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 μ L 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.     

Flooding, gas exchange and hydraulic root conductivity of highbush blueberry

Highbush blueberry plants ( Vaccinium corymbosum L. cv. Bluecrop) growing in containers were flooded in the laboratory for various durations to determine the effect of flooding on carbon assimilation, photosynthetic response to varying CO₂ and O₂ concentrations and apparent quantum yield as measured in an open flow gas analysis system. Hydraulic conductivity of the root was also measured using a pressure chamber. Root conductivity was lower and the effect of increasing CO₂ levels on carbon assimilation less for flooded than unflooded plants after short-(i-2 days), intermediate-(10–14 days) and long-term (35–40 days) flooding. A reduction in O₂ levels surrounding the leaves from 21 to 2% for unflooded plants increased carbon assimilation by 33% and carboxylation efficiency from 0.012 to 0.021 mol CO₂ fixed (mol CO₂)⁻¹. Carboxylation efficiency of flooded plants, however, was unaffected by a decrease in percentage O₂, averaging 0.005 mol CO₂ fixed (mol CO₂)⁻¹. Apparent quantum yield decreased from 2.2 × 10⁻¹ mol of CO₂ fixed (mol light)⁻¹ for unflooded plants to 2.0 × 10⁻³ and 9.0 × 10⁻⁴ for intermediate- and long-term flooding durations, respectively. Shortterm flooding reduced carbon assimilation via a decrease in stomatal conductance, while longer flooding durations also decreased the carboxylation efficiency of the leaf.     

Effects of manganese toxicity on photosynthesis of white birch (Betula platyphylla var. japonica) seedlings

The effects of manganese (Mn) toxicity on photosynthesis in white birch ( Betula platyphylla var. japonica ) leaves were examined by the measurement of gas exchange and chlorophyll fluorescence in hydroponically cultured plants. The net photosynthetic rate at saturating light and ambient CO₂ (C_a) of 35 Pa decreased with increasing leaf Mn concentrations. The carboxylation efficiency, derived from the difference in CO₂ assimilation rate at intercellular CO₂ pressures attained at C_a of 13 Pa and O Pa, decreased with greater leaf Mn accumulation. Net photosynthetic rate at saturating light and saturating CO₂ (5%) also declined with leaf Mn accumulation while the maximum quantum yield of O₂ evolution at saturating CO₂ was not affected. The maximum efficiency of PSII photochemistry (F_v/F_m) was little affected by Mn accumulation in white birch leaves over a wide range of leaf Mn concentrations (2–17 mg g₋₁ dry weight). When measured in the steady state of photosynthesis under ambient air at 430 μmol quanta m₋₂ s₋₁, the levels of photochemical quenching (qP) and the excitation capture efficiency of open PSII (F'^v/F'^m) declined with Mn accumulation in leaves. The present results suggest that excess Mn in leaves affects the activities of the CO² reduction cycle rather than the potential efficiency of photochemistry, leading to increases in Q_A reduction state and thermal energy dissipation, and a decrease in quantum yield of PSII in the steady state.     

Effect of elevated CO2 on the stomatal distribution and leaf physiology of Alnus glutinosa

Variation in stomatal development and physiology of mature leaves from Alnus glutinosa plants grown under reference (current ambient, 360 μmol mol⁻¹ CO₂) and double ambient (720 μmol mol⁻¹ CO₂) carbon dioxide (CO₂) mole fractions is assessed in terms of relative plant growth, stomatal characters (i.e. stomatal index and density) and leaf photosynthetic characters. This is the first study to consider the effects of elevated CO₂ concentration on the distribution of stomata and epidermal cells across the whole leaf and to try to ascertain the cause of intraleaf variation. In general, a doubling of the atmospheric CO₂ concentration enhanced plant growth and significantly increased stomatal index. However, there was no significant change in relative stomatal density. Under elevated CO₂ concentration there was a significant decrease in stomatal conductance and an increase in assimilation rate. However, no significant differences were found for the maximum rate of carboxylation ( V _cmax) and the light saturated rate of electron transport ( J _max) between the control and elevated CO₂ treatment.     

