Growth In Elevated CO2 Research Articles

Effects of mycorrhizal colonization and elevated atmospheric carbon dioxide on carbon fixation and below-ground carbon partitioning in Plantago lanceolata

Plantago lanceolata with or without the mycorrhizal fungus Glomus mosseae were grown over a 100 d period under ambient (380 ± 50 µmol mol-1) and elevated (600 ± 150 Émol mol-1) atmospheric CO2 conditions. To achieve similar growth, non-mycorrhizal plants received phosphorus in solution whereas mycorrhizal plants were supplied with bonemeal. Measures of plant growth, photosynthesis and carbon input to the soil were obtained. Elevated CO2 stimulated plant growth to the same extent in mycorrhizal and non-mycorrhizal plants, but had no effect on the partitioning of carbon between shoots and roots or on shoot tissue phosphorus concentration. Mycorrhizal colonization was low, but unaffected by CO2 treatment. Net photosynthesis was stimulated both by mycorrhizal colonization and elevated CO2, and there was a more than additive effect of the two treatments on net photosynthesis. Colonization by mycorrhizal fungi inhibited acclimation, in terms of net carbon assimilation, of plants to elevated CO2. 13C natural abundance techniques were used to measure carbon input into the soil, although the results were not conclusive. Direct measurements of below-ground root biomass showed that elevated CO2 did stimulate carbon flow below-ground and this was higher in mycorrhizal than non-mycorrhizal plants. For the four treatment combinations, the observed relative differences in amount of below-ground carbon were compared with those expected from the differences in net photosynthesis. A considerable amount of the extra carbon fixed both as a result of mycorrhizal colonization and growth in elevated CO2 did not reveal itself as increased plant biomass. As there was no evidence for a substantial increase in soil organic matter, most of this extra carbon must have been respired by the mycorrhizal fungus and the roots or by the the plants as dark-respiration. The need for detailed studies in this area is emphasized.

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 Amax 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.

Photosynthetic enhancement and conductance to water vapor of field-grown Solanum tuberosum (L.) in response to CO2 enrichment

The photosynthetic responses of potato [Solanum tuberosum(L.)] to CO2 enrichment were studied in open-topped field chambers. Plants were raised in 2.4 m 2 plastic enclosures over three growing seasons from 1996 to 1998. Plots were continuously fertilized with 1, 1.5 and 2 times ambient daytime CO2. These were the low (L), medium (M) and high (H) CO2 treatments, respectively. Tuber dry matter yields were increased 9 and 40%, respectively, in the M and H treatments compared to the L CO2 treatment. Net photosynthesis (Pn) and conductance to water vapor (gs) of upper canopy leaves were measured at 1 or 2-week intervals at the growth CO2 partial pressure and then Pn of plants in the L treatment was determined at 70 Pa CO2 (L70). Leaflet P n rates averaged over all measurement dates were 28, 49 and 84% greater, respectively, in the M, H and L70 CO2 treatments, compared to plants in the L treatment. Changes of Pn in response to the L, M and H CO2 treatments were proportional to increases of internal CO2 (Ci) and at low leaf-to-air vapor pressure deficits mid-day gs was inversely related to growth CO2. The ratio of Pn at H compared to L70 was 0.81 when averaged over all measurement dates. Leaf soluble protein, Rubisco protein and chlorophyll ( a+b ) levels were unaffected by CO2 treatment. Total Rubisco activity was decreased by CO2 enrichment in 1998, but percent activation was similar in the L, M and H plots. Leaf starch was increased but sucrose, glucose and fructose were unaffected by CO2 treatment. The above findings indicated that a down regulation of Pn in response to elevated CO2 was consistently observed in field-grown potato. This was attributed to a decrease of total Rubisco activity that was potentially due to the presence of inhibitory compounds bound to the active site of the enzyme. The amount of photosynthetic acclimation observed here did not preclude a persistent enhancement of Pn under the elevated CO2 growth conditions.

