A field study of the effects of elevated CO2 on carbon assimilation, stomatal conductance and leaf and branch growth of Pinus taeda trees

A study was conducted in 21-year-old loblolly pine (Pinus taeda L.) trees growing in plantation in north central Georgia, USA. The experiment used branch chambers to impose treatments of ambient, ambient +165 and ambient + 330 μmol mol^?1 CO₂. After one growing season there was no indication of acclimation to elevated CO₂. In August and September, carbon assimilation, measured by two different methods, was twice as high at ambient +330 μmol mol^?1 CO₂ than at ambient. Dark respiration was suppressed by 6% at ambient +165 and by 14% at ambient + 330 μmol mol^?1 CO₂. This suppression was immediate, and not an effect of exposure to elevated CO₂ during growth, since respiration was reduced by the same amount in all treatments when measured at a high CO₂ concentration. Elevated CO₂ increased the growth of foliage and woody tissue. It also increased instantaneous transpiration efficiency, but it had no effect on stomatal conductance. Since the soil at the study site had low to moderate fertility, these results suggest that the growth potential of forests on many sites may be enhanced by global increases in atmospheric CO₂, concentration.     

Combined effects of elevated CO2 and air temperature on carbon assimilation of Pinus taeda trees

Branches of 22-year-old loblolly pine (Pinus taeda, L.) trees growing in a plantation were exposed to ambient CO₂, ambient + 165 μmol mol^?1 CO₂ or ambient + 330 μmol mol^?1 CO₂ concentrations in combination with ambient or ambient + 2°C air temperatures for 3 years. Field measurements in the third year indicated that net carbon assimilation was enhanced in the elevated CO₂ treatments in all seasons. On the basis of A/C_i, curves, there was no indication of photosynthetic down-regulation. Branch growth and leaf area also increased significantly in the elevated CO₂ treatments. The imposed 2°C increase in air temperature only had slight effects on net assimilation and growth. Compared with the ambient CO₂ treatment, rates of net assimilation were ～1·6 times greater in the ambient + 165 μmol mol^?1 CO₂ treatment and 2·2 times greater in the ambient + 330 μmol mol^?1 CO₂ treatment. These ratios did not change appreciably in measurements made in all four seasons even though mean ambient air temperatures during the measurement periods ranged from 12·6 to 28·2°C. This indicated that the effect of elevated CO₂ concentrations on net assimilation under field conditions was primarily additive. The results also indicated that the effect of elevated CO₂ (+ 165 or + 330 μmol mol^?1) was much greater than the effect of a 2°C increase in air temperature on net assimilation and growth in this species.     

Direct and indirect inhibition of single leaf respiration by elevated CO2 concentrations: Interaction with temperature

Two herbaceous perennials, alfalfa (Medicago sativa L. cv. Arc) and orchard grass (Dactylus glomerata L. cv. Potomac), were grown at ambient (367 μmol mol⁻¹) and elevated (729 μmol mol⁻¹) CO₂ concentrations at constant temperatures of 15, 20, 25 and 30°C in order to examine direct and indirect changes in nighttime CO₂ efflux rate (respiration) of single leaves. Direct (biochemical) effects of CO₂ on nighttime respiration were determined for each growth condition by brief (<30 min) exposure to each CO₂ concentration. If no direct inhibition of respiration was observed, then long-term reductions in CO₂ efflux between CO₂ treatments were presumed to be due to indirect inhibition, probably related to long-term changes in leaf composition. By this criterion, indirect effects of CO₂ on leaf respiration were observed at 15 and 20°C for M. sativa on a weight basis, but not on a leaf area or protein basis. Direct effects however, were observed at 15, 20 and 25°C in D. glomerata; therefore the observed reductions in respiration for leaves grown and measured at elevated relative to ambient CO₂ concentrations could not be distinguished as indirect inhibition. No inhibition of respiration at elevated CO₂ was observed at the highest growth temperature (30°C) in either species. CO₂ efflux increased with measurement and growth temperature for M. sativa at both CO₂ concentrations; however, CO₂ efflux in D. glomerata showed complete acclimation to growth temperature. Stimulation of leaf area and weight by elevated CO₂ levels declined with growth temperature in both species. Data from the present study suggest that both direct and indirect inhibition of respiration are possible with future increases in atmospheric CO₂, and that the degree of each type of respiratory inhibition is a function of growth temperature.     

