Herbivory by an Outbreaking Moth Increases Emissions of Biogenic Volatiles and Leads to Enhanced Secondary Organic Aerosol Formation Capacity

Atmospheric Degradation of Ecologically Important Biogenic Volatiles: Investigating the Ozonolysis of (E)-β-Ocimene, Isomers of α and β-Farnesene, α-Terpinene and 6-Methyl-5-Hepten-2-One, and Their Gas-Phase Products

Biogenic volatile organic compounds (bVOCs), synthesised by plants, are important mediators of ecological interactions that can also undergo a series of reactions in the atmosphere. Ground-level ozone is a secondary pollutant generated through sunlight-driven reactions between nitrogen oxides (NOx) and VOCs. Its levels have increased since the industrial revolution and reactions involving ozone drive many chemical processes in the troposphere. While ozone precursors often originate in urban areas, winds may carry these hundreds of kilometres, causing ozone formation to also occur in less populated rural regions. Under elevated ozone conditions, ozonolysis of bVOCs can result in quantitative and qualitative changes in the gas phase, reducing the concentrations of certain bVOCs and resulting in the formation of other compounds. Such changes can result in disruption of bVOC-mediated behavioural or ecological interactions. Through a series of gas-phase experiments using Gas Chromatography Mass Spectrometry (GC-MS) and Proton Transfer Reaction Mass Spectrometry (PTR-MS), we investigated the products and their yields from the ozonolysis of a range of ubiquitous bVOCs, which were selected because of their importance in mediating ecological interactions such as pollinator and natural enemy attraction and plant-to-plant communication, namely: (E)-β-ocimene, isomers of α and β-farnesene, α-terpinene and 6-methyl-5-hepten-2-one. New products from the ozonolysis of these compounds were identified, and the formation of these compounds is consistent with terpene-ozone oxidation mechanisms. We present the degradation mechanism of our model bVOCs and identify their reaction products. We discuss the potential ecological implications of the degradation of each bVOC and of the formation of reaction products.

Ozone stress response of leaf BVOC emission and photosynthesis in mountain birch (Betula pubescens spp. czerepanovii) depends on leaf age

Oxidative stress from ozone (O3) causes plants to alter their emission of biogenic volatile organic compounds (BVOC) and their photosynthetic rate. Stress reactions from O3 on birch trees can result in prohibited plant growth and lead to increased BVOC emission rates as well as changes in their compound blend to emit more monoterpenes (MT) and sesquiterpenes (SQT). BVOCs take part in atmospheric reactions such as enhancing the production of secondary organic aerosols (SOA). As the compound blend and emission rate change with O3 stress, this can influence the atmospheric conditions by affecting the production of SOA. Studying the stress responses of plants provides important information on how these reactions might change, which is vital to making better predictions of the future climate. In this study, measurements were taken to find out how the leaves of mature mountain birch trees (Betula pubescens ssp. czerepanovii) respond to different levels of elevated O3 exposure in situ depending on leaf age. We found that leaves from both early and late summers responded with induced SQT emission after exposure to 120 ppb O3. Early leaves were, however, more sensitive to increased O3 concentrations, with enhanced emission of green leaf volatiles (GLV) and tendencies of both induced leaf senescence as well as poor recovery in the photosynthetic rate between exposures. Late leaves had more stable photosynthetic rates throughout the experiment and responded less to exposure at different O3 levels.

Volatile responses of dwarf birch to mimicked insect herbivory and experimental warming at two elevations in Greenlandic tundra

Plants release a complex blend of volatile organic compounds (VOCs) in response to stressors. VOC emissions vary between contrasting environments and increase with insect herbivory and rising temperatures. However, the joint effects of herbivory and warming on plant VOC emissions are understudied, particularly in high latitudes, which are warming fast and facing increasing herbivore pressure. We assessed the individual and combined effects of chemically mimicked insect herbivory, warming, and elevation on dwarf birch (Betula glandulosa) VOC emissions in high-latitude tundra ecosystems in Narsarsuaq, South Greenland. We hypothesized that VOC emissions and compositions would respond synergistically to warming and herbivory, with the magnitude differing between elevations. Warming increased emissions of green leaf volatiles (GLVs) and isoprene. Herbivory increased the homoterpene, (E)-4,8-dimethyl-1,3,7-nonatriene, emissions, and the response was stronger at high elevation. Warming and herbivory had synergistic effects on GLV emissions. Dwarf birch emitted VOCs at similar rates at both elevations, but the VOC blends differed between elevations. Several herbivory-associated VOC groups did not respond to herbivory. Harsher abiotic conditions at high elevations might not limit VOC emissions from dwarf birch, and high-elevation plants might be better at herbivory defense than assumed. The complexity of VOC responses to experimental warming, elevation, and herbivory are challenging our understanding and predictions of future VOC emissions from dwarf birch-dominated ecosystems.

