The stomatal CO<Subscript>2</Subscript> proxy does not saturate at high atmospheric CO<Subscript>2</Subscript> concentrations: evidence from stomatal index responses of Araucariaceae conifers

The inverse relationship between the number of stomata on a leaf surface and the atmospheric carbon dioxide concentration ([CO₂]) in which the leaf developed allows plants to optimise water-use efficiency (WUE), but it also permits the use of fossil plants as proxies of palaeoatmospheric [CO₂]. The ancient conifer family Araucariaceae is often represented in fossil floras and may act as a suitable proxy of palaeo-[CO₂], yet little is known regarding the stomatal index (SI) responses of extant Araucariaceae to [CO₂]. Four Araucaria species (Araucaria columnaris, A. heterophylla, A. angustifolia and A. bidwillii) and Agathis australis displayed no significant relationship in SI to [CO₂] below current ambient levels (~380 ppm). However, representatives of the three extant genera within the Araucariaceae (A. bidwillii, A. australis and Wollemia nobilis) all exhibited significant reductions in SI when grown in atmospheres of elevated [CO₂] (1,500 ppm). Stomatal conductance was reduced and WUE increased when grown under elevated [CO₂]. Stomatal pore length did not increase alongside reduced stomatal density (SD) and SI in the three araucariacean conifers when grown at elevated [CO₂]. These pronounced SD and SI reductions occur at higher [CO₂] levels than in other species with more recent evolutionary origins, and may reflect an evolutionary legacy of the Araucariaceae in the high [CO₂] world of the Mesozoic Era. Araucariacean conifers may therefore be suitable stomatal proxies of palaeo-[CO₂] during periods of “greenhouse” climates and high [CO₂] in the Earth’s history.     

Effects of elevated CO<Subscript>2</Subscript> and nitrogen on wood structure related to water transport in seedlings of two deciduous broad-leaved tree species

Wood structure might be altered through the physiological responses to atmospheric carbon dioxide concentration ([CO₂]) and nitrogen (N) deposition. We investigated growth, water relations and wood structure of 1-year-old seedlings of two deciduous broad-leaved tree species, Quercus mongolica (oak, a ring-porous species) and Alnus hirsuta (alder, a diffuse-porous species and N₂–fixer), grown under a factorial combination of two levels of [CO₂] (36 and 72 Pa) and nitrogen supply (N; low and high) for 141 days in phytotron chambers. In oak, there was no significant effect of [CO₂] on wood structure, although elevated [CO₂] tended to decrease stomatal conductance (g _s) and increased water use efficiency regardless of the N treatment. However, high N supply increased root biomass and induced wider earlywood and larger vessels in the secondary xylem in stems, leading to increased hydraulic conductance. In alder, there was significant interactive effect of [CO₂] and N on vessel density, and high N supply increased the mean vessel area. Our results suggest that wood structures related to water transport were not markedly altered, although elevated [CO₂] induced changes in physiological parameters such as g _s and biomass allocation, and that N fertilization had more pronounced effects on non-N₂-fixing oak than on N₂-fixing alder.     

Effect of panicle removal on photosynthetic acclimation under elevated CO<Subscript>2</Subscript> in rice

To examine the role of sink size on photosynthetic acclimation under elevated atmospheric CO₂ concentrations ([CO₂]), we tested the effects of panicle-removal (PR) treatment on photosynthesis in rice (Oryza sativa L.). Rice was grown at two [CO₂] levels (ambient and ambient + 200 μmol mol⁻¹) throughout the growing season, and at full-heading stage, at half the plants, a sink-limitation treatment was imposed by the removal of the panicles. The PR treatment alleviated the reduction of green leaf area, the contents of chlorophyll (Chl) and Rubisco after the full-heading stage, suggesting delay of senescence. Nonetheless, elevated [CO₂] decreased photosynthesis (measured at current [CO₂]) of plants exposed to the PR treatment. No significant [CO₂] × PR interaction on photosynthesis was observed. The decrease of photosynthesis by elevated [CO₂] of plants was associated with decreased leaf Rubisco content and N content. Leaf glucose content was increased by the PR treatment and also by elevated [CO₂]. In conclusion, a sink-limitation in rice improved N status in the leaves, but this did not prevent the photosynthetic down-regulation under elevated [CO₂].     