LIMITATIONS OF PHOTOSYNTHESIS IN DIFFERENT REGIONS OF THE ZEA MAYS LEAF

The progressive development of the photosynthetic apparatus occurring along the length of the Zea mays leaf offers a convenient system with which to examine the limitations to photosynthetic CO₂ assimilation during biogenesis of a C₄ leaf. Changes in light-induced O₂ evolution and CO₂ assimilation, chlorophyll content, activity of PEP-carboxylase, NADP-malic enzyme and the 'R5P system' (consisting of d -ribose-5-phosphate-keto isomerase, ATP- d -ribulose-5 phosphate 1-phosphotransferase and d -ribulose-1,5-bisphosphate carboxylase) and fluorescence emission characteristics were examined along the length of the second leaf of 7-day-old plants grown under a diurnal light regime. The results suggest that the major limitation to CO₂ assimilation in the leaf sheath lies within the chlorenchyma and is either energy supply for carboxylation or the capacity of key photosynthetic enzymes. In the leaf blade stomatal resistance to CO₂ diffusion constitutes a major fraction of the total leaf resistance to CO₂ assimilation implicating the stoma as the major limiting factor to photosynthetic CO₂ assimilation.     

Changes in leaf nitrogen and carbohydrates underlie temperature and CO2 acclimation of dark respiration in five boreal tree species

We tested the hypothesis that acclimation of foliar dark respiration to CO₂ concentration and temperature is associated with adjustments in leaf structure and chemistry. Populus tremuloides Michx. , Betula papyrifera Marsh. , Larix laricina (Du Roi) K. Koch , Pinus banksiana Lamb., and Picea mariana (Mill.) B.S.P. were grown from seed in combined CO₂ (370 or 580 μ mol mol^–1) and temperature treatments (18/12, 24/18, or 30/24 °C). Temperature and CO₂ effects were predominately independent. Specific respiration rates partially acclimated to warmer thermal environments through downward adjustment in the intercept, but not Q ₁₀ of the temperature–response functions. Temperature acclimation of respiration was larger for conifers than broad-leaved species and was associated with pronounced reductions in leaf nitrogen concentrations in conifers at higher growth temperatures. Short-term increases in CO₂ concentration did not inhibit respiration. Growth in the elevated CO₂ concentration reduced leaf nitrogen and increased non-structural carbohydrate concentrations. However, for a given nitrogen concentration, respiration was higher in leaves grown in the elevated CO₂ concentration, as rates increased with increasing carbohydrates. Across species and treatments, respiration rates were a function of both leaf nitrogen and carbohydrate concentrations ( R ² = 0·71, P < 0·0001). Long-term acclimation of foliar dark respiration to temperature and CO₂ concentration is largely associated with changes in nitrogen and carbohydrate concentrations.     

Elevated CO2 concentration, nitrogen use, and seed production in annual plants

Elevated atmospheric CO₂ concentration ([CO₂]) stimulates seed mass production in many species, but the extent of stimulation shows large variation among species. We examined (1) whether seed production is enhanced more in species with lower seed nitrogen concentrations, and (2) whether seed production is enhanced by elevated [CO₂] when the plant uses more N for seed production. We grew 11 annuals in open top chambers that have different [CO₂] conditions (ambient: 370 μmol mol⁻¹, elevated: 700 μmol mol⁻¹). Elevated [CO₂] significantly increased seed production in six out of 11 species with a large interspecific variation (0.84–2.12, elevated/ambient [CO₂]). Seed nitrogen concentration was not correlated with the enhancement of seed production by elevated [CO₂]. The enhancement of seed production was strongly correlated with the enhancement of seed nitrogen per plant caused by increased N acquisition during the reproductive period. In particular, legume species tended to acquire more N and produced more seeds at elevated [CO₂] than non-nitrogen fixing species. Elevated [CO₂] little affected seed [N] in all species. We conclude that seed production is limited primarily by nitrogen availability and will be enhanced by elevated [CO₂] only when the plant is able to increase nitrogen acquisition.     

Effect of CO2-enrichnient on seedling physiology and growth of two tropical tree species

Seedlings of two tree species from the Atlantic lowlands of Costa Rica, Ochroma la-gopus Swartz, a fast-growing pioneer species, and Pentaclethra macroloba (Willd.) Kuntze, a slower-growing climax species, were grown under enriched atmospheric CO₂ in controlled environment chambers. Carbon dioxide concentrations were maintained at 350 and 675 μl 1⁻¹ under photosynthetic photon flux densities of 500 μol m⁻² s⁻¹ and temperatures of 26°C day and 20°C night. Total biomass of both species increased significantly in the elevated CO₂ treatment; the increase in biomass was greatest for the pioneer species, O. lagopus . Both species had greater leaf areas and specific leaf weights with increased atmospheric CO₂. However, the ratio of non-pho-tosynthetic tissue to leaf area also increased in both species leading to decreased leaf area ratios. Plants of both species grown at 675 μl 1⁻¹ CO₂ had lower chlorophyll contents and photosynthesis on a leaf area basis than those grown at 350 μl 1⁻¹. Reductions in net photosynthesis occurred despite increased internal CO₂ concentrations in the CO₂-enriched treatment. Stomatal conductances of both species decreased with CO₂-enrichment resulting in significant increases in water use efficiency.     

