Climate change‐induced vegetation change as a driver of increased subarctic biogenic volatile organic compound emissions

Emissions of biogenic volatile organic compounds (BVOCs) have been earlier shown to be highly temperature sensitive in subarctic ecosystems. As these ecosystems experience rapidly advancing pronounced climate warming, we aimed to investigate how warming affects the BVOC emissions in the long term (up to 13 treatment years). We also aimed to assess whether the increased litterfall resulting from the vegetation changes in the warming subarctic would affect the emissions. The study was conducted in a field experiment with factorial open‐top chamber warming and annual litter addition treatments on subarctic heath in Abisko, northern Sweden. After 11 and 13 treatment years, BVOCs were sampled from plant communities in the experimental plots using a push–pull enclosure technique and collection into adsorbent cartridges during the growing season and analyzed with gas chromatography–mass spectrometry. Plant species coverage in the plots was analyzed by the point intercept method. Warming by 2 °C caused a 2‐fold increase in monoterpene and 5‐fold increase in sesquiterpene emissions, averaged over all measurements. When the momentary effect of temperature was diminished by standardization of emissions to a fixed temperature, warming still had a significant effect suggesting that emissions were also indirectly increased. This indirect increase appeared to result from increased plant coverage and changes in vegetation composition. The litter addition treatment also caused significant increases in the emission rates of some BVOC groups, especially when combined with warming. The combined treatment had both the largest vegetation changes and the highest BVOC emissions. The increased emissions under litter addition were probably a result of a changed vegetation composition due to alleviated nutrient limitation and stimulated microbial production of BVOCs. We suggest that the changes in the subarctic vegetation composition induced by climate warming will be the major factor indirectly affecting the BVOC emission potentials and composition.     

Non-methane biogenic volatile organic compound emissions from boreal peatland microcosms under warming and water table drawdown

Boreal peatlands have significant emissions of non-methane biogenic volatile organic compounds (BVOCs). Climate warming is expected to affect these ecosystems both directly, with increasing temperature, and indirectly, through water table drawdown following increased evapotranspiration. We assessed the combined effect of warming and water table drawdown on the BVOC emissions from boreal peatland microcosms. We also assessed the treatment effects on the BVOC emissions from the peat soil after the 7-week long experiment. Emissions of isoprene, monoterpenes, sesquiterpenes, other reactive VOCs and other VOCs were sampled using a conventional chamber technique, collected on adsorbent and analyzed by GC–MS. Carbon emitted as BVOCs was less than 1% of the CO₂ uptake and up to 3% of CH₄ emission. Water table drawdown surpassed the direct warming effect and significantly decreased the emissions of all BVOC groups. Only isoprene emission was significantly increased by warming, parallel to the increased leaf number of the dominant sedge Eriophorum vaginatum. BVOC emissions from peat soil were higher under the control and warming treatments than water table drawdown, suggesting an increased activity of anaerobic microbial community. Our results suggest that boreal peatlands could have concomitant negative and positive radiative forcing effects on climate warming following the effect of water table drawdown. The observed decrease in CH₄ emission causes a negative radiative forcing while the increase in CO₂ emission and decrease in reactive BVOC emissions, which could reduce the cooling effect induced by the lower formation rate of secondary organic aerosols, both contribute to increased radiative forcing.     

Isoprene emission from a subarctic peatland under enhanced UV-B radiation

Isoprene is a reactive hydrocarbon with an important role in atmospheric chemistry, and emissions from vegetation contribute to atmospheric carbon fluxes. The magnitude of isoprene emissions from arctic peatlands is not known, and it may be altered by increasing UV-B radiation. Isoprene emission was measured with the dynamic chamber method from a subarctic peatland under long-term enhancement of UV-B radiation targeted to correspond to a 20% loss in the stratospheric ozone layer. The site type of the peatland was a flark fen dominated by the moss Warnstorfia exannulata and sedges Eriophorum russeolum and Carex limosa. The relationship between species densities and the emission was also assessed. Isoprene emissions were significantly increased by enhanced UV-B radiation during the second (2004) and the fourth (2006) growing seasons under the UV-B exposure. Emissions were related to the density of E. russeolum. The dominant moss, W. exannulata, proved to emit small amounts of isoprene in a laboratory trial. Subarctic fens, even without Sphagnum moss, are a significant source of isoprene to the atmosphere, especially under periods of warm weather. Warming of the Arctic together with enhanced UV-B radiation may substantially increase the emissions.     

