A stomatal optimization theory to describe the effects of atmospheric CO2 on leaf photosynthesis and transpiration

Background and Aims

Global climate models predict decreases in leaf stomatal conductance and transpiration due to increases in atmospheric CO₂. The consequences of these reductions are increases in soil moisture availability and continental scale run-off at decadal time-scales. Thus, a theory explaining the differential sensitivity of stomata to changing atmospheric CO₂ and other environmental conditions must be identified. Here, these responses are investigated using optimality theory applied to stomatal conductance.

Methods

An analytical model for stomatal conductance is proposed based on: (a) Fickian mass transfer of CO₂ and H₂O through stomata; (b) a biochemical photosynthesis model that relates intercellular CO₂ to net photosynthesis; and (c) a stomatal model based on optimization for maximizing carbon gains when water losses represent a cost. Comparisons between the optimization-based model and empirical relationships widely used in climate models were made using an extensive gas exchange dataset collected in a maturing pine (Pinus taeda) forest under ambient and enriched atmospheric CO₂.

Key Results and Conclusion

In this interpretation, it is proposed that an individual leaf optimally and autonomously regulates stomatal opening on short-term (approx. 10-min time-scale) rather than on daily or longer time-scales. The derived equations are analytical with explicit expressions for conductance, photosynthesis and intercellular CO₂, thereby making the approach useful for climate models. Using a gas exchange dataset collected in a pine forest, it is shown that (a) the cost of unit water loss λ (a measure of marginal water-use efficiency) increases with atmospheric CO₂; (b) the new formulation correctly predicts the condition under which CO₂-enriched atmosphere will cause increasing assimilation and decreasing stomatal conductance.     

On the complementary relationship between marginal nitrogen and water-use efficiencies among Pinus taeda leaves grown under ambient and CO2-enriched environments

Background and Aims

Water and nitrogen (N) are two limiting resources for biomass production of terrestrial vegetation. Water losses in transpiration (E) can be decreased by reducing leaf stomatal conductance (g_s) at the expense of lowering CO₂ uptake (A), resulting in increased water-use efficiency. However, with more N available, higher allocation of N to photosynthetic proteins improves A so that N-use efficiency is reduced when g_s declines. Hence, a trade-off is expected between these two resource-use efficiencies. In this study it is hypothesized that when foliar concentration (N) varies on time scales much longer than g_s, an explicit complementary relationship between the marginal water- and N-use efficiency emerges. Furthermore, a shift in this relationship is anticipated with increasing atmospheric CO₂ concentration (c_a).

Methods

Optimization theory is employed to quantify interactions between resource-use efficiencies under elevated c_a and soil N amendments. The analyses are based on marginal water- and N-use efficiencies, λ = (∂A/∂g_s)/(∂E/∂g_s) and η = ∂A/∂N, respectively. The relationship between the two efficiencies and related variation in intercellular CO₂ concentration (c_i) were examined using A/c_i curves and foliar N measured on Pinus taeda needles collected at various canopy locations at the Duke Forest Free Air CO₂ Enrichment experiment (North Carolina, USA).

Key Results

Optimality theory allowed the definition of a novel, explicit relationship between two intrinsic leaf-scale properties where η is complementary to the square-root of λ. The data support the model predictions that elevated c_a increased η and λ, and at given c_a and needle age-class, the two quantities varied among needles in an approximately complementary manner.

Conclusions

The derived analytical expressions can be employed in scaling-up carbon, water and N fluxes from leaf to ecosystem, but also to derive transpiration estimates from those of η, and assist in predicting how increasing c_a influences ecosystem water use.     

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%.     