Atmospheric CO2 concentration does not directly affect leaf respiration in bean or poplar

It is a matter of debate if there is a direct (short‐term) effect of elevated atmospheric CO₂ concentration (C_a) on plant respiration in the dark. When C_a doubles, some authors found no (or only minor) changes in dark respiration, whereas most studies suggest a respiratory inhibition of 15–20%. The present study shows that the measurement artefacts – particularly leaks between leaf chamber gaskets and leaf surface, CO₂ memory and leakage effects of gas exchange systems as well as the water vapour (‘water dilution’) effect on DCO₂ measurement caused by transpiration – may result in larger errors than generally discussed. A gas exchange system that was used in three different ways – as a closed system in which C_a increased continuously from 200 to 4200 mmol (CO₂) mol^‐1 (air) due to respiration of the enclosed leaf; as an intermittently closed system that was repeatedly closed and opened during C_a periods of either 350 or 2000 mmol mol^‐1, and as an open system in which C_a varied between 350 and 2000 mmol mol^‐1– is described. In control experiments (with an empty leaf chamber), the respective system characteristics were evaluated carefully. When all relevant system parameters were taken into account, no effects of short‐term changes in CO₂ on dark CO₂ efflux of bean and poplar leaves were found, even when C_a increased to 4200 mmol mol^‐1. It is concluded that the leaf respiration of bean and poplar is not directly inhibited by elevated atmospheric CO₂.     

Elevated atmospheric CO2 increases fine root production, respiration, rhizosphere respiration and soil CO2 efflux in Scots pine seedlings

In this study, we investigated the impact of elevated atmospheric CO₂ (ambient + 350 μmol mol^–1) on fine root production and respiration in Scots pine (Pinus sylvestris L.) seedlings. After six months exposure to elevated CO₂, root production measured by root in-growth bags, showed significant increases in mean total root length and biomass, which were more than 100% greater compared to the ambient treatment. This increased root length may have lead to a more intensive soil exploration. Chemical analysis of the roots showed that the roots in the elevated treatment accumulated more starch and had a lower C/N-ratio. Specific root respiration rates were significantly higher in the elevated treatment and this was probably attributed to increased nitrogen concentrations in the roots. Rhizospheric respiration and soil CO₂ efflux were also enhanced in the elevated treatment. These results clearly indicate that under elevated atmospheric CO₂ root production and development in Scots pine seedlings is altered and respiratory carbon losses through the root system are increased.     

Acclimation of photosynthesis and respiration to elevated atmospheric CO2 in two Scrub Oaks

Abstract For two species of oak, we determined whether increasing atmospheric CO₂ concentration (C_a) would decrease leaf mitochondrial respiration (R) directly, or indirectly owing to their growth in elevated C_a, or both. In particular, we tested whether acclimatory decreases in leaf‐Rubisco content in elevated C_a would decrease R associated with its maintenance. This hypothesis was tested in summer 2000 on sun and shade leaves of Quercus myrtifolia Willd. and Quercus geminata Small. We also measured R on five occasions between summer 1999 and 2000 on leaves of Q. myrtifolia. The oaks were grown in the field for 4 years, in either current ambient or elevated (current ambient + 350 µmol mol^?1) C_a, in open‐top chambers (OTCs). For Q. myrtifolia, an increase in C_a from 360 to 710 µmol mol^?1 had no direct effect on R at any time during the year. In April 1999, R in young Q. myrtifolia leaves was significantly higher in elevated C_a—the only evidence for an indirect effect of growth in elevated C_a. Leaf R was significantly correlated with leaf nitrogen (N) concentration for the sun and shade leaves of both the species of oak. Acclimation of photosynthesis in elevated C_a significantly reduced maximum RuBP‐saturated carboxylation capacity (V_{c max}) for both the sun and shade leaves of only Q. geminata. However, we estimated that only 11–12% of total leaf N was invested in Rubisco; consequently, acclimation in this plant resulted in a small effect on N and an insignificant effect on R. In this study measurements of respiration and photosynthesis were made on material removed from the field; this procedure had no effect on gas exchange properties. The findings of this study were applicable to R expressed either per unit leaf area or unit dry weight, and did not support the hypothesis that elevated C_a decreases R directly, or indirectly owing to acclimatory decreases in Rubisco content.     