Methane emissions by alpine plant communities in the Qinghai-Tibet Plateau

For the first time to our knowledge, we report here methane emissions by plant communities in alpine ecosystems in the Qinghai-Tibet Plateau. This has been achieved through long-term field observations from June 2003 to July 2006 using a closed chamber technique. Strong methane emission at the rate of 26.2+/-1.2 and 7.8+/-1.1microg CH4 m-2h-1 was observed for a grass community in a Kobresia humilis meadow and a Potentilla fruticosa meadow, respectively. A shrub community in the Potentilla meadow consumed atmospheric methane at the rate of 5.8+/-1.3microg CH4 m-2h-1 on a regional basis; plants from alpine meadows contribute at least 0.13Tg CH4 yr-1 in the Tibetan Plateau. This finding has important implications with regard to the regional methane budget and species-level difference should be considered when assessing methane emissions by plants.     

Biogenic volatile organic compound emissions in four vegetation types in high arctic Greenland

Biogenic volatile organic compounds (BVOCs) emitted from terrestrial vegetation participate in oxidative reactions in the atmosphere, leading to the formation of secondary organic aerosols and longer lifetime of methane. Global models of BVOC emissions have assumed minimal emissions from the high latitudes. However, measurements from this region are lacking, and studies from the high arctic are yet to be published. This study aimed to obtain estimates for BVOC emissions from the high arctic, and hereby to add new knowledge to the understanding of global BVOC emissions. Measurements were conducted in four vegetation types dominated by Cassiope tetragona, Salix arctica, Vaccinium uliginosum and a mixture of Kobresia myosuroides, Dryas spp. and Poa arctica. Emissions were measured by an enclosure technique and collection of volatiles into adsorbent cartridges in August. Volatiles were analyzed by gas chromatography–mass spectrometry following thermal desorption. Isoprene showed highest emissions in S. arctica heath. Monoterpene and sesquiterpene emissions were especially associated with C. tetragona heath. Total observed emissions were comparable in magnitude to emissions previously found in the subarctic, whereas isoprene emissions were lower. This study shows that considerable amounts of BVOCs are emitted from the high arctic. The results are also of importance as the emissions from this region are expected to increase in the future as a result of the predicted climate warming in the high arctic. We suggest further studies to assess the effects of climate changes in the region in order to gain new knowledge and understanding of future global BVOC emissions.     

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.     

Isoprene synthesis in plants: lessons from a transgenic tobacco model

Isoprene is a highly reactive gas, and is emitted in such large quantities from the biosphere that it substantially affects the oxidizing potential of the atmosphere. Relatively little is known about the control of isoprene emission at the molecular level. Using transgenic tobacco lines harbouring a poplar isoprene synthase gene, we examined control of isoprene emission. Isoprene synthase required chloroplastic localization for catalytic activity, and isoprene was produced via the methyl erythritol (MEP) pathway from recently assimilated carbon. Emission patterns in transgenic tobacco plants were remarkably similar to naturally emitting plants under a wide variety of conditions. Emissions correlated with photosynthetic rates in developing and mature leaves, and with the amount of isoprene synthase protein in mature leaves. Isoprene synthase protein levels did not change under short-term increase in heat/light, despite an increase in emissions under these conditions. A robust circadian pattern could be observed in emissions from long-day plants. The data support the idea that substrate supply and changes in enzyme kinetics (rather than changes in isoprene synthase levels or post-translational regulation of activity) are the primary controls on isoprene emission in mature transgenic tobacco leaves.     

