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.     

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].     

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.     

Carbon dioxide enrichment alleviates heat stress by improving cellular redox homeostasis through an ABA‐independent process in tomato plants

Plant responses to elevated CO₂ and high temperature are critically regulated through a complex network of phytohormones and redox homeostasis. However, the involvement of abscisic acid (ABA) in plant adaptation to heat stress under elevated CO₂ conditions has not been thoroughly studied. This study investigated the interactive effects of elevated CO₂ (800 μmol·mol^?1) and heat stress (42 °C for 24 h) on the endogenous level of ABA and the cellular redox state of two genotypes of tomato with different ABA biosynthesis capacities. Heat stress significantly decreased maximum photochemical efficiency of PSII (Fv/Fm) and leaf water potential, but also increased levels of malondialdehyde (MDA) and electrolyte leakage (EL) in both genotypes. Heat‐induced damage was more severe in the ABA‐deficient mutant notabilis (not) than in its parental cultivar Ailsa Craig (Ailsa), suggesting that a certain level of endogenous ABA is required to minimise the heat‐induced oxidative damage to the photosynthetic apparatus. Irrespective of genotype, the enrichment of CO₂ remarkably stimulated Fv/Fm, MDA and EL in heat‐stressed plants towards enhanced tolerance. In addition, elevated CO₂ significantly strengthened the antioxidant capacity of heat‐stressed tomato seedlings towards a reduced cellular redox state for a prolonged period, thereby mitigating oxidative stress. However, elevated CO₂ and heat stress did not alter the endogenous level of ABA or the expression of its biosynthetic gene NCED2 in either genotype, indicating that ABA is not involved in elevated CO₂‐induced heat stress alleviation. The results of this study suggest that elevated CO₂ alleviated heat stress through efficient regulation of the cellular redox poise in an ABA‐independent manner in tomato plants.     

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.     

Within‐plant isoprene oxidation confirmed by direct emissions of oxidation products methyl vinyl ketone and methacrolein

Isoprene is emitted from many terrestrial plants at high rates, accounting for an estimated 1/3 of annual global volatile organic compound emissions from all anthropogenic and biogenic sources combined. Through rapid photooxidation reactions in the atmosphere, isoprene is converted to a variety of oxidized hydrocarbons, providing higher order reactants for the production of organic nitrates and tropospheric ozone, reducing the availability of oxidants for the breakdown of radiatively active trace gases such as methane, and potentially producing hygroscopic particles that act as effective cloud condensation nuclei. However, the functional basis for plant production of isoprene remains elusive. It has been hypothesized that in the cell isoprene mitigates oxidative damage during the stress‐induced accumulation of reactive oxygen species (ROS), but the products of isoprene‐ROS reactions in plants have not been detected. Using pyruvate‐2‐¹³C leaf and branch feeding and individual branch and whole mesocosm flux studies, we present evidence that isoprene (i) is oxidized to methyl vinyl ketone and methacrolein (i_ox) in leaves and that i_ox/i emission ratios increase with temperature, possibly due to an increase in ROS production under high temperature and light stress. In a primary rainforest in Amazonia, we inferred significant in plant isoprene oxidation (despite the strong masking effect of simultaneous atmospheric oxidation), from its influence on the vertical distribution of i_ox uptake fluxes, which were shifted to low isoprene emitting regions of the canopy. These observations suggest that carbon investment in isoprene production is larger than that inferred from emissions alone and that models of tropospheric chemistry and biota–chemistry–climate interactions should incorporate isoprene oxidation within both the biosphere and the atmosphere with potential implications for better understanding both the oxidizing power of the troposphere and forest response to climate change.     

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.     

The capacity to emit isoprene differentiates the photosynthetic temperature responses of tropical plant species

Experimental research shows that isoprene emission by plants can improve photosynthetic performance at high temperatures. But whether species that emit isoprene have higher thermal limits than non‐emitting species remains largely untested. Tropical plants are adapted to narrow temperature ranges and global warming could result in significant ecosystem restructuring due to small variations in species' thermal tolerances. We compared photosynthetic temperature responses of 26 co‐occurring tropical tree and liana species to test whether isoprene‐emitting species are more tolerant to high temperatures. We classified species as isoprene emitters versus non‐emitters based on published datasets. Maximum temperatures for net photosynthesis were ~1.8°C higher for isoprene‐emitting species than for non‐emitters, and thermal response curves were 24% wider; differences in optimum temperatures (T_opt) or photosynthetic rates at T_opt were not significant. Modelling the carbon cost of isoprene emission, we show that even strong emission rates cause little reduction in the net carbon assimilation advantage over non‐emitters at supraoptimal temperatures. Isoprene emissions may alleviate biochemical limitations, which together with stomatal conductance, co‐limit photosynthesis above T_opt. Our findings provide evidence that isoprene emission may be an adaptation to warmer thermal niches, and that emitting species may fare better under global warming than co‐occurring non‐emitting species.     