Growth and maintenance respiration in stems of Quercus alba after four years of CO2 enrichment

Atmospheric CO₂ enrichment is increasingly being reported to inhibit leaf and whole-plant respiration. It is not known, however, whether this response is unique to foliage or whether woody-tissue respiration might be affected as well. This was examined for mid-canopy stem segments of white oak (Quercus alba L.) trees that had been grown in open-top field chambers and exposed to either ambient or ambient + 300 µmol mol^?1 CO₂ over a 4-year period. Stem respiration measurements were made throughout 1992 by using an infrared gas analyzer and a specially designed in situ cuvette. Rates of woody-tissue respiration were similar between CO₂ treatments prior to leaf initiation and after leaf senescence, but were several fold greater for saplings grown at elevated concentrations of CO₂ during much of the growing season. These effects were most evident on 7 July when stem respiration rates for trees exposed to elevated CO₂ concentrations were 7.25 compared to 3.44 µmol CO₂ m^?2 s^?1 for ambient-grown saplings. While other explanations must be explored, greater rates of stem respiration for saplings grown at elevated CO₂ concentrations were consistent with greater rates of stem growth and more stem-wood volume present at the time of measurement. When rates of stem growth were at their maximum (7 July to 3 August), growth respiration accounted for about 80 to 85% of the total respiratory costs of stems at both CO₂ treatments, while 15 to 20% supported the costs of stem-wood maintenance. Integrating growth and maintenance respiration throughout the season, taking into account treatment differences in stem growth and volume, indicated that there were no significant effects of elevated CO₂ concentration on either respiratory process. Quantitative estimates that could be used in modeling the costs of woody-tissue growth and maintenance respiration are provided.     

Photosynthetic downregulation in leaves of the Japanese white birch grown under elevated CO2 concentration does not change their temperature‐dependent susceptibility to photoinhibition

To determine the effects of elevated CO₂ concentration ([CO₂]) on the temperature‐dependent photosynthetic properties, we measured gas exchange and chlorophyll fluorescence at various leaf temperatures (15, 20, 25, 30, 35 and 40°C) in 1‐year‐old seedlings of the Japanese white birch (Betula platyphylla var. japonica), grown in a phytotron under natural daylight at two [CO₂] levels (ambient: 400 µmol mol^?1 and elevated: 800 µmol mol^?1) and limited N availability (90 mg N plant^?1). Plants grown under elevated [CO₂] exhibited photosynthetic downregulation, indicated by a decrease in the carboxylation capacity of Rubisco. At temperatures above 30°C, the net photosynthetic rates of elevated‐CO₂‐grown plants exceeded those grown under ambient [CO₂] when compared at their growth [CO₂]. Electron transport rates were significantly lower in elevated‐CO₂‐grown plants than ambient‐CO₂‐grown ones at temperatures below 25°C. However, no significant difference was observed in the fraction of excess light energy [(1 ? q_P)× F_v′/F_m′] between CO₂ treatments across the temperature range. The quantum yield of regulated non‐photochemical energy loss was significantly higher in elevated‐CO₂‐grown plants than ambient, when compared at their respective growth [CO₂] below 25°C. These results suggest that elevated‐CO₂‐induced downregulation might not exacerbate the temperature‐dependent susceptibility to photoinhibition, because reduced energy consumption by electron transport was compensated for by increased thermal energy dissipation at low temperatures.     

The influence of elevated temperature,elevated atmospheric CO2 concentration and water stress on net photosynthesis of loblolly pine (Pinus taeda L.) at northern,central and southern sites in its native range