Combination of elevated CO<Subscript>2</Subscript> concentration and elevated temperature and elevated temperature only promote photosynthesis of <Emphasis Type="Italic">Quercus mongolica</Emphasis> seedlings

One-year-old oak (Quercus mongolica Fisch.) seedlings were grown in growth chambers for 30 days to investigate the effects of the combination of elevated CO₂ concentration ([CO₂], 700 μmol/mol) and temperature (ambient T + 4°C) and only elevated temperature (ambient T +4°C) on leaf gas exchange, chlorophyll a fluorescence, and chlorophyll content. In the growth chambers, natural conditions of the Maoershan mountain regions of Heilongjiang Province (45–46°N, 127–128°E) of China for the average growth season were simulated. The results showed that the maximum net photosynthetic rate (P _Nmax) was ≈ 1.64 times greater at elevated temperature than at ambient temperature. The irradiance saturation point (I _s), apparent quantum yield (AQY), maximum photosystem II efficiency (F _v/F _m), and chlorophyll content significantly increased, while irradiance compensation point (I _c) was not affected by elevated temperature. The combination of elevated [CO₂] and temperature also significantly increased P _Nmax by approximately 34% but much lower than that under elevated temperature only. In the case of factor combination, dark respiration (R _d), I _c, F _v/F _m, and total chlorophyll content increased significantly, while I _s and AQY were not affected. Moreover, under elevated [CO₂] and temperature, R _d and I _c, F _v/F _m were significantly higher than under elevated temperature only. The results indicated that the combination of elevated [CO₂] and temperature expected in connection with the further global climate change may affect carbon storage of the coenotype of Q. mongolica in this region of China. This text was submitted by the authors in English.     

Stem CO<Subscript>2</Subscript> efflux of a <Emphasis Type="Italic">Populus nigra</Emphasis> stand: effects of elevated CO<Subscript>2</Subscript>, fertilization,and shoot size

To determine whether long-term growth in elevated atmospheric CO₂ concentration [CO₂] and nitrogen fertilization affects woody tissue CO₂ efflux, we measured stem CO₂ efflux as a function of temperature in three different size classes of shoots of Populus nigra L. (clone Jean Pourtet) on two occasions in 2004. Trees were growing in a short rotation coppice in ambient (370 μmol mol⁻¹) and elevated (550 μmol mol⁻¹, realised by a Free Air Carbon dioxide Enrichment system) [CO₂], and measurements were performed during the third growing season of the second rotation. Elevated CO₂ did not affect Q₁₀ or specific stem CO₂ efflux (E₁₀) of overall poplar shoots. The lack of any effect of N on stem CO₂ efflux indicated that nutrients were sufficient. Specific stem CO₂ efflux differed significantly between shoot sizes, emphasizing the importance of tree size when scaling-up respiration measurements to the stand level. Variation in stem CO₂ efflux could not be satisfactorily explained by temperature as the only driving variable. We hypothesize that transport of CO₂ with the sapflow might have confounded our results and could explain the high Q₁₀ values reported here. Predicting the respiratory carbon loss in a future elevated [CO₂] world must therefore move beyond the single-factor temperature dependent respiration model and involve multiple factors affecting stem CO₂ efflux rate.     

The effects of elevated atmospheric CO<Subscript>2</Subscript> and nitrogen amendments on subsurface CO<Subscript>2</Subscript> production and concentration dynamics in a maturing pine forest