Impact of rising CO2 on emissions of volatile organic compounds: isoprene emission from Phragmites australis growing at elevated CO2 in a natural carbon dioxide spring†

Isoprene basal emission (the emission of isoprene from leaves exposed to a light intensity of 1000 µmol m^?2 s^?1 and maintained at a temperature of 30 °C) was measured in Phragmites australis plants growing under elevated CO₂ in the Bossoleto CO₂ spring at Rapolano Terme, Italy, and under ambient CO₂ at a nearby control site. Gas exchange and biochemical measurements were concurrently taken. Isoprene emission was lower in the plants growing at elevated CO₂ than in those growing at ambient CO₂. Isoprene emission and isoprene synthase activity (IsoS) were very low in plants growing at the bottom of the spring under very rich CO₂ and increased at increasing distance from the spring (and decreasing CO₂ concentration). Distance from the spring did not significantly affect photosynthesis making it therefore unlikely that there is carbon limitation to isoprene formation. The isoprene emission rate was very quickly reduced after rapid switches from elevated to ambient CO₂ in the gas‐exchange cuvette, whereas it increased when switching from ambient to elevated CO₂. The rapidity of the response may be consistent with post‐translational modifications of enzymes in the biosynthetic pathway of isoprene formation. Reduction of IsoS activity is interpreted as a long‐term response. Basal emission of isoprene was not constant over the day but showed a diurnal course opposite to photosynthesis, with a peak during the hottest hours of the day, independent of stomatal conductance and probably dependent on external air temperature or temporary reduction of CO₂ concentration. The present experiments show that basal emission rate of isoprene is likely to be reduced under future elevated CO₂ levels and allow improvement in the modelling of future isoprene emission rates.     

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.     

Alternatives for natural‐gas‐based heating systems: A quantitative GIS‐based analysis of climate impacts and financial feasibility

The heating of buildings currently produces 6% of global greenhouse gas emissions. Sustainable heating technologies can reduce heating‐related CO₂ emissions by up to 90%. We present a Python‐based GIS model to analyze the environmental and financial impact of strategies to reduce heating‐related CO₂ emissions of residential buildings. The city‐wide implementation of three alternatives to natural gas are evaluated: high‐temperature heating networks, low‐temperature heating networks, and heat pumps. We find that both lowering the demand for heat and providing more sustainable sources of heat will be necessary to achieve significant CO₂‐emission reductions. Of the studied alternatives, only low‐temperature heating networks and heat pumps have the potential to reduce CO₂ emissions by 90%. A CO₂ tax and an increase in tax on the use of natural gas are potent policy tools to accelerate the adoption of low‐carbon heating technologies.     

Effect of heat wave on N2 fixation and N remobilisation of lentil (Lens culinaris MEDIK) grown under free air CO2 enrichment in a mediterranean‐type environment

• The stimulatory effect of elevated [CO₂] (e[CO₂]) on crop production in future climates is likely to be cancelled out by predicted increases in average temperatures. This effect may become stronger through more frequent and severe heat waves, which are predicted to increase in most climate change scenarios. Whilst the growth and yield response of some legumes grown under the interactive effect of e[CO₂] and heat waves has been studied, little is known about how N₂ fixation and overall N metabolism is affected by this combination.
• To address these knowledge gaps, two lentil genotypes were grown under ambient [CO₂] (a[CO₂], ~400 µmol·mol^?1) and e[CO₂] (~550 µmol·mol^?1) in the Australian Grains Free Air CO₂ Enrichment facility and exposed to a simulated heat wave (3‐day periods of high temperatures ~40 °C) at flat pod stage. Nodulation and concentrations of water‐soluble carbohydrates (WSC), total free amino acids, N and N₂ fixation were assessed following the imposition of the heat wave until crop maturity.
• Elevated [CO₂] stimulated N₂ fixation so that total N₂ fixation in e[CO₂]‐grown plants was always higher than in a[CO₂], non‐stressed control plants. Heat wave triggered a significant decrease in active nodules and WSC concentrations, but e[CO₂] had the opposite effect. Leaf N remobilization and grain N improved under interaction of e[CO₂] and heat wave.
• These results suggested that larger WSC pools and nodulation under e[CO₂] can support post‐heat wave recovery of N₂ fixation. Elevated [CO₂]‐induced accelerated leaf N remobilisation might contribute to restore grain N concentration following a heat wave.

