Temperature effects on the photosynthetic response of C3 plants to long-term CO2 enrichment

To assess the long-term effect of increased CO₂ and temperature on plants possessing the C₃ photosynthetic pathway, Chenopodium album plants were grown at one of three treatment conditions: (1) 23 °C mean day temperature and a mean ambient partial pressure of CO₂ equal to 350 bar; (2) 34 °C and 350 bar CO₂; and (3) 34 °C and 750 bar CO₂. No effect of the growth treatments was observed on the CO₂ reponse of photosynthesis, the temperature response of photosynthesis, the content of Ribulose-1,5-bisphosphate carboxylase (Rubisco), or the activity of whole chain electron transport when measurements were made under identical conditions. This indicated a lack of photosynthetic acclimation in C. album to the range of temperature and CO₂ used in the growth treatments. Plants from every treatment exhibited similar interactions between temperature and CO₂ on photosynthetic activity. At low CO₂ (< 300 bar), an increase in temperature from 25 to 35 °C was inhibitory for photosynthesis, while at elevated CO₂ (> 400 bar), the same increase in temperature enhanced photosynthesis by up to 40%. In turn, the stimulation of photosynthesis by CO₂ enrichment increased as temperature increased. Rubisco capacity was the primary limitation on photosynthetic activity at low CO₂ (195 bar). As a consequence, the temperature response of A was relatively flat, reflecting a low temperature response of Rubisco at CO₂ levels below its k_m for CO₂. At elevated CO₂ (750 bar), the temperature response of electron transport appeared to control the temperature dependency of photosynthesis above 18 °C. These results indicate that increasing CO₂ and temperature could substantially enhance the carbon gain potential in tropical and subtropical habitats, unless feedbacks at the whole plant or ecosystem level limit the long-term response of photosynthesis to an increase in CO₂ and temperature.Abbreviations A net CO₂ assimilation rate - C _a ambient partial pressure of CO₂ - C _i intercellular partial pressure of CO₂ - Rubisco Ribulose-1,5-bisphosphate carboxylase - VPD vapor pressure difference between leaf and air     

The effect of different night conditions on the CO2 fixation in a lichen Xanthoria parietina

CO₂ fixation was studied in a lichen, Xanthoria parietina, kept in continuous light, and with cyclic changes in light intensity, dark period or temperature. The diurnal and seasonal courses of CO₂ exchange were followed. The rate of net photosynthesis was observed to fall from morning to evening, and this decline was more pronounced in winter than in summer. The maximal net photosynthetic rate, 223 ng CO₂g^-1dws^-1, occured in winter and the minimum, 94 ng CO₂g^-1dws^-1, late in spring. The light compensation point in summer was four times as high as in winter. In continuous light (180 or 90 mol photons m^-2s^-1, 15°C) net photosynthesis decreased noticeably during one week, falling below the level maintained in a 12 h light: 12 h dark cycle. Photosynthetic activity did not decrease, however, in lichens held in continuous light (90 mol photons m^-2s^-1) with cyclic changes of temperature (12 h 20 °C: 12 h 5 °C). Active photosynthesis was also maintained in light of cyclically changing intensity (12 h: 12 h, 15 °C) when night-time light was at least 75% lower than illumination by day. A dark period of 4 hours in a 24-h light:dark cycle was sufficient to keep CO₂ fixation at the control level. It seems that plants need an unproductive period during the day to survive and this can be induced by fluctuations in light and/or temperature.     

Elevated atmospheric partial pressure of CO2 and plant growth

Summary Cotton and maize plants were grown under full sunlight in glass houses containing normal ambient partial pressure of CO₂ (330±20 bar) and enriched partial pressure of CO₂ (640 ±15 bar) with four levels of nitrogen nutrient. In 40 day old cotton plants grown in high CO₂, there was a 2-fold increase in day weight and a 1.6-fold increase in leaf area compared with plants grown in ambient CO₂. In 30 day old maize plants there was only 20% increase in dry weight in plants grown in 640 bar CO₂ compared with plants grown in 330 bar and no significant increase in leaf area. In both species, at both CO₂ treatments, dry weight and leaf area decreased in similar proportion with decreased nitrogen nutrient.The increase of leaf area in cotton plants at high CO₂ caused a reduction of total nitrogen on a dry weight basis. In cotton assimilation rate increased 1.5 fold when plants were grown with high nitrogen and high CO₂. The increase was less at lower levels of nitrate nutrient. There was a 1.2 fold increase in assimilation rate in maize grown at high CO₂ with high nitrate nutrient.Cotton and maize grown in high CO₂ had a lower assimilation rate in ambient CO₂ compared to plants grown in normal ambient air. This difference was due to the reduction in RuBP carboxylase activity. Water use efficiency was doubled in both cotton and maize plants grown at high CO₂ in all nutrient treatments. However, this increase in water use efficiency was due primarily to reduced transpiration in some treatments and to increased assimilation in others. These data show that plant responses to elevated atmospheric partial pressure of CO₂ depend on complex of partially compensatory processes which are not readily predictable.     