Changes of leaf water potential and gas exchange during and after drought in triticale and maize genotypes differing in drought tolerance

Influence of drought (D) on changes of leaf water potential (Ψ) and parameters of gas exchange in D-resistant and D-sensitive genotypes of triticale and maize was compared. Soil D (from −0.01 to −2.45 MPa) was simulated by mannitol solutions. At −0.013 MPa significant differences in Ψ, net photosynthetic rate (P _N), transpiration rate (E), stomatal conductance (g _s), and internal CO₂ concentration (C _i) of D-resistant and D-sensitive triticale and maize genotypes were not found. Together with the increase in concentration of the mannitol solution the impact of D on E and g _s for D-sensitive genotypes (CHD-12, Ankora) became lower than for the D-resistant ones (CHD-247, Tina). Inversely, impact of D on Ψ was higher in D-sensitive than D-resistant genotypes. From 1 to 3 d of D, a higher decrease in P _N was observed in D-resistant genotypes than in the D-sensitive ones. Under prolonged D (5–14 d) and simultaneous more severe D the decrease in P _N was lower in D-resistant than in D-sensitive genotypes. Changes in Ψ, P _N, E, and g _s caused by D in genotypes differing in the drought susceptibility were similar for triticale and maize. Compared to control plants, increase of C _i was different for triticale and maize genotypes. Hence one of the physiological reasons of different susceptibility to D between sensitive and resistant genotypes is more efficient protection of tissue water status in resistant genotypes reflected in higher decrease in g _s and limiting E compared to the sensitive ones. Other reason, observed in D-resistant genotypes during the recovery from D-stress, was more efficient removal of detrimental effects of D.     

Soybean leaf hydraulic conductance does not acclimate to growth at elevated [CO2] or temperature in growth chambers or in the field

Background and Aims

Leaf hydraulic properties are strongly linked with transpiration and photosynthesis in many species. However, it is not known if gas exchange and hydraulics will have co-ordinated responses to climate change. The objective of this study was to investigate the responses of leaf hydraulic conductance (K_leaf) in Glycine max (soybean) to growth at elevated [CO₂] and increased temperature compared with the responses of leaf gas exchange and leaf water status.

Methods

Two controlled-environment growth chamber experiments were conducted with soybean to measure K_leaf, stomatal conductance (g_s) and photosynthesis (A) during growth at elevated [CO₂] and temperature relative to ambient levels. These results were validated with field experiments on soybean grown under free-air elevated [CO₂] (FACE) and canopy warming.

Key results

In chamber studies, K_leaf did not acclimate to growth at elevated [CO₂], even though stomatal conductance decreased and photosynthesis increased. Growth at elevated temperature also did not affect K_leaf, although g_s and A showed significant but inconsistent decreases. The lack of response of K_leaf to growth at increased [CO₂] and temperature in chamber-grown plants was confirmed with field-grown soybean at a FACE facility.

Conclusions

Leaf hydraulic and leaf gas exchange responses to these two climate change factors were not strongly linked in soybean, although g_s responded to [CO₂] and increased temperature as previously reported. This differential behaviour could lead to an imbalance between hydraulic supply and transpiration demand under extreme environmental conditions likely to become more common as global climate continues to change.     

Assessing the impact of elevated CO2 on soil microbial activity in a Mediterranean model ecosystem

The fate, as well as the consequence for plant nutrition, of the additional carbon entering soil under elevated CO₂ is largely determined by the activity of soil microorganisms. However, most elevated CO₂ studies have documented changes (generally increases) in microbial biomass and total infection by symbiotic organisms, which is only a first step in the understanding of the modification of soil processes. Using a Mediterranean model ecosystem, we complemented these variables by analyzing changes in enzymatic activities, hyphal lengths, and bacterial substrate assimilation, to tentatively identify the specific components affected under elevated CO₂ and those which suggest changes in soil organic matter pools. We also investigated changes in the functional structures of arbuscular mycorrhizas. Most of the microbial variables assessed showed significant and substantial increase under elevated CO₂, of the same order or less than those observed for root mass and length. The increase in dehydrogenase activity indicates that the larger biomass of microbes was accompanied by an increase in their activity. The increase in hyphal length (predominantly of saprophytic fungi), and xylanase, cellulase and phosphatase activities, suggests an overall stimulation of organic matter decomposition. The higher number of substrates utilized by microorganisms from the soil under elevated CO₂ was significant for the amine/amide group. Total arbuscular and vesicular mycorrhizal infection of roots was higher under elevated CO₂, but the proportion of functional structures was not modified. These insights into the CO₂-induced changes in soil biological activity point towards potential areas of investigation complementary to a direct analysis of the soil organic matter pools.     

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.     

剪叶疏花条件下高浓度CO2对汕优63生长和产量的影响

利用稻田FACE(Free Air CO_2Enrichment)系统平台,以杂交稻汕优63为供试材料,二氧化碳设环境CO_2浓度(Ambient)和高CO_2浓度(Ambient+200μmol/mol),抽穗期源库改变设剪叶(剪除剑叶)和疏花处理(相间剪除1次枝梗),以不处理为对照(CK),研究大气CO_2浓度升高对不同源库处理水稻产量形成及物质生产的影响。结果表明:CK条件下,大气CO_2浓度升高使汕优63籽粒产量显著增加32%,这主要与单位面积总颖花量大幅增加(+26%)有关,结实能力亦呈增加趋势但未达显著水平。大气CO_2浓度升高使抽穗期剪叶处理水稻的籽粒产量平均增加55%,明显大于对照水稻,这主要与受精率(+28%)、饱粒率(+23%)和所有籽粒平均粒重(+19%)大幅增加有关。相反,对抽穗期疏花处理水稻而言,高CO_2浓度环境下籽粒产量的增幅(+25%,P=0.07)明显小于对照水稻,这主要与结实能力的响应略有下调有关。与产量响应类似,大气CO_2浓度升高使对照、剪叶和疏花条件下最终生物量分别增加39%、43%和28%,除疏花处理外均达显著水平。抽穗期剪叶和疏花处理本身使水稻籽粒产量分别降低40%和45%,前者主要是结实能力大幅下降所致,而后者与总颖花量减半相关。以上结果表明,大气CO_2浓度升高使杂交水稻生产力大幅增加,人为减小源库比(如剪叶)可增强CO_2肥料效应,而增加源库比(如疏花)则可使这种肥料效应减弱。     

Impact of elevated CO2 concentration on ultrastructure of pericarp and composition of grain in three Triticum species of different ploidy levels

Triticum species of three ploidies were grown under ambient (375 μl/l) or elevated (FACE, 550 μl/l) CO₂ concentration [CO₂] to evaluate their response to CO₂ enrichment. The consistent effect of elevated CO₂ was an increase in concentration of starch and decrease in concentration of protein in the grain. Transmission electron micrographs revealed an increase in width and area of chloroplasts, and change in shape from elliptical in ambient to round in elevated [CO₂]. There was a corresponding increase in starch grain size and number in chloroplasts. The large starch grains distributed among thylakoids resulted in separation and distortion of internal membrane system in chloroplasts. The level of response was different in species of different ploidy levels. Maximum increase in starch concentration, and least decrease in protein concentration, was observed in Triticum dicoccoides, which also proved the most suitable species in terms of C:N ratio.     

Activities of carboxylating enzymes in the CAM species Opuntia ficus-indica grown under current and elevated CO2 concentrations

Responses of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPCase) to an elevated atmospheric CO₂ concentration were determined along with net CO₂ uptake rates for the Crassulacean acid metabolism species Opuntia ficus-indica growing in open-top chambers. During the spring 13 months after planting, total daily net CO₂ uptake of basal and first-order daughter cladodes was 28% higher at 720 than at 360 l CO₂ l^-1. The enhancement, caused mainly by higher CO₂ assimilation during the early part of the night, was also observed during late summer (5 months after planting) and the following winter. The activities of Rubisco and PEPCase measured in vitro were both lower at the elevated CO₂ concentration, particularly under the more favorable growth conditions in the spring and late summer. Enzyme activity in second-order daughter cladodes increased with cladode age, becoming maximal at 6 to 10 days. The effect ofelevated CO₂ on Rubisco and PEPCase activity declined with decreasing irradiance, especially for Rubisco. Throughout the 13-month observation period, O. ficus-indica thus showed increased CO₂ uptake when the atmospheric CO₂ concentration was doubled despite lower activities of both carboxylating enzymes.     

Cotton bracts are adapted to a microenvironment of concentrated CO2 produced by rapid fruit respiration

Background and Aims

Elucidation of the mechanisms by which plants adapt to elevated CO₂ is needed; however, most studies of the mechanisms investigated the response of plants adapted to current atmospheric CO₂. The rapid respiration rate of cotton (Gossypium hirsutum) fruits (bolls) produces a concentrated CO₂ microenvironment around the bolls and bracts. It has been observed that the intercellular CO₂ concentration of a whole fruit (bract and boll) ranges from 500 to 1300 µmol mol⁻¹ depending on the irradiance, even in ambient air. Arguably, this CO₂ microenvironment has existed for at least 1·1 million years since the appearance of tetraploid cotton. Therefore, it was hypothesized that the mechanisms by which cotton bracts have adapted to elevated CO₂ will indicate how plants will adapt to future increased atmospheric CO₂ concentration. Specifically, it is hypothesized that with elevated CO₂ the capacity to regenerate ribulose-1,5-bisphosphate (RuBP) will increase relative to RuBP carboxylation.

Methods

To test this hypothesis, the morphological and physiological traits of bracts and leaves of cotton were measured, including stomatal density, gas exchange and protein contents.

Key results

Compared with leaves, bracts showed significantly lower stomatal conductance which resulted in a significantly higher water use efficiency. Both gas exchange and protein content showed a significantly greater RuBP regeneration/RuBP carboxylation capacity ratio (J_max/V_cmax) in bracts than in leaves.

Conclusions

These results agree with the theoretical prediction that adaptation of photosynthesis to elevated CO₂ requires increased RuBP regeneration. Cotton bracts are readily available material for studying adaption to elevated CO₂.     

Biomass production in a nitrogen-fertilized,tallgrass prairie ecosystem exposed to ambient and elevated levels of CO2

Increased biomass production in terrestrial ecosystems with elevated atmospheric CO₂ may be constrained by nutrient limitations as a result of increased requirement or reduced availability caused by reduced turnover rates of nutrients. To determine the short-term impact of nitrogen (N) fertilization on plant biomass production under elevated CO₂, we compared the response of N-fertilized tallgrass prairie at ambient and twice-ambient CO₂ levels over a 2-year period. Native tallgrass prairie plots (4.5 m diameter) were exposed continuously (24 h) to ambient and twice-ambient CO₂ from 1 April to 26 October. We compared our results to an unfertilized companion experiment on the same research site. Above- and belowground biomass production and leaf area of fertilized plots were greater with elevated than ambient CO₂ in both years. The increase in biomass at high CO₂ occurred mainly aboveground in 1991, a dry year, and belowground in 1990, a wet year. Nitrogen concentration was lower in plants exposed to elevated CO₂, but total standing crop N was greater at high CO₂. Increased root biomass under elevated CO₂ apparently increased N uptake. The biomass production response to elevated CO₂ was much greater on N-fertilized than unfertilized prairie, particularly in the dry year. We conclude that biomass production response to elevated CO₂ was suppressed by N limitation in years with below-normal precipitation. Reduced N concentration in above- and belowground biomass could slow microbial degradation of soil organic matter and surface litter, thereby exacerbating N limitation in the long term.     

Environmental and stomatal control of photosynthetic enhancement in the canopy of a sweetgum (Liquidambar styraciflua L.) plantation during 3 years of CO2 enrichment

Light‐saturated photosynthetic and stomatal responses to elevated CO₂ were measured in upper and mid‐canopy foliage of a sweetgum (Liquidambar styraciflua L) plantation exposed to free‐air CO₂ enrichment (FACE) for 3 years, to characterize environmental interactions with the sustained CO₂ effects in an intact deciduous forest stand. Responses were evaluated in relation to one another, and to seasonal patterns and natural environmental stresses, including high  temperatures, vapour pressure deficits (VPD), and drought. Photosynthetic CO₂ assimilation (A) averaged 46% higher in the +200 µmol mol^?1 CO₂ treatment, in mid‐ and upper canopy foliage. Stomatal conductance (g_s) averaged 14% (mid‐canopy) and 24% (upper canopy) lower under CO₂ enrichment. Variations in the relative responses of A and g_s were linked, such that greater relative stimulation of A was observed on dates when relative reductions in g_s were slight. Dry soils and high VPD reduced g_s and A in both treatments, and tended to diminish treatment differences. The absolute effects of CO₂ on A and g_s were minimized whenever g_s was low (<0·15 mol m^?2 s^?1), but relative effects, as the ratio of elevated to ambient rates, varied greatly under those conditions. Both stomatal and non‐stomatal limitations of A were involved during late season droughts. Leaf temperature had a limited influence on A and g_s, and there was no detectable relationship between prevailing temperature and CO₂ effects on A or g_s. The responsiveness of A and g_s to elevated CO₂, both absolute and relative, was maintained through time and within the canopy of this forest stand, subject to seasonal constraints and variability associated with limiting air and soil moisture.     

Urea additions and defoliation affect plant responses to elevated CO2 in a C3 grass from Yellowstone National Park

A common grass from Yellowstone National Park, Stipa occidentalis, was grown in a factorial experiment to determine if its response to the direct effects of elevated CO₂ would be affected by defoliation, and urea additions simulating the N in a urine hit. Plants were grown in tall pots (to mimic rooting depth in the field) in growth chambers under elevated (700 ppm) and ambient (370 ppm) CO₂, were defoliated or left undefoliated, and given N-supply rates based on field mineralization rates (untreated) or with an additional 40 g N/m². Growth increases in response to elevated CO₂ were largest when plants remained unclipped and received urea additions, and were found primarily in crowns and roots (storage organs). Aboveground biomass, which is the part of the plant consumed by grazing mammals, was not affected by elevated CO₂. The elevated CO₂ treatment caused a reduction in leaf percent N. However, there was a significant interaction between the CO₂ and urea treatments, resulting in a larger difference in leaf percent N between urea-treated and control plants under elevated than under ambient CO₂. Hence, elevations in atmospheric CO₂ may cause an increase in the amount of urine-hit-induced spatial variability in temperate grasslands. Since food quantity remained largely unchanged in response to elevated CO₂, and forage N content went down, grazing mammals may be negatively affected by increases in atmospheric CO₂.     

Effect of elevated atmospheric CO2 concentration on soil and root respiration in winter wheat by using a respiration partitioning chamber

Soil respiration in a cropland is the sum of heterotrophic (mainly microorganisms) and autotrophic (root) respiration. The contribution of both these types to soil respiration needs to be understood to evaluate the effects of environmental change on soil carbon cycling and sequestration. In this paper, the effects of free-air CO₂ enrichment (FACE) on hetero- and autotrophic respiration in a wheat field were differentiated and evaluated by a novel split-root growth and gas collection system. Elevated atmospheric pCO₂ of approximately 200 μmol mol⁻¹ above the ambient pCO₂ significantly increased soil respiration by 15.1 and 14.8% at high nitrogen (HN) and low nitrogen (LN) application rates, respectively. The effect of elevated atmospheric pCO₂ on root respiration was not consistent across the wheat growth stages. Elevated pCO₂ significantly increased and decreased root respiration at the booting-heading stage (middle stage) and the late-filling stage (late stage), respectively, in HN and LN treatments; however, no significant effect was found at the jointing stage (early stage). Thus, the effect of increased pCO₂ on cumulative root respiration for the entire wheat growing season was not significant. Cumulative root respiration accounted for approximately 25–30% of cumulative soil respiration in the entire wheat growing season. Consequently, cumulative microbial respiration (soil respiration minus root respiration) increased by 22.5 and 21.1% due to elevated pCO₂ in HN and LN, respectively. High nitrogen application significantly increased root respiration at the late stage under both elevated pCO₂ and ambient pCO₂; however, no significant effects were found on cumulative soil respiration, root respiration, and microbial respiration. These findings suggest that heterotrophic respiration, which is influenced by increased substrate supplies from the plant to the soil, is the key process to determine C emission from agro-ecosystems with regard to future scenarios of enriched pCO₂.     

Elevated CO<Subscript>2</Subscript> mitigates the adverse effects of drought on daytime net ecosystem CO<Subscript>2</Subscript> exchange and photosynthesis in a Florida scrub-oak ecosystem

Drought is a normal, recurrent feature of climate. In order to understand the potential effect of increasing atmospheric CO₂ concentration (C _a) on ecosystems, it is essential to determine the combined effects of drought and elevated C _a (EC) under field conditions. A severe drought occurred in Central Florida in 1998 when precipitation was 88 % less than the average between 1984 and 2002. We determined daytime net ecosystem CO₂ exchange (NEE) before, during, and after the drought in the Florida scrub-oak ecosystem exposed to doubled C _a in open-top chamber since May 1996. We measured diurnal leaf net photosynthetic rate (P _N) of Quercus myrtifolia Willd, the dominant species, during and after the drought. Drought caused a midday depression in NEE and P _N at ambient CO₂ concentration (AC) and EC. EC mitigated the midday depression in NEE by about 60 % compared to AC and the effect of EC on leaf P _N was similar to its effect on NEE. Growth in EC lowered the sensitivity of NEE to air vapor pressure deficit under drought. Thus EC would help the scrub-oak ecosystem to survive the consequences of the effects of rising atmospheric CO₂ on climate change, including increased frequency of drought, while simultaneously sequestering more anthropogenic carbon.     

Distinct responses of soil microbial communities to elevated CO2 and O3 in a soybean agro-ecosystem

The concentrations of atmospheric carbon dioxide (CO₂) and tropospheric ozone (O₃) have been rising due to human activities. However, little is known about how such increases influence soil microbial communities. We hypothesized that elevated CO₂ (eCO₂) and elevated O₃ (eO₃) would significantly affect the functional composition, structure and metabolic potential of soil microbial communities, and that various functional groups would respond to such atmospheric changes differentially. To test these hypotheses, we analyzed 96 soil samples from a soybean free-air CO₂ enrichment (SoyFACE) experimental site using a comprehensive functional gene microarray (GeoChip 3.0). The results showed the overall functional composition and structure of soil microbial communities shifted under eCO₂, eO₃ or eCO₂+eO₃. Key functional genes involved in carbon fixation and degradation, nitrogen fixation, denitrification and methane metabolism were stimulated under eCO₂, whereas those involved in N fixation, denitrification and N mineralization were suppressed under eO₃, resulting in the fact that the abundance of some eO₃-supressed genes was promoted to ambient, or eCO₂-induced levels by the interaction of eCO₂+eO₃. Such effects appeared distinct for each treatment and significantly correlated with soil properties and soybean yield. Overall, our analysis suggests possible mechanisms of microbial responses to global atmospheric change factors through the stimulation of C and N cycling by eCO₂, the inhibition of N functional processes by eO₃ and the interaction by eCO₂ and eO₃. This study provides new insights into our understanding of microbial functional processes in response to global atmospheric change in soybean agro-ecosystems.     

Water relations in Norway spruce trees growing at ambient and elevated CO2 concentrations

Water relations were studied in Norway spruce [Picea abies (L.) Karst.] trees grown at ambient (AC, 350 μmol mol⁻¹) and elevated (EC, 700 μmol mol⁻¹) CO₂ concentrations under temperate water stress. The results suggested that both crown position and variability in atmospheric CO₂ concentration are responsible for different patterns of crown water relations. Mean hourly sap flux density (FSA) showed higher values in upper crown position in comparison with the whole crown in both AC and EC treatments. Mean soil-to-leaf hydraulic conductance (G_Tsa) was 1.4 times higher for the upper crown than that calculated across the whole crown for the trees in AC. However, G_Tsa did not vary significantly with crown position in EC trees, suggesting that elevated CO₂ may mitigate differences in hydraulic supply for different canopy layers. The trees in EC treatment exhibited significantly higher values of F_SA measured on the whole crown level and slightly higher soil water content compared to AC treatment, suggesting more economical use of soil water and therefore an advantage under water-limited conditions.     

Diurnal and seasonal variations in stomatal conductance of rice at elevated atmospheric CO2 under fully open‐air conditions

Understanding of leaf stomatal responses to the atmospheric CO₂ concentration, [CO₂], is essential for accurate prediction of plant water use under future climates. However, limited information is available for the diurnal and seasonal changes in stomatal conductance (g_s) under elevated [CO₂]. We examined the factors responsible for variations in g_s under elevated [CO₂] with three rice cultivars grown in an open‐field environment under flooded conditions during two growing seasons (a total of 2140 individual measurements). Conductance of all cultivars was generally higher in the morning and around noon than in the afternoon, and elevated [CO₂] decreased g_s by up to 64% over the 2 years (significantly on 26 out of 38 measurement days), with a mean g_s decrease of 23%. We plotted the g_s variations against three parameters from the Ball‐Berry model and two revised versions of the model, and all parameters explained the g_s variations well at each [CO₂] in the morning and around noon (R² > 0.68), but could not explain these variations in the afternoon (R² < 0.33). The present results provide an important basis for modelling future water use in rice production.     

Stomatal density and aperture in non-vascular land plants are non-responsive to above-ambient atmospheric CO2 concentrations

Background and Aims Following the consensus view for unitary origin and conserved function of stomata across over 400 million years of land plant evolution, stomatal abundance has been widely used to reconstruct palaeo-atmospheric environments. However, the responsiveness of stomata in mosses and hornworts, the most basal stomate lineages of extant land plants, has received relatively little attention. This study aimed to redress this imbalance and provide the first direct evidence of bryophyte stomatal responsiveness to atmospheric CO₂.Methods A selection of hornwort (Anthoceros punctatus, Phaeoceros laevis) and moss (Polytrichum juniperinum, Mnium hornum, Funaria hygrometrica) sporophytes with contrasting stomatal morphologies were grown under different atmospheric CO₂ concentrations ([CO₂]) representing both modern (440 p.p.m. CO₂) and ancient (1500 p.p.m. CO₂) atmospheres. Upon sporophyte maturation, stomata from each bryophyte species were imaged, measured and quantified.Key Results Densities and dimensions were unaffected by changes in [CO₂], other than a slight increase in stomatal density in Funaria and abnormalities in Polytrichum stomata under elevated [CO₂].Conclusions The changes to stomata in Funaria and Polytrichum are attributed to differential growth of the sporophytes rather than stomata-specific responses. The absence of responses to changes in [CO₂] in bryophytes is in line with findings previously reported in other early lineages of vascular plants. These findings strengthen the hypothesis of an incremental acquisition of stomatal regulatory processes through land plant evolution and urge considerable caution in using stomatal densities as proxies for paleo-atmospheric CO₂ concentrations.