Acquisition and allocation of carbon and nitrogen by Danthonia richardsonii in response to restricted nitrogen supply and CO2 enrichment

Dry weight (DW) and nitrogen (N) accumulation and allocation were measured in isolated plants of Danthonia richardsonii (Wallaby Grass) for 37 d following seed imbibition. Plants were grown at ≈ 365 or 735 μL L–1 CO2 with N supply of 0·05, 0·2 or 0·5 mg N plant–1 d–1. Elevated CO2 increased DW accumulation by 28% (low‐N) to 103% (high‐N), following an initial stimulation of relative growth rate. Net assimilation rate and leaf nitrogen productivity were increased by elevated CO2, while N concentration was reduced. N uptake per unit root surface area was unaffected by CO2 enrichment. The ratio of leaf area to root surface area was decreased by CO2 enrichment. Allometric analysis revealed a decrease in the shoot‐N to root‐N ratio at elevated CO2, while the shoot‐DW to root‐DW ratio was unchanged. Allometric analysis showed leaf area was reduced, while root surface area was unchanged by elevated CO2, indicating a down‐regulation of total plant capacity for carbon gain rather than a stimulation of mineral nutrient acquisition capacity. Overall, growth in elevated CO2 resulted in changes in plant morphology and nitrogen use, other than those associated simply with changing plant size and non‐structural carbohydrate content.

Ontogeny affects response of northern red oak seedlings to elevated CO2 and water stress: II. Recent photosynthate distribution and growth.

Northern red oak in the western Lake States area of the USA exists on the most xeric edge of its distribution range. Future climate-change scenarios for this area predict decreased water availability along with increased atmospheric CO2 . We examined recent photosynthate distribution and growth in seedlings as a function of CO2 mole fraction (400, 530 and 700 μmol mol-1 CO2 ), water regime (well watered and water-stressed), and ontogenic stage. Water stress effects on growth were largely offset by elevated CO2 . Water stress increased root mass ratio without concurrently increasing allocation of recent photosynthate to the roots. However, apparent sink strength of water-stressed seedlings at the completion of the third growth stage tended to be greater than that of well watered seedlings, as shown by continued high export, which may contribute carbon reserves to support preferential root growth under water-stressed conditions. Elevated CO2 decreased apparent shoot sink strength associated with the rapid expansion of the third flush. Carbon resources for the observed enhanced growth under elevated CO2 could be provided by enhanced photosynthetic rate over an increased leaf area (Anderson & Tomlinson, 1998, this volume). Increased sink strength of LG seedlings under water-stressed conditions, together with decreased apparent shoot sink strength associated with growth in elevated CO2 provide mechanisms for offsetting water stress effects by growth in elevated CO2 . Careful control of ontogeny was necessary to discern these changes and provides further evidence of the need for such careful control in mechanistic studies.

The effects of elevated atmospheric CO2 on lipid metabolism in leaves from mature wheat (Triticum aestivum cv. Hereward) plants

Winter wheat (Triticum aestivum L. cv. Hereward) plants were grown for 35 d either at 350 μmol mol–1 CO2 or at 650 μmol mol–1 CO2. Lipid synthesis was studied in these plants by incubating the 5th leaf on the main stem with [1–14C]acetate. Increased CO2 concentrations did not significantly affect the total incorporation of radiolabel into lipids of whole leaf tissue, but altered the distribution for individual lipid classes. Most noticeable amongst acyl lipids was the reduction in labelling of diacylglycerol and a corresponding increase in the proportion of phosphatidylcholine labelling. In the basal regions, there were similar changes and, in addition, phosphatidylglycerol labelling was particularly increased following growth in an enriched CO2 atmosphere. The stimulation of labelling of the mitochondrial‐specific lipid, diphosphatidylglycerol, prompted an examination of the mitochondrial population in wheat plants. Mitochondria were localized in intact wheat sections by immunolabelling for the mitochondrial‐specific chaperonin probe. Growth in elevated CO2 doubled the number of mitochondria compared to growth in ambient CO2. Fatty acid labelling was also significantly influenced following growth at elevated CO2 concentrations. Most noticeable were the changes in 16C:18C ratios for the membrane lipids, phosphatidylcholine, phosphatidylglycerol and monogalactosyldiacylglycerol. These data imply a change in the apportioning of newly synthesized fatty acids between the ‘eukaryotic’ and ‘prokaryotic’ pathways of metabolism under elevated CO2.