Insect Herbivory Caused Plant Stress Emissions Increases the Negative Radiative Forcing of Aerosols

Plant stress in a changing climate is predicted to increase plant volatile organic compound (VOC) emissions and thus can affect the formed secondary organic aerosol (SOA) concentrations, which in turn affect the radiative properties of clouds and aerosol. However, global aerosol-climate models do not usually consider plant stress induced VOCs in their emission schemes. In this study, we modified the monoterpene emission factors in biogenic emission model to simulate biotic stress caused by insect herbivory on needleleaf evergreen boreal and broadleaf deciduous boreal trees and studied the consequent effects on SOA formation, aerosol-cloud interactions as well as direct radiative effects of formed SOA. Simulations were done altering the fraction of stressed and healthy trees in the latest version of ECHAM-HAMMOZ (ECHAM6.3-HAM2.3-MOZ1.0) global aerosol-climate model. Our simulations showed that increasing the extent of stress to the aforementioned tree types, substantially increased the SOA burden especially over the areas where these trees are located. This indicates that increased VOC emissions due to increasing stress enhance the SOA formation via oxidation of VOCs to low VOCs. In addition, cloud droplet number concentration at the cloud top increased with increasing extent of biotic stress. This indicates that as SOA formation increases, it further enhances the number of particles acting as cloud condensation nuclei. The increase in SOA formation also decreased both all-sky and clear-sky radiative forcing. This was due to a shift in particle size distributions that enhanced aerosol reflecting and scattering of incoming solar radiation.

Synergistic effects of insect herbivory and changing climate on plant volatile emissions in the subarctic tundra

Climate change increases the insect abundance, especially in Arctic ecosystems. Insect herbivory also significantly increases plant emissions of volatile organic compounds (VOCs), which are highly reactive in the atmosphere and play a crucial role in atmospheric chemistry and physics. However, it is unclear how the effects of insect herbivory on VOC emissions interact with climatic changes, such as warming and increased cloudiness. We assessed how experimental manipulations of temperature and light availability in subarctic tundra, that had been maintained for 30 years at the time of the measurements, affect the VOC emissions from a widespread dwarf birch (Betula nana) when subjected to herbivory by local geometrid moth larvae, the autumnal moth (Epirrita autumnata) and the winter moth (Operophtera brumata). Warming and insect herbivory on B. nana stimulated VOC emission rates and altered the VOC blend. The herbivory-induced increase in sesquiterpene and homoterpene emissions were climate-treatment-dependent. Many herbivory-associated VOCs were more strongly induced in the shading treatment than in other treatments. We showed generally enhanced tundra VOC emissions upon insect herbivory and synergistic effects on the emissions of some VOC groups in a changing climate, which can have positive feedbacks on cloud formation. Furthermore, the acclimation of plants to long-term climate treatments affects VOC emissions and strongly interacts with plant responses to herbivory. Such acclimation complicates predictions of how climate change, together with interacting biotic stresses, affects VOC emissions in the high latitudes.

BVOC Emissions From a Subarctic Ecosystem, as Controlled by Insect Herbivore Pressure and Temperature