Dark CO2-fixation and diurnal malic acid fluctuations in the submerged-aquatic Isoetes storkii

Summary In the leaves (but not corms) of the submerged aquatic Isoetes storkii malic acid concentration fluctuated from 22 eg g FW^-1 in the evening to 171 eg g FW^-1 in the morning. Associated with this was a change in titratable acidity of 152 eg g FW^-1 between morning and evening. ¹⁴C carbon was fixed in both the light and the dark, though the amount of carbon fixed in the light was more than that fixed in the dark. Autoradiographs show 88% of ¹⁴CO₂ fixed in the dark is recovered after 1 h, in malic acid and the remainder in one other unidentified product, whereas these two products contain less than 15% of the ¹⁴C fixed after 1 h exposure to ¹⁴CO₂ in the light. It is suggested that CAM metabolism in this aquatic species may be related to the low availability of CO₂ for photosynthesis during the day in its aquatic environment and that this metabolic pathway may prove common in the genus Isoetes.     

Leaf cavity CO2 concentrations and CO2 exchange in onion,Allium cepa L.

Onion (Allium cepa L.) plants were examined to determine the photosynthetic role of CO₂ that accumulates within their leaf cavities. Leaf cavity CO₂ concentrations ranged from 2250 L L^–1 near the leaf base to below atmospheric (<350 L L^–1) near the leaf tip at midday. There was a daily fluctuation in the leaf cavity CO₂ concentrations with minimum values near midday and maximum values at night. Conductance to CO₂ from the leaf cavity ranged from 24 to 202 mol m^–2 s^–1 and was even lower for membranes of bulb scales. The capacity for onion leaves to recycle leaf cavity CO₂ was poor, only 0.2 to 2.2% of leaf photosynthesis based either on measured CO₂ concentrations and conductance values or as measured directly by ¹⁴CO₂ labeling experiments. The photosynthetic responses to CO₂ and O₂ were measured to determine whether onion leaves exhibited a typical C₃-type response. A linear increase in CO₂ uptake was observed in intact leaves up to 315 L L^–1 of external CO₂ and, at this external CO₂ concentration, uptake was inhibited 35.4±0.9% by 210 mL L^–1 O₂ compared to 20 mL L^–1 O₂. Scanning electron micrographs of the leaf cavity wall revealed degenerated tissue covered by a membrane. Onion leaf cavity membranes apparently are highly impermeable to CO₂ and greatly restrict the refixation of leaf cavity CO₂ by photosynthetic tissue.Abbreviations C_a external CO₂ concentration - C_i intercellular CO₂ concentration - CO₂ compensation concentration - PPFR photosynthetic photon fluence rate     

Growth,photosynthesis and photorespiration of Lemna gibba: response to variations in CO2 and O2 concentrations and photon flux density

Dry weight and Relative Growth Rate of Lemna gibba were significantly increased by CO₂ enrichment up to 6000 l CO₂ l^–1. This high CO₂ optimum for growth is probably due to the presence of nonfunctional stomata. The response to high CO₂ was less or absent following four days growth in 2% O₂. The Leaf Area Ratio decreased in response to CO₂ enrichment as a result of an increase in dry weight per frond. Photosynthetic rate was increased by CO₂ enrichment up to 1500 l CO₂ l^–1 during measurement, showing only small increases with further CO₂ enrichment up to 5000 l CO₂ l^–1 at a photon flux density of 210 mol m^–2 s^–1 and small decreases at 2000 mol m^–1 s^–1. The actual rate of photosynthesis of those plants cultivated at high CO₂ levels, however, was less than the air grown plants. The response of photosynthesis to O₂ indicated that the enhancement of growth and photosynthesis by CO₂ enrichment was a result of decreased photorespiration. Plants cultivated in low O₂ produced abnormal morphological features and after a short time showed a reduction in growth.     