Yellowing and photosynthetic decline of barley primary leaves in response to atmospheric CO2 enrichment

The photosynthetic response of barley (Hordeum vulgare L. cv. Brant) primary leaves was studied as a function of chlorosis induced by CO2 enrichment. Leaf yellowing, measured as changes of chlorophyll a and b, was more extensive in controlled environments at elevated (680 ± 17 µl l−1) than at ambient (380 ± 21 µl l−1) CO2. Stomatal conductance of primary leaves was decreased by growth in elevated CO2 between 11 and 18 days after sowing (DAS) when measured at both 380 and 680 µl l−1 CO2. Internal leaf CO2 concentration (Ci) was also lower for elevated‐ compared to ambient‐CO2‐grown primary leaves between 11 and 14 DAS. Results suggest that non‐stomatal factors were responsible for the decreased photosynthetic rates of elevated‐ compared to ambient‐CO2‐grown primary leaves 18 DAS. Various photochemical measurements, including quantum absorptance (α), minimal (F0), maximal (Fm), and variable (Fv) chlorophyll fluorescence, as well as the Fv/Fm ratio, were significantly decreased 18 DAS in the elevated‐ compared to ambient‐CO2 treatment. Photochemical (qP) and nonphotochemical (qN) chlorophyll fluorescence quenching coefficients of 18‐day‐old primary leaves did not differ between CO2 treatments. Photosynthetic electron transport rates of photosystem II were slightly lower for elevated‐ compared to ambient‐CO2‐grown primary leaves 18 DAS. Concentrations of α‐amino N (i.e. free amino acids) in barley primary leaves were increased by CO2 enrichment 10 DAS, but subsequently, α‐amino N decreased in association with photosynthetic decline. Total acid protease activity was greater in elevated‐ than in ambient‐CO2‐grown leaves 18 DAS. The above findings suggest that photoinhibition and premature senescence were factors in the CO2‐dependent yellowing of barley primary leaves.

Elevated CO2 Effects during Leaf Ontogeny (A New Perspective on Acclimation).

For many plants growth in elevated CO2 leads to reduced rates of photosynthesis. To examine the role that leaf ontogeny plays in the acclimation response, we monitored photosynthesis and some related parameters at short intervals throughout the ontogenetic development of tobacco (Nicotiana tabacum L.) leaves under ambient (350 [mu]L L-1)- and high (950 [mu]L L-1)-CO2 conditions. The pattern of photosynthetic rate over time was similar between the two treatments and consistent with the expected pattern for a typical dicot leaf. However, the photosynthesis pattern in high-CO2-grown tobacco was shifted temporally to an earlier maximum and subsequent senescent decline. Ribulose-1,5-biphosphate carboxylase/oxygenase activity appeared to be the main factor regulating photosynthetic rates in both treatments. Therefore, we propose a new model for interpreting the acclimation response. Lowered photosynthetic rates observed during acclimation appear to be the result of a shift in the timing of the normal photosynthetic stages of leaf ontogeny to an earlier onset of the natural decline in photosynthetic rates associated with senescence.

The anthrax toxin activator gene atxA is associated with CO2-enhanced non-toxin gene expression in Bacillus anthracis.

The Bacillus anthracis toxin genes, cya, lef, and pag, can be viewed as a regulon, in which transcription of all three genes is activated in trans by the same regulatory gene, atxA, in response to the same signal, CO2. In atxA+ strains, toxin gene expression is increased 5- to 20-fold in cells grown in 5% CO2 relative to cells grown in air. CO2-enhanced toxin gene transcription is not observed in atx4-null mutants. Here, we used two independent techniques to obtain evidence for additional CO2-induced atxA-regulated genes. First, total protein preparations from atxA4+ and atxA isolates grown in 5% CO2 and in air were examined by two-dimensional electrophoresis. Comparison of the resulting protein patterns indicated that synthesis of non-toxin proteins is influenced by growth in elevated CO2 and the toxin gene regulator, atxA. Second, we generated random transcriptional lacZ fusions in B. anthracis with transposon Tn917-LTV3. Transposon-insertion libraries were screened for mutants expressing CO2-enhanced atxA-dependent beta-galactosidase activity. DNA sequence analysis of transposon insertion sites in 17 mutants carrying CO2- and atxA-regulated fusions revealed 10 mutants carrying independent insertions on the 185-kb toxin plasmid pXO1 which did not map to the toxin genes. The tcr-lacZ fusion mutants (tcr for toxin coregulated) were Tox+, indicating that these genes may not be involved in anthrax toxin gene activation. Our data indicate a clear association of atxA with CO2-enhanced gene expression in B. anthracis and provide evidence that atxA regulates genes other than the structural genes for the anthrax toxin proteins.