Abstract

The biogenic volatile organic compounds, BVOCs have a central role in ecosystem–atmosphere interactions. High-latitude ecosystems are facing increasing temperatures and insect herbivore pressure, which may affect their BVOC emission rates, but evidence and predictions of changes remain scattered. We studied the long-term effects of + 3 °C warming and reduced insect herbivory (achieved through insecticide sprayings) on mid- and late summer BVOC emissions from field layer vegetation, supplemented with birch saplings, and the underlying soil in Subarctic mountain birch forest in Finland in 2017–2018. Reduced insect herbivory decreased leaf damage by 58–67% and total ecosystem BVOC emissions by 44–72%. Of the BVOC groups, total sesquiterpenes had 70–80% lower emissions with reduced herbivory, and in 2017 the decrease was greater in warmed plots (89% decrease) than in ambient plots (34% decrease). While non-standardized total BVOC, monoterpene, sesquiterpene and GLV emissions showed instant positive responses to increasing chamber air temperature in midsummer samplings, the long-term warming treatment effects on standardized emissions mainly appeared as changes in the compound structure of BVOC blends and varied with compounds and sampling times. Our results suggest that the effects of climate warming on the total quantity of BVOC emissions will in Subarctic ecosystems be, over and above the instant temperature effects, mediated through changes in insect herbivore pressure rather than plant growth. If insect herbivore numbers will increase as predicted under climate warming, our results forecast herbivory-induced increases in the quantity of Subarctic BVOC emissions.

Graphic Abstract

Crops for Carbon Farming

Agricultural cropping systems and pasture comprise one third of the world's arable land and have the potential to draw down a considerable amount of atmospheric CO2 for storage as soil organic carbon (SOC) and improving the soil carbon budget. An improved soil carbon budget serves the dual purpose of promoting soil health, which supports crop productivity, and constituting a pool from which carbon can be converted to recalcitrant forms for long-term storage as a mitigation measure for global warming. In this perspective, we propose the design of crop ideotypes with the dual functionality of being highly productive for the purposes of food, feed, and fuel, while at the same time being able to facilitate higher contribution to soil carbon and improve the below ground ecology. We advocate a holistic approach of the integrated plant-microbe-soil system and suggest that significant improvements in soil carbon storage can be achieved by a three-pronged approach: (1) design plants with an increased root strength to further allocation of carbon belowground; (2) balance the increase in belowground carbon allocation with increased source strength for enhanced photosynthesis and biomass accumulation; and (3) design soil microbial consortia for increased rhizosphere sink strength and plant growth-promoting (PGP) properties.

Potential of Climate Change and Herbivory to Affect the Release and Atmospheric Reactions of BVOCs from Boreal and Subarctic Forests

Compared to most other forest ecosystems, circumpolar boreal and subarctic forests have few tree species, and are prone to mass outbreaks of herbivorous insects. A short growing season with long days allows rapid plant growth, which will be stimulated by predicted warming of polar areas. Emissions of biogenic volatile organic compounds (BVOC) from soil and vegetation could be substantial on sunny and warm days and biotic stress may accelerate emission rates. In the atmosphere, BVOCs are involved in various gas-phase chemical reactions within and above forest canopies. Importantly, the oxidation of BVOCs leads to secondary organic aerosol (SOA) formation. SOA particles scatter and absorb solar radiation and grow to form cloud condensation nuclei (CCN) and participate in cloud formation. Through BVOC and moisture release and SOA formation and condensation processes, vegetation has the capacity to affect the abiotic environment at the ecosystem scale. Recent BVOC literature indicates that both temperature and herbivory have a major impact on BVOC emissions released by woody species. Boreal conifer forest is the largest terrestrial biome and could be one of the largest sources of biogenic mono- and sesquiterpene emissions due to the capacity of conifer trees to store terpene-rich resins in resin canals above and belowground. Elevated temperature promotes increased diffusion of BVOCs from resin stores. Moreover, insect damage can break resin canals in needles, bark, and xylem and cause distinctive bursts of BVOCs during outbreaks. In the subarctic, mountain birch forests have cyclic outbreaks of Geometrid moths. During outbreaks, trees are often completely defoliated leading to an absence of BVOC-emitting foliage. However, in the years following an outbreak there is extended shoot growth, a greater number of leaves, and greater density of glandular trichomes that store BVOCs. This can lead to a delayed chemical defense response resulting in the highest BVOC emission rates from subarctic forest in the 1-3 years after an insect outbreak. Climate change is expected to increase insect outbreaks at high latitudes due to warmer seasons and arrivals of invasive herbivore species. Increased BVOC emission will affect tropospheric ozone (O3) formation and O3 induced oxidation of BVOCs. Herbivore-induced BVOC emissions from deciduous and coniferous trees are also likely to increase the formation rate of SOA and further growth of the particles in the atmosphere. Field experiments measuring the BVOC emission rates, SOA formation rate and particle concentrations within and above the herbivore attacked forest stands are still urgently needed.