Fruit effects on photosynthesis in Prunus persica

Seasonal measurements of net CO₂ assimilation, leaf conductance and mesophyll conductance were made in the field on mature, fruiting and defruited Prunus persica L. Batsch trees. During early stages of fruit growth there were no significant differences in leaf gas exchange characteristics between fruiting and defruited trees. During the early part of the last stage of fruit growth, CO₂ assimilation rates were 11–15% higher in fruiting trees than defruited trees. These increased assimilation rates corresponded with approximately 30% increases in leaf conductance and only minor changes in mesophyll conductances or leaf CO₂ assimilation capacity as indicated by leaf nitrogen content. It is concluded that under the field conditions of this study the fruit effect on photosynthesis is primarily related to stomatal behavior.     

Does down‐regulation of photosynthetic capacity by elevated CO2 depend on N supply in Dactylis glomerata?

Dactylis glomerata was grown hydroponically in a controlled environment at ambient (360 μl l⁻¹) or elevated (680 μl l⁻¹) CO₂ and four concentrations of nitrogen (0.15, 0.6, 1.5 and 6.0 m M NO₃⁻), to test the hypothesis that reduction of photosynthetic capacity at elevated [CO₂] is dependent on N availability and mediated by a build-up of non-structural carbohydrates. Photosynthetic capacity of the youngest fully expanded leaf (leaf 5, 2 days after full expansion) was reduced in CO₂-enriched plants at low, but not high N supply and so the stimulation of net photosynthesis by CO₂ enhancement was less at low than at high N supply. CO₂ enrichment resulted in a decrease in ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) content on a leaf area basis at 0.6 and 1.5 m M NO₃⁻, but not at 0.15 and 6.0 m M NO₃⁻, and had no effect on the total N content of the leaf on an area basis. However, decreases in Rubisco content could be primarily accounted for by a decrease in total N content of leaves, independent of [CO₂]. A doubling of the Rubisco content by increasing the N supply beyond 0.6 m M had only a marginal effect on the maximum carboxylation velocity in vivo, suggesting that the fraction of inactive Rubisco increased with increasing N supply. Although CO₂-enriched plants accumulated more non-structural carbohydrates in the leaf, the reduction of photosynthetic capacity at low N supply was not mediated simply by a build-up of carbohydrates. In D . glomerata , the photosynthetic capacity was mainly determined by the total N content of the leaf.     

The response of nodulated alfalfa to water supply, temperature and elevated CO2: photosynthetic downregulation

Plants grown in an environment of elevated CO₂ and temperature often show reduced CO₂ assimilation capacity, providing evidence of photosynthetic downregulation. The aim of this study was to analyse the downregulation of photosynthesis in elevated CO₂ (700 µmol mol⁻¹) in nodulated alfalfa plants grown at different temperatures (ambient and ambient + 4°C) and water availability regimes in temperature gradient tunnels. When the measurements were taken in growth conditions, a combination of elevated CO₂ and temperature enhanced the photosynthetic rate; however, when they were carried out at the same CO₂ concentration (350 and 700 µmol mol⁻¹), elevated CO₂ induced photosynthetic downregulation, regardless of temperature and drought. Intercellular CO₂ concentration measurements revealed that photosynthetic acclimation could not be accounted for by stomatal limitations. Downregulation of plants grown in elevated CO₂ was a consequence of decreased carboxylation efficiency as a result of reduced rubisco activity and protein content; in plants grown at ambient temperature, downregulation was also induced by decreased quantum efficiency. The decrease in rubisco activity was associated with carbohydrate accumulation and depleted nitrogen availability. The root nodules were not sufficiently effective to balance the source–sink relation in elevated CO₂ treatments and to provide the required nitrogen to counteract photosynthetic acclimation.     