Net ecosystem exchange and whole plant isoprenoid emissions by a mediterranean shrubland exposed to experimental climate change

We tested the effect of forecasted soil drought and warming climate conditions for the next decades on emission rates of isoprenoids by mediterranean shrublands. We measured isoprenoid emissions by whole dominant mediterranean woody plants (Erica multiflora L. and Globularia alypum L.) inhabiting the studied shrublands. Monoterpene emissions were detected in both species, but isoprene was emitted only by E. multiflora. Maximum emission rates were found during the hottest periods (except for G. alypum, in which they occurred in autumn), and minimum emission rates in winter in E. multiflora. Terpene emission rates ranged from 0.08 μg/(g dry wt h) in winter in E. multiflora to 8.8 μg/(g dry wt h) in G. alypum in autumn. In E. multiflora, the terpene emission rates decreased in response to soil drought only in summer, but increased in response to warming in spring and autumn. Isoprene emissions ranged from 0.1 μg/(g dry wt h) in spring to 4.4 μg/(g dry wt h) in summer. The effect of the treatments was only detected in autumn when soil drought and warming had a negative effect on isoprene emission rates. These data might improve our knowledge of isoprenoid emissions at the canopy level and in response to climate change, soil drought, or warming. Published in Russian in Fiziologiya Rastenii, 2009, Vol. 56, No. 1, pp. 35–45. The text was submitted by the authors in English.     

Enhanced summer warming reduces fungal decomposer diversity and litter mass loss more strongly in dry than in wet tundra

Many Arctic regions are currently experiencing substantial summer and winter climate changes. Litter decomposition is a fundamental component of ecosystem carbon and nutrient cycles, with fungi being among the primary decomposers. To assess the impacts of seasonal climatic changes on litter fungal communities and their functioning, Betula glandulosa leaf litter was surface‐incubated in two adjacent low Arctic sites with contrasting soil moisture regimes: dry shrub heath and wet sedge tundra at Disko Island, Greenland. At both sites, we investigated the impacts of factorial combinations of enhanced summer warming (using open‐top chambers; OTCs) and deepened snow (using snow fences) on surface litter mass loss, chemistry and fungal decomposer communities after approximately 1 year. Enhanced summer warming significantly restricted litter mass loss by 32% in the dry and 17% in the wet site. Litter moisture content was significantly reduced by summer warming in the dry, but not in the wet site. Likewise, fungal total abundance and diversity were reduced by OTC warming at the dry site, while comparatively modest warming effects were observed in the wet site. These results suggest that increased evapotranspiration in the OTC plots lowered litter moisture content to the point where fungal decomposition activities became inhibited. In contrast, snow addition enhanced fungal abundance in both sites but did not significantly affect litter mass loss rates. Across sites, control plots only shared 15% of their fungal phylotypes, suggesting strong local controls on fungal decomposer community composition. Nevertheless, fungal community functioning (litter decomposition) was negatively affected by warming in both sites. We conclude that although buried soil organic matter decomposition is widely expected to increase with future summer warming, surface litter decay and nutrient turnover rates in both xeric and relatively moist tundra are likely to be significantly restricted by the evaporative drying associated with warmer air temperatures.     

Stimulation of isoprene emissions and electron transport rates as key mechanisms of thermal tolerance in the tropical species Vismia guianensis

Tropical forests absorb large amounts of atmospheric CO₂ through photosynthesis, but high surface temperatures suppress this absorption while promoting isoprene emissions. While mechanistic isoprene emission models predict a tight coupling to photosynthetic electron transport (ETR) as a function of temperature, direct field observations of this phenomenon are lacking in the tropics and are necessary to assess the impact of a warming climate on global isoprene emissions. Here we demonstrate that in the early successional species Vismia guianensis in the central Amazon, ETR rates increased with temperature in concert with isoprene emissions, even as stomatal conductance (g_s) and net photosynthetic carbon fixation (P_n) declined. We observed the highest temperatures of continually increasing isoprene emissions yet reported (50°C). While P_n showed an optimum value of 32.6 ± 0.4°C, isoprene emissions, ETR, and the oxidation state of PSII reaction centers (q_L) increased with leaf temperature with strong linear correlations for ETR (? = 0.98) and q_L (? = 0.99) with leaf isoprene emissions. In contrast, other photoprotective mechanisms, such as non‐photochemical quenching, were not activated at elevated temperatures. Inhibition of isoprenoid biosynthesis repressed P_n at high temperatures through a mechanism that was independent of stomatal closure. While extreme warming will decrease g_s and P_n in tropical species, our observations support a thermal tolerance mechanism where the maintenance of high photosynthetic capacity under extreme warming is assisted by the simultaneous stimulation of ETR and metabolic pathways that consume the direct products of ETR including photorespiration and the biosynthesis of thermoprotective isoprenoids. Our results confirm that models which link isoprene emissions to the rate of ETR hold true in tropical species and provide necessary “ground‐truthing” for simulations of the large predicted increases in tropical isoprene emissions with climate warming.     

Rapid leaf development drives the seasonal pattern of volatile organic compound (VOC) fluxes in a ‘coppiced’ bioenergy poplar plantation

Leaves of fast‐growing, woody bioenergy crops often emit volatile organic compounds (VOC). Some reactive VOC (especially isoprene) play a key role in climate forcing and may negatively affect local air quality. We monitored the seasonal exchange of VOC using the eddy covariance technique in a ‘coppiced’ poplar plantation. The complex interactions of VOC fluxes with climatic and physiological variables were also explored by using an artificial neural network (Self Organizing Map). Isoprene and methanol were the most abundant VOC emitted by the plantation. Rapid development of the canopy (and thus of the leaf area index, LAI) was associated with high methanol emissions and high rates of gross primary production (GPP) since the beginning of the growing season, while the onset of isoprene emission was delayed. The highest emissions of isoprene, and of isoprene photo‐oxidation products (Methyl Vinyl Ketone and Methacrolein, i_ox), occurred on the hottest and sunniest days, when GPP and evapotranspiration were highest, and formaldehyde was significantly deposited. Canopy senescence enhanced the exchange of oxygenated VOC. The accuracy of methanol and isoprene emission simulations with the Model of Emissions of Gases and Aerosols from Nature increased by applying a function to modify their basal emission factors, accounting for seasonality of GPP or LAI.     

The Influence of Light and Temperature on Isoprene Emission Rates from Live Oak

There is a growing awareness of the role of vegetation as a source of reactive hydrocarbons that may serve as photochemical oxidant precursors. A study was designed to assess independently the influence of variable light and temperature on isoprene emissions from live oak (Quercus virginiana Mill.). Plants were conditioned in a growth chamber and then transferred to an environmentally controlled gas-exchange chamber. Samples of the chamber atmosphere were collected; isoprene was concentrated cryogenically and measured by gas chromatography. A logistic function was used to model isoprene emission rates. Under regimes of low temperature (20°C) or darkness, isoprene emissions were lowest. With increasing temperature or light intensity, the rate of isoprene emission increased, reaching maxima at 800 μE m^-2 s^-1 and 40–44°C, respectively. Higher temperatures caused a large decrease in emissions. Since the emissions of isoprene were light-saturated at moderate intensities, temperature appeared to be the main factor controlling emissions during most of the day. Carbon lost through isoprene emissions accounted for 0.1 to 2% of the carbon fixed during photosynthesis depending on light intensity and temperature.     

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).     

