasing external NaCl concentration while the root/shoot ratio increased. The Cl^- concentration of leaf tissue increased linearly with increasing external NaCl concentration, as did K⁺ concentration, although to a lesser degree. Increases in leaf Na⁺ concentration occurred only at the higher external NaCl concentrations (100 mM). Increases in leaf Cl^- were primarily balanced by increases in K⁺ and Na⁺. X-ray microanalysis of leaf cells from salinized plants showed that Cl^- concentration was high in both the cell vacuole and chloroplast-cytoplasm (250–300 mM in both compartments for the most stressed plants), indicating a lack of effective intracellular ion compartmentation in this species. Salinity had little effect on the total nitrogen and ribulose-1,5-bisphosphate (RuBP) carboxylase (EC 4.1.1.39) content per unit leaf area. Chlorophyll per unit leaf area was reduced considerably by salt stress, however. Stomatal conductance declined substantially with salt stress such that the intercellular CO₂ concentration (C _i) was reduced by up to 30%. Salinization of plants was found to alter the ¹³C value of leaves of Phaseolus by up to 5 and this change agreed quantitatively with that predicted by the theory relating carbon-isotope fractionation to the corresponding measured intercellular CO₂ concentration. Salt stress also brought about a reduction in photosynthetic CO₂ fixation independent of altered diffusional limitations. The initial slope of the photosynthesis versus C _i response declined with salinity stress, indicating that the apparent in-vivo activity of RuBP carboxylase was decreased by up to 40% at high leaf Cl^- concentrations. The quantum yield for net CO₂ uptake was also reduced by salt stress.Abbreviations and symbols A net CO₂ assimilation rate - C _a ambient CO₂ concentration - C _i intercellular CO₂ concentration - RuBP ribulose-1,5-bisphosphate - ¹³C ratio of ¹³C to ¹²C relative to standard limestone     

Effects of elevated CO2 and increased nitrogen deposition on photosynthesis and growth of understory plants in spruce model ecosystems

We studied the effects of atmospheric CO₂ enrichment (280, 420 and 560 l CO₂ l^–1) and increased N deposition (0,30 and 90 kg ha^–1 year^–1) on the spruce-forest understory species Oxalis acetosella, Homogyne alpina and Rubus hirtus. Clones of these species formed the ground cover in nine 0.7 m² model ecosystems with 5-year-old Picea abies trees (leaf area index of approx 2.2). Communities grew on natural forest soil in a simulated montane climate. Independently of N deposition, the rate of light-saturated net photosynthesis of leaves grown and measured at 420 l CO₂ l^–1 was higher in Oxalis and in Homogyne, but was not significantly different in Rubus compared to leaves grown and measured at the pre-industrial CO₂ concentration of 280 l l^–1. Remarkably, further CO₂ enrichment to 560 l l^–1 caused no additional increase of CO₂ uptake. With increasing CO₂ supply concentrations of non-structural carbohydrates in leaves increased and N concentrations decreased in all species, whereas N deposition had no significant effect on these traits. Above-ground biomass and leaf area production were not significantly affected by elevated CO₂ in the more vigorously growing species O. acetosella and R. hirtus, but the slow growing H. alpina produced almost twice as much biomass and 50% more leaf area per plant under 420 l CO₂ l^–1 compared to 280 l l^–1 (again no further stimulation at 560 l l^–1). In contrast, increased N addition stimulated growth in Oxalis and Rubus but had no effect on Homogyne. In Oxalis (only) biomass per plant was positively correlated with microhabitat quantum flux density at low CO₂, but not at high CO₂ indicating carbon saturation. On the other hand, the less shade-tolerant Homogyne profited from CO₂ enrichment at all understory light levels facilitating its spread into more shady micro-habitats under elevated CO₂. These species-specific responses to CO₂ and N deposition will affect community structure. The non-linear responses to elevated CO₂ of several of the traits studied here suggest that the largest responses to rising atmospheric CO₂ are under way now or have already occurred and possible future responses to further increases in CO₂ concentration are likely to be much smaller in these understory species.     