Elevated atmospheric CO2 and soil N fertility effects on growth, mycorrhizal colonization, and xylem water potential of juvenile ponderosa pine in a field soil

Interactive effects of atmospheric CO2 enrichment and soil N fertility on above- and below-ground development and water relations of juvenile ponderosa pine (Pinus ponderosa Dougl. ex Laws.) were examined. Open-top field chambers permitted creation of atmospheres with 700 µL L-1, 525 µL L-1, or ambient CO2 concentrations. Seedlings were reared from seed in field soil with a total N concentration of approximately 900 µg g-1 or in soil amended with sufficient (NH4)2SO4 to increase total N by 100 µg g-1 or 200 µg g-1. The 525 µL L-1 CO2 treatment within the intermediate N treatment was excluded from the study. Following each of three consecutive growing seasons, whole seedlings of each combination of CO2 and N treatment were harvested to permit assessment of shoot and root growth and ectomycorrhizal colonization. In the second and third growing seasons, drought cycles were imposed by withholding irrigation during which predawn and midday xylem water potential and soil water potential were measured. The first harvest revealed that shoot weight and coarse and fine root weights were increased by growth in elevated CO2. Shoot and root volume and weights were increased by CO2 enrichment at the second harvest, but growth stimulation by the 525 µL L-1 CO2 concentration exceeded that in 700 µL L-1 CO2 during the first two growing seasons. At the third harvest, above- and below-ground growth increases were largely confined to the 700 µL L-1 CO2 treatment, an effect accentuated by high soil N but evident in all N treatments. Ectomycorrhizal formation was reduced by elevated CO2 after one growing season, but thereafter was not significantly affected by CO2 and was unaffected by soil N throughout the study. Results of the xylem water potential measurements were variable, as water potentials in seedlings grown in elevated CO2 were intermittently higher on some measurement days but lower on others than that of seedlings grown in the ambient atmosphere. These results suggest that elevated CO2 exerts stimulatory effects on shoot and root growth of juvenile ponderosa pine under field conditions which are somewhat dependent on N availability, but that temporal variation may periodically result in a greater response to a moderate rise in atmospheric CO2 than to a doubling of the current ambient concentration.

Decomposition of litter produced under elevated CO2: Dependence on plant species and nutrient supply

We investigated the effect of CO2 concentration and soilnutrient availability during growth on the subsequent decomposition andnitrogen (N) release from litter of four annual grasses that differ inresource requirements and native habitat. Vulpia microstachys isa native grass found on California serpentine soils, whereas Avenafatua, Bromus hordaceus, and Lolium multiflorum areintroduced grasses restricted to more fertile sandstone soils (Hobbs & Mooney 1991). Growth in elevated CO2 altered litter C:N ratio,decomposition, and N release, but the direction and magnitude of thechanges differed among plant species and nutrient treatments. ElevatedCO2 had relatively modest effects on C:N ratio of litter,increasing this ratio in Lolium roots (and shoots at high nutrients),but decreasing C:N ratio in Avena shoots. Growth of plants underelevated CO2 decreased the decomposition rate of Vulpialitter, but increased decomposition of Avena litter from the high-nutrient treatment. The impact of elevated CO2 on N loss fromlitter also differed among species, with Vulpia litter from high-CO2 plants releasing N more slowly than ambient-CO2litter, whereas growth under elevated CO2 caused increased Nloss from Avena litter. CO2 effects on N release in Lolium and Bromus depended on the nutrient regime in whichplants were grown. There was no overall relationship between litter C:Nratio and decomposition rate or N release across species and treatments.Based on our study and the literature, we conclude that the effects ofelevated CO2 on decomposition and N release from litter arehighly species-specific. These results do not support the hypothesis thatCO2 effects on litter quality consistently lead to decreasednutrient availability in nutrient-limited ecosystems exposed to elevatedCO2.