We investigated the effect of elevated [CO₂] (700 μmol mol^?1), elevated temperature (+2 °C above ambient) and decreased soil water availability on net photosynthesis (A_net) and water relations of one‐year old potted loblolly pine (Pinus taeda L.) seedlings grown in treatment chambers with high fertility at three sites along a north‐south transect covering a large portion of the species native range. At each location (Blairsville, Athens and Tifton, GA) we constructed four treatment chambers and randomly assigned each chamber one of four treatments: ambient [CO₂] and ambient temperature, elevated [CO₂] and ambient temperature, ambient [CO₂] and elevated temperature, or elevated [CO₂] and elevated temperature. Within each chamber half of the seedlings were well watered and half received much less water (1/4 that of the well watered). Measurements of net photosynthesis (A_net), stomatal conductance (g_s), leaf water potential and leaf fluorescence were made in June and September, 2008. We observed a significant increase in A_net in response to elevated [CO₂] regardless of site or temperature treatment in June and September. An increase in air temperature of over 2 °C had no significant effect on A_net at any of the sites in June or September despite over a 6 °C difference in mean annual temperature between the sites. Decreased water availability significantly reduced A_net in all treatments at each site in June. The effects of elevated [CO₂] and temperature on g_s followed a similar trend. The temperature, [CO₂] and water treatments did not significantly affect leaf water potential or chlorophyll fluorescence. Our findings suggest that predicted increases in [CO₂] will significantly increase A_net, while predicted increases in air temperature will have little effect on A_net across the native range of loblolly pine. Potential decreases in precipitation will likely cause a significant reduction in A_net, though this may be mitigated by increased [CO₂].     

First-year growth response of trees in an intact forest exposed to elevated CO2

Although elevated atmospheric CO₂ has been shown to increase growth of tree seedlings and saplings, the response of intact forest ecosystems and established trees is unclear. We report results from the first large-scale experimental system designed to study the effects of elevated CO₂ on an intact forest with the full complement of species interactions and environmental stresses. During the first year of exposure to ^ 1.5 Ë ambient CO₂, canopy loblolly pine (Pinus taeda, L.) trees increased basal area growth rate by 24% but understorey trees of loblolly pine, sweetgum (Liquidambar styraciflua L.), and red maple (Acer rubrum L.) did not respond. Winged elm (Ulmus alata Michx.) had a marginally significant increase in growth rate (P = 0.069). These data suggest that this ecosystem has the capacity to respond immediately to a step increase in atmospheric CO₂; however, as exposure time increases, nutrient limitations may reduce this initial growth stimulation.     

Effects of elevated atmospheric CO2 on fine root production and activity in an intact temperate forest ecosystem

We investigated the effects of elevated atmospheric CO₂ concentrations (ambient + 200 ppm) on fine root production and soil carbon dynamics in a loblolly pine (Pinus taeda) forest subject to free‐air CO₂ enrichment (FACE) near Durham, NC (USA). Live fine root mass (LFR) showed less seasonal variation than dead fine root mass (DFR), which was correlated with seasonal changes in soil moisture and soil temperature. LFR mass increased significantly (by 86%) in the elevated CO₂ treatment, with an increment of 37 g(dry weight) m^?2 above the control plots after two years of CO₂ fumigation. There was no long‐term increment in DFR associated with elevated CO₂, but significant seasonal accumulations of DFR mass occurred during the summer of the second year of fumigation. Overall, root net primary production (RNPP) was not significantly different, but annual carbon inputs were 21.7 gC m^?2 y^?1 (68%) higher in the elevated CO₂ treatment compared to controls. Specific root respiration was not altered by the CO₂ treatment during most of the year; however, it was significantly higher by 21% and 13% in September 1997 and May 1998, respectively, in elevated CO₂. We did not find statistically significant differences in the C/N ratio of the root tissue, root decomposition or phosphatase activity in soil and roots associated with the treatment. Our data show that the early response of a loblolly pine forest ecosystem subject to CO₂ enrichment is an increase in its fine root population and a trend towards higher total RNPP after two years of CO₂ fumigation.     