Modelling the effect of the 2018 summer heatwave and drought on isoprene emissions in a UK woodland

Projected future climatic extremes such as heatwaves and droughts are expected to have major impacts on emissions and concentrations of biogenic volatile organic compounds (bVOCs) with potential implications for air quality, climate and human health. While the effects of changing temperature and photosynthetically active radiation (PAR) on the synthesis and emission of isoprene, the most abundant of these bVOCs, are well known, the role of other environmental factors such as soil moisture stress are not fully understood and are therefore poorly represented in land surface models. As part of the Wytham Isoprene iDirac Oak Tree Measurements campaign, continuous measurements of isoprene mixing ratio were made throughout the summer of 2018 in Wytham Woods, a mixed deciduous woodland in southern England. During this time, the United Kingdom experienced a prolonged heatwave and drought, and isoprene mixing ratios were observed to increase by more than 400% at Wytham Woods under these conditions. We applied the state‐of‐the‐art FORest Canopy‐Atmosphere Transfer canopy exchange model to investigate the processes leading to these elevated concentrations. We found that although current isoprene emissions algorithms reproduced observed mixing ratios in the canopy before and after the heatwave, the model underestimated observations by ~40% during the heatwave–drought period implying that models may substantially underestimate the release of isoprene to the atmosphere in future cases of mild or moderate drought. Stress‐induced emissions of isoprene based on leaf temperature and soil water content (SWC) were incorporated into current emissions algorithms leading to significant improvements in model output. A combination of SWC, leaf temperature and rewetting emission bursts provided the best model‐measurement fit with a 50% improvement compared to the baseline model. Our results highlight the need for more long‐term ecosystem‐scale observations to enable improved model representation of atmosphere–biosphere interactions in a changing global climate.     

Litter decomposition in grasslands of Central North America (US Great Plains)

One of the major concerns about global warming is the potential for an increase in decomposition and soil respiration rates, increasing CO₂ emissions and creating a positive feedback between global warming and soil respiration. This is particularly important in ecosystems with large belowground biomass, such as grasslands where over 90% of the carbon is allocated belowground. A better understanding of the relative influence of climate and litter quality on litter decomposition is needed to predict these changes accurately in grasslands. The Long‐Term Intersite Decomposition Experiment Team (LIDET) dataset was used to evaluate the influence of climatic variables (temperature, precipitation, actual evapotranspiration, and climate decomposition index), and litter quality (lignin content, carbon : nitrogen, and lignin : nitrogen ratios) on leaf and root decomposition in the US Great Plains. Wooden dowels were used to provide a homogeneous litter quality to evaluate the relative importance of above and belowground environments on decomposition. Contrary to expectations, temperature did not explain variation in root and leaf decomposition, whereas precipitation partially explained variation in root decomposition. Percent lignin was the best predictor of leaf and root decomposition. It also explained most variation in root decomposition in models which combined litter quality and climatic variables. Despite the lack of relationship between temperature and root decomposition, temperature could indirectly affect root decomposition through decreased litter quality and increased water deficits. These results suggest that carbon flux from root decomposition in grasslands would increase, as result of increasing temperature, only if precipitation is not limiting. However, where precipitation is limiting, increased temperature would decrease root decomposition, thus likely increasing carbon storage in grasslands. Under homogeneous litter quality, belowground decomposition was faster than aboveground and was best predicted by mean annual precipitation, which also suggests that the high moisture in soil accelerates decomposition belowground.     

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.     

The shift in plant species composition in a subarctic mountain birch forest floor due to climate change would modify the biogenic volatile organic compound emission profile

Background and aims

Mountain birch forests dominate in the Subarctic but little is known of their non-methane biogenic volatile organic compound (BVOC) emissions. The dwarf shrubs Empetrum hermaphroditum, Vaccinium myrtillus and Vaccinium uliginosum co-dominate in the forest floors of these forests. The abundance of these three dwarf shrubs relative to each other could be affected by climate warming expected to increase nutrient availability by accelerating litter decomposition and nutrient mineralization. We 1) compared the BVOC emission profiles of vegetation covers dominated by E. hermaphroditum and V. myrtillus plus V. uliginosum in a subarctic mountain birch forest floor, 2) distinguished the BVOCs emitted from plants and soil and 3) measured how the BVOC emissions from the different vegetation covers differed under darkness.

