Effects of elevated CO2 and nitrogen on growth ofPoa pratensis (L.)

The growth responses of a grass,Poa pratensis, to elevated CO₂ and nitrogen were investigated. Light-saturated photosynthetic rate per unit leaf area increased with exposure to elevated CO₂, while dry weight did not respond to increased CO₂. Patterns of biomass allocation within plants, including leaf area, leaf area ratio, specific leaf area, and root to shoot ratios, were not altered by elevated CO₂, but changed considerably with N treatment Shoot and whole-plant tissue N concentrations were significantly diluted by elevated CO₂ (Tukey test, P < 0.05). Total N content did not differ significantly among CO₂ treatments. The absence of a concomitant increase in N uptake under elevated CO₂ may have caused a dilution in plant tissue [N], probably negating the positive effects of increased photosynthesis on biomass accumulation.     

Ponderosa Pine Responses to Elevated CO2and Nitrogen Fertilization

The effects of elevated CO₂ (ambient, +175, and +350 μl l⁻¹) and nitrogen fertilization (0, 100, and 200 kg N ha⁻¹ yr⁻¹ as ammonium sulfate) on C and N accumulations in biomass and soils planted with ponderosa pine (Pinus ponderosa Laws) over a 6-year study period are reported. Both nitrogen fertilization and elevated CO₂ caused increases in C and N contents of vegetation over the study period. The pattern of responses varied over time. Responses to CO₂ decreased in the +175 μl l⁻¹ and increased in the +350 μl l⁻¹ after the first year, whereas responses to N decreased after the first year and became non-significant by year six. Foliar N concentrations were lower and tree C:N ratios were higher with elevated CO₂ in the early years, but this was offset by the increases in biomass, resulting in substantial increases in N uptake with elevated CO₂. Nitrogen budget estimates showed that the major source of the N for unfertilized trees, with or without elevated CO₂, was likely the soil organic N pool. There were no effects of elevated CO₂ on soil C, but a significant decrease in soil N and an increase in soil C:N ratio in year six. Nitrogen fertilization had no significant effect on tree C:N ratios, foliar N concentrations, soil C content, soil N content, or soil C:N ratios. There were no significant interactions between CO₂ and N treatments, indicating that N fertilization had no effect on responses to CO₂ and that CO₂ treatments had no effect on responses to N fertilization. These results illustrate the importance of long-term studies involving more than one level of treatment to assess the effects of elevated CO₂.     

Contrasting growth response of an N2-fixing and non-fixing forb to elevated CO2: dependence on soil N supply

With the ability to symbiotically fix atmospheric N₂, legumes may lack the N-limitations thought to constrain plant response to elevated concentrations of atmospheric CO₂. The growth and photosynthetic responses of two perennial grassland species were compared to test the hypotheses that (1) the CO₂ response of wild species is limited at low N availability, (2) legumes respond to a greater extent than non-fixing forbs to elevated CO₂, and (3) elevated CO₂ stimulates symbiotic N₂ fixation, resulting in an increased amount of N derived from the atmosphere. This study investigated the effects of atmospheric CO₂ concentration (365 and 700 mol mol^–1) and N addition on whole plant growth and C and N acquisition in an N₂-fixing legume (Lupinus perennis) and a non-fixing forb (Achillea millefolium) in controlled-chamber environments. To evaluate the effects of a wide range of N availability on the CO₂ response, we incorporated six levels of soil N addition starting with native field soil inherently low in N (field soil + 0, 4, 8, 12, 16, or 20 g N m^–2 yr^–1). Whole plant growth, leaf net photosynthetic rates (A), and the proportion of N derived from N₂ fixation were determined in plants grown from seed over one growing season. Both species increased growth with CO₂enrichment, but this response was mediated by N supply only for the non-fixer, Achillea. Its response depended on mineral N supply as growth enhancements under elevated CO₂ increased from 0% in low N soil to +25% at the higher levels of N addition. In contrast, Lupinus plants had 80% greater biomass under elevated CO₂ regardless of N treatment. Although partial photosynthetic acclimation to CO₂ enrichment occurred, both species maintained comparably higher A in elevated compared to ambient CO₂ (+38%). N addition facilitated increased A in Achillea, however, in neither species did additional N availability affect the acclimation response of A to CO₂. Elevated CO₂ increased plant total N yield by 57% in Lupinus but had no effect on Achillea. The increased N in Lupinus came from symbiotic N₂ fixation, which resulted in a 47% greater proportion of N derived from fixation relative to other sources of N. These results suggest that compared to non-fixing forbs, N₂-fixers exhibit positive photosynthetic and growth responses to increased atmospheric CO₂ that are independent of soil N supply. The enhanced amount of N derived from N₂ fixation under elevated CO₂ presumably helps meet the increased N demand in N₂-fixing species. This response may lead to modified roles of N₂-fixers and N₂-fixer/non-fixer species interactions in grassland communities, especially those that are inherently N-poor, under projected rising atmospheric CO₂.     