Effects of age and ontogeny on photosynthetic responses of a determinate annual plant to elevated CO2 concentrations

Plant responses to elevated CO₂ concentrations ([CO₂]) may be regulated by both accelerated ontogeny and allocational changes as plants grow. However, isolating ontogeny‐related effects from age‐related effects are difficult because these factors are often confounded. In this study, the roles of age and ontogeny in photosynthetic responses to elevated [CO₂] were examined on Xanthium strumarium L. grown at ambient (365 µmol mol^?1) and elevated (730 µmol mol^?1) [CO₂]. To examine age‐related effects, six cohorts were planted at 5‐day intervals. To examine ontogeny‐related effects, all plants were induced to flower at the same time; ontogeny in Xanthium is relatively unaffected by growth in elevated [CO₂]. Growth in elevated [CO₂] increased net photosynthetic rates by approximately 30% throughout vegetative growth (i.e. active carbohydrate sinks), approximately 10% during flowering (i.e. minimal sink activity), and approximately 20% during fruit production (i.e. active sinks). At the harvest, the ratio of source to sink tissue significantly decreased with increasing plant age and was correlated with leaf soluble sugar concentration. Leaf soluble sugar concentration was negatively correlated with the relative photosynthetic response to elevated [CO₂]. These results suggest that age and ontogeny independently affect photosynthetic responses to elevated [CO₂] and the effects are mediated by reversible changes in source : sink balance.     

Does elevated atmospheric carbon dioxide affect internal nitrogen allocation in the temperate trees Alnus glutinosa and Pinus sylvestris?

Nitrogen‐fixing plant species growing in elevated atmospheric carbon dioxide concentration ([CO₂]) should be able to maintain a high nutrient supply and thus grow better than other species. This could in turn engender changes in internal storage of nitrogen (N) and remobilisation during periods of growth. In order to investigate this one‐year‐old‐seedlings of Alnus glutinosa (L.) Gaertn and Pinus sylvestris (L.) were exposed to ambient [CO₂] (350 µ mol mol ^{? 1}) and elevated [CO₂] (700 µ mol mol ^{? 1}) in open top chambers (OTCs). This constituted a main comparison between a nitrogen‐fixing tree and a nonfixer, but also between an evergreen and a deciduous species. The trees were supplied with a full nutrient solution and in July 1994, the trees were given a pulse of ¹⁵N‐labelled fertiliser. The allocation of labelled N to different tissues (root, leaves, shoots) was followed from September 1994 to June 1995. While N allocation in P. sylvestris (Scots pine) showed no response to elevated [CO₂], A. glutinosa (common alder) responded in several ways. During the main nutrient uptake period of June–August, trees grown in elevated [CO₂] had a higher percentage of N derived from labelled fertiliser than trees grown in ambient [CO₂]. Remobilisation of labelled N for spring growth was significantly higher in A. glutinosa grown in elevated [CO₂] (9.09% contribution in ambient vs. 29.93% in elevated [CO₂] leaves). Exposure to elevated [CO₂] increased N allocation to shoots in the winter of 1994–1995 (12.66 mg in ambient vs. 43.42 mg in elevated 1993 shoots; 4.81 mg in ambient vs. 40.00 mg in elevated 1994 shoots). Subsequently significantly more labelled N was found in new leaves in April 1995. These significant increases in movement of labelled N between tissues could not be explained by associated increases in tissue biomass, and there was a significant shift in C‐biomass allocation away from the leaves towards the shoots (all above‐ground material except leaves) in A. glutinosa. This experiment provides the first evidence that not only are shifts in C allocation affected by elevated [CO₂], but also internal N resource utilisation in an N₂‐fixing tree.     

Photosynthetic responses to elevated CO2and O3 in field-grown potato(Solanum tuberosum)

Potato plants (Solanum tuberosum L. cv. Bintje) were grown to maturity in open-top chambers under three carbon dioxide (CO₂; ambient and 24 h d⁻¹ seasonal mean concentrations of 550 and 680 μmol mol⁻¹) and two ozone levels (O₃; ambient and an 8 h d⁻¹ seasonal mean of 50 nmol mol⁻¹). Chlorophyll content, photosynthetic characteristics, and stomatal responses were determined to test the hypothesis that elevated atmospheric CO₂ may alleviate the damaging influence of O₃ by reducing uptake by the leaves. Elevated O₃ had no detectable effect on photosynthetic characteristics, leaf conductance, or chlorophyll content, but did reduce SPAD values for leaf 15, the youngest leaf examined. Elevated CO₂ also reduced SPAD values for leaf 15, but not for older leaves; destructive analysis confirmed that chlorophyll content was decreased. Leaf conductance was generally reduced by elevated CO₂, and declined with time in the youngest leaves examined, as did assimilation rate (A). A generally increased under elevated CO₂, particularly in the older leaves during the latter stages of the season, thereby increasing instantaneous transpiration efficiency. Exposure to elevated CO₂ and/or O₃ had no detectable effect on dark-adapted fluorescence, although the values decreased with time. Analysis of the relationships between assimilation rate and intercellular CO₂ concentration and photosynthetically active photon flux density showed there was initially little treatment effect on CO₂-saturated assimilation rates for leaf 15. However, the values for plants grown under 550 μmol mol⁻¹ CO₂ were subsequently greater than in the ambient and 680 μmol mol⁻¹ treatments, although the beneficial influence of the former treatment declined sharply towards the end of the season. Light-saturated assimilation was consistently greater under elevated CO₂, but decreased with time in all treatments. The values decreased sharply when leaves grown under elevated CO₂ were measured under ambient CO₂, but increased when leaves grown under ambient CO₂ were examined under elevated CO₂. The results obtained indicate that, although elevated CO₂ initially increased assimilation and growth, these beneficial effects were not necessarily sustained to maturity as a result of photosynthetic acclimation and the induction of earlier senescence.     