Herbivore Gender Effects on Volatile Induction in Aspen and on Olfactory Responses in Leaf Beetles

Hybrid aspen (Populus tremula × tremuloides Michx.) is a fast-growing tree species used for short-rotation forestry in northern latitudes. Aspen species have a rich herbivore fauna, including defoliating leaf beetles that induce emissions of volatile organic compounds (VOCs) when feeding on aspen leaves. We investigated the differential induction of VOCs by male and female Phratora laticollis leaf beetles feeding on hybrid aspen and the differences in the orientation of beetles in response to gender-specific induced VOCs. The hypotheses for the study were (1) the VOCs in the headspace of plants infested with beetles of the two genders individually and in mixed aggregates would vary subtly, and (2) foraging adult beetles would be able to detect differences in VOC blends and use them to fine-tune their orientation choices. In Y-tube bioassays, both females and males preferred VOCs from leaves damaged by one gender (females or males) over undamaged leaves. However, if leaves were damaged by a two-gender population, neither females nor males indicated a preference over volatiles of undamaged leaves. Leaves damaged by both beetle genders simultaneously had significantly increased green leaf volatile (GLV), benzenoid and homoterpene emissions compared to undamaged leaves. Emissions of these compounds possibly indicate higher herbivore pressure and a higher risk of attack by parasitoids and predators and could thus be the cause of the lack of beetle preference. Our findings provide new basic information on gender-based host plant selection by herbivores and may be helpful in the development of sustainable biogenic VOC-based herbivore-control methods for intensive short-rotation hybrid aspen production.

Dynamics of plant responses to combinations of air pollutants

The focus of this review is on how plants respond to combinations of multiple air pollutants. Global pollution trends, plant physiological responses and ecological perspectives in natural and agricultural systems are all discussed. In particular, we highlight the importance of studying sequential or simultaneous exposure of plants to pollutants, rather than exposure to individual pollutants in isolation, and explore how these responses may interfere with the way plants interact with their biotic community. Air pollutants can alter the normal physiology and metabolic functioning of plants. Here we describe how the phenotypic and molecular changes in response to multiple pollutants can differ compared to those elicited by single pollutants, and how different responses have been observed between plants in the field and in controlled laboratory conditions and between trees and crop plants. From an ecological perspective, we discuss how air pollution can result in greater susceptibility to biotic stressors and in direct or indirect effects on interactions with organisms that occupy higher trophic levels. Finally, we provide an overview of the potential uses of plants to mitigate air pollution, exploring the feasibility for pollution removal via the processes of bio-accumulation and phytoremediation. We conclude by proposing some new directions for future research in the field.

What makes a volatile organic compound a reliable indicator of insect herbivory?

Plants that are subject to insect herbivory emit a blend of so-called herbivore-induced plant volatiles (HIPVs), of which only a few serve as cues for the carnivorous enemies to locate their host. We lack understanding which HIPVs are reliable indicators of insect herbivory. Here, we take a modelling approach to elucidate which physicochemical and physiological properties contribute to the information value of a HIPV. A leaf-level HIPV synthesis and emission model is developed and parameterized to poplar. Next, HIPV concentrations within the canopy are inferred as a function of dispersion, transport and chemical degradation of the compounds. We show that the ability of HIPVs to reveal herbivory varies from almost perfect to no better than chance and interacts with canopy conditions. Model predictions matched well with leaf-emission measurements and field and laboratory assays. The chemical class a compound belongs to predicted the signalling ability of a compound only to a minor extent, whereas compound characteristics such as its reaction rate with atmospheric oxidants, biosynthesis rate upon herbivory and volatility were much more important predictors. This study shows the power of merging fields of plant-insect interactions and atmospheric chemistry research to increase our understanding of the ecological significance of HIPVs.