Involvement of nitrogen and cytokinins in photosynthetic acclimation to elevated CO2 of spring wheat

Acclimation of photosynthetic capacity to elevated CO₂ involves a decrease of the leaf Rubisco content. In the present study, it was hypothesized that nitrogen uptake and partitioning within the leaf and among different aboveground organs affects the down-regulation of Rubisco. Given the interdependence of nitrogen and cytokinin signals at the whole plant level, it is also proposed that cytokinins affect the nitrogen economy of plants under elevated CO₂, and therefore the acclimatory responses. Spring wheat received varying levels of nitrogen and cytokinin in field chambers with ambient (370 μmol mol⁻¹) or elevated (700 μmol mol⁻¹) atmospheric CO₂. Gas exchange, Rubisco, soluble protein and nitrogen contents were determined in the top three leaves in the canopy, together with total nitrogen contents per shoot. Growth in elevated CO₂ induced decreases in photosynthetic capacity only when nitrogen supply was low. However, the leaf contents of Rubisco, soluble protein and total nitrogen on an area basis declined in elevated CO₂ regardless of nitrogen supply. Total nitrogen in the shoot was no lower in elevated than ambient CO₂, but the fraction of this nitrogen located in flag and penultimate leaves was lower in elevated CO₂. Decreased Rubisco: chlorophyll ratios accompanied losses of leaf Rubisco with CO₂ enrichment. Cytokinin applications increased nitrogen content in all leaves and nitrogen allocation to senescing leaves, but decreased Rubisco contents in flag leaves at anthesis and in all leaves 20 days later, together with the amount of Rubisco relative to soluble protein in all leaves at both growth stages. The results suggest that down regulation of Rubisco in leaves at elevated CO₂ is linked with decreased allocation of nitrogen to the younger leaves and that cytokinins cause a fractional decrease of Rubisco and therefore do not alleviate acclimation to elevated CO₂.     

Mycorrhizal dynamics under elevated CO2 and nitrogen fertilization in a warm temperate forest

We examined the response of mycorrhizal fungi to free-air CO₂ enrichment (FACE) and nitrogen (N) fertilization in a warm temperate forest to better understand potential influences over plant nutrient uptake and soil carbon (C) storage. In particular, we hypothesized that mycorrhizal fungi and glomalin would become more prevalent under elevated CO₂ but decrease under N fertilization. In addition, we predicted that N fertilization would mitigate any positive effects of elevated CO₂ on mycorrhizal abundance. Overall, we observed a 14% increase in ectomycorrhizal (ECM) root colonization under CO₂ enrichment, which implies that elevated CO₂ results in greater C investments in these fungi. Arbuscular mycorrhizal (AM) hyphal length and glomalin stocks did not respond substantially to CO₂ enrichment, and effects of CO₂ on AM root colonization varied by date. Nitrogen effects on AM fungi were not consistent with our hypothesis, as we found an increase in AM colonization under N fertilization. Lastly, neither glomalin concentrations nor ECM colonization responded significantly to N fertilization or to an N-by-CO₂ interaction. A longer duration of N fertilization may be required to detect effects on these parameters.     

Defense characteristics of seral deciduous broad-leaved tree seedlings grown under differing levels of CO2 and nitrogen

Mixed broad-leaved forests are abundantly populated by several kinds of herbivore species in northern Japan. The life of herbivores depends strongly on the quality of food leaves. Leaf quality is changing with increasing atmospheric CO₂ and nitrogen deposition. Four seral species of deciduous broad-leaved tree seedlings (alder, birch, oak and maple) were raised in all four combinations of two levels of CO₂ and two levels of nutrient, to examine foliar defense traits. To evaluate the relative defense capacity of the four tree species, we used wild silkworms (Erisan; a generalist herbivore) as a bioassay material. Except with alder, the survival rate and longevity (ML₅₀) of the silkworms were least when they were fed with leaves of seedlings raised under high CO₂ and infertile soil conditions, and longest on a diet of leaves grown in ambient CO₂ and fertile soil, especially in birch and maple. The longevity of Erisan decreased in the order birch, oak, and maple. The longevity of Erisan fed with alder leaves was independent of CO₂ levels and was longer on alder seedlings grown in infertile soil conditions. Alder is an actinorhizal plant that can fix atmospheric nitrogen in root nodules formed by the actinomycetes Frankia sp. The activity of symbiotic microbes would have been enhanced by the greater amount of photosynthates received from the host plants at high CO₂, improving the food quality for the silkworms. It was concluded that in all but alder, leaf chemical traits, especially C/N ratio, affect the ML₅₀ of Erisan larvae.     

Root response to CO2 enrichment and nitrogen supply in loblolly pine

This paper examines how elevated CO₂ and nitrogen (N) supply affect plant characteristics of loblolly pine (Pinus taeda L.) with an emphasis on root morphology. Seedlings were grown in greenhouses from seeds during one growing season at two atmospheric CO₂ concentrations (375 and 710 μL L^-1) and two N levels (High and Low). Root morphological characteristics were determined using a scanner and an image analysis program on a Macintosh computer. In the high N treatment, elevated CO₂ increased total plant dry weight by 80% and did not modify root to shoot (R/S) dry weight ratio, and leaf and plant N concentration at the end of the growing season. In the low N treatment, elevated CO₂ increased total dry weight by 60%. Plant and leaf N concentration declined and R/S ratio tended to increase. Nitrogen uptake rate on both a root length and a root dry weight basis was greater at elevated CO₂ in the high N treatment and lower in the low N treatment. We argue that N stress resulting from short exposures to nutrients might help explain the lower N concentrations observed at high CO₂ in other experiments; Nitrogen and CO₂ levels modified root morphology. High N increased the number of secondary lateral roots per length of first order lateral root and high CO₂ increased the length of secondary lateral roots per length of first order lateral root. Number and length of first order lateral roots were not modified by either treatment. Specific root length of main axis, and to a lower degree, of first order laterals, declined at high CO₂, especially at high N. Basal stem diameter and first order root diameters increased at high CO₂, especially at high N. Elevated CO₂ increased the proportion of upper lateral roots within the root system.     

Canopy N and P dynamics of a southeastern US pine forest under elevated CO2

Forest production is strongly nutrient limited throughout the southeastern US. If nutrient limitations constrain plant acquisition of essential resources under elevated CO₂, reductions in the mass or nutrient content of forest canopies could constrain C assimilation from the atmosphere. We tested this idea by quantifying canopy biomass, foliar concentrations of N and P, and the total quantity of N and P in a loblolly pine (Pinus taeda) canopy subject to 4 years of free-air CO₂ enrichment. We also used N:P ratios to detect N versus P limitation to primary production under elevated CO₂. Canopy biomass was significantly higher under elevated CO₂ during the first 4 years of this experiment. Elevated CO₂ significantly reduced the concentration of N in loblolly pine foliage (5% relative to ambient CO₂) but not P. Despite the slight reduction foliage N concentrations, there were significant increases in canopy N and P contents under elevated CO₂. Foliar N:P ratios were not altered by elevated CO₂ and were within a range suggesting forest production is N limited not P limited. Despite the clear limitation of NPP by N under ambient and elevated CO₂ at this site, there is no evidence that the mass of N or P in the canopy is declining through the first 4 years of CO₂ fumigation. As a consequence, whole-canopy C assimilation is strongly stimulated by elevated CO₂ making this forest a larger net C sink under elevated CO₂ than under ambient CO₂. We discuss the potential for future decreases in canopy nutrient content as a result of limited changes in the size of the plant-available pools of N under elevated CO₂.     

Controls of biomass partitioning between roots and shoots: Atmospheric CO2 enrichment and the acquisition and allocation of carbon and nitrogen in wild radish

Summary The effects of CO₂ enrichment on plant growth, carbon and nitrogen acquisition and resource allocation were investigated in order to examine several hypotheses about the mechanisms that govern dry matter partitioning between shoots and roots. Wild radish plants (Raphanus sativus × raphanistrum) were grown for 25 d under three different atmospheric CO₂ concentrations (200 ppm, 330 ppm and 600 ppm) with a stable hydroponic 150 mol 1^–1 nitrate supply. Radish biomass accumulation, photosynthetic rate, water use efficiency, nitrogen per unit leaf area, and starch and soluble sugar levels in leaves increased with increasing atmospheric CO₂ concentration, whereas specific leaf area and nitrogen concentration of leaves significantly decreased. Despite substantial changes in radish growth, resource acquisition and resource partitioning, the rate at which leaves accumulated starch over the course of the light period and the partitioning of biomass between roots and shoots were not affected by CO₂ treatment. This phenomenon was consistent with the hypothesis that root/shoot partitioning is related to the daily rate of starch accumulation by leaves during the photoperiod, but is inconsistent with hypotheses suggesting that root/shoot partitioning is controlled by some aspect of plant C/N balance.     