Gypsy moth feeding in the canopy of a CO2-enriched mature forest

Rising atmospheric carbon dioxide (CO₂) concentration is expected to change plant tissue quality with important implications for plant–insect interactions. Taking advantage of canopy access by a crane and long‐term CO₂ enrichment (530 μ mol mol^?1) of a natural old‐growth forest (web‐free air carbon dioxide enrichment), we studied the responses of a generalist insect herbivore feeding in the canopy of tall trees. We found that relative growth rates (RGR) of gypsy moth (Lymantria dispar) were reduced by 30% in larvae fed on high CO₂‐exposed Quercus petraea, but increased by 29% when fed on high CO₂‐grown Carpinus betulus compared with control trees at ambient CO₂ (370 μ mol mol^?1). In Fagus sylvatica, there was a nonsignificant trend for reduced RGR under elevated CO₂. Tree species‐specific changes in starch to nitrogen ratio, water, and the concentrations of proteins, condensed and hydrolyzable tannins in response to elevated CO₂ were identified to correlate with altered RGR of gypsy moth larvae. Our data suggest that rising atmospheric CO₂ will have strong species‐specific effects on leaf chemical composition of canopy trees in natural forests leading to contrasting responses of herbivores such as those reported here. A future change in host tree preference seems likely with far‐ranging consequences for forest community dynamics.     

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

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

Dynamic changes of canopy-scale mesophyll conductance to CO₂ diffusion of sunflower as affected by CO₂ concentration and abscisic acid

Leaf‐level measurements have shown that mesophyll conductance (g_m) can vary rapidly in response to CO₂ and other environmental factors, but similar studies at the canopy‐scale are missing. Here, we report the effect of short‐term variation of CO₂ concentration on canopy‐scale g_m and other CO₂ exchange parameters of sunflower (Helianthus annuus L.) stands in the presence and absence of abscisic acid (ABA) in their nutrient solution. g_m was estimated from gas exchange and on‐line carbon isotope discrimination (Δ_obs) in a ¹³CO₂/¹²CO₂ gas exchange mesocosm. The isotopic contribution of (photo)respiration to stand‐scale Δ_obs was determined with the experimental approach of Tcherkez et al. Without ABA, short‐term exposures to different CO₂ concentrations (C_a 100 to 900 µmol mol^?1) had little effect on canopy‐scale g_m. But, addition of ABA strongly altered the CO₂‐response: g_m was high (approx. 0.5 mol CO₂ m^?2 s^?1) at C_a < 200 µmol mol^?1 and decreased to <0.1 mol CO₂ m^?2 s^?1 at C_a >400 µmol mol^?1. In the absence of ABA, the contribution of (photo)respiration to stand‐scale Δ_obs was high at low C_a (7.2‰) and decreased to <2‰ at C_a > 400 µmol mol^?1. Treatment with ABA halved this effect at all C_a.     