Elevated atmospheric partial pressure of carbon dioxide and dry matter production of konjak (Amorphophallus konjac K. Koch)

Konjak (Amorphophallus konjac K. Koch) was grown under normal (350 bar) or enriched (700 bar) CO₂ partial pressure in glasshouses kept at 33/26 °C. Doubling the CO₂ partial pressure resulted in twice the yield of corm because the net CO₂ assimilation rate doubled and, due to the simple source-sink relationship, the increased production was partitioned to the corm. The response to CO₂ of assimilation by konjak is discussed in relation to its original habitat in the tropics.     

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.     

Effect of Elevated CO2 Concentration on Leaf Structure of Brassica Juncea under Water Stress

Study on the effect of elevated CO₂ concentration on leaf structure of Brassica juncea L. cv. Bio-141 (95) under moisture stress revealed, that CO₂ elevated to 600 mol mol^–1 increased the length of epidermal cel and length of palisade parenchyma cells, and induced larger chloroplasts and more oval shaped starch granules in comparison with plants grown at ambient CO₂ concentration. This increase in structural sink size helped in check feedback inhibition by excessive photoassimilate which was subsequently used to compensate the adverse moisture stress effect in B. juncea leaves.     

Gas exchange of ears of cereals in response to carbon dioxide and light

One cultivar each of spring wheat (Triticum aestivum L. cv. Arkas), oat (Avena sativa L. cv. Lorenz), and barley (Hordeum vulgare L. cv. Aramir) was chosen in order to study the relative contributions of individual bracts to the gas exchange of whole ears. The distribution and frequency of the stomata on the bracts were examined. Gas exchange was measured at normal atmospheric CO₂ (330 bar) and at high CO₂ (2000 bar) on intact ears and on ears from which glumes or lemmas and pleae (wheat and oat) or awns (barley) had been removed.The relative contribution to the gas exchange of the whole organ is highest for the awns of barley ears. In wheat, the contribution of the glumes is slightly higher than that of the inner bracts before anthesis. Two weeks after anthesis the inner bracts contribute more than the glumes. This tendency of increasing importance of the inner bracts is also found in oat ears, but the relative amount of CO₂ uptake by the glumes is higher than in wheat. These changes during ontogeny result from the better supply of light to the inner bracts caused by opening of the ears' structures during grain filling, which in part compensates for the decreasing photosynthetic capacity.The ratio of the photosynthesis rate at high CO₂ to that at normal CO₂ is lower for the glumes of oat and for the awns of barley than for the other bracts.Abbreviations A³³⁰, A²⁰⁰⁰ net photosynthesis rate, A³³⁰ at normal atmospheric CO₂ (330 bar), A²⁰⁰⁰ at high CO₂ (2000 bar) - PPFD photosynthetic photon flux density - p^c intercellular partial pressure of CO₂     

Short-term CO2 exchange response to temperature,irradiance, and CO2 concentration in strawberry

Relative importance of short-term environmental interaction and preconditioning to CO₂ exchange response was examined in Fragaria ananasa (strawberry, cv. Quinault). Tests included an orthogonal comparison of 15 to 60-min and 6 to 7-h exposures to different levels of temperature (16 to 32°C), photosynthetically active radiation (PAR, 200 to 800 E m² s^-1), and CO₂ (300 to 600 l/l) on successive days of study. Plants were otherwise maintained at 21°C, 300 E m² s^-1 PAR and 300–360 l/l CO₂ as standard conditions. Treatment was restricted to the mean interval of 14 h daily illumination and the first 3–4 days of each test week over a 12-week cultivation period. CO₂ exchange rates were followed with each step-change in environmental level including ascending/descending temperature/PAR within a test period, initial response at standard conditions on successive days of testing, and measurement at reduced O₂. Response generally supported prior concepts of leaf biochemical modeling in identifying CO₂ fixation as the major site of environmental influence, while overall patterns of whole plant CO₂ exchange suggested additional effects for combined environmental factors and preconditioning. These included a positive interaction between temperature and CO₂ concentration on photosynthesis at high irradiance and a greater contribution by dark respiration at lower PAR than previously indicated. The further importance of estimating whole plant CO₂ exchange from repetitive tests and measurements was evidenced by a high correlation of response to prior treatment both during the daily test period and on consecutive days of testing.Abbreviations C₃ plant a plant in which the product of CO₂ fixation is a 3-carbon acid (3-phosphoglyceric acid) - IRGA intra-red gas analyzer - PAR photosynthetically active radiation - RH relative humidity - RuBisCO ribulose-1,5-bisphosphate carboxylase/oxygenase Reference to a company and/or product named by the Department is only for purposes of information and does not imply approval or recommendation of the product to the exclusion of others which may also be suitable.     