Long-term response of Sitka spruce (Picea sitchensis (Bong.) Carr.) to CO2 enrichment and nitrogen supply. I. Growth,biomass allocation and physiology

ABSTRACT

After a 3-year exposure to elevated CO₂, young trees of Sitka spruce (Picea sitchensis (Bong.) Carr.) were planted in native, nutrient-deficient forest soil and grown for two more years with three CO₂ treatments in open-top chambers, and with two nutrient treatments (with and without supplied N). Elevated CO₂ resulted in larger fresh mass, dry mass, leaf area and leaf thickness in two-year old needles, but had no effect on one-year old and current needles. Tree height, basal diameter and biomass production also increased, regardless of N supply. In trees without added N, elevated CO₂ resulted in higher root-to-shoot and absorbing roots-to-stump ratios. Regardless of N supply, trees grown in elevated CO₂ had lower photosynthetic rates on a leaf area basis. Photosynthesis reduction was accompanied by a decline in Rubisco activity and leaf N concentration. Under elevated CO₂, added N elevated photosynthesis and Rubisco activity, suggesting a dependence on N availability of the photosynthetic response to elevated CO₂. Stomatal conductance of trees grown with added N decreased in response to elevated CO₂. This may account for the larger reduction in intercellular CO₂ concentration, and hence photosynthesis, in the trees supplied with N than in those without N supply.     

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     

Response of Forest Trees to Increased Atmospheric CO 2

The CO ₂ fertilization hypothesis stipulates that rising atmospheric CO ₂ has a positive effect on tree growth due to increasing availability of carbon. The objective of this paper is to compare the recent literature related to both field CO ₂ -enriched experiments with trees and empirical dendrochronological studies detecting CO ₂ fertilization effects in tree-rings. This will allow evaluation of tree growth responses to atmospheric CO ₂ enrichment by combining evidence from both ecophysiology and tree-ring research. Based on considerable experimental evidence of direct CO ₂ fertilization effect (increased photosynthesis, water use efficiency, and above- and belowground biomass), and predications from the interactions of enriched CO ₂ with temperature, nitrogen and drought, we propose that warm, moderately drought-stressed ecosystems with an ample nitrogen supply might be the most CO ₂ responsive ecosystems. Empirical tree-ring studies took the following three viewpoints on detecting CO ₂ fertilization effect in tree-rings: 1) finding evidence of CO ₂ fertilization effect in tree-rings, 2) attributing growth enhancement to favorable climate rather than atmospheric CO ₂ enrichment, and 3) considering that tree growth enhancement might be caused by synergistic effects of several factors such as favorable climate change, CO ₂ fertilization, and anthropogenic atmospheric deposition (e.g., nitrogen). At temperature-limiting sites such as high elevations, nonfindings of CO ₂ fertilization evidence could be ascribed to the following possibilities: 1) cold temperatures, a short season of cambial division, and nitrogen deficiency that preclude a direct CO ₂ response, 2) old trees past half of their maximum life expectancy and consequently only a small increase in biomass increment due to CO ₂ fertilization effect might be diminished, 3) the elimination of age/size-related trends by statistical detrending of tree-ring series that might remove some long-term CO ₂ -related trends in tree-rings, and 4) carbon partitioning and growth within a plant that is species-specific. Our review supports the atmospheric CO ₂ fertilization effect hypothesis, at least in trees growing in semi-arid or arid conditions because the drought-stressed trees could benefit from increased water use efficiency to enhance growth.     

Modifications of the carbon and nitrogen allocations in the plant (Triticum aestivum L.) soil system in response to increased atmospheric CO2 concentration

The aim of this work was to examine the response of wheat plants to a doubling of the atmospheric CO₂ concentration on: (1) carbon and nitrogen partitioning in the plant; (2) carbon release by the roots; and (3) the subsequent N uptake by the plants. The experiment was performed in controlled laboratory conditions by exposing fast-growing spring wheat plants, during 28 days, to a ¹⁴CO₂ concentration of 350 or 700 L L^–1 at two levels of soil nitrogen fertilization. Doubling CO₂ availability increased total plant production by 34% for both N treatment. In the N-fertilized soil, the CO₂ enrichment resulted in an increase in dry mass production of 41% in the shoots and 23% in the roots; without N fertilization this figure was 33% and 37%, respectively. In the N-fertilized soil, the CO₂ increase enhanced the total N uptake by 14% and lowered the N concentration in the shoots by 23%. The N concentration in the roots was unchanged. In the N-fertilized soil, doubling CO₂ availability increased N uptake by 32% but did not change the N concentrations, in either shoots or roots. The CO₂ enrichment increased total root-derived carbon by 12% with N fertilization, and by 24% without N fertilization. Between 85 and 90% of the total root derived-¹⁴C came from respiration, leaving only 10 to 15% in the soil as organic ¹⁴C. However, when total root-derived ¹⁴C was expressed as a function of root dry weight, these differences were only slightly significant. Thus, it appears that the enhanced carbon release from the living roots in response to increased atmospheric CO₂, is not due to a modification of the activity of the roots, but is a result of the increased size of the root system. The increase of root dry mass also resulted in a stimulation of the soil N mineralization related to the doubling atmospheric CO₂ concentration. The discussion is focused on the interactions between the carbon and nitrogen allocation, especially to the root system, and the implications for the acquisition of nutrients by plants in response to CO₂ increase.Abbreviations N soil fertilization without nitrogen - N soil fertilization with nitrogen     

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.     

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     

Acclimation of photosynthesis to increasing atmospheric CO2: The gas exchange perspective

The nature of photosynthetic acclimation to elevated CO₂ is evaluated from the results of over 40 studies focusing on the effect of long-term CO₂ enrichment on the short-term response of photosynthesis to intercellular CO₂ (the A/C_i response). The effect of CO₂ enrichment on the A/C_i response was dependent on growth conditions, with plants grown in small pots (< 5 L) or low nutrients usually exhibiting a reduction of A at a given C_i, while plants grown without nutrient deficiency in large pots or in the field tended to exhibit either little reduction or an enhancement of A at a given C_i following a doubling or tripling of atmospheric CO₂ during growth. Using theoretical interpretations of A/C_i curves to assess acclimation, it was found that when pot size or nutrient deficiency was not a factor, changes in the shape of A/C_i curves which are indicative of a reallocation of resources within the photosynthetic apparatus typically were not observed. Long-term CO₂ enrichment usually had little effect or increased the value of A at all C_i. However, a minority of species grown at elevated CO₂ exhibited gas exchange responses indicative of a reduced amount of Rubisco and an enhanced capacity to metabolize photosynthetic products. This type of response was considered beneficial because it enhanced both photosynthetic capacity at high CO₂ and reduced resource investment in excessive Rubisco capacity. The ratio of intercellular to ambient CO₂ (the C_i/C_a ratio) was used to evaluate stomatal acclimation. Except under water and humidity stress, C_i/C_a exhibited no consistent change in a variety of C₃ species, indicating no stomatal acclimation. Under drought or humidity stress, C_i/C_a declined in high-CO₂ grown plants, indicating stomata will become more conservative during stress episodes in future high CO₂ environments.Abbreviations A net CO₂ assimilation rate - C_i (C_a) intercellular (ambient) partial pressure of CO₂ - operational C_i intercellular partial pressure of CO₂ at a given ambient partial pressure of CO₂ - g_s stomatal conductance - normal CO₂ current atmospheric mole fraction of CO₂ (330 to 355 mol mol^–1) - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase     

Hemiparasite abundance in an alpine treeline ecotone increases in response to atmospheric CO2 enrichment

Populations of the annual hemiparasites Melampyrum pratense L. and Melampyrum sylvaticum L. were studied at the treeline in the Swiss Alps after 3 years of in situ CO₂ enrichment. The total density of Melampyrum doubled to an average of 44 individuals per square meter at elevated CO₂ compared to ambient CO₂. In response to elevated CO₂, the height of the more abundant and more evenly distributed M. pratense increased by 20%, the number of seeds per fruit by 21%, and the total seed dry mass per fruit by 27%, but the individual seed size did not change. These results suggest that rising atmospheric CO₂ may stimulate the reproductive output and increase the abundance of Melampyrum in the alpine treeline ecotone. Because hemiparasites can have important effects on community dynamics and ecosystem processes, notably the N cycle, changing Melampyrum abundance may potentially influence the functioning of alpine ecosystems in a future CO₂-rich atmosphere.     