Secondary Organic Aerosol Formation from Healthy and Aphid-Stressed Scots Pine Emissions

One barrier to predicting biogenic secondary organic aerosol (SOA) formation in a changing climate can be attributed to the complex nature of plant volatile emissions. Plant volatile emissions are dynamic over space and time, and change in response to environmental stressors. This study investigated SOA production from emissions of healthy and aphid-stressed Scots pine saplings via dark ozonolysis and photooxidation chemistry. Laboratory experiments using a batch reaction chamber were used to investigate SOA production from different plant volatile mixtures. The volatile mixture from healthy plants included monoterpenes, aromatics, and a small amount of sesquiterpenes. The biggest change in the volatile mixture for aphid-stressed plants was a large increase (from 1.4 to 7.9 ppb) in sesquiterpenes-particularly acyclic sesquiterpenes, such as the farnesene isomers. Acyclic sesquiterpenes had different effects on SOA production depending on the chemical mechanism. Farnesenes suppressed SOA formation from ozonolysis with a 9.7-14.6% SOA mass yield from healthy plant emissions and a 6.9-10.4% SOA mass yield from aphid-stressed plant emissions. Ozonolysis of volatile mixtures containing more farnesenes promoted fragmentation reactions, which produced higher volatility oxidation products. In contrast, plant volatile mixtures containing more farnesenes did not appreciably change SOA production from photooxidation. SOA mass yields ranged from 10.8 to 23.2% from healthy plant emissions and 17.8-26.8% for aphid-stressed plant emissions. This study highlights the potential importance of acyclic terpene chemistry in a future climate regime with an increased presence of plant stress volatiles.

Petiole gall aphid (Pemphigus spyrothecae) infestation of Populus × petrovskiana leaves alters foliage photosynthetic characteristics and leads to enhanced emissions of both constitutive and stress-induced volatiles

Poplar spiral gall aphid (Pemphigus spyrothecae) forms galls on the petiole in poplars (Populus) and mass infestations are frequent in poplar stands, but how these parasite gall infestations can affect the leaf lamina structure, photosynthetic rate and constitutive and stress volatile emissions is unknown. We investigated how the infestation by the petiole gall aphids affects lamina photosynthetic characteristics (net assimilation rate, stomatal conductance), C and N contents, and constitutive isoprene and induced volatile emissions in Populus × petrovskiana. The dry gall mass per leaf dry mass (M _g/M _l) was used as a quantitative measure of the severity of gall infestation. Very high fraction of leaf biomass was invested in gall formation with M _g/M _l varying between 0.5-2. Over the whole range of the infestation severities, net assimilation rate per area, leaf dry mass per unit area and N content decreased with increasing the severity of infestation. In contrast, stomatal conductance, leaf dry mass per fresh mass, constitutive isoprene emissions, and induced green leaf volatile (GLV), monoterpene, sesquiterpene and benzenoid emissions increased with increasing the severity of gall infestation. The rates of induced emissions were low and these emissions were associated with methyl jasmonate release from leaf laminas. The data demonstrate that petiole gall infestations lead to major changes in leaf lamina sink-source relationships and leaf water relations, thereby significantly altering lamina photosynthesis. Modifications in stress-induced emissions likely indicated systemic signaling triggered by jasmonate transported from the petiole galls to the lamina where jasmonate elicited a cascade of volatile emission responses. Enhance isoprene emissions and induced volatile emissions can play a major role in indirect defense against other herbivores, securing the food source for the gall aphids. In conclusion, a massive infestation by petiole gall aphids can profoundly modify the foliage photosynthetic performance and volatile emission profiles in poplars.

New Perspectives on CO2, Temperature, and Light Effects on BVOC Emissions Using Online Measurements by PTR-MS and Cavity Ring-Down Spectroscopy

Volatile organic compounds (VOC) play important roles in atmospheric chemistry, plant ecology, and physiology, and biogenic VOC (BVOC) emitted by plants is the largest VOC source. Our knowledge about how environmental drivers (e.g., carbon, light, and temperature) may regulate BVOC emissions is limited because they are often not controlled. We combined a greenhouse facility to manipulate atmospheric CO₂ ([CO₂]) with proton-transfer-reaction mass spectrometry (PTR-MS) and cavity ring-down spectroscopy to investigate the regulation of BVOC in Norway spruce. Our results indicate a direct relationship between [CO₂] and methanol and acetone emissions, and their temperature and light dependencies, possibly related to substrate availability. The composition of monoterpenes stored in needles remained constant, but emissions of mono-(linalool) and sesquiterpenes (β-farnesene) increased at lower [CO₂], with the effects being most pronounced at the highest air temperature. Pulse-labeling suggested an immediate incorporation of recently assimilated carbon into acetone, mono- and sesquiterpene emissions even under 50 ppm [CO₂]. Our results provide new perspectives on CO₂, temperature and light effects on BVOC emissions, in particular how they depend on stored pools and recent photosynthetic products. Future studies using smaller but more seedlings may allow sufficient replication to examine the physiological mechanisms behind the BVOC responses.