The role of temperature in determining the stimulation of CO2 assimilation at elevated carbon dioxide concentration in soybean seedlings

Soybean ( Glycine max cv. Clark) was grown at both ambient (ca 350 μmol mol⁻¹) and elevated (ca 700 μmol mol⁻¹) CO₂ concentration at 5 growth temperatures (constant day/night temperatures of 20, 25, 30, 35 and 40°C) for 17–22 days after sowing to determine the interaction between temperature and CO₂ concentration on photosynthesis (measured as A, the rate of CO₂ assimilation per unit leaf area) at both the single leaf and whole plant level. Single leaves of soybean demonstrated increasingly greater stimulation of A at elevated CO₂ as temperature increased from 25 to 35°C (i.e. optimal growth rates). At 40°C, primary leaves failed to develop and plants eventually died. In contrast, for both whole plant A and total biomass production, increasing temperature resulted in less stimulation by elevated CO₂ concentration. For whole plants, increased CO₂ stimulated leaf area more as growth temperature increased. Differences between the response of A to elevated CO₂ for single leaves and whole plants may be related to increased self-shading experienced by whole plants at elevated CO₂ as temperature increased. Results from the present study suggest that self-shading could limit the response of CO₂ assimilation rate and the growth response of soybean plants if temperature and CO₂ increase concurrently, and illustrate that light may be an important consideration in predicting the relative stimulation of photosynthesis by elevated CO₂ at the whole plant level.     

Photosynthesis, growth and nutrient changes in non-nodulated Phaseolus vulgaris grown under atmospheric and elevated carbon dioxide conditions

The response of Phaseolus vulgaris L. cv. Contender grown under controlled environment at either ambient or elevated (360 and 700 μmol mol^-1, respectively) CO₂ concentrations ([CO₂]), was monitored from 10 days after germination (DAG) until the onset of senescence. Elevated CO₂ had a pronounced effect on total plant height (TPH), leaf area (LA), leaf dry weight (LD), total plant biomass (TB) accumulation and specific leaf area (SLA). All of these were significantly increased under elevated carbon dioxide with the exception of SLA which was significantly reduced. Other than high initial growth rates in CO₂-enriched plants, relative growth rates remained relatively unchanged throughout the growth period. While the trends in growth parameters were clearly different between [CO₂], some physiological processes were largely transient, in particular, net assimilation rate (NAR) and foliar nutrient concentrations of N, Mg and Cu. CO₂ enrichment significantly increased NAR, but from 20 DAG, a steady decline to almost similar levels to those measured in plants grown under ambient CO₂ occurred. A similar trend was observed for leaf N content where the loss of leaf nitrogen in CO₂-enriched plants after 20 DAG, was significantly greater than that observed for ambient-CO₂ plants. Under enhanced CO₂, the foliar concentrations of K and Mn were increased significantly whilst P, Ca, Fe and Zn were reduced significantly. Changes in Mg and Cu concentrations were insignificant. In addition. high CO₂ grown plants exhibited a pronounced leaf discoloration or chlorosis, coupled with a significant reduction in leaf longevity.     

Does photosynthetic acclimation to elevated CO2 increase photosynthetic nitrogen-use efficiency? A study of three native UK grassland species in open-top chambers

1. The photosynthetic response to elevated CO₂ and nutrient stress was investigated in Agrostis capillaris, Lolium perenne and Trifolium repens grown in an open-top chamber facility for 2 years under two nutrient regimes. Acclimation was evaluated by measuring the response of light-saturated photosynthesis to changes in the substomatal CO₂ concentration.
2. Growth at elevated CO₂ resulted in reductions in apparent Rubisco activity in vivo in all three species, which were associated with reductions of total leaf nitrogen content on a unit area basis for A. capillaris and L. perenne . Despite this acclimation, photosynthesis was significantly higher at elevated CO₂ for T. repens and A. capillaris , the latter exhibiting the greatest increase of carbon uptake at the lowest nutrient supply.
3. The photosynthetic nitrogen-use efficiency (the rate of carbon assimilation per unit leaf nitrogen) increased at elevated CO₂, not purely owing to higher values of photosynthesis at elevated CO₂, but also as a result of lower leaf nitrogen contents.
4. Contrary to most previous studies, this investigation indicates that elevated CO₂ can stimulate photosynthesis under a severely limited nutrient supply. Changes in photosynthetic nitrogen-use efficiency may be a critical determinant of competition within low nutrient ecosystems and low input agricultural systems.     