Methods

BVOCs were sampled during two growing seasons using a conventional ecosystem chamber-based method, collected on adsorbent and analyzed with gas chromatography–mass spectrometry.

Results

High abundance of E. hermaphroditum increased the sesquiterpene emissions. Soil released fewer different BVOCs than controls (i.e. natural vegetation) but the total emission rates were similar. Darkness did not affect the emissions. Carbon emitted as BVOCs was less than 0.2% of the CO₂ exchange.

Conclusions

Our results suggest that sesquiterpene emissions from subarctic mountain birch forest floors would be reduced following an increased abundance of V. myrtillus and V. uliginosum with climate change because these species respond rapidly to increased nutrient availability.     

Relationships among Isoprene Emission Rate, Photosynthesis, and Isoprene Synthase Activity as Influenced by Temperature

Isoprene emissions from the leaves of velvet bean (Mucuna pruriens L. var utilis) plants exhibited temperature response patterns that were dependent on the plant's growth temperature. Plants grown in a warm regimen (34/28°C, day/night) exhibited a temperature optimum for emissions of 45°C, whereas those grown in a cooler regimen (26/20°C, day/night) exhibited an optimum of 40°C. Several previous studies have provided evidence of a linkage between isoprene emissions and photosynthesis, and more recent studies have demonstrated that isoprene emissions are linked to the activity of isoprene synthase in plant leaves. To further explore this linkage within the context of the temperature dependence of isoprene emissions, we determined the relative temperature dependencies of photosynthetic electron transport, CO₂ assimilation, and isoprene synthase activity. When measured over a broad range of temperatures, the temperature dependence of isoprene emission rate was not closely correlated with either the electron transport rate or the CO₂ assimilation rate. The temperature optima for electron transport rate and CO₂ assimilation rate were 5 to 10°C lower than that for the isoprene emission rate. The dependence of isoprene emissions on photon flux density was also affected by measurement temperature in a pattern independent of those exhibited for electron transport rate and CO₂ assimilation rate. Thus, despite no change in the electron transport rate or CO₂ assimilation rate at 26 and 34°C, the isoprene emission rate changed markedly. The quantum yield of isoprene emissions was stimulated by a temperature increase from 26 to 34°C, whereas the quantum yield for CO₂ assimilation was inhibited. In greenhouse-grown aspen leaves (Populus tremuloides Michaux.), the high temperature threshold for inhibition of isoprene emissions was closely correlated with the high temperature-induced decrease in the in vitro activity of isoprene synthase. When taken together, the results indicate that although there may be a linkage between isoprene emission rate and photosynthesis, the temperature dependence of isoprene emission is not determined solely by the rates of CO₂ assimilation or electron transport. Rather, we propose that regulation is accomplished primarily through the enzyme isoprene synthase.     

Circadian control of isoprene emissions from oil palm (Elaeis guineensis)

The emission of isoprene from the biosphere to the atmosphere has a profound effect on the Earth's atmospheric system. Until now, it has been assumed that the primary short-term controls on isoprene emission are photosynthetically active radiation and temperature. Here we show that isoprene emissions from a tropical tree (oil palm, Elaeis guineensis) are under strong circadian control, and that the circadian clock is potentially able to gate light-induced isoprene emissions. These rhythms are robustly temperature compensated with isoprene emissions still under circadian control at 38 degrees C. This is well beyond the acknowledged temperature range of all previously described circadian phenomena in plants. Furthermore, rhythmic expression of LHY/CCA1, a genetic component of the central clock in Arabidopsis thaliana, is still maintained at these elevated temperatures in oil palm. Maintenance of the CCA1/LHY-TOC1 molecular oscillator at these temperatures in oil palm allows for the possibility that this system is involved in the control of isoprene emission rhythms. This study contradicts the accepted theory that isoprene emissions are primarily light-induced.