Profiles of subsurface soil CO₂ concentration, soil temperature, and soil moisture, and throughfall were measured continuously during the years 2005 and 2006 in 16 locations at the free air CO₂ enrichment facility situated within a temperate loblolly pine (Pinus taeda L.) stand. Sampling at these locations followed a 4 by 4 replicated experimental design comprised of two atmospheric CO₂ concentration levels (ambient [CO₂]^a, ambient + 200 ppmv, [CO₂]^e) and two soil nitrogen (N) deposition levels (ambient, ambient + fertilization at 11.2 g_N m⁻² year⁻¹). The combination of these measurements permitted indirect estimation of belowground CO₂ production and flux profiles in the mineral soil. Adjacent to the soil CO₂ profiles, direct (chamber-based) measurements of CO₂ fluxes from the soil–litter complex were simultaneously conducted using the automated carbon efflux system. Based on the measured soil CO₂ profiles, neither [CO₂]^e nor N fertilization had a statistically significant effect on seasonal soil CO₂, CO₂ production, and effluxes from the mineral soil over the study period. Soil moisture and temperature had different effects on CO₂ concentration depending on the depth. Variations in CO₂ were mostly explained by soil temperature at deeper soil layers, while water content was an important driver at the surface (within the first 10 cm), where CO₂ pulses were induced by rainfall events. The soil effluxes were equal to the CO₂ production for most of the time, suggesting that the site reached near steady-state conditions. The fluxes estimated from the CO₂ profiles were highly correlated to the direct measurements when the soil was neither very dry nor very wet. This suggests that a better parameterization of the soil CO₂ diffusivity is required for these soil moisture extremes.     

Elevated CO<Subscript>2</Subscript> reduces stomatal and metabolic limitations on photosynthesis caused by salinity in <Emphasis Type="Italic">Hordeum vulgare</Emphasis>

The future environment may be altered by high concentrations of salt in the soil and elevated [CO₂] in the atmosphere. These have opposite effects on photosynthesis. Generally, salt stress inhibits photosynthesis by stomatal and non-stomatal mechanisms; in contrast, elevated [CO₂] stimulates photosynthesis by increasing CO₂ availability in the Rubisco carboxylating site and by reducing photorespiration. However, few studies have focused on the interactive effects of these factors on photosynthesis. To elucidate this knowledge gap, we grew the barley plant, Hordeum vulgare (cv. Iranis), with and without salt stress at either ambient or elevated atmospheric [CO₂] (350 or 700 μmol mol⁻¹ CO₂, respectively). We measured growth, several photosynthetic and fluorescence parameters, and carbohydrate content. Under saline conditions, the photosynthetic rate decreased, mostly because of stomatal limitations. Increasing salinity progressively increased metabolic (photochemical and biochemical) limitation; this included an increase in non-photochemical quenching and a reduction in the PSII quantum yield. When salinity was combined with elevated CO₂, the rate of CO₂ diffusion to the carboxylating site increased, despite lower stomatal and internal conductance. The greater CO₂ availability increased the electron sink capacity, which alleviated the salt-induced metabolic limitations on the photosynthetic rate. Consequently, elevated CO₂ partially mitigated the saline effects on photosynthesis by maintaining favorable biochemistry and photochemistry in barley leaves.     

Elevated CO<Subscript>2</Subscript> Concentration and Drought Stress Exert Opposite Effects on Plant Biomass,Nitrogen, and Phosphorus Allocation in <Emphasis Type="Italic">Bothriochloa ischaemum</Emphasis>

Increased concentrations of atmospheric carbon dioxide (CO₂) and drought stress have greatly influenced plant growth, the status of nitrogen (N) and phosphorus (P), and N:P ratios. We identified the plant biomass, N and P distributional patterns, and N:P stoichiometry of a grass species on the Loess Plateau in China under elevated CO₂ concentration and drought stress conditions. Bothriochloa ischaemum, a C4 perennial herbaceous grass, was grown in pots at CO₂ concentrations of 400 (ambient) and 800 (elevated) μmol mol^?1 and at 60 ± 5 and 40 ± 5 % of field capacity. The elevated CO₂ concentration significantly increased plant total biomass, N concentration, N and P content, allocation of biomass to roots, and allocation of N to shoots, and increased the N:P ratios of whole plants and the shoots, especially under well-watered conditions. Drought stress significantly decreased plant biomass and plant N and P content, especially under elevated CO₂. Drought stress decreased the N:P ratios, but was only significant in the roots under ambient CO₂. Drought stress may attenuate the stimulation of plant growth and N and P acquisition by CO₂ enrichment, and projected elevated CO₂ concentrations may partially offset the negative effects of increased drought by increasing the assimilation of N and P.     