Understorey Rhododendron tomentosum and Leaf Trichome Density Affect Mountain Birch VOC Emissions in the Subarctic

Subarctic vegetation is composed of mountain birch [Betula pubescens ssp. czerepanovii (MB)] forests with shrubs and other species growing in the understorey. The effects of the presence and density of one understorey shrub, Rhododendron tomentosum (RT), on the volatile emissions of MB, were investigated in a Finnish subarctic forest site in early and late growing season. Only MB trees with an RT-understorey emitted the RT-specific sesquiterpenoids, palustrol, ledol and aromadendrene. Myrcene, which is the most abundant RT-monoterpene was also emitted in higher quantities by MB trees with an RT-understorey. The effect of RT understorey density on the recovery of RT compounds from MB branches was evident only during the late season when sampling temperature, as well as RT emissions, were higher. MB sesquiterpene and total emission rates decreased from early season to late season, while monoterpene emission rate increased. Both RT and MB terpenoid emission rates were linked to density of foliar glandular trichomes, which deteriorated over the season on MB leaves and emerged with new leaves in the late season in RT. We show that sesquiterpene and monoterpene compounds emitted by understorey vegetation are adsorbed and re-released by MB, strongly affecting the MB volatile emission profile.

VOC emissions and carbon balance of two bioenergy plantations in response to nitrogen fertilization: A comparison of Miscanthus and Salix

Energy crops are an important renewable source for energy production in future. To ensure high yields of crops, N fertilization is a common practice. However, knowledge on environmental impacts of bioenergy plantations, particularly in systems involving trees, and the effects of N fertilization is scarce. We studied the emission of volatile organic compounds (VOC), which negatively affect the environment by contributing to tropospheric ozone and aerosols formation, from Miscanthus and willow plantations. Particularly, we aimed at quantifying the effect of N fertilization on VOC emission. For this purpose, we determined plant traits, photosynthetic gas exchange and VOC emission rates of the two systems as affected by N fertilization (0 and 80 kg ha^-1 yr^-1). Additionally, we used a modelling approach to simulate (i) the annual VOC emission rates as well as (ii) the OH^. reactivity resulting from individual VOC emitted. Total VOC emissions from Salix was 1.5- and 2.5-fold higher compared to Miscanthus in non-fertilized and fertilized plantations, respectively. Isoprene was the dominating VOC in Salix (80-130 μg g^-1 DW h^-1), whereas it was negligible in Miscanthus. We identified twenty-eight VOC compounds, which were released by Miscanthus with the green leaf volatile hexanal as well as dimethyl benzene, dihydrofuranone, phenol, and decanal as the dominant volatiles. The pattern of VOC released from this species clearly differed to the pattern emitted by Salix. OH^. reactivity from VOC released by Salix was ca. 8-times higher than that of Miscanthus. N fertilization enhanced stand level VOC emissions, mainly by promoting the leaf area index and only marginally by enhancing the basal emission capacity of leaves. Considering the higher productivity of fertilized Miscanthus compared to Salix together with the considerably lower OH^. reactivity per weight unit of biomass produced, qualified the C₄-perennial grass Miscanthus as a superior source of future bioenergy production.

Terpene Composition Complexity Controls Secondary Organic Aerosol Yields from Scots Pine Volatile Emissions

Secondary organic aerosol (SOA) impact climate by scattering and absorbing radiation and contributing to cloud formation. SOA models are based on studies of simplified chemical systems that do not account for the chemical complexity in the atmosphere. This study investigated SOA formation from a mixture of real Scots pine (Pinus sylvestris) emissions including a variety of monoterpenes and sesquiterpenes. SOA generation was characterized from different combinations of volatile compounds as the plant emissