Interactive effects of elevated CO2 and soil fertility on isoprene emissions from Quercus robur

The effects of global change on the emission rates of isoprene from plants are not clear. A factor that can influence the response of isoprene emission to elevated CO₂ concentrations is the availability of nutrients. Isoprene emission rate under standard conditions (leaf temperature: 30°C, photosynthetically active radiation (PAR): 1000 μmol photons m^?2 s^?1), photosynthesis, photosynthetic capacity, and leaf nitrogen (N) content were measured in Quercus robur grown in well‐ventilated greenhouses at ambient and elevated CO₂ (ambient plus 300 ppm) and two different soil fertilities. The results show that elevated CO₂ enhanced photosynthesis but leaf respiration rates were not affected by either the CO₂ or nutrient treatments. Isoprene emission rates and photosynthetic capacity were found to decrease with elevated CO₂, but an increase in nutrient availability had the converse effect. Leaf N content was significantly greater with increased nutrient availability, but unaffected by CO₂. Isoprene emission rates measured under these conditions were strongly correlated with photosynthetic capacity across the range of different treatments. This suggests that the effects of CO₂ and nutrient levels on allocation of carbon to isoprene production and emission under near‐saturating light largely depend on the effects on photosynthetic electron transport capacity.     

Profiles of isoprene emission and photosynthetic parameters in hybrid poplars exposed to free‐air CO2 enrichment†

Poplar (Populus × euroamericana) saplings were grown in the field to study the changes of photosynthesis and isoprene emission with leaf ontogeny in response to free air carbon dioxide enrichment (FACE) and soil nutrient availability. Plants growing in elevated [CO₂] produced more leaves than those in ambient [CO₂]. The rate of leaf expansion was measured by comparing leaves along the plant profile. Leaf expansion and nitrogen concentration per unit of leaf area was similar between nutrient treatment, and this led to similar source–sink functional balance. Consequently, soil nutrient availability did not cause downward acclimation of photosynthetic capacity in elevated [CO₂] and did not affect isoprene synthesis. Photosynthesis assessed in growth [CO₂] was higher in plants growing in elevated than in ambient [CO₂]. After normalizing for the different number of leaves over the profile, maximal photosynthesis was reached and started to decline earlier in elevated than in ambient [CO₂]. This may indicate a [CO₂]‐driven acceleration of leaf maturity and senescence. Isoprene emission was adversely affected by elevated [CO₂]. When measured on the different leaves of the profile, isoprene peak emission was higher and was reached earlier in ambient than in elevated [CO₂]. However, a larger number of leaves was emitting isoprene in plant growing in elevated [CO₂]. When integrating over the plant profile, emissions in the two [CO₂] levels were not different. Normalization as for photosynthesis showed that profiles of isoprene emission were remarkably similar in the two [CO₂] levels, with peak emissions at the centre of the profile. Only the rate of increase of the emission of young leaves may have been faster in elevated than in ambient [CO₂]. Our results indicate that elevated [CO₂] may overall have a limited effect on isoprene emission from young seedlings and that plants generally regulate the emission to reach the maximum at the centre of the leaf profile, irrespective of the total leaf number. In comparison with leaf expansion and photosynthesis, isoprene showed marked and repeatable differences among leaves of the profile and may therefore be a useful trait to accurately monitor changes of leaf ontogeny as a consequence of elevated [CO₂].     

Enhanced isoprene emission capacity and altered light responsiveness in aspen grown under elevated atmospheric CO2 concentration

Controversial evidence of CO₂‐responsiveness of isoprene emission has been reported in the literature with the response ranging from inhibition to enhancement, but the reasons for such differences are not understood. We studied isoprene emission characteristics of hybrid aspen (Populus tremula x P. tremuloides) grown under ambient (380 μmol mol^?1) and elevated (780 μmol mol^?1) [CO₂] to test the hypothesis that growth [CO₂] effects on isoprene emission are driven by modifications in substrate pool size, reflecting altered light use efficiency for isoprene synthesis. A novel in vivo method for estimation of the pool size of the immediate isoprene precursor, dimethylallyldiphosphate (DMADP) and the activity of isoprene synthase was used. Growth at elevated [CO₂] resulted in greater leaf thickness, more advanced development of mesophyll and moderately increased photosynthetic capacity due to morphological “upregulation”, but isoprene emission rate under growth light and temperature was not significantly different among ambient‐ and elevated‐[CO₂]‐grown plants independent of whether measured at 380 μmol mol^?1 or 780 μmol mol^?1 CO₂. However, DMADP pool size was significantly less in elevated‐[CO₂]‐grown plants, but this was compensated by increased isoprene synthase activity. Analysis of CO₂ and light response curves of isoprene emission demonstrated that the [CO₂] for maximum isoprene emission was shifted to lower [CO₂] in elevated‐[CO₂]‐grown plants. The light‐saturated isoprene emission rate (I_max,Q) was greater, but the quantum efficiency at given I_max,Q was less in elevated‐[CO₂]‐grown plants, especially at higher CO₂ measurement concentration, reflecting stronger DMADP limitation at lower light and higher [CO₂]. These results collectively demonstrate important shifts in light and CO₂‐responsiveness of isoprene emission in elevated‐[CO₂]‐acclimated plants that need consideration in modeling isoprene emissions in future climates.     

Effects of environmental conditions on isoprene emission from live oak

Live-oak plants (Quercus virginiana Mill.) were subjected to various levels of CO₂, water stress or photosynthetic photon flux density to test the hypothesis that isoprene biosynthesis occurred only under conditions of restricted CO₂ availability. Isoprene emission increases as the ambient CO₂ concentration decreased, independent of the amount of time that plants had photosynthesized at ambient CO₂ levels. When plants were water-stressed over a 4-d period photosynthesis and leaf conductance decreased 98 and 94%, respectively, while isoprene emissions remained constant. Significant isoprene emissions occurred when plants were saturated with CO₂, i.e., below the light compensation level for net photosynthesis (100 mol m^-2 s^-1). Isoprene emission rates increased with photosynthetic photon flux density and at 25 and 50 mol m^-2 s^-1 were 7 and 18 times greater than emissions in the dark. These data indicate that isoprene is a normal plant metabolite and not — as has been suggested — formed exclusively in response to restricted CO₂ or various stresses.Abbreviation PPFD photosynthetic photon flux density     

Isoprene emissions from plants are mediated by atmospheric CO2 concentrations

The tropical African tree species Acacia nigrescens Oliv. was grown in environmentally controlled growth chambers at three CO₂ concentrations representative of the Last Glacial Maximum (～180 ppmv), the present day (～380 ppmv), and likely mid‐21st century (～600 ppmv) CO₂ concentrations. Isoprene (C₅H₈) emissions, per unit leaf area, were greater at lower‐than‐current CO₂ levels and lower at higher‐than‐current CO₂ levels relative to controls grown at 380 ppmv CO₂. Changes in substrate availability and isoprene synthase (IspS) activity were identified as the mechanisms behind the observed leaf‐level emission response. In contrast, canopy‐scale emissions remained unaltered between the treatments as changes in leaf‐level emissions were offset by changes in biomass and leaf area. Substrate concentration and IspS activity‐CO₂ responses were used in a biochemical model, coupled to existing isoprene emission algorithms, to model isoprene emissions from A. nigrescens grown for over 2 years at three different CO₂ concentrations. The addition of the biochemical model allowed for the use of emission factors measured under present day CO₂ concentrations across all three CO₂ treatments. When isoprene emissions were measured from A. nigrescens in response to instantaneous changes in CO₂ concentration, the biochemical model satisfactorily represented the observed response. Therefore, the effect of changes in atmospheric CO₂ concentration on isoprene emission at any timescale can be modelled and predicted.     

Isoprenoid emission in trees of Quercus pubescens and Quercus ilex with lifetime exposure to naturally high CO2 environment†

The long‐term effect of elevated atmospheric CO₂ on isoprenoid emissions from adult trees of two Mediterranean oak species (the monoterpene‐emitting Quercus ilex L. and the isoprene‐emitting Quercus pubescens Willd.) native to a high‐CO₂ environment was investigated. During two consecutive years, isoprenoid emission was monitored both at branch level, measuring the actual emissions under natural conditions, and at leaf level, measuring the basal emissions under the standard conditions of 30 °C and at light intensity of 1000 µmol m^?2 s^?1. Long‐term exposure to high atmospheric levels of CO₂ did not significantly affect the actual isoprenoid emissions. However, when leaves of plants grown in the control site were exposed for a short period to an elevated CO₂ level by rapidly switching the CO₂ concentration in the gas‐exchange cuvette, both isoprene and monoterpene basal emissions were clearly inhibited. These results generally confirm the inhibitory effect of elevated CO₂ on isoprenoid emission. The absence of a CO₂ effect on actual emissions might indicate higher leaf temperature at elevated CO₂, or an interaction with multiple stresses some of which (e.g. recurrent droughts) may compensate for the CO₂ effect in Mediterranean ecosystems. Under elevated CO₂, isoprene emission by Q. pubescens was also uncoupled from the previous day's air temperature. In addition, pronounced daily and seasonal variations of basal emission were observed under elevated CO₂ underlining that correction factors may be necessary to improve the realistic estimation of isoprene emissions with empirical algorithms in the future. A positive linear correlation of isoprenoid emission with the photosynthetic electron transport and in particular with its calculated fraction used for isoprenoid synthesis was found. The slope of this relationship was different for isoprene and monoterpenes, but did not change when plants were grown in either ambient or elevated CO₂. This suggests that physiological algorithms may usefully predict isoprenoid emission also under rising CO₂ levels.     

Isoprene emission from aspen leaves : influence of environment and relation to photosynthesis and photorespiration

Isoprene emission rates from quaking aspen (Populus tremuloides Michx.) leaves were measured simultaneously with photosynthesis rate, stomatal conductance, and intercellular CO₂ partial pressure. Isoprene emission required the presence of CO₂ or O₂, but not both. The light response of isoprene emission rate paralleled that of photosynthesis. Isoprene emission was inhibited by decreasing ambient O₂ from 21% to 2%, only when there was oxygen insensitive photosynthesis. Mannose (10 millimolar) fed through cut stems resulted in strong inhibition of isoprene emission rate and is interpreted as evidence that isoprene biosynthesis requires either the export of triose phosphates from the chloroplast, or the continued synthesis of ATP. Light response experiments suggest that photosynthetically generated reductant or ATP is required for isoprene biosynthesis. Isoprene biosynthesis and emission are not directly linked to glycolate production through photorespiration, contrary to previous reports. Isoprene emission rate was inhibited by above-ambient CO₂ partial pressures (640 microbar outside and 425 microbar inside the leaf). The inhibition was not due to stomatal closure. This was established by varying ambient humidity at normal and elevated CO₂ partial pressures to measure isoprene emission rates over a range of stomatal conductances. Isoprene emission rates were inhibited at elevated CO₂ despite no change in stomatal conductance. Addition of abscisic acid to the transpiration stream dramatically inhibited stomatal conductance and photosynthesis rate, with a slight increase in isoprene emission rate. Thus, isoprene emission is independent of stomatal conductance, and may occur through the cuticle. Temperature had an influence on isoprene emission rate, with the Q₁₀ being 1.8 to 2.4 between 35 and 45°C. At these high temperatures the amount of carbon lost through isoprene emission was between 2.5 and 8% of that assimilated through photosynthesis. This represents a significant carbon cost that should be taken into account in determining midsummer carbon budgets for plants that are isoprene emitters.     

Effects of Elevated CO2 and Defoliation on Compensatory Growth and Photosynthesis of Seedlings in a Tropical Tree,Copaifera aromatica1

After defoliation by herbivores, some plants exhibit enhanced rates of photosynthesis and growth that enable them to compensate for lost tissue, thus maintaining their fitness relative to competing, undefoliated plants. Our aim was to determine whether compensatory photosynthesis and growth would be altered by increasing concentrations of atmospheric CO₂. Defoliation of developing leaflets on seedlings of a tropical tree, Copaifera aromatica, caused increases in photosynthesis under ambient CO₂, but not under elevated CO₂. An enhancement in the development of buds in the leaf axils followed defoliation at ambient levels of CO₂. In contrast, under elevated CO₂, enhanced development of buds occurred in undefoliated plants with no further enhancement in bud development due to exposure to elevated CO₂. Growth of leaf area after defoliation was increased, particularly under elevated CO₂. Despite this increase, defoliated plants grown under elevated CO₂ were further from compensating for tissue lost during defoliation after 5¹/₂ weeks than those grown under ambient CO₂ concentrations.     

The effect of elevated atmospheric CO2 and drought on sources and sinks of isoprene in a temperate and tropical rainforest mesocosm

Isoprene is the most abundant volatile hydrocarbon emitted by many tree species and has a major impact on tropospheric chemistry, leading to formation of pollutants and enhancing the lifetime of methane, a powerful greenhouse gas. Reliable estimates of global isoprene emission from different ecosystems demand a clear understanding of the processes of both production and consumption. Although the biochemistry of isoprene production has been studied extensively and environmental controls over its emission are relatively well known, the study of isoprene consumption in soil has been largely neglected. Here, we present results on the production and consumption of isoprene studied by measuring the following different components: (1) leaf and soil and (2) at the whole ecosystem level in two distinct enclosed ultraviolet light‐depleted mesocosms at the Biosphere 2 facility: a cottonwood plantation with trees grown at ambient and elevated atmospheric CO₂ concentrations and a tropical rainforest, under well watered and drought conditions. Consumption of isoprene by soil was observed in both systems. The isoprene sink capacity of litter‐free soil of the agriforest stands showed no significant response to different CO₂ treatments, while isoprene production was strongly depressed by elevated atmospheric CO₂ concentrations. In both mesocosms, drought suppressed the sink capacity, but the full sink capacity of dry soil was recovered within a few hours upon rewetting. We conclude that soil uptake of atmospheric isoprene is likely to be modest but significant and needs to be taken into account for a comprehensive estimate of the global isoprene budget. More studies investigating the capacity of soils to uptake isoprene in natural conditions are clearly needed.     

Infection with the parasitic angiosperm Striga hermonthica influences the response of the C3 cereal Oryza sativa to elevated CO2

Upland rice (Oryza sativa L.) was grown at both ambient (350 μmol mol^?1) and elevated (700 μmol mol^?1) CO₂ in either the presence or absence of the root hemi‐parasitic angiosperm Striga hermonthica (Del) Benth. Elevated CO₂ alleviated the impact of the parasite on host growth: biomass of infected rice grown at ambient CO₂ was 35% that of uninfected, control plants, while at elevated CO₂, biomass of infected plants was 73% that of controls. This amelioration occurred despite the fact that O. sativa grown at elevated CO₂ supported both greater numbers and a higher biomass of parasites per host than plants grown at ambient CO₂. The impact of infection on host leaf area, leaf mass, root mass and reproductive tissue mass was significantly lower in plants grown at elevated as compared with ambient CO₂. There were significant CO₂ and Striga effects on photosynthetic metabolism and instantaneous water‐use efficiency of O. sativa. The response of photosynthesis to internal [CO₂] (A/C_i curves) indicated that, at 45 days after sowing (DAS), prior to emergence of the parasites, uninfected plants grown at elevated CO₂ had significantly lower CO₂ saturated rates of photosynthesis, carboxylation efficiencies and ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) contents than uninfected, ambient CO₂‐grown O. sativa. In contrast, infection with S. hermonthica prevented down‐regulation of photosynthesis in O. sativa grown at elevated CO₂, but had no impact on photosynthesis of hosts grown at ambient CO₂. At 76 DAS (after parasites had emerged), however, infected plants grown at both elevated and ambient CO₂ had lower carboxylation efficiencies and Rubisco contents than uninfected O. sativa grown at ambient CO₂. The reductions in carboxylation efficiency (and Rubisco content) were accompanied by similar reductions in nitrogen concentration of O. sativa leaves, both before and after parasite emergence. There were no significant CO₂ or infection effects on the concentrations of soluble sugars in leaves of O. sativa, but starch concentration was significantly lower in infected plants at both CO₂ concentrations. These results demonstrate that elevated CO₂ concentrations can alleviate the impact of infection with Striga on the growth of C₃ hosts such as rice and also that infection can delay the onset of photosynthetic down‐regulation in rice grown at elevated CO₂.     

The effects on Arbutus unedo L. of long-term exposure to elevated CO2

Arbutus unedo is a sclerophyllous evergreen, characteristic of Mediterranean coastal scrub vegetation. In Italy, trees of A. unedo have been found close to natural CO₂ vents where the mean atmospheric carbon dioxide concentration is about 2200 μmol mol^?1. Comparisons were made between trees growing in elevated and ambient CO₂ concentrations to test for evidence of adaptation to long-term exposure to elevated CO₂. Leaves formed at elevated CO₂ have a lower stomatal density and stomatal index and higher specific leaf area than those formed at ambient CO₂, but there was no change in carbon to nitrogen ratios of the leaf tissue. Stomatal conductance was lower at elevated CO₂ during rapid growth in the spring. In mid-summer, under drought stress, stomatal closure of all leaves occurred and in the autumn, when stress was relieved, the conductance of leaves at both elevated and ambient CO₂ increased. In the spring, the stomatal conductance of the new flush of leaves at ambient CO₂ was higher than the leaves at elevated CO₂, increasing instantaneous water use efficiency at elevated CO₂. Chlorophyll fluorescence measurements suggested that elevated CO₂ provided some protection against photoinhibition in mid-summer. Analysis of A/C_i curves showed that there was no evidence of either upward or downward regulation of photosynthesis at elevated CO₂. It is therefore anticipated that A. unedo will have higher growth rates as the ambient CO₂ concentrations increase.     

Isoprene is more affected by climate drivers than monoterpenes: A meta‐analytic review on plant isoprenoid emissions

Isoprene and monoterpenes (MTs) are among the most abundant and reactive volatile organic compounds produced by plants (biogenic volatile organic compounds). We conducted a meta‐analysis to quantify the mean effect of environmental factors associated to climate change (warming, drought, elevated CO₂, and O₃) on the emission of isoprene and MTs. Results indicated that all single factors except warming inhibited isoprene emission. When subsets of data collected in experiments run under similar change of a given environmental factor were compared, isoprene and photosynthesis responded negatively to elevated O₃ (?8% and ?10%, respectively) and drought (?15% and ?42%), and in opposite ways to elevated CO₂ (?23% and +55%) and warming (+53% and ?23%, respectively). Effects on MTs emission were usually not significant, with the exceptions of a significant stimulation caused by warming (+39%) and by elevated O₃ (limited to O₃‐insensitive plants, and evergreen species with storage organs). Our results clearly highlight individual effects of environmental factors on isoprene and MT emissions, and an overall uncoupling between these secondary metabolites produced by the same methylerythritol 4‐phosphate pathway. Future results from manipulative experiments and long‐term observations may help untangling the interactive effects of these factors and filling gaps featured in the current meta‐analysis.     

SHORT‐ AND LONG‐TERM EFFECTS OF ELEVATED CO2 ON PHOTOSYNTHESIS AND RESPIRATION IN THE MARINE MACROALGA HIZIKIA FUSIFORMIS (SARGASSACEAE,PHAEOPHYTA) GROWN AT LOW AND HIGH N SUPPLIES1

The short‐term and long‐term effects of elevated CO₂ on photosynthesis and respiration were examined in cultures of the marine brown macroalga Hizikia fusiformis (Harv.) Okamura grown under ambient (375 μL · L^?1) and elevated (700 μL · L^?1) CO₂ concentrations and at low and high N availability. Short‐term exposure to CO₂ enrichment stimulated photosynthesis, and this stimulation was maintained with prolonged growth at elevated CO₂, regardless of the N levels in culture, indicating no down‐regulation of photosynthesis with prolonged growth at elevated CO₂. However, the photosynthetic rate of low‐N‐grown H. fusiformis was more responsive to CO₂ enrichment than that of high‐N‐grown algae. Elevation of CO₂ concentration increased the value of K_1/2(Ci) (the half‐saturation constant) for photosynthesis, whereas high N supply lowered it. Neither short‐term nor long‐term CO₂ enrichment had inhibitory effects on respiration rate, irrespective of the N supply, under which the algae were grown. Under high‐N growth, the Q₁₀ value of respiration was higher in the elevated‐CO₂‐grown algae than the ambient‐CO₂‐grown algae. Either short‐ or long‐term exposure to CO₂ enrichment decreased respiration as a proportion of gross photosynthesis (Pg) in low‐N‐grown H. fusiformis. It was proposed that in a future world of higher atmospheric CO₂ concentration and simultaneous coastal eutrophication, the respiratory carbon flux would be more sensitive to changing temperature.     

The effects of glacial atmospheric CO2 concentrations and climate on isoprene emissions by vascular plants

Isoprene (C₅H₈) emissions by terrestrial vegetation vary with temperature and light intensity, and play an important role in biosphere–chemistry–climate interactions. Such interactions were probably substantially modified by Pleistocene climate and CO₂ cycles. Central to understanding the nature of these modifications is assessment and analysis of how emissions changed under glacial environmental conditions. Currently, even the net direction of change is difficult to predict because a CO₂‐depleted atmosphere may have stimulated emissions compensating for the negative impacts of a cooler climate. Here, we address this issue and attempt to determine the direction of change from an experimental standpoint by investigating the interaction between isoprene emissions and plant growth of two known isoprene‐emitting herbaceous species (Mucuna pruriens and Arundo donax) grown at glacial (180 ppm) to present (366 ppm) CO₂ levels. We found a significant enhancement of isoprene emissions per unit leaf area in M. pruriens under subambient CO₂ concentrations relative to ambient controls but not for A. donax. In contrast, canopy emissions remained unaltered for both plant species because enhanced leaf emissions were offset by reductions in biomass and leaf area. Separate growth experiments with M. pruriens revealed that lowering day/night temperatures by 5°C decreased canopy isoprene emissions irrespective of the CO₂ level. Incorporation of these results into a simple canopy emissions model highlights their potential to attenuate reductions in the total isoprene flux from forests under glacial conditions predicted by standard models.     

Elevated CO2 alters carbon fluxes in early successional Mediterranean ecosystems

Annual carbon budgets of ecosystems are central to our understanding of the biotic control of atmospheric composition, but they are not available under elevated CO₂ for most vegetation types. Using gas exchange techniques, we assessed carbon fluxes of four early successional Mediterranean model communities, consisting of grasses, legumes and composites. The assemblages were grown on the same monoliths for three consecutive years in greenhouses tracking field conditions except for CO₂ maintained at ambient (370 μmol mol^?1) or elevated (700 μmol mol^?1) concentration. During the third year of study, CO₂ enrichment consistently shifted the annual carbon balance towards lower efflux, with displacements between 4.3 and 26.2 mol m^?2 y^?1 (one assemblage became a net CO₂ sink, another just reached equilibrium, and the remaining two remained as a CO₂ source). At least 50% of the shift under elevated CO₂ originated from a decrease in belowground respiration. This indicates that, during this year, CO₂ enrichment did not predominantly enhance C‐cycling, but on the contrary inhibited root respiration or microbial C‐utilization. Although elevated‐CO₂‐grown systems acted as a net CO₂ sink during a longer period of the year (4–7 months) compared with ambient‐CO₂‐grown systems (3–3.5 months), gross canopy photosynthesis was modified only to a limited extent (between ?5.9 and + 14.8%). Interaction between the carbon and the water cycle was apparently responsible for this weak stimulation. In particular, reduced evapotranspiration under elevated CO₂ coincided with inhibited canopy photosynthesis in early spring, most likely resulting from water saturation of the soil. In addition, only the earliest‐planted assemblages had an increased gross canopy photosynthesis during late autumn and early winter. This suggests that a longer summer drought, by delaying the establishment of such an annual type of vegetation, would reduce the positive impact of elevated CO₂ on productivity. Water regime appears to strongly govern the influence of CO₂ on the carbon fluxes in Mediterranean ecosystems with annual herbaceous vegetation.     

The interacting effects of elevated atmospheric CO<Subscript>2</Subscript> concentration,drought and leaf-to-air vapour pressure deficit on ecosystem isoprene fluxes

Isoprene is the most abundant biogenic hydrocarbon released from vegetation and it plays a major role in tropospheric chemistry. Because of its link to climate change, there is interest in understanding the relationship between CO₂, water availability and isoprene emission. We explored the effect of atmospheric elevated CO₂ concentration and its interaction with vapour pressure deficit (VPD) and water stress, on gross isoprene production (GIP) and net ecosystem exchange of CO₂ (NEE) in two Populus deltoides plantations grown at ambient and elevated atmospheric CO₂ concentration in the Biosphere 2 Laboratory facility. Although GIP and NEE showed a similar response to light and temperature, their responses to CO₂ and VPD were opposite; NEE was stimulated by elevated CO₂ and depressed by high VPD, while GIP was inhibited by elevated CO₂ and stimulated by high VPD. The difference in response between isoprene production and photosynthesis was also evident during water stress. GIP was stimulated in the short term and declined only when the stress was severe, whereas NEE started to decrease from the beginning of the experiment. This contrasting response led the carbon lost as isoprene in both the ambient and the elevated CO₂ treatments to increase as water stress progressed. Our results suggest that water limitation can override the inhibitory effect of elevated CO₂ leading to increased global isoprene emissions in a climate change scenario with warmer and drier climate.     

Growth in elevated CO2 protects photosynthesis against high-temperature damage

We present evidence that plant growth at elevated atmospheric CO₂ increases the high‐temperature tolerance of photosynthesis in a wide variety of plant species under both greenhouse and field conditions. We grew plants at ambient CO₂ (~ 360 μ mol mol ^{? 1}) and elevated CO₂ (550–1000 μ mol mol ^{? 1}) in three separate growth facilities, including the Nevada Desert Free‐Air Carbon Dioxide Enrichment (FACE) facility. Excised leaves from both the ambient and elevated CO₂ treatments were exposed to temperatures ranging from 28 to 48 °C. In more than half the species examined (4 of 7, 3 of 5, and 3 of 5 species in the three facilities), leaves from elevated CO₂‐grown plants maintained PSII efficiency (F_v/F_m) to significantly higher temperatures than ambient‐grown leaves. This enhanced PSII thermotolerance was found in both woody and herbaceous species and in both monocots and dicots. Detailed experiments conducted with Cucumis sativus showed that the greater F_v/F_m in elevated versus ambient CO₂‐grown leaves following heat stress was due to both a higher F_m and a lower F_o, and that F_v/F_m differences between elevated and ambient CO₂‐grown leaves persisted for at least 20 h following heat shock. Cucumis sativus leaves from elevated CO₂‐grown plants had a critical temperature for the rapid rise in F_o that averaged 2·9 °C higher than leaves from ambient CO₂‐grown plants, and maintained a higher maximal rate of net CO₂ assimilation following heat shock. Given that photosynthesis is considered to be the physiological process most sensitive to high‐temperature damage and that rising atmospheric CO₂ content will drive temperature increases in many already stressful environments, this CO₂‐induced increase in plant high‐temperature tolerance may have a substantial impact on both the productivity and distribution of many plant species in the 21st century.     

Photosynthetic acclimation to elevated CO2 in Phaseolus vulgaris L. is altered by growth response to nitrogen supply

Abstract Long‐term exposure of plants to elevated CO₂ often leads to downward photosynthetic acclimation. Nitrogen (N) deficiency could potentially exacerbate this response by reducing growth rate and the sink for photosynthates, but this has not always been observed. Experimentally, the interpretation of N effects on CO₂ responses can be confounded by increasing severity of tissue N deficiency over time when N supply is not adjusted as demand increases. In this study, N supply ranged from sub‐ to supra‐optimal (20–540 kgN ha^–l equivalent), and relatively stable levels of tissue N concentration were obtained in all treatments by varying twice‐weekly application rates in proportion to plant growth. The effects of N on photosynthesis and growth of beans (Phaseolus vulgaris L.) raised at ambient (35 Pa) and three elevated (70, 105, 140 Pa) CO₂ partial pressures (pCO₂) were evaluated. Averaging across N treatments, leaf total non‐structural carbohydrates (TNC) were 2.5‐ to 3‐fold higher and leaf N concentrations were 31–35% lower at elevated compared to ambient pCO₂. Light‐saturated net CO₂ assimilation rates measured at growth pCO₂ (A_satg) were significantly higher (26–40% depending on N supply) in plants grown at elevated compared to ambient pCO₂. When measured at a common pCO₂ of 35 Pa, the A_sat of plants grown at elevated CO₂ was 15–29% less than that of plants grown at 35 Pa, indicative of downward photosynthetic acclimation. The magnitude of downward photosynthetic acclimation to elevated CO₂ was greater in plants grown at high (180 and 540 kgN ha^–l) compared to low (20 and 60 kgN ha^–l) N supply, and this was associated with a higher A_sat at growth pCO₂, higher leaf area ratio (leaf area/total biomass), and higher TNC in leaves of high‐N plants. Our results indicate that the effect of N on acclimation to CO₂ will depend on the balance between supply and demand for N during the growing period, and the effect this has on biomass allocation and source‐sink C balance at the whole‐plant level.     

Sources of increased N uptake in forest trees growing under elevated CO2: results of a large‐scale 15N study

Nitrogen availability in terrestrial ecosystems strongly influences plant productivity and nutrient cycling in response to increasing atmospheric carbon dioxide (CO₂). Elevated CO₂ has consistently stimulated forest productivity at the Duke Forest free‐air CO₂ enrichment experiment throughout the decade‐long experiment. It remains unclear how the N cycle has changed with elevated CO₂ to support this increased productivity. Using natural‐abundance measures of N isotopes together with an ecosystem‐scale ¹⁵N tracer experiment, we quantified the cycling of ¹⁵N in plant and soil pools under ambient and elevated CO₂ over three growing seasons to determine how elevated CO₂ changed N cycling between plants, soil, and microorganisms. After measuring natural‐abundance ¹⁵N differences in ambient and CO₂‐fumigated plots, we applied inorganic ¹⁵N tracers and quantified the redistribution of ¹⁵N for three subsequent growing seasons. The natural abundance of leaf litter was enriched under elevated compared to ambient CO₂, consistent with deeper rooting and enhanced N mineralization. After tracer application, ¹⁵N was initially retained in the organic and mineral soil horizons. Recovery of ¹⁵N in plant biomass was 3.5 ± 0.5% in the canopy, 1.7 ± 0.2% in roots and 1.7 ± 0.2% in branches. After two growing seasons, ¹⁵N recoveries in biomass and soil pools were not significantly different between CO₂ treatments, despite greater total N uptake under elevated CO₂. After the third growing season, ¹⁵N recovery in trees was significantly higher in elevated compared to ambient CO₂. Natural‐abundance ¹⁵N and tracer results, taken together, suggest that trees growing under elevated CO₂ acquired additional soil N resources to support increased plant growth. Our study provides an integrated understanding of elevated CO₂ effects on N cycling in the Duke Forest and provides a basis for inferring how C and N cycling in this forest may respond to elevated CO₂ beyond the decadal time scale.     

Seasonal patterns of tissue water relations in three Mediterranean shrubs co‐occurring at a natural CO2 spring

Seasonal changes in tissue water relations of Erica arborea L., Myrtus communis L. and Juniperus communis L., grown in a Mediterranean environment, were analysed under field conditions over a 12 month period by comparing plants grown in the proximity of a natural CO₂ spring (about 700 μ mol mol ^{? 1} atmospheric CO₂ concentration, [CO₂]) with plants in ambient conditions. Tissue water relations varied in response to changes in water availability, but the seasonal course of tissue water relations parameters was also related to ontogeny. Tissue water relations of these co‐occurring shrubs were not alike. Osmotic potentials and saturated mass/dry mass ratio were lowest during peak drought stress periods. Diurnal changes in osmotic potential at the point of turgor loss were least early in the season, maximal in mid‐season, and decreased again in autumn. Turgor potentials decreased as drought progressed and were highest in late fall and mid‐winter. Symplastic water fraction was highest in mid‐spring for E. arborea and M. communis and decreased during the summer, while the opposite was observed for J. communis. Common to all species, under elevated [CO₂], was an increase of turgor pressure, particularly during the summer months. Other parameters showed species‐specific responses to long‐term elevated [CO₂]. In particular, exposure to elevated [CO₂] increased osmotic potentials in E. arborea under drought, while the opposite was the case for J. communis. Site differences in predawn to midday shifts were not strong in any of the species. Differences in tissue water relations suggest that the coexistence of these shrubs in the same environment with similar water availability are partially based on differential water relations strategies and water use patterns. Regardless of the mechanisms, growth of these shrubs in elevated [CO₂] may be either less, similarly or more affected by drought stress than plants in ambient [CO₂] depending on the species and season.