Photosynthesis and fluorescence responses of C<Subscript>4</Subscript> plant <Emphasis Type="Italic">Andropogon gerardii</Emphasis> acclimated to temperature and carbon dioxide

Increase in both atmospheric CO₂ concentration [CO₂] and associated warming are likely to alter Earths’ carbon balance and photosynthetic carbon fixation of dominant plant species in a given biome. An experiment was conducted in sunlit, controlled environment chambers to determine effects of atmospheric [CO₂] and temperature on net photosynthetic rate (P _N) and fluorescence (F) in response to internal CO₂ concentration (C _i) and photosynthetically active radiation (PAR) of the C₄ species, big bluestem (Andropogon gerardii Vitman). Ten treatments were comprised of two [CO₂] of 360 (ambient, AC) and 720 (elevated, EC) μmol mol⁻¹ and five day/night temperature of 20/12, 25/17, 30/22, 35/27 and 40/32 °C. Treatments were imposed from 15 d after sowing (DAS) through 130 DAS. Both F-P _N/C _i and F-P _N/PAR response curves were measured on top most fully expanded leaves between 55 and 75 DAS. Plants grown in EC exhibited significantly higher CO₂-saturated net photosynthesis (P _sat), phosphoenolpyruvate carboxylase (PEPC) efficiency, and electron transport rate (ETR). At a given [CO₂], increase in temperature increased P _sat, PEPC efficiency, and ETR. Plants grown at EC did not differ for dark respiration rate (R _D), but had significantly higher maximum photosynthesis (P _max) than plants grown in AC. Increase in temperature increased Pmax, R _D, and ETR, irrespective of the [CO₂]. The ability of PEPC, ribulose-1,5-bisphosphate carboxylase/oxygenase, and photosystem components, derived from response curves to tolerate higher temperatures (>35 °C), particularly under EC, indicates the ability of C₄ species to sustain photosynthetic capacity in future climates.     

Decades-long effects of high CO<Subscript>2</Subscript> concentration on soil nitrogen dynamics at a natural CO<Subscript>2</Subscript> spring

The effects of high atmospheric CO₂ concentration ([CO₂]) on ecosystem processes have been explored using temporal facilities such as open-top-chambers and free-air CO₂ enrichment. However, the effects of high [CO₂] on soil properties takes decades and may not be captured by short-term experiments. Natural CO₂ springs provide a unique opportunity to study the long-term effects of high [CO₂]. In this study, we investigated soil properties at a natural CO₂ spring. We found that the amounts of total carbon (C) and nitrogen (N) stored in the soil at the high [CO₂] site exceeded those in the reference site by 60 and 30%, respectively. The effects of high [CO₂] were large in the upper slope position where the canopy openness was high and plants grew faster, but no effects were detected in the lowest position where the canopy openness was lower (half of that at the upper slope position). In contrast, effects of high [CO₂] on soil N dynamics, such as N mineralization and nitrification rates, did not exhibit a slope gradient. This suggests that effects of high [CO₂] differed among soil stoichiometric characteristics and N dynamics. These complicated effects of high [CO₂] imply that the future effects of high [CO₂] on ecosystems could vary widely in conjunction with environmental conditions such as light availability and/or topographic conditions.     

Elevated CO<Subscript>2</Subscript>, litter chemistry,and decomposition: a synthesis