Construction cost of loblolly and ponderosa pine leaves grown with varying carbon and nitrogen availability

ABSTRACTWe grew loblolly and ponderosa pine seedlings in a factorial experiment with two CO2 partial pressures (35 and 70 Pa), and two nitrogen treatments (1.0 and 3.5 mol m−3 NH4+), for one growing season to examine the effects of carbon and nitrogen availability on leaf construction cost. Growth in elevated CO2 reduced leaf nitrogen concentrations by 17 to 40%, and increased C:N by 22 to 68%. Elevated N availability increased leaf N concentrations and decreased C:N. Non‐structural carbohydrates increased in high‐CO2‐grown loblolly seedlings, except in fascicles from low N, and in ponderosa primary and fascicle leaves grown in high N. In loblolly, increases in starch were nearly 2‐fold greater than the increases in soluble sugars. In ponderosa, only the soluble sugars were affected by CO2. Leaf construction cost (g glucose g−1 dm) varied by 9.3% across all treatments. All of the variation in loblolly leaf construction cost could be explained by changes in non‐structural carbohydrates. A model of the response of construction cost to changes in the mass of different biochemical fractions suggests that the remainder of the variation in ponderosa, not explained by non‐structural carbohydrates, is probably attributable to changes in lignin, phenolic or protein concentrations.

Responses of Antioxidative Systems to Drought Stress in Pendunculate Oak and Maritime Pine as Modulated by Elevated CO2.

The aim of the present study was to investigate the effects of an enhanced CO2 concentration alone or in combination with drought stress on antioxidative systems of a deciduous (oak; Quercus robur) and an evergreen (pine; Pinus pinaster) tree species. The seedlings were grown for one season in a greenhouse in tunnels supplied with 350 or 700 [mu]L L-1 CO2. The experiment was repeated in a second year. Antioxidants, protective enzymes, soluble protein, and pigments showed considerable fluctuations in different years. Elevated CO2 caused significant reductions in the activities of superoxide dismutases in both oak and pine. The activities of ascorbate peroxidase and catalase were also reduced in most cases. The activities of dehydroascorbate reductase, monodehydroascorbate radical reductase, glutathione reductase, and guaiacol peroxidase were affected little or not at all by elevated CO2. When the trees were subjected to drought stress by withholding water, the activities of antioxidative enzymes decreased in leaves of pine and oak grown at ambient CO2 and increased in plants grown at elevated CO2 concentrations. The present results suggest that growth in elevated CO2 might reduce oxidative stress to which leaf tissues are normally exposed and enhance metabolic flexibility to encounter increased stress by increases in antioxidative capacity.

Mechanism for increased leaf growth in elevated CO2

The effect of exposure to elevated CO2 on the processes of leaf cell production and leaf cell expansion was studied using primary leaves of Phaseolus vulgaris L. Cell division and expansion were separated temporally by exposing seedlings to dim red light for 10 d (when leaf cell division was completed) followed by exposure to bright white light for 14 d (when leaf growth was entirely dependent on cell expansion). When plants were exposed to elevated CO2 during the phase of cell expansion, epidermal cell size and leaf area development were stimulated. Three pieces of evidence suggest that this occurred as a result of increased cell wall loosening and extensibility, (i) cell wall extensibility (WEx, measured as tensiometric extension using an Instron) was significantly increased, (ii) cell wall yield turgor (V, MPa) was reduced and (iii) xyloglucan endotransglycosylase (XET) enzyme activity was significantly increased. When plants were exposed to elevated CO2 during the phase of cell division, the number of epidermal cells was increased whilst final cell size was significantly reduced and this was associated with reduced final leaf area, WEx and XET activity. When plants were exposed to elevated CO2 during both phases of cell division and expansion, leaf area development was not affected. For this treatment, however, the number of epidermal cells was increased, but cell expansion was inhibited, despite exposure to elevated CO2 during the expansion phase. Assessments were also made of the spatial patterns of WEx across the expanding leaf lamina and the data suggest that exposure to elevated CO2 during the phase of leaf expansion may lead to enhanced extensibility particularly at basal leaf margins which may result in altered leaf shape. The data show that both cell production and expansion were stimulated by elevated CO2, but that leaf growth was only enhanced by exposure to elevated CO2 in the cell expansion phase of leaf development. Increased leaf cell expansion is, therefore, an important mechanism for enhanced leaf growth in elevated CO2, whilst the importance of increased leaf cell production in elevated CO2 remains to be elucidated.