Overcoming drought-induced decreases in soybean leaf photosynthesis by measuring with CO2-enriched air

Soybean [Glycine max (L.) Merr. cv. Williams 82 and A3127] plants were grown in the field under long-term soil moisture deficit and irrigation to determine the effects of severe drought stress on the photosynthetic capacity of soybean leaves. Afternoon leaf water potentials, stomatal conductances, intercellular CO₂ concentrations and CO₂-assimilation rates for the two soil moisture treatments were compared during the pod elongation and seed enlargement stages of crop development. Leaf CO₂-assimilation rates were measured with either ambient (340 l CO₂ l^–1) or CO₂-enriched (1800 l CO₂ l^–1) air. Although seed yield and leaf area per plant were decreased an average of 48 and 31%, respectively, as a result of drought stress, leaf water potentials were reduced only an average of 0.27 MPa during the sampling period. Afternoon leaf CO₂-assimilation rates measured with ambient air were decreased an average of 56 and 49% by soil moisture deficit for Williams 82 and A3127, respectively. The reductions in leaf photosynthesis of both cultivars were associated with similar decreases in leaf stomatal conductance and with small increases in leaf intercellular CO₂ concentration. When the CO₂-enriched air was used, similar afternoon leaf CO₂-assimilation rates were found between the soil moisture treatments at each stage of crop development. These results suggest that photosynthetic capacity of soybean leaves is not reduced by severe soil moisture deficit when a stress develops gradually under field conditions.Abbreviations C_i intercellular CO₂ concentrations - A_a rates of CO₂ assimilation measured with ambient air - A_e rates of CO₂ assimilation measured with CO₂-enriched air - g_s stomatal conductances - RuBPCase ribulose-1,5-bisphosphate carboxylase     

Involvement of ethylene in the morphological and developmental response of rice to elevated atmospheric CO2 concentrations

We tested the hypothesis that increased carbohydrate flux under elevatedCO₂ regulates accelerated development using rice (Oryzasativa L. cv. Jarrah). Plants were grown either in flooded soil orsolution culture at either 360 or 700 L CO₂L^–1. Total dry mass, shoot elongation rates (SER),tiller appearance rates (TAR) and ethylene release from intact rice seedlingswere measured from 5 to 42 days after planting (DAP). At maturity, shoot andsheath length, tiller number and grain mass were also measured. ElevatedCO₂ had a profound effect on growth, morphology and development andthe effects were more pronounced during the early growth phase. Total aboveground biomass increased at elevated CO₂ and this was accounted for by enhanced tiller number. Grain yield was increased by 56% under elevated CO₂mainly due to increased tiller number and hence panicle number. TAR and SERwereenhanced at elevated CO₂ but SER increased only untill 25 DAP.Elevated CO₂ stimulated a 2-3-fold increase in endogenous andACC-mediated ethylene release but the ACC concentration in the leaves waslittleaffected showing that rates of ACC synthesis matched its oxidation. Inhibitionof ethylene action by 1-aminocyclopropane (1-MCP) had a more pronouncedinhibitory effect on ethylene release in plants that were grown at 700 ascompared to 360 L CO₂ L^–1. Feedingsucrose to intact plants enhanced ethylene synthesis and these results areconsistent with the hypothesis that increased accumulation of sucrose atelevated CO₂ may enhance expression of genes in the ethylenebiosynthetic pathway. We conclude that increase in ethylene release may becentral in promoting accelerated development under elevated CO₂ andthis coincides with the release of auxiliary buds and accelerated rates oftiller appearance hence increased grain yield at elevated CO₂.     

Activities of carboxylating enzymes in the CAM species Opuntia ficus-indica grown under current and elevated CO2 concentrations

Responses of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPCase) to an elevated atmospheric CO₂ concentration were determined along with net CO₂ uptake rates for the Crassulacean acid metabolism species Opuntia ficus-indica growing in open-top chambers. During the spring 13 months after planting, total daily net CO₂ uptake of basal and first-order daughter cladodes was 28% higher at 720 than at 360 l CO₂ l^-1. The enhancement, caused mainly by higher CO₂ assimilation during the early part of the night, was also observed during late summer (5 months after planting) and the following winter. The activities of Rubisco and PEPCase measured in vitro were both lower at the elevated CO₂ concentration, particularly under the more favorable growth conditions in the spring and late summer. Enzyme activity in second-order daughter cladodes increased with cladode age, becoming maximal at 6 to 10 days. The effect ofelevated CO₂ on Rubisco and PEPCase activity declined with decreasing irradiance, especially for Rubisco. Throughout the 13-month observation period, O. ficus-indica thus showed increased CO₂ uptake when the atmospheric CO₂ concentration was doubled despite lower activities of both carboxylating enzymes.     

Effects of extreme high temperature,drought and elevated CO2 on photosynthesis of the Mojave Desert evergreen shrub,Larrea tridentata

The interaction of extreme temperature events with future atmospheric CO₂ concentrations may have strong impacts on physiological performance of desert shrub seedlings, which during the critical establishment phase often endure temperature extremes in conjunction with pronounced drought. To evaluate the interaction of drought and CO₂ on photosynthesis during heat stress, one-year-old Larrea tridentata[DC] Cov. seedlings were exposed to nine days of heat with midday air temperature maxima reaching 53 °C under three atmospheric CO₂ concentrations (360, 550 and 700 mol mol^–1) and two water regimes (well-watered and droughted). Photosynthetic gas exchange, chlorophyll fluorescence and water potential responses were measured prior to, during and one week following the high temperature stress event. Heat stress markedly decreased net photosynthetic rate (A _net), stomatal conductance (g _s), and the photochemical efficiency of photosystem II (F _v/F _m) in all plants except for well-watered L. tridentata grown in 700 mol mol^–1 CO₂. A _net and g _s remained similar to pre-stress levels in these plants. In droughted L. tridentata, A _net was ca. 2× (in 550 mol mol^–1 CO₂) to 3× (in 700 mol mol^–1 CO₂) higher than in ambient-CO₂-grown plants, while g _s and F _v/F _m were similar and low in all CO₂ treatments. Following heat stress, g _s in all well-watered plants rose dramatically, exceeding pre-stress levels by up to 100%. In droughted plants, g _s and A _net rose only in plants grown at elevated CO₂ following release from heat. This recovery response was strongest at 700 mol mol^–1 CO₂, which returned to A _net and g _s values similar to pre-heat following several days of recovery. Extreme heat diminished the photosynthetic down-regulation response to growth at elevated CO₂ under well-watered conditions, similar to the action of drought. Ambient-CO₂-grown L. tridentata did not show significant recovery of photosynthetic capacity (A _\max and CE) after alleviation of temperature stress, especially when exposed to drought, while plants exposed to elevated CO₂ appeared to be unaffected. These findings suggest that elevated CO₂ could promote photosynthetic activity during critical periods of seedling establishment, and enhance the potential for L. tridentata to survive extreme high temperature events.     

Effect at the ecosystem level of elevated atmospheric CO2in an aquatic microcosm

We studied the responses of an aquatic microcosm in two different eutrophic conditions to elevated atmospheric CO₂concentration. We used microcosms, consisting of Escherichia coli(bacteria), Tetrahymena thermophila(protozoa) and Euglena gracilis(algae), in salt solution with 50 and 500 mg l^–1of proteose peptone (eutrophic and hypereutrophic conditions, respectively) under ambient and elevated CO₂(1550±100 l l^–1) conditions. The density of E. gracilisincreased significantly under elevated CO₂in both eutrophic and hypereutrophic microcosms. In the eutrophic microcosm, the other elements were not affected by elevated CO₂. In the hypereutrophic microcosm, however, the concentrations of ammonium and phosphate decreased significantly under elevated CO₂. Furthermore, the density of T. thermophilawas maintained in higher level than that in the microcosm with ambient CO₂and the density of E. coliwas decreased by CO₂enrichment. Calculating the carbon biomasses of T. thermophilaand E. colifrom their densities, the changes in their biomasses by CO₂enrichment were little as compared with large increase of E. graciliscarbon biomass converted from chlorophyll a. From the responses to elevated CO₂in the subsystems of the hypereutrophic microcosm consisting of either one or two species, the increase of E. graciliswas a direct effect of elevated CO₂, whereas the changes in the density of E. coliand T. thermophilaand the decreases in the concentration of ammonium and phosphate are considered to be indirect effects rather than direct effects of elevated CO₂. The indirect effects of elevated CO₂were prominent in the hypereutrophic microcosm.     

Interactive effects of atmospheric CO2 enrichment,salinity and flooding on growth of C3 (Elymus athericus) and C4 (Spartina anglica) salt marsh species

The growth response of Dutch salt marsh species (C₃ and C₄) to atmospheric CO₂ enrichment was investigated. Tillers of the C₃ speciesElymus athericus were grown in combinations of 380 and 720 11^-1 CO₂ and low (O) and high (300 mM NaCl) soil salinity. CO₂ enrichment increased dry matter production and leaf area development while both parameters were reduced at high salinity. The relative growth response to CO₂ enrichment was higher under saline conditions. Growth increase at elevated CO₂ was higher after 34 than 71 days. A lower response to CO₂ enrichment after 71 days was associated with a decreased specific leaf area (SLA). In two other experiments the effect of CO₂ (380 and 720 11^-1) on growth of the C₄ speciesSpartina anglica was studied. In the first experiment total plant dry weight was reduced by 20% at elevated CO₂. SLA also decreased at high CO₂. The effect of elevated CO₂ was also studied in combination with soil salinity (50 and 400 mM NaCl) and flooding. Again plant weight was reduced (10%) at elevated CO₂, except under the combined treatment high salinity/non-flooded. But these effects were not significant. High salinity reduced total plant weight while flooding had no effect. Causes of the salinity-dependent effect of CO₂ enrichment on growth and consequences of elevated CO₂ for competition between C₃ and C₄ species are discussed.     

Leaf and canopy responses of Lolium perenne to long-term elevated atmospheric carbon-dioxide concentration

The relationship between leaf photosynthetic capacity (p _{n, max}), net canopy CO₂- and H₂O-exchange rate (NCER and E _t, respectively) and canopy dry-matter production was examined in Lollium perenne L. cv. Vigor in ambient (363±30 l· l^-1) and elevated (631±43 l·l^-1) CO₂ concentrations. An open system for continuous and simultaneous regulation of atmospheric CO₂ concentration and NCER and E _t measurement was designed and used over an entire growth cycle to calculate a carbon and a water balance. While NCER_max of full-grown canopies was 49% higher at elevated CO₂ level, stimulation of p _n, max was only 46% (in spite of a 50% rise in one-sided stomatal resistance for water-vapour diffusion), clearly indicating the effect of a higher leaf-area index under high CO₂ (approx. 10% in one growing period examined). A larger amount of CO₂-deficient leaves resulted in higher canopy dark-respiration rates and higher canopy light compensation points. The structural component of the high-CO₂ effect was therefore a disadvantage at low irradiance, but a far greater benefit at high irradiance. Higher canopy darkrespiration rates under elevated CO₂ level and low irradiance during the growing period are the primary causes for the increase in dry-matter production (19%) being much lower than expected merely based on the NCER_max difference. While total water use was the same under high and low CO₂ levels, water-use efficiency increased 25% on the canopy level and 87% on a leaf basis. In the course of canopy development, allocation towards the root system became greater, while stimulation of shoot dry-matter accumulation was inversely affected. Over an entire growing season the root/shoot production ratio was 22% higher under high CO₂ concentration.Abbreviations and symbols C350 ambient CO₂, 363±30 l·l^-1 - C600 high CO₂, 631±43 l·l^-1 - c _a atmospheric CO₂ level - c _i CO₂ concentration in the intracellular spaces of the leaf - E_t canopy evapotranspiration - I _o canopy light compensation point - NCER canopy CO₂-exchange rate - p _n leaf photosynthetic rate - PPFD photosynthetic photon flux density - r _a leaf boundary-layer resistance - RD canopy dark-respiration rate - r _s stomatal resistance - WUE water use efficiency     

Effect of CO2 and light on survival and growth of rice regenerants grownin vitro on sugar-free medium

Rice (Oryza sativa L.) plantlets regenerated from callus (rice regenerants) were grownin vitro during the preparation stage either on a 1/4 strength N6 gellan gum (4 g l^-1) medium without sucrose (SFM) or with 30 g l^-1 sucrose (SCM), and under CO₂ concentrations of 0.4, 2, 10, 50 or 100 mmol mol^-1, a photoperiod of 24 h and a photosynthetic photon flux density (PPFD) of 125 mol m^-2 s^-1. Rice regenerants were also grownin vitro on SFM or SCM under CO₂ concentration of 50 mmol mol^-1, a photoperiod of 12 or 24 h and a PPFD of 80 or 125 mol m^-2 s^-1. All rice regenerants grew successfully on SFM under CO₂ concentrations of 50 or 100 mmol mol^-1. Increasing the CO₂ concentration increased the survival percentage, shoot length and shoot and root dry weights of rice regenerants grown on SFM. Increasing CO₂ concentration had no significant effect on the survival or growth of rice regenerants grown on SCM. Survival percentages of rice regenerants grown on SCM were less than 80% for each of the CO₂ concentrations. A photoperiod of 24 h under CO₂ enrichment improved the survival and growth of rice regenerants grown on SFM, and increased the survival percentage and shoot dry weight of rice regenerants grown on SCM.     

Interactions of elevated CO2, NH3 and O3 on mycorrhizal infection,gas exchange and N metabolism in saplings of Scots pine

Four-year-old saplings of Scots pine (Pinus sylvestris) (L.) were exposed for 11 weeks in controlled-environment chambers to charcoad-filtered air, or to charcoal-filtered air supplemented with NH₃ (40 g m^–3), O₃ (110 g m^–3 during day/ 40 g m^–3 during night) or NH₃+O₃. All treatments were carried out at ambient (259 L L^–1) and at elevated CO₂ concentration (700 L L^–1). Total tree biomass, mycorrhizal infection, net CO₂ assimilation (P_n), stomatal conductance (g_s), transpiration of the shoots and NH₃ metabolization of the needles were measured. In ambient CO₂ (1) gaseous NH₃ decreased mycorrhizal infection, without significantly affecting tree biomass or N concentration and it enhanced the activity of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) in one-year-old needles; (2) ozone decreased mycorrhizal infection and the acitivity of GS in the needles, while it increased the activity og GDH; (3) exposure to NH₃+O₃ lessened the effects of single exposures to NH₃ and O₃ on reduction of mycorrhizal infection and on increase in GDH activity. Similar lessing effects on mycorrhizal infection as observed in trees exposed to NH₃+O₃ at ambient CO₂, were measured in trees exposed to NH₃+O₃ at elevated CO₂. Exposure to elevated CO₂ without pollutants did not significantly affect any of the parameters studied, except for a decrease in the concentration of soluble proteins in the needles. Elevated CO₂ _NH₃ strongly decreased root branching and mycorrhizal infection and temporarily stimulated P_n and g_s. The exposure to elevated CO₂+NH₃+O₃ also transiently stimulated P_n. The possible mechanisms underlying and integrating these effects are discussed. Elevated CO₂ clearly did not alleviate the negative effects of NH₃ and O₃ mycoorhiral infection. The significant reduction of mycorrhizal infection after exposure to NH₃ or O₃, observed before significant changes in gas exchange or growth occurred, suggest the use of mycorrhizal infection as an early indicator for NH₃ and O₃ induced stress.Abbreviations DW dry weight - FA filtered air - FA_a filtered air at ambient CO₂ - FA_e filtered air at elevated CO₂ - FW fresh weight - GDH glutamate dehydrogenase - GS glutamine synthetase - g_s stomatal conductance - P_n net CO₂ assimilation - RWR root weight ratio - SRL specific root length     

Effect of elevated atmospheric CO2 on mycorrhizal colonization, external mycorrhizal hyphal production and phosphorus inflow in Plantago lanceolata and Trifolium repens in association with the arbuscular mycorrhizal fungus Glomus mosseae

Plantago lanceolata and Trifolium repens were grown under ambient (400 μmol mol^–1) and elevated (650 μmol mol^–1) atmospheric CO₂ conditions. Plants were inoculated with the arbuscular mycorrhizal fungus Glomus mosseae and given a phosphorus supply in the form of bonemeal. Six sequential harvests were taken in order to determine whether the effect of elevated CO₂ on internal mycorrhizal colonization and external hyphal production was independent of the stimulatory effect of elevated CO₂ on plant growth. At a given time, elevated CO₂ increased the percentage of root length colonized (RLC), the total length of colonized root and the external mycorrhizal hyphal (EMH) density and decreased the ratio of EMH to total length of colonized root. When plant size was taken into account, the CO₂ effect on RLC and total length of colonized root was greatly reduced (and only apparent for early harvests in T. repens) and the effects on the EMH parameters disappeared. Root tissue P concentration was unchanged at elevated CO₂, but there was a decrease in shoot P at the later harvests. There was no direct effect of elevated CO₂ on P inflow for the earlier period (< 50 d) of the experiment. However, over the last period, there was a significant negative effect of elevated CO₂ on P inflow for both species, independent of plant size. It is concluded that elevated CO₂ had no direct effect on mycorrhizal colonization or external hyphal production, and that any observed effects on a time basis were due to faster growing plants at elevated CO₂. However, for older plants, elevated CO₂ had a direct negative effect on P inflow. This decrease in P inflow coincides with the observed decrease in shoot P concentration. This is discussed in terms of downregulation of photosynthesis often seen in elevated CO₂ grown plants, and the potential for mycorrhizas (via external hyphal turnover) to alleviate the phenomenon. The direction for future research is highlighted, especially in relation to carbon flow to and storage in the soil.     

Gas exchange of Ginkgo biloba leaves at different CO2 concentration levels

One and a half year-old Ginkgo saplings were grown for 2 years in 7 litre pots with medium fertile soil at ambient air CO₂ concentration and at 700 μmol mol⁻¹ CO₂ in temperature and humidity-controlled cabinets standing in the field. In the middle of the 2nd season of CO₂ enrichment, CO₂ exchange and transpiration in response to CO₂ concentration was measured with a mini-cuvette system. In addition, the same measurements were conducted in the crown of one 60-year-old tree in the field. Number of leaves/tree was enhanced by elevated CO₂ and specific leaf area decreased significantly.CO₂ compensation points were reached at 75–84 μmol mol⁻¹ CO₂. Gas exchange of Ginkgo saplings reacted more intensively upon CO₂ than those of the adult Ginkgo. On an average, stomatal conductance decreased by 30% as CO₂ concentration increased from 30 to 1000 μmol mol⁻¹ CO₂. Water use efficiency of net photosynthesis was positively correlated with CO₂ concentration levels. Saturation of net photosynthesis and lowest level of stomatal conductance was reached by the leaves of Ginkgo saplings at >1000 μmol mol⁻¹ CO₂. Acclimation of leaf net CO₂ assimilation to the elevated CO₂ concentration at growth occurred after 2 years of exposure. Maximum of net CO₂ assimilation was 56% higher at ambient air CO₂ concentration than at 700 μmol mol⁻¹ CO₂.     

Leaf and canopy responses to elevated CO2 in a pine forest under free-air CO2 enrichment

Physiological responses to elevated CO₂ at the leaf and canopy-level were studied in an intact pine (Pinus taeda) forest ecosystem exposed to elevated CO₂ using a free-air CO₂ enrichment (FACE) technique. Normalized canopy water-use of trees exposed to elevated CO₂ over an 8-day exposure period was similar to that of trees exposed to current ambient CO₂ under sunny conditions. During a portion of the exposure period when sky conditions were cloudy, CO₂-exposed trees showed minor (7%) but significant reductions in relative sap flux density compared to trees under ambient CO₂ conditions. Short-term (minutes) direct stomatal responses to elevated CO₂ were also relatively weak (5% reduction in stomatal aperture in response to high CO₂ concentrations). We observed no evidence of adjustment in stomatal conductance in foliage grown under elevated CO₂ for nearly 80 days compared to foliage grown under current ambient CO₂, so intrinsic leaf water-use efficiency at elevated CO₂ was enhanced primarily by direct responses of photosynthesis to CO₂. We did not detect statistical differences in parameters from photosynthetic responses to intercellular CO₂ (A _net-C _i curves) for Pinus taeda foliage grown under elevated CO₂ (550 mol mol^–1) for 50–80 days compared to those for foliage grown under current ambient CO₂ from similar-sized reference trees nearby. In both cases, leaf net photosynthetic rate at 550 mol mol^–1 CO₂ was enhanced by approximately 65% compared to the rate at ambient CO₂ (350 mol mol^–1). A similar level of enhancement under elevated CO₂ was observed for daily photosynthesis under field conditions on a sunny day. While enhancement of photosynthesis by elevated CO₂ during the study period appears to be primarily attributable to direct photosynthetic responses to CO₂ in the pine forest, longer-term CO₂ responses and feedbacks remain to be evaluated.     

Soil-atmosphere exchange of CH4, CO2, NOx,and N2O in the Colorado shortgrass steppe under elevated CO2

In late March 1997, an open-top-chamber (OTC) CO₂ enrichment study was begun in the Colorado shortgrass steppe. The main objectives of the study were to determine the effect of elevated CO₂ (720 mol mol^–1) on plant production, photosynthesis, and water use of this mixed C₃/C₄ plant community, soil nitrogen (N) and carbon (C) cycling and the impact of changes induced by CO₂ on trace gas exchange. From this study, we report here our weekly measurements of CO₂, CH₄, NO_x and N₂O fluxes within control (unchambered), ambient CO₂ and elevated CO₂ OTCs. Soil water and temperature were measured at each flux measurement time from early April 1997, year round, through October 2000. Even though both C₃ and C₄ plant biomass increased under elevated CO₂ and soil moisture content was typically higher than under ambient CO₂ conditions, none of the trace gas fluxes were significantly altered by CO₂ enrichment. Over the 43 month period of observation NO_x and N₂O flux averaged 4.3 and 1.7 in ambient and 4.1 and 1.7 g N m^–2 hr ^–1 in elevated CO₂ OTCs, respectively. NO_x flux was negatively correlated to plant biomass production. Methane oxidation rates averaged –31 and –34 g C m^–2 hr^–1 and ecosystem respiration averaged 43 and 44 mg C m^–2 hr^–1 under ambient and elevated CO₂, respectively, over the same time period.     

Influence of elevated CO2 and nitrogen nutrition on rice plant growth,soil microbial biomass,dissolved organic carbon and dissolved CH4

Rice (Oryza sativa) was grown in six sunlit, semi-closed growth chambers for two seasons at 350 L L^–1 (ambient) and 650 L L^–1 (elevated) CO₂ and different levels of nitrogen (N) supplement. The objective of this research was to study the influence of CO₂ enrichment and N nutrition on rice plant growth, soil microbial biomass, dissolved organic carbon (DOC) and dissolved CH₄. Elevated CO₂ concentration ([CO₂]) demonstrated a wide range of enhancement to both above- and below-ground plant biomass, in particular to stems and roots (for roots when N was not limiting) in the mid-season (80 days after transplanting) and stems/ears at the final harvest, depending on season and the level of N supplement. Elevated [CO₂] significantly increased microbial biomass carbon in the surface 5 cm soil when N (90 kg ha^–1) was in sufficient supply. Low N supplement (30 kg ha^–1) limited the enhancement of root growth by elevated [CO₂], leading consequently to diminished response of soil microbial biomass carbon to CO₂ enrichment. The concentration of dissolved CH₄ (as well as soil DOC, but to a lesser degree) was observed to be positively related to elevated [CO₂], especially at high rate of N application (120 kg ha^–1) or at 10 cm depth (versus 5 cm depth) in the later half of the growing season (at 80 kg N ha^–1). Root senescence in the late season complicated the assessment of the effect of elevated [CO₂] on root growth and soil organic carbon turnover and thus caution should be taken when interpreting respective high CO₂ results.     

Reproductive allocation of an annual,<Emphasis Type="Italic"> Xanthium canadense</Emphasis>, at an elevated carbon dioxide concentration

Stimulation of vegetative growth by an elevated CO₂ concentration does not always lead to an increase in reproductive yield. This is because reproductive yield is determined by the fraction of biomass allocated to the reproductive part as well as biomass production. We grew Xanthium canadense at low N (LN) and high N levels (HN) under an ambient (360 mol mol^-1) and elevated (700 mol mol^-1) CO₂ concentration ([CO₂]) in open-top chambers. Reproductive yield was analysed as the product of: (1) the duration of the reproductive period, (2) the rate of dry mass acquisition in the reproductive period, and (3) the fraction of acquired biomass allocated to the reproductive part. Elevated [CO₂] increased the total amount of biomass that was allocated to reproductive structures, but this increase was caused by increased capsule mass without a significant increase in seed production. The increase in total reproductive mass was due mainly to an increase in the rate of dry mass acquisition in the reproductive period with a delay in leaf senescence. This positive effect was partly offset by a reduction in biomass allocation to the reproductive part at elevated [CO₂] and HN. The duration of the reproductive period was not affected by elevated [CO₂] but increased by HN. Seed production was strongly constrained by the availability of N for seed growth. The seed [N] was very high in X. canadense and did not decrease significantly at elevated [CO₂]. HN increased seed [N] without a significant increase in seed biomass production. Limited seed growth caused a reduction in biomass allocation to the reproductive part even though dry mass production was increased due to increased [CO₂] and N availability.     

Competition and patterns of resource use among seedlings of five tropical trees grown at ambient and elevated CO2

Summary Seedlings of five tropical trees, Cecropia obtusifolia, Myriocarpa longipes, Piper auritum, Senna multijuga and Trichospermum mexicanum, were grown both as individuals, and in competition with each other at ambient (350) and two levels of elevated CO₂ (525 and 700 l l^-1) for a period of 111 days. Growth, allocation, canopy architecture, mid-day leaf water potential and soil moisture content were assessed three times over this period for individually grown plants, and at the end of the experiment for competitively grown plants. In addition, leaf photosynthesis and conductance were assessed for the individually grown plants midway through the experiment, and light profile curves were determined for the competitive arrays at three stages of development. Elevated CO₂ did not affect photosynthesis or overall growth of the individually-grown plants but did affect canopy architecture; mean canopy height increased with CO₂ in Piper and Trichospermum and decreased in Senna. Stomatal conductance decreased slightly as CO₂ increased from 350 to 525 l l^-1 but this had no significant effect upon whole plant water use of leaf water potential. Soil moisture content for the individuals increased marginally as CO₂ increased, but this did not occur in the competitive arrays. There was a marked effect of CO₂ upon species composition of the competitive arrays; Senna decreased in importance as CO₂ increased while Cecropia, Trichospermum and Piper increased in importance. Stepwise regression analysis using competitive performance as the independent variable, and the various morphological and physiological parameters measured on the individually grown plants as independent variables, suggested that canopy height was the single most important variable determining competitive ability. Also significant were photosynthetic rate (particularly at low light levels) and allocation to roots early in the experiment. Light profiles in the canopy revealed that less than 15% of incident light penetrated to the level of mean canopy height. Results suggest that competition for light was the major factor determining community composition, and that CO₂ affected competitive outcome through its affect upon canopy architecture.This study was supported by a grant from the US Department of Energy