The results of published and unpublished experiments investigating the impacts of elevated [CO₂] on the chemistry of leaf litter and decomposition of plant tissues are summarized. The data do not support the hypothesis that changes in leaf litter chemistry often associated with growing plants under elevated [CO₂] have an impact on decomposition processes. A meta-analysis of data from naturally senesced leaves in field experiments showed that the nitrogen (N) concentration in leaf litter was 7.1% lower in elevated [CO₂] compared to that in ambient [CO₂]. This statistically significant difference was: (1) usually not significant in individual experiments, (2) much less than that often observed in green leaves, and (3) less in leaves with an N concentration indicative of complete N resorption. Under ideal conditions, the efficiency with which N is resorbed during leaf senescence was found not to be altered by CO₂ enrichment, but other environmental influences on resorption inevitably increase the variability in litter N concentration. Nevertheless, the small but consistent decline in leaf litter N concentration in many experiments, coupled with a 6.5% increase in lignin concentration, would be predicted to result in a slower decomposition rate in CO₂-enriched litter. However, across the assembled data base, neither mass loss nor respiration rates from litter produced in elevated [CO₂] showed any consistent pattern or differences from litter grown in ambient [CO₂]. The effects of [CO₂] on litter chemistry or decomposition were usually smallest under experimental conditions similar to natural field conditions, including open-field exposure, plants free-rooted in the ground, and complete senescence. It is concluded that any changes in decomposition rates resulting from exposure of plants to elevated [CO₂] are small when compared to other potential impacts of elevated [CO₂] on carbon and N cycling. Reasons for experimental differences are considered, and recommendations for the design and execution of decomposition experiments using materials from CO₂-enrichment experiments are outlined.     

Stomatal conductance and not stomatal density determines the long-term reduction in leaf transpiration of poplar in elevated CO<Subscript>2</Subscript>

Using a free-air CO₂ enrichment (FACE) experiment, poplar trees (Populus × euramericana clone I214) were exposed to either ambient or elevated [CO₂] from planting, for a 5-year period during canopy development, closure, coppice and re-growth. In each year, measurements were taken of stomatal density (SD, number mm⁻²) and stomatal index (SI, the proportion of epidermal cells forming stomata). In year 5, measurements were also taken of leaf stomatal conductance (g _s, μmol m⁻² s⁻¹), photosynthetic CO₂ fixation (A, mmol m⁻² s⁻¹), instantaneous water-use efficiency (A/E) and the ratio of intercellular to atmospheric CO₂ (C_i:C_a). Elevated [CO₂] caused reductions in SI in the first year, and in SD in the first 2 years, when the canopy was largely open. In following years, when the canopy had closed, elevated [CO₂] had no detectable effects on stomatal numbers or index. In contrast, even after 5 years of exposure to elevated [CO₂], g _s was reduced, A/E was stimulated, and C_i:C_a was reduced relative to ambient [CO₂]. These outcomes from the long-term realistic field conditions of this forest FACE experiment suggest that stomatal numbers (SD and SI) had no role in determining the improved instantaneous leaf-level efficiency of water use under elevated [CO₂]. We propose that altered cuticular development during canopy closure may partially explain the changing response of stomata to elevated [CO₂], although the mechanism for this remains obscure.     

Down-regulation of Rubisco activity under combined increases of CO<Subscript>2</Subscript> and temperature minimized by changes in Rubisco k<Subscript>cat</Subscript> in wheat

Increases in growth temperature have been observed to affect photosynthesis differently under long-term exposure to ambient- and twice ambient-air CO₂ concentrations. This study investigates the causes of this interaction in wheat (Triticum aestivum L.) grown in the field over two consecutive years under temperature gradient chambers in ambient (370 μmol mol⁻¹) or elevated (700 μmol mol⁻¹) atmospheric CO₂ concentrations and at ambient or ambient +4°C temperatures, with either a low or a high nitrogen supply. The photosynthesis-internal CO₂ response curves and the activity, activation state, k_cat and amount of Ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) were measured, as well as the soluble protein concentration in flag leaves at ear emergence and 8–15 days after anthesis. A high nitrogen supply increased V_cmax, the Rubisco amount and activity and soluble protein contents, but did not significantly change the Rubisco k_cat. Both elevated CO₂ and above ambient temperatures had negative effects on V_cmax and Rubisco activity, but at elevated CO₂, an increase in temperature did not decrease V_cmax or Rubisco activity in relation to ambient temperature. The amounts of Rubisco and soluble protein decreased with elevated CO₂ and temperature. The negative impact of elevated CO₂ on Rubisco properties was somewhat counteracted at elevated temperatures by an increase in k_cat. This effect can diminish the detrimental effects on photosynthesis of combined increases of CO₂ and temperature.     