Tissue culture ofKarwinskia humboldtiana—a plant producing toxins with antitumoural effects

Isolated embryos ofKarwinskia humboldtiana were cultured in vitro. The growth of embryos and development to plantlets on woody plant medium supplemented with indole-3-acetic acid 6.10^-2 mol l^–1, gibberellic acid (GA₃) 3.10^-2 mol l^–1, and 6-benzylaminopurine (BA) 2 mol l^–1 was obtained. Multiplication of shoots and rooting of excised shoots has been achieved. Callus formation on modified Murashige-Skoog medium supplemented with 1-naphthaleneacetic acid 10 mol l^–1, GA₃ 14 mol l^–1, and kinetin 5 mol l^–1 on hypocotyls, or on root cultures on medium supplemented with 2.4-dichlorophenoxyacetic acid 10 mol l^–1 and BA 10 mol l^–1 was induced.Abbreviations BA 6-benzylaminopurine - 2,4-d 2,4-dichlorophenoxyacetic acid - GA₃ gibberellic acid - IAA indole-3-acetic acid - NAA 1-naphthaleneacetic acid - TEM transmission electron microscopy     

Photosynthesis in Plants of Four Tropical Species Growing under Elevated CO2

We studied the responses of leaf gas exchange and growth to an increase in atmospheric CO₂ concentration in four tropical deciduous species differing in carbon fixation metabolism: Alternanthera crucis, C3-C4; Ipomoea carnea, C3; Jatropha gossypifolia, C3; and Talinum triangulare, inducible-CAM. In the first stage, plants were grown in one open-top chamber at a CO₂ concentration of 560±40 mol mol^-1 (EC), one ambient CO₂ concentration chamber (AC), and one unenclosed plot (U). In the second stage, plants were grown in five EC chambers (CO₂ concentration = 680±30 mol mol^-1), five AC chambers, and five unenclosed plots. During the first weeks under EC in the first stage, plants of all the species had a very marked increase in their maximal net photosynthetic rates (P _max) of 3.5 times on average; this stimulatory effect was maintained for 11-15 weeks, rates dampening afterward to values still higher than controls for 37 weeks. After a suspension of CO₂ enrichment for 6 weeks, an increase in P _max of EC plants over the controls was found in plants of all the species until week 82 of the experiment. Stomatal conductance (g) showed no response to EC. Carboxylation efficiency decreased in all the species under EC     

Elevated CO2 increases belowground respiration in California grasslands

This study was designed to identify potential effects of elevated CO₂ on belowground respiration (the sum of root and heterotrophic respiration) in field and microcosm ecosystems and on the annual carbon budget. We made three sets of respiration measurements in two CO₂ treatments, i.e., (1) monthly in the sandstone grassland and in microcosms from November 1993 to June 1994; (2) at the annual peak of live biomass (March and April) in the serpentine and sandstone grasslands in 1993 and 1994; and (3) at peak biomass in the microcosms with monocultures of seven species in 1993. To help understand ecosystem carbon cycling, we also made supplementary measurements of belowground respiration monthly in sandstone and serpentine grasslands located within 500 m of the CO₂ experiment site. The seasonal average respiration rate in the sandstone grassland was 2.12 mol m^-2 s^-1 in elevated CO₂, which was 42% higher than the 1.49 mol m^-2 s^-1 measured in ambient CO₂ (P=0.007). Studies of seven individual species in the microcosms indicated that respiration was positively correlated with plant biomass and increased, on average, by 70% with CO₂. Monthly measurements revealed a strong seasonality in belowground respiration, being low (0–0.5 mol CO₂ m^-2 s^-1 in the two grasslands adjacent to the CO₂ site) in the summer dry season and high (2–4 mol CO₂ m^-2 s^-1 in the sandstone grassland and 2–7 mol CO₂ m^-2 s^-1 in the microcosms) during the growing season from the onset of fall rains in November to early spring in April and May. Estimated annual carbon effluxes from the soil were 323 and 440 g C m^-2 year^-1 for the sandstone grasslands in ambient and elevated CO₂. That CO₂-stimulated increase in annual soil carbon efflux is more than twice as big as the increase in aboveground net primary productivity (NPP_a) and approximately 60% of NPP_a in this grassland in the current CO₂ environment. The results of this study suggest that below-ground respiration can dissipate most of the increase in photosynthesis stimulated by elevated CO₂.CIWDPB Publication # 1271     

Production and diurnal utilization of assimilates in leaves of spinach (Spinacia oleracea L.) and barley (Hordeum vulgare L.)