Whole plant respiration and photosynthesis of wheat under increased CO2 concentration and temperature: long‐term vs. short‐term distinctions for modelling

AbstractShort‐ and long‐term effects of elevated CO2 concentration and temperature on whole plant respiratory relationships are examined for wheat grown at four constant temperatures and at two CO2 concentrations. Whole plant CO2 exchange was measured on a 24 h basis and measurement conditions varied both to observe short‐term effects and to determine the growth respiration coefficient (rg), dry weight maintenance coefficient (rm), basal (i.e. dark acclimated) respiration coefficient (rg), and 24 h respiration:photosynthesis ratio (R:P). There was no response of rg to short‐term variation in CO2 concentration. For plants with adequate N supply, rg was unaffected by the growth‐CO2 despite a 10% reduction in the plant's N concentration (%N). However, rm was decreased 13%, and rb was decreased 20% by growth in elevated CO2 concentration relative to ambient. Nevertheless, R:P was not affected by growth in elevated CO2. Whole plant respiration responded to short‐term variation of ± 5 °C around the growth temperature with low sensitivity (Q10= 1.8 at 15 °C, 1.3 at 30 °C). The shape of the response of whole plant respiration to growth temperature was different from that of the short term response, being a slanted S‐shape declining between 25 and 30 °C. While rm, increased, rg decreased when growth temperature increased between 15 and 20 °C. Above 20 °C rm became temperature insensitive while rg increased with growth temperature. Despite these complex component responses, R:P increased only from 0.40 to 0.43 between 15° and 30 °C growth temperatures. Giving the plants a step increase in temperature caused a transient increase in R:P which recovered to the pre‐transient value in 3 days. It is concluded that use of a constant R:P with respect to average temperature and CO2 concentration may be a more simple and accurate way to model the responses of wheat crop respiration to ‘climate change’ than the more complex and mechanistically dubious functional analysis into growth and maintenance components.

Growth and Topological Changes of Citrus limon (L.) Burm. f. `Eureka' in Response to High Temperatures and Elevated Atmospheric Carbon Dioxide

Growth and topological indices of `Eureka' lemon were measured after 6 months in well-watered and well-fertilized conditions and factorial combinations of moderate (29/21C day/night) or high (42/32C day/night) temperatures and ambient (350 to 380 μmol·mol) or elevated (constant 680 μmol·mol-1) CO2. In high temperatures, plants were smaller and had higher levels of leaf chlorophyll a than in moderate temperatures. Moreover, plants in high temperatures and elevated CO2 had about 15 % higher levels of leaf chlorophyll a than those in high temperatures and ambient CO2. In high temperatures, plant growth in elevated CO2 was about 87% more than in ambient CO2. Thus, high CO2 reduced the negative effect of high temperature on shoot growth. In moderate temperatures, plant growth in elevated CO2 was only about 21% more than in ambient CO2. Irrespective of temperature treatments, shoot branch architecture in elevated CO2 was more hierarchical than those in ambient CO2. Specific shoot extension, a topological measure of branch frequency, was not affected by elevated CO2 in moderate temperatures, but was increased by elevated CO2 enrichment in high temperatures-an indication of decreased branch frequency and increased apical dominance. In moderate temperatures, plants in elevated CO2 had fibrous root branch patterns that were less hierarchical than at ambient CO2. The lengths of exterior and interior fibrous roots between branch points and the length of second-degree adventitious lateral branches were increased >50% by high temperatures compared with moderate temperatures. Root length between branch points was not affected by CO2 levels.

Nitrogen, phosphorus and potassium uptake and demand in Agrostis capillaris: the influence of elevated CO2 and nutrient supply.

Responses to elevated CO2 have been studied using Agrostis capillaris L., an upland grass which is abundant on nutrient-poor soils. Plants were grown in sand culture with a wide range of nitrogen, phosphorus and potassium concentrations, and the impact of CO2 on the demand for nutrients was determined using isotopic root bioassays. Plants grown with the smallest concentrations of N and P showed typical foliar symptoms associated with deficiency of these elements. However, even when supplies of N and P were limiting to growth, additional CO2 (250 ppm above ambient) influenced neither total N nor total P in above-ground tissues, nor nutrient demands as indicated by the bioassay. The estimates of the demand of the plants for K from the 86 Rb bioassay indicated an appreciable increase when plants were raised in elevated CO2 . For plants of the same size with the same nutrient supply, those grown in elevated CO2 consistently displayed an increased internal demand for K. Uptake of K was not however, enhanced by elevated CO2 even in non-limiting conditions and it might therefore be limited by a factor other than K supply. The overall conclusion from the experiments is that when A. capillaris is grown in elevated CO2 , uptake of N, P and K fails to increase proportionally with dry mass. This was true even when nutrient supplies were adequate, and it appears that nutrient-use-efficiency might increase to enable the plants to maintain growth in elevated CO2 .

Canopy gas exchange and growth of upland pasture swards in elevated CO2

summaryVegetation monoliths (450 × 450 mm) from two contrasting upland grassland communities were grown in Solardomes in either ambient air or ambient air enriched with 250 ppm CO2 During the first two growing seasons measurements of canopy gas exchange showed that rates of photosynthesis of limestone swards were enhanced by growth in elevated CO2, by approx. 50% during spring and early summer. Although canopy respiration was also greater in elevated CO2 the overall effect was an average increase of 33% in net CO2 assimilation. Enhanced respiration rates persisted into the autumn, whereas the effect on photosynthesis diminished through the growing season, so that in September swards growing in high CO2 had net photosynthesis rates similar to, or even lower than those in ambient air. This response varied between swards of differing species composition. In acidic grassland no significant effects of CO2 on respiration or net CO2 uptake rates were detected at any time.The above ground productivity of limestone grassland was measured in several harvests throughout both seasons, and was not affected by CO2 concentration at any time. Similarly, the acidic grassland, harvested at the end of the second season, showed no significant effect of CO2 on above‐ground biomass. The results suggest that increasing atmospheric CO2 concentration is unlikely to cause large changes in net primary productivity in these grasslands.

The growth and gas exchange response of soil-planted Norway spruce [Picea abies (L.) Karst.] and red oak (Quercus rubra L.) exposed to elevated CO2 and to naturally occurring drought.

Norway spruce and red oak trees were planted directly into the soil and exposed to 700 μmol mol-1 CO2 in open-top chambers. There were large inter-specific differences in response to naturally occurring drought during the second year of exposure to elevated CO2 . Both species had decreased assimilation rates. CO2 -treated red oak had no loss of photosynthetic enhancement when undroughted, whereas CO2 -treated Norway spruce showed a relative increase in assimilation rates only when draughted. The effect of CO2 on radial growth of both species was less marked in the second growing season, but this may have been a result of different biomass partitioning as Norway spruce shoot extension had a different pattern of growth in elevated CO2 . Stomatal density and chlorophyll content were largely unaffected by the CO2 treatment. A precise method for measuring Norway spruce needle surface area was also developed.

Significant Changes in Cell and Chloroplast Development in Young Wheat Leaves (Triticum aestivum cv Hereward) Grown in Elevated CO2.

Cell and chloroplast development were characterized in young Triticum aestivum cv Hereward leaves grown at ambient (350 [mu]L L-1) or at elevated (650 [mu]L L-1) CO2. In elevated CO2, cell and chloroplast expansion was accelerated by 10 and 25%, respectively, in the first leaf of 7-d-old wheat plants without disruption to the leaf developmental pattern. Elevated CO2 did not affect the number of chloroplasts in relation to mesophyll cell size or the linear relationship between chloroplast number or size and mesophyll cell size. No major changes in leaf anatomy or in chloroplast ultrastructure were detected as a result of growth in elevated CO2, but there was a marked reduction in starch accumulation. In leaf sections fluorescently tagged antisera were used to visualize and quantitate the amount of cytochrome f, the [alpha]- and [beta]-subunits of the coupling factor 1 in ATP synthase, D1 protein of the photosystem II reaction center, the 33-kD protein of the extrinsic oxygen-evolving complex, subunit II of photosystem I, and ribulose-1,5-bisphosphate carboxylase/oxygenase. A significant finding was that in 10 to 20% of the mesophyll cells grown in elevated CO2 the 33-kD protein of the extrinsic oxygen-evolving complex of photosystem II and cytochrome f were deficient by 75%, but the other proteins accumulated normally.