     

Detection of plant volatiles after leaf wounding and darkening by proton transfer reaction "time-of-flight" mass spectrometry (PTR-TOF)

Proton transfer reaction-time of flight (PTR-TOF) mass spectrometry was used to improve detection of biogenic volatiles organic compounds (BVOCs) induced by leaf wounding and darkening. PTR-TOF measurements unambiguously captured the kinetic of the large emissions of green leaf volatiles (GLVs) and acetaldehyde after wounding and darkening. GLVs emission correlated with the extent of wounding, thus confirming to be an excellent indicator of mechanical damage. Transient emissions of methanol, C5 compounds and isoprene from plant species that do not emit isoprene constitutively were also detected after wounding. In the strong isoprene-emitter Populus alba, light-dependent isoprene emission was sustained and even enhanced for hours after photosynthesis inhibition due to leaf cutting. Thus isoprene emission can uncouple from photosynthesis and may occur even after cutting leaves or branches, e.g., by agricultural practices or because of abiotic and biotic stresses. This observation may have important implications for assessments of isoprene sources and budget in the atmosphere, and consequences for tropospheric chemistry.     

Contribution of volatile organic compound fluxes to the ecosystem carbon budget of a poplar short‐rotation plantation

Biogenic volatile organic compounds (BVOCs) are major precursors of both ozone and secondary organic aerosols (SOA) in the troposphere and represent a non‐negligible portion of the carbon fixed by primary producers, but long‐term ecosystem‐scale measurements of their exchanges with the atmosphere are lacking. In this study, the fluxes of 46 ions corresponding to 36 BVOCs were continuously monitored along with the exchanges of mass (carbon dioxide and water vapor) and energy (sensible and latent heat) for an entire year in a poplar (Populus) short‐rotation crop (SRC), using the eddy covariance methodology. BVOC emissions mainly consisted of isoprene, acetic acid, and methanol. Total net BVOC emissions were 19.20 kg C ha^?1 yr^?1, which represented 0.63% of the net ecosystem exchange (NEE), resulting from ?23.59 Mg C ha^?1 yr^?1 fixed as CO₂ and 20.55 Mg C ha^?1 yr^?1 respired as CO₂ from the ecosystem. Isoprene emissions represented 0.293% of NEE, being emitted at a ratio of 1 : 1709 mol isoprene per mol of CO₂ fixed. Based on annual ecosystem‐scale measurements, this study quantified for the first time that BVOC carbon emissions were lower than previously estimated in other studies (0.5–2% of NEE) on poplar trees. Furthermore, the seasonal and diurnal emission patterns of isoprene, methanol, and other BVOCs provided a better interpretation of the relationships with ecosystem CO₂ and water vapor fluxes, with air temperature, vapor pressure deficit, and photosynthetic photon flux density.     

Aphid and host‐plant genotype × genotype interactions under elevated CO2

1. Elevated CO₂ can alter plant physiology and morphology, and these changes are expected to impact diet quality for insect herbivores. While the plastic responses of insect herbivores have been well studied, less is known about the propensity of insects to adapt to such changes. Genetic variation in insect responses to elevated CO₂ and genetic interactions between insects and their host plants may exist and provide the necessary raw material for adaptation. 2. We used clonal lines of Rhopalosiphum padi (L.) aphids to examine genotype‐specific responses to elevated CO₂. We used the host plant Schedonorus arundinaceus (tall fescue; Schreb), which is capable of asexual reproduction, to investigate host plant genotype‐specific effects and possible host plant‐by‐insect genotype interactions. The abundance and density of three R. padi genotypes on three tall fescue genotypes under three concentrations of CO₂ (ambient, 700, and 1000 ppm) in a controlled greenhouse environment were examined. 3. Aphid abundance decreased in the 700 ppm CO₂ concentration, but increased in the 1000 ppm concentration relative to ambient. The effect of CO₂ on aphid density was dependent on host plant genotype; the density of aphids in high CO₂ decreased for two plant genotypes but was unchanged in one. No interaction between aphid genotype and elevated CO₂ was found, nor did we find significant genotype‐by‐genotype interactions. 4. This study suggests that the density of R. padi aphids feeding on tall fescue may decrease under elevated CO₂ for some plant genotypes. The likely impact of genotype‐specific responses on future changes in the genetic structure of plant and insect populations is discussed.     