Gas exchange parameters, water relations and carbohydrate partitioning in leaves of field-grown Prunus domestica following fruit removal

The effect of fruit removal on gas exchange, water relations, chlorophyll and non-structural carbohydrate content of leaves from mature, field-grown plum trees ( Prunus domestica L. cv. Stanley) was determined over 2 consecutive growing seasons. Removal of fruits during stage II of fruit development decreased CO₂ assimilation rate within 24 h from 12.6 to 8.5 μmol m^-2 s^-1 in 1986, and from 12.1 to 10.2 μmol m^-2 s^-1 in 1987. Depression of net photosynthesis persisted for at least 5 days and was greatest in the early afternoon. Recovery of the CO₂ assimilation rate to pretreatment levels coincided in defruited trees with vegetative growth that was more than 5-fold that of fruiting trees in the first 6 weeks after fruit removal in 1986. Estimated photorespiration was similar in both fruiting and defruited trees. The stomatal contribution to the decrease of CO₂ assimilation rate, calculated from assimilation/intercellular CO₂ curves, ranged from 31 to 46%. Defruiting did not affect leaf water potential, but decreased leaf osmotic potential. Leaf levels of chlorophyll, fructose, glucose, sorbitol and sucrose were not affected by defruiting, whereas starch content increased up to 51% in leaves of defruited trees within 24 h after fruit removal. However, because of the small starch pool present in plum leaves (<1.9% dry weight) it is unlikely that starch accumulation was responsible for the observed decline in CO₂ assimilation rate after fruit removal. The decrease of CO₂ assimilation rate is discussed in relation to the hypothesis of assimilate demand regulating photosynthesis through a feedback mechanism.     

Low vapour pressure deficit reduces the beneficial effect of elevated CO2 on growth of N2-fixing alfalfa plants

Plant responses to elevated CO₂ can be modified by many environmental factors, but very little attention has been paid to the interaction between CO₂ and changes in vapour pressure deficit (VPD). Thirty-day-old alfalfa plants ( Medicago sativa L. cv. Aragón), which were inoculated with Sinorhizobium meliloti 102F78 strain, were grown for 1 month in controlled environment chambers at 25/15°C, 14 h photoperiod, and 600 µmol m⁻² s⁻¹ photosynthetic photon flux (PPF), using a factorial combination of CO₂ concentration (400 µmol mol⁻¹ or 700 µmol mol⁻¹) and vapour pressure deficit (0.48 kPa or 1.74 kPa, which corresponded to relative humidities of 85% and 45% at 25°C, respectively). Elevated CO₂ strongly stimulated plant growth under high VPD conditions, but this beneficial effect was not observed under low VPD. Under low VPD, elevated CO₂ also did not enhance plant photosynthesis, and plant water stress was greatest for plants grown at elevated CO₂ and low VPD. Moreover, plants grown under elevated CO₂ and low VPD had a lower leaf soluble protein and photosynthetic activity (photosynthetic rate and carboxylation efficiency) than plants grown under elevated CO₂ and high VPD. Elevated CO₂ significantly increased leaf adaxial and abaxial temperatures. Because the effects of elevated CO₂ were dependent on vapour pressure deficit, VPD needs to be controlled in experiments studying the effect of elevated CO₂ as well as considered in the extrapolations of results to a warmer, high-CO₂ world.     

Effects of nitrogen deficiency on leaf photosynthetic response of tall fescue to water deficit

Abstract. The objective of the present work was to study the effect of nitrogen deficiency on drought sensitivity of tall fescue plants. The authors compared photosynthetic and stomatal behaviour of plants grown at either high (8 mol m⁻³) or low (0.5 mol m⁻³) nitrogen levels during a drought cycle followed by rehydration. Other processes investigated were stomatal and non-stomatal inhibition of leaf photosynthesis, water use efficiency and leaf rolling. Plants were grown in pots in controlled conditions on expanded clay. A Wescor in situ hygrometer placed on the leaf base outside the assimilation chamber permitted, simultaneously to leaf gas exchange measurements, monitoring of leaf water potential. Drought was imposed by withholding water from the pot. CO₂ uptake and stomatal conductance decreased and leaves started to roll at a lower leaf water potential in the high-N than in the low-N grown plants. Stomatal inhibition of leaf photosynthesis seemed larger in the low-N than in the high-N plants. Water-use efficiency increased more in the high-N than in the low-N grown plants during the drought. The decrease of photosynthesis was largely reversible after rehydration in low-N but not in high-N leaves. The authors suggest that low-N plants avoid water deficit rather than tolerate it.     