Elevated CO<Subscript>2</Subscript> ameliorated oxidative stress induced by elevated O<Subscript>3</Subscript> in <Emphasis Type="Italic">Quercus mongolica</Emphasis>

Using open top chambers, the effects of elevated O₃ (80 nmol mol⁻¹) and elevated CO₂ (700 μmol mol⁻¹), alone and in combination, were studied on young trees of Quercus mongolica. The results showed that elevated O₃ increased malondialdehyde content and decreased photosynthetic rate after 45 days of exposure, and prolonged exposure (105 days) induced significant increase in electrolyte leakage and reduction of chlorophyll content. All these changes were alleviated by elevated CO₂, indicating that oxidative stress on cell membrane and photosynthesis was ameliorated. After 45 days of exposure, elevated O₃ stimulated activities of superoxide dismutase (SOD, EC 1.15.1.1) and ascorbate peroxidase (APX, EC 1.11.1.11), but the stimulation was dampened under elevated CO₂ exposure. Furthermore, ascorbate (AsA) and total phenolics contents were not higher in the combined gas treatment than those in elevated O₃ treatment. It indicates that the protective effect of elevated CO₂ against O₃ stress was achieved hardly by enhancing ROS scavenging ability after 45 days of exposure. After 105 days of exposure, elevated O₃ significantly decreased activities of SOD, catalase (CAT, EC 1.11.1.6) and APX and AsA content. Elevated CO₂ suppressed the O₃-induced decrease, which could ameliorate the oxidative stress in some extent. In addition, elevated CO₂ increased total phenolics content in the leaves both under ambient O₃ and elevated O₃ exposure, which might contribute to the protection against O₃-induced oxidative stress as well.     

CO<Subscript>2</Subscript> enrichment,nitrogen fertilization and development of freezing tolerance in Norway spruce

Plant growth and adaptation to cold and freezing temperatures in a CO₂-enriched atmosphere have received little attention despite the predicted effects of elevated CO₂ on plant distribution and productivity. Norway spruce [Picea abies (L.) Karst.] seedlings from latitudinally distinct seed sources (66°N and 60°N) were grown for one simulated growth season under controlled conditions in an atmosphere enriched in CO₂ (70 Pa) and at ambient CO₂ (40 Pa), combined factorially with low (3.6 mM) or high (15.7 mM) concentrations of nitrogen fertilization. There was a clear difference between the two provenances in height growth, in the timing of bud set, and in freezing tolerance. Nitrogen fertilization increased height growth in both provenances, while CO₂ enrichment stimulated height growth only in the southern provenance. We found no significant effects of elevated CO₂ or nitrogen fertilization on the timing of bud set. During cold acclimation, freezing tolerance increased from –10°C to –35°C, and there was a marked increase in all soluble sugars except inositol. Elevated CO₂ in combination with high nitrogen led to a slight increased freezing tolerance in both provenances during the early stages of cold acclimation. However, towards the end of cold acclimation, elevated CO₂ and high nitrogen led to reduced freezing tolerance in the southern provenance, while elevated CO₂ and low nitrogen reduced freezing tolerance in the northern provenance. These results suggest that CO₂ enrichment influences the development of freezing tolerance, and that these responses differ with available nitrogen and between provenances.     

Gas exchange and photosynthetic performance of the tropical tree <Emphasis Type="Italic">Acacia nigrescens</Emphasis> when grown in different CO<Subscript>2</Subscript> concentrations

The photosynthetic responses of the tropical tree species Acacia nigrescens Oliv. grown at different atmospheric CO₂ concentrations—from sub-ambient to super-ambient—have been studied. Light-saturated rates of net photosynthesis (A _sat) in A. nigrescens, measured after 120 days exposure, increased significantly from sub-ambient (196 μL L⁻¹) to current ambient (386 μL L⁻¹) CO₂ growth conditions but did not increase any further as [CO₂] became super-ambient (597 μL L⁻¹). Examination of photosynthetic CO₂ response curves, leaf nitrogen content, and leaf thickness showed that this acclimation was most likely caused by reduction in Rubisco activity and a shift towards ribulose-1,5-bisphosphate regeneration-limited photosynthesis, but not a consequence of changes in mesophyll conductance. Also, measurements of the maximum efficiency of PSII and the carotenoid to chlorophyll ratio of leaves indicated that it was unlikely that the pattern of A _sat seen was a consequence of growth [CO₂] induced stress. Many of the photosynthetic responses examined were not linear with respect to the concentration of CO₂ but could be explained by current models of photosynthesis.     