Response of isoprene emission to ambient CO2 changes and implications for global budgets

We explore the potential role of atmospheric carbon dioxide (CO₂) on isoprene emissions using a global coupled land–atmosphere model [Community Atmospheric Model–Community Land Model (CAM–CLM)] for recent (year 2000, 365 ppm CO₂) and future (year 2100, 717 ppm CO₂) conditions. We incorporate an empirical model of observed isoprene emissions response to both ambient CO₂ concentrations in the long‐term growth environment and short‐term changes in intercellular CO₂ concentrations into the MEGAN biogenic emission model embedded within the CLM. Accounting for CO₂ inhibition has little impact on predictions of present‐day global isoprene emission (increase from 508 to 523 Tg C yr^?1). However, the large increases in future isoprene emissions typically predicted in models, which are due to a projected warmer climate, are entirely offset by including the CO₂ effects. Projected global isoprene emissions in 2100 drop from 696 to 479 Tg C yr^?1 when this effect is included, maintaining future isoprene sources at levels similar to present day. The isoprene emission response to CO₂ is dominated by the long‐term growth environment effect, with modulations of 10% or less due to the variability in intercellular CO₂ concentration. As a result, perturbations to isoprene emissions associated with changes in ambient CO₂ are largely aseasonal, with little diurnal variability. Future isoprene emissions increase by more than a factor of two in 2100 (to 1242 Tg C yr^?1) when projected changes in vegetation distribution and leaf area density are included. Changing land cover and the role of nutrient limitation on CO₂ fertilization therefore remain the largest source of uncertainty in isoprene emission prediction. Although future projections suggest a compensatory balance between the effects of temperature and CO₂ on isoprene emission, the enhancement of isoprene emission due to lower ambient CO₂ concentrations did not compensate for the effect of cooler temperatures over the last 400 thousand years of the geologic record (including the Last Glacial Maximum).     

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     

Stress-induced changes in carbon sources for isoprene production in Populus deltoides

Isoprene is emitted from leaves of numerous plant species and has important implications for plant metabolism and atmospheric chemistry. The ability to use stored carbon (alternative carbon sources), as opposed to recently assimilated photosynthate, for isoprene production may be important as plants routinely experience photosynthetic depression in response to environmental stress. A CO₂‐labelling study was performed and stable isotopes of carbon were used to examine the role of alternative carbon sources in isoprene production in Populus deltoides during conditions of water stress and high leaf temperature. Isotopic fractionation during isoprene production was higher in heat‐ and water‐stressed leaves (?8.5 and ?9.3‰, respectively) than in unstressed controls (?2.5 to ?3.2‰). In unstressed plants, 84–88% of the carbon in isoprene was derived from recently assimilated photosynthate. A significant shift in the isoprene carbon composition from photosynthate to alternative carbon sources was observed only under severe photosynthetic limitation (stomatal conductance < 0.05 mol m^?2 s^?1). The contribution of photosynthate to isoprene production decreased to 77 and 61% in heat‐ and water‐stressed leaves, respectively. Across water‐ and heat‐stress experiments, allocation of photosynthate was negatively correlated to the ratio of isoprene emission to photosynthesis. In water‐stressed plants, the use of alternative carbon was also related to stomatal conductance. It has been proposed that isoprene emission may be regulated by substrate availability. Thus, understanding carbon partitioning to isoprene production from multiple sources is essential for building predictive models of isoprene emission.     

Modelling changes in VOC emission in response to climate change in the continental United States

The alteration of climate is driven not only by anthropogenic activities, but also by biosphere processes that change in conjunction with climate. Emission of volatile organic compounds (VOCs) from vegetation may be particularly sensitive to changes in climate and may play an important role in climate forcing through their influence on the atmospheric oxidative balance, greenhouse gas concentration, and the formation of aerosols. Using the VEMAP vegetation database and associated vegetation responses to climate change, this study examined the independent and combined effects of simulated changes in temperature, CO₂ concentration, and vegetation distribution on annual emissions of isoprene, monoterpenes, and other reactive VOCs (ORVOCs) from potential vegetation of the continental United States. Temperature effects were modelled according to the direct influence of temperature on enzymatic isoprene production and the vapour pressure of monoterpenes and ORVOCs. The effect of elevated CO₂ concentration was modelled according to increases in foliar biomass per unit of emitting surface area. The effects of vegetation distribution reflects simulated changes in species spatial distribution and areal coverage by 21 different vegetation classes. Simulated climate warming associated with a doubled atmospheric CO₂ concentration enhanced total modelled VOC emission by 81.8% (isoprene + 82.1%, monoterpenes + 81.6%, ORVOC + 81.1%), whereas a simulated doubled CO₂ alone enhanced total modelled VOC emission by only + 11.8% (isoprene + 13.7%, monoterpenes + 4.1%, ORVOC + 11.7%). A simulated redistribution of vegetation in response to altered temperatures and precipitation patterns caused total modelled VOC emission to decline by 10.4% (isoprene – 11.7%, monoterpenes – 18.6%, ORVOC 0.0%) driven by a decline in area covered by vegetation classes emitting VOCs at high rates. Thus, the positive effect of leaf-level adjustments to elevated CO₂ (i.e. increases in foliar biomass) is balanced by the negative effect of ecosystem-level adjustments to climate (i.e. decreases in areal coverage of species emitting VOC at high rates).