Regulation of hormonal responses of sweet pepper as affected by salinity and elevated CO2 concentration

This study examines the extent to which the predicted CO₂‐protective effects on the inhibition of growth, impairment of photosynthesis and nutrient imbalance caused by saline stress are mediated by an effective adaptation of the endogenous plant hormonal balance. Therefore, sweet pepper plants (Capsicum annuum, cv. Ciclón) were grown at ambient or elevated [CO₂] (400 or 800 µmol mol^–1) with a nutrient solution containing 0 or 80 mM NaCl. The results show that, under saline conditions, elevated [CO₂] increased plant dry weight, leaf area, leaf relative water content and net photosynthesis compared with ambient [CO₂], whilst the maximum potential quantum efficiency of photosystem II was not modified. In salt‐stressed plants, elevated [CO₂] increased leaf NO₃^– concentration and reduced Cl^– concentration. Salinity stress induced ABA accumulation in the leaves but it was reduced in the roots at high [CO₂], being correlated with the stomatal response. Under non‐stressed conditions, IAA was dramatically reduced in the roots when high [CO₂] was applied, which resulted in greater root DW and root respiration. Additionally, the observed high CK concentration in the roots (especially tZR) could prevent downregulation of photosynthesis at high [CO₂], as the N level in the leaves was increased compared with the ambient [CO₂], under salt‐stress conditions. These results demonstrate that the hormonal balance was altered by the [CO₂], which resulted in significant changes at the growth, gas exchange and nutritional levels.     

Elevated atmospheric CO2 in open top chambers increases net nitrification and potential denitrification

The control of soil nitrogen (N) availability under elevated atmospheric CO₂ is central to predicting changes in ecosystem carbon (C) storage and primary productivity. The effects of elevated CO₂ on belowground processes have so far attracted limited research and they are assumed to be controlled by indirect effects through changes in plant physiology and chemistry. In this study, we investigated the effects of a 4‐year exposure to elevated CO₂ (ambient + 400 µmol mol^?1) in open top chambers under Scots pine (Pinus sylvestris L) seedlings on soil microbial processes of nitrification and denitrification. Potential denitrification (DP) and potential N₂O emissions were significantly higher in soils from the elevated CO₂ treatment, probably regulated indirectly by the changes in soil conditions (increased pH, C availability and NO₃^– production). Net N mineralization was mainly accounted for by nitrate production. Nitrate production was significantly larger for soil from the elevated CO₂ treatment in the field when incubated in the laboratory under elevated CO₂ (increase of 100%), but there was no effect when incubated under ambient CO₂. Net nitrate production of the soil originating from the ambient CO₂ treatment in the field was not influenced by laboratory incubation conditions. These results indicate that a direct effect of elevated atmospheric CO₂ on soil microbial processes might take place. We hypothesize that physiological adaptation or selection of nitrifiers could occur under elevated CO₂ through higher soil CO₂ concentrations. Alternatively, lower microbial NH₄ assimilation under elevated CO₂ might explain the higher net nitrification. We conclude that elevated atmospheric CO₂ has a major direct effect on the soil microbial processes of nitrification and denitrification despite generally higher soil CO₂ concentrations compared to atmospheric concentrations.     

N2 fixation by Acacia species increases under elevated atmospheric CO2

In the present study the effect of elevated CO₂ on growth and nitrogen fixation of seven Australian Acacia species was investigated. Two species from semi‐arid environments in central Australia (Acacia aneura and A. tetragonophylla) and five species from temperate south‐eastern Australia (Acacia irrorata, A. mearnsii, A. dealbata, A. implexa and A. melanoxylon) were grown for up to 148 d in controlled greenhouse conditions at either ambient (350 µmol mol^?1) or elevated (700 µmol mol^?1) CO₂ concentrations. After establishment of nodules, the plants were completely dependent on symbiotic nitrogen fixation. Six out of seven species had greater relative growth rates and lower whole plant nitrogen concentrations under elevated versus normal CO₂. Enhanced growth resulted in an increase in the amount of nitrogen fixed symbiotically for five of the species. In general, this was the consequence of lower whole‐plant nitrogen concentrations, which equate to a larger plant and greater nodule mass for a given amount of nitrogen. Since the average amount of nitrogen fixed per unit nodule mass was unaltered by atmospheric CO₂, more nitrogen could be fixed for a given amount of plant nitrogen. For three of the species, elevated CO₂ increased the rate of nitrogen fixation per unit nodule mass and time, but this was completely offset by a reduction in nodule mass per unit plant mass.