Daily and seasonal CO2 exchange in Scots pine grown under elevated O3 and CO2: experiment and simulation

Starting in early spring of 1994, naturally regenerated, 30-year-old Scots pine (Pinus sylvestris L.) trees were grown in open-top chambers and exposed in situ to doubled ambient O₃,doubled ambient CO₂ and a combination of O₃ and CO₂ from 15 April to 15 September. To investigate daily and seasonal responses of CO₂ exchange to elevated O₃ and CO₂, the CO₂ exchange of shoots was measured continuously by an automatic system for measuring gas exchange during the course of one year (from 1 Januray to 31 December 1996). A process-based model of shoot photosynthesis was constructed to quantify modifications in the intrinsic capacity of photosynthesis and stomatal conductance by simulating the daily CO₂ exchange data from the field. Results showed that on most days of the year the model simulated well the daily course of shoot photosynthesis. Elevated O₃ significantly decreased photosynthetic capacity and stomatal conductance during the whole photosynthetic period. Elevated O₃ also led to a delay in onset of photosynthetic recovery in early spring and an increase in the sensitivity of photosynthesis to environmental stress conditions. The combination of elevated O₃ and CO₂ had an effect on photosynthesis and stomatal conductance similar to that of elevated O₃ alone, but significantly reduced the O₃-induced depression of photosynthesis. Elevated CO₂ significantly increased the photosynthetic capacity of Scots pine during the main growing season but slightly decreased it in early spring and late autumn. The model calculation showed that, compared to the control treatment, elevated O₃ alone and the combination of elevated O₃ and CO₂ decreased the annual total of net photosynthesis per unit leaf area by 55% and 38%, respectively. Elevated CO₂ increased the annual total of net photosynthesis by 13%.     

CO2 transported in xylem sap affects CO2 efflux from Liquidambar styraciflua and Platanus occidentalis stems, and contributes to observed wound respiration phenomena

The [CO₂] in the xylem of tree stems is typically two to three orders of magnitude greater than atmospheric [CO₂]. In this study, xylem [CO₂] was experimentally manipulated in saplings of sycamore (Platanus occidentalis L.) and sweetgum (Liquidambar styraciflua L.) by allowing shoots severed from their root systems to absorb water containing [CO₂] ranging from 0.04% to 14%. The effect of xylem [CO₂] on CO₂ efflux to the atmosphere from uninjured and mechanically injured, i.e., wounded, stems was examined. In both wounded and unwounded stems, and in both species, CO₂ efflux was directly proportional to xylem [CO₂], and increased 5-fold across the range of xylem [CO₂] produced by the [CO₂] treatment. Xylem [CO₂] explained 76–77% of the variation in pre-wound efflux. After wounding, CO₂ efflux increased substantially but remained directly proportional to internal stem [CO₂]. These experiments substantiated our previous finding that stem CO₂ efflux was directly related to internal xylem [CO₂] and expanded our observations to two new species. We conclude that CO₂ transported in the xylem may confound measurements of respiration based on CO₂ efflux to the atmosphere. This study also provided evidence that the rapid increase in CO₂ efflux observed after tissues are excised or injured is likely the result of the rapid diffusion of CO₂ from the xylem, rather than an actual increase in the rate of respiration of wounded tissues.     

Photosynthetic characteristics of coffee (Coffea arabusta) plantlets in vitro in response to different CO2 concentrations and light intensities

The photosynthetic characteristics of coffee ( Coffea arabusta) plantlets cultured in vitro in response to different CO₂ concentrations inside the culture vessel and photosynthetic photon flux (PPF) were investigated preliminarily. The estimation of net photosynthetic rate (P_n) of coffee plantlets involved three methods: (1) estimating time courses of actual P_n in situ based on measuring CO₂ concentrations inside and outside the vessel during a 45-day period, (2) estimating P_n in situ at different CO₂ concentrations and PPFs using the above measuring approach for 10-day and 30-day old in vitro plantlets, and (3) estimating P_n of a single leaf at different CO₂ concentrations and PPFs by using a portable photosynthesis measurement system for 45-day old in vitro coffee plantlets. The results showed that coffee plantlets in vitro had relatively high photosynthetic ability and that the P_n increased with the increase in CO₂ concentration inside the vessel. The CO₂ saturation point of in vitro coffee plantlets was high (4500–5000 μmol mol^-1); on the other hand, the PPF saturation point was not so high as compared to some other species, though it increased with increasing CO₂ concentration inside the vessel. This revised version was published online in June 2006 with corrections to the Cover Date.