Effects of elevated CO<Subscript>2</Subscript> and nitrogen on wheat growth and photosynthesis

The effects of nitrogen [75 and 150 kg (N) ha⁻¹] and elevated CO₂ on growth, photosynthetic rate, contents of soluble leaf proteins and activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and nitrate reductase (NR) were studied on wheat (Triticum aestivum L. cv. HD-2285) grown in open top chambers under either ambient (AC) or elevated (EC) CO₂ concentration (350 ± 50, 600 ± 50 μmol mol⁻¹) and analyzed at 40, 60 and 90 d after sowing. Plants grown under EC showed greater photosynthetic rate and were taller and attained greater leaf area along with higher total plant dry mass at all growth stages than those grown under AC. Total soluble and Rubisco protein contents decreased under EC but the activation of Rubisco was higher at EC with higher N supply. Nitrogen increased the NR activity whereas EC reduced it. Thus, EC causes increased growth and P_N ability per unit uptake of N in wheat plants, even if N is limiting.     

Phosphorus supply and arbuscular mycorrhizas increase growth and net gas exchange responses of two Citrus spp. grown at elevated [CO2]

We hypothesized that greater photosynthate supply at elevated [CO₂] could compensate for increased below-ground C demands of arbuscular mycorrhizas. Therefore, we investigated plant growth, mineral nutrition, starch, and net gas exchange responses of two Citrus spp. to phosphorus (P) nutrition and mycorrhizas at elevated atmospheric [CO₂]. Half of the seedlings of sour orange (C. aurantium L.) and ‘Ridge Pineapple’ sweet orange (C. sinensis L. Osbeck) were inoculated with the arbuscular mycorrhizal (AM) fungus, Glomus intraradices Schenck and Smith and half were non-mycorrhizal (NM). Plants were grown at ambient or 2X ambient [CO₂] in unshaded greenhouses for 11 weeks and fertilized daily with nutrient solution either without added P or with 2 mM P in a low-P soil. High P supply reduced AM colonization whereas elevated [CO₂] counteracted the depressive effect of P on intraradical colonization and vesicle development. Seedlings grown at either elevated [CO₂], high P or with G. intraradices had greater growth, net assimilation of CO₂ (A _CO2) in leaves, leaf water-use efficiency, leaf dry wt/area, leaf starch and carbon/nitrogen (C/N) ratio. Root/whole plant dry wt ratio was decreased by elevated [CO₂], P, and AM colonization. Mycorrhizal seedlings had higher leaf-P status but lower leaf N and K concentrations than nonmycorrhizal seedlings which was due to growth dilution effects. Starch in fibrous roots was increased by elevated [CO₂] but reduced by G. intraradices, especially at low-P supply. In fibrous roots, elevated [CO₂] had no effect on C/N, but AM colonization decreased C/N in both Citrus spp. grown at low-P supply. Overall, there were no species differences in growth or A _CO2. Mycorrhizas did not increase plant growth at ambient [CO₂]. At elevated [CO₂], however, mycorrhizas stimulated growth at both P levels in sour orange, the more mycorrhiza-dependent species, but only at low-P in sweet orange, the less dependent species. At low-P and elevated [CO₂], colonization by the AM fungus increased A _CO2 in both species but more so in sour orange than in sweet orange. Leaf P and root N concentrations were increased more and root starch level was decreased less by AM in sour orange than in sweet orange. Thus, the additional [CO₂] availability to mycorrhizal plants increased CO₂ assimilation, growth and nutrient uptake over that of NM plants especially in sour orange under P limitation. This revised version was published online in June 2006 with corrections to the Cover Date.