Interaction of elevated CO2 and O3 on growth, photosynthesis and respiration of three perennial species grown in low and high nitrogen

Seedlings of three species native to central North America, a C₃ tree, Populus tremuloides Michx., a C₃ grass, Agropyron smithii Rybd., and a C₄ grass, Bouteloua curtipendula Michx., were grown in all eight combinations of two levels each of CO₂, O₃ and nitrogen (N) for 58 days in a controlled environment. Treatment levels consisted of 360 or 674 μmol mol^-1 CO₂, 3 or 92 nmol mol^-1 O₃, and 0.5 or 6.0 m M N. In situ photosynthesis and relative growth rate (RGR) and its determinants were obtained at each of three sequential harvests, and leaf dark respiration was measured at the second and third harvests. In all three species, plants grown in high N had significantly greater whole-plant mass, RGR and photosynthesis than plants grown in low N. Within a N treatment, elevated CO₂ did not significantly enhance any of these parameters nor did it affect leaf respiration. However, plants of all three species grown in elevated CO₂ had lower stomatal conductance compared to ambient CO₂-exposed plants. Seedlings of P. tremuloides (in both N treatments) and B. curtipendula (in high N) had significant ozone-induced reductions in whole-plant mass and RGR in ambient but not under elevated CO_2. This negative O₃ impact on RGR in ambient CO₂ was related to increased leaf dark respiration, decreased photosynthesis and/or decreased leaf area ratio, none of which were noted in high O₃ treatments in the elevated CO₂ environment. In contrast, A. smithii was marginally negatively affected by high O_3.     

Gas exchange and carbon isotope composition of Ananas comosus in response to elevated CO2 and temperature

Ananas comosus L. (Merr.) (pineapple) was grown at three day/night temperatures and 350 (ambient) and 700 (elevated) μ mol mol^–1 CO₂ to examine the interactive effects of these factors on leaf gas exchange and stable carbon isotope discrimination ( Δ ,‰). All data were collected on the youngest mature leaf for 24 h every 6 weeks. CO₂ uptake (mmol m^–2 d^–1) at ambient and elevated CO₂, respectively, were 306 and 352 at 30/20 °C, 175 and 346 at 30/25 °C and 187 and 343 at 35/25 °C. CO₂ enrichment enhanced CO₂ uptake substantially in the day in all environments. Uptake at night at elevated CO₂, relative to that at ambient CO₂, was unchanged at 30/20 °C, but was 80% higher at 30/25 °C and 44% higher at 35/25 °C suggesting that phosphoenolpyruvate carboxylase was not CO₂-saturated at ambient CO₂ levels and a 25 °C night temperature. Photosynthetic water use efficiency (WUE) was higher at elevated than at ambient CO₂. Leaf Δ -values were higher at elevated than at ambient CO₂ due to relatively higher assimilation in the light. Leaf Δ was significantly and linearly related to the fraction of total CO₂ assimilated at night. The data suggest that a simultaneous increase in CO₂ level and temperature associated with global warming would enhance carbon assimilation, increase WUE, and reduce the temperature dependence of CO₂ uptake by A. comosus .     

The impact of nitrogen supply on the potential response of a noxious, invasive weed, Canada thistle (Cirsium arvense) to recent increases in atmospheric carbon dioxide

A recognized invasive weed, Canada thistle ( Cirsium arvense L. Scop.) was grown at ambient and pre-ambient concentrations of atmospheric carbon dioxide [CO₂] (373 and 287 μmol mol⁻¹, respectively) at three levels of supplemental nitrogen (N) (3, 6 and 14.5 m M ) from seeding until flowering [77 days after sowing (DAS)]. The primary objective of the study was to determine if N supply limited the potential photosynthetic and growth response of this species to the increase in atmospheric [CO₂] which occurred during the 20th century (i.e. approximately 290 to 370 μmol mol⁻¹ CO₂). Leaf photosynthesis increased both as a function of growth [CO₂] and N supply during the first 46 DAS. Although by 46 DAS photosynthetic acclimation was observed relative to a common measurement CO₂ concentration, there was no interaction with N supply. Both [CO₂] and N increased biomass, relative growth rates and leaf area whereas root : shoot ratio was increased by CO₂ and decreased by increasing N; however, N supply did not effect the relative response to [CO₂] for any measured vegetative parameter up to 77 DAS. Due to the relative stimulation of shoot biomass, total above-ground N increased at elevated [CO₂] for all levels of supplemental N, but nitrogen use efficiency (NUE) did not differ as a function of [CO₂]. Overall, these data suggest that any potential response to increased atmospheric [CO₂] in recent decades for this noxious weedy species was probably not limited by nitrogen supply.