Rates of CO₂ fixation during the light period and the rates of CO₂ release during the night period were measured using mature leaves from 39- to 49-d-old spinach (Spinacia oleracea L., US Hybrid 424; grown in 9 h light, 15 h darkness, daily) and mature leaves from 21-d-old barley (Hordeum vulgare L., cv. Apex; grown in 14 h light, 10 h darkness, daily). At certain times during the light and dark periods leaves were harvested for assay of their contents of soluble carbohydrates, starch, malate and the various amino acids. Evaluation of the results of these measurements shows that in spinach and barley leaves 46% and 26%, respectively, of the carbon assimilated during the light period is deposited in the leaves for export during the night period. Taking into account the carbon consumption in the source leaves by dark respiration, it is evaluated that rates of assimilate export during the light period from spinach and barley leaves [38 and 42 atom C · (mg Chl)^–1 · h^–1] are reduced in the dark period to 16 atom C · (mg Chl)^–1 · h^–1 in both species. The calculated C/N ratios of the photoassimilates exported during the dark period were 0.029 and 0.015 for spinach and barley leaves, respectively.This work was supported by the Deutsche Forschungsgemeinschaft. We thank Dr. Dieter Heineke for stimulating discussions and Mrs. Petra Hoferichter and Mrs. Marita Feldkämper for their technical assistance.     

Flavonoid Responses in Wheat Grown at Elevated CO2: Green Versus Senescent Leaves

We compared flavonoids in green, mature, and senescing flag leaves of wheat grown under ambient (AC - 370 mol mol^-1) and elevated (EC - 550 mol mol^-1) concentrations of CO₂ in a FACE (Free Air CO₂ Enrichment) system. The concentrations of flag leaf flavonoids (e.g., isoorientin and tricin) decreased to one third in mature leaves, and the majoritary isoorientin almost disappeared in senescing leaves. Flavonoid concentrations increased in green well-developed flag leaves under EC (46 % isoorientin and 55 % tricin), whereas the differences disappeared in mature and senescing flag leaves. Predictions of changes in litter phenolic concentrations and their effects on decomposition rates under EC based on changes in green leaves need to be revised.     

Effect of soil characteristics on sequential reduction and methane production in sixteen rice paddy soils from China, the Philippines, and Italy

The potentials for sequential reduction of inorganic electron acceptors and production of methane have been examined in sixteen rice soils obtained from China, the Philippines, and Italy. Methane, CO₂, Fe(II), NO ₃ ^- , SO ₄ ² , pH, E_h, H₂ and acetate were monitored during anaerobic incubation at 30 °C for 120 days. Based on the accumulation patterns of CO₂ and CH₄, the reduction process was divided into three distinct phases: (1) an initial reduction phase during which most of the inorganic electron acceptors were depleted and CO₂ production was at its maximum, (2) a methanogenic phase during which CH₄ production was initiated and reached its highest rate, and (3) a steady state phase with constant production rates of CH₄ and CO₂. The reduction phases lasted for 19 to 75 days with maximum CO₂ production of 2.3 to 10.9 mol d^-1 g^-1 dry soil. Methane production started after 2 to 87 days and became constant after about 38--68 days (one soil >120 days). The maximum CH₄ production rates ranged between 0.01 and 3.08 mol d^-1 g^-1. During steady state the constant CH₄ and CO₂ production rates varied from 0.07 to 0.30 mol d^-1 g^-1 and 0.02 and 0.28 mol d^-1 g^-1, respectively. Within the 120 d of anaerobic incubation only 6--17% of the total soil organic carbon was released into the gas phase. The gaseous carbon released consisted of 61--100% CO₂, <0.1--35% CH₄, and <5% nonmethane hydrocarbons. Associated with the reduction of available Fe(III) most of the CO₂ was produced during the reduction phase. The electron transfer was balanced between total CO₂ produced and both CH₄ formed and Fe(III), sulfate and nitrate reduced. Maximum CH₄ production rate (r = 0.891) and total CH₄ produced (r = 0.775) correlated best with the ratio of soil nitrogen to electron acceptors. Total nitrogen content was a better indicator for available organic substrates than the total organic carbon content. The redox potential was not a good predictor of potential CH₄ production. These observations indicate that the availability of degradable organic substrates mainly controls the CH₄ production in the absence of inorganic electron acceptors.     

Long-term effects of CO2 enrichment and temperature increase on a temperate grass sward

Perennial ryegrass swards were grown in large containers on a soil and were exposed during two years to elevated (700 L L^-1) or ambient atmospheric CO₂ concentration at outdoor temperature and to a 3 °C increase in air temperature in elevated CO₂. The nitrogen nutrition of the grass sward was studied at two sub-optimal (160 and 530 kg N ha^-1 y^-1) and one non-limiting (1000 kg N ha^-1 y^-1) N fertilizer supplies. At cutting date, elevated CO₂ reduced by 25 to 33%, on average, the leaf N concentration per unit mass. Due to an increase in the leaf blade weight per unit area in elevated CO₂, this decline did not translate for all cuts in a lower N concentration per unit leaf blade area. With the non-limiting N fertilizer supply, the leaf N concentration (% N) declined with the shoot dry-matter (DM) according to highly significant power models in ambient (% N=4.9 DM^-0.38) and in elevated (%N=5.3 DM^-0.52) CO₂. The difference between both regressions was significant and indicated a lower critical leaf N concentration in elevated than in ambient CO₂ for high, but not for low values of shoot biomass. With the sub-optimal N fertilizer supplies, the nitrogen nutrition index of the grass sward, calculated as the ratio of the actual to the critical leaf N concentration, was significantly lowered in elevated CO₂. This indicated a lower inorganic N availability for the grass plants in elevated CO₂, which was also apparent from the significant declines in the annual nitrogen yield of the grass sward and in the nitrate leaching during winter. For most cuts, the harvested fraction of the plant dry-matter decreased in elevated CO₂ due, on average, to a 45–52% increase in the root phytomass. In the same way, a smaller share of the plant total nitrogen was harvested by cutting, due, on average, to a 25–41% increase in the N content of roots. The annual means of the DM and N harvest indices were highly correlated to the annual means of the nitrogen nutrition index. Changes in the harvest index and in the nitrogen nutrition index between ambient and elevated CO₂ were also positively correlated. The possible implication of changes in the soil introgen cycle and of a limitation in the shoot growth potential of the grass in elevated. CO₂ is discussed.Abbreviations 350 outdoor climate - 700 outdoor climate +350 L L^-1[CO₂] - 700+ outdoor climate +350 L L^-1 (CO₂) and +3 °C - N-- low N fertilizer supply - N+ high N fertilizer supply - N++ non-limiting N fertilizer supply - DM dry-matter     

Light response characteristics of net CO2 exchange in brackish wetland plant communities

Summary Photosynthetic responses to incident photon flux density (400–700 nm; PPFD) was studied in a grass community consisting of Spartina patens and Distichlis spicata and a mixed community having the two grasses and a sedge, Scirpus Olneyi. Net community CO₂ exchange and incident PPFD were monitored from dawn to dusk in a large open gas exchange system, and a hyperbolic light response model was fit to the data for each day. Light response curves from five growing seasons were evaluated for seasonal trends in the compensation value, initial slope, and maximum net CO₂ exchange rate calculated from the model at PPFD=1670 mol m^-2s^-1.All response curves were curvilinear. Data from approximately 30% of the 113 days studied fit saturation curves which occurred primarily in spring and fall. Approximately 5% of all curves constructed required a different response curve for the morning and afternoon. These occurred during mid-summer and were interpreted to be evidence of water stress.The compensation flux density was very high early in the growing season, but rapidly decreased and during the months June, July and August, it averaged near 100 and 120 mol m^-2s^-1 in the mixed and grass communities. The initial slope and maximum net CO₂ exchange rate increased from early May to maxima in July and declined thereafter. Mid-summer mean values for the mixed and grass communities respectively were 34.3±10.3 mmol mol^-1 and 39.1±9.1 mmol mol^-1 for the initial slope and 20.3±4.2 mol m^-2s^-1 and 23.0±3.8 mol m^-2s^-1 for maximum net CO₂ exchange. Daytime respiration accounted for approximately 20% of maximum gross photosynthesis in both communities.Photosynthetic efficiency, CO₂ assimilated per unit total incident solar radiation, was approximately 4.1% and 4.7% at dawn or dusk and 2.3% and 2.6% at midday for the mixed and grass community. Gross photosynthesis, maximum photosynthesis plus midday respiration, accounted for 2.7% and 3.0% of total incident solar radiation in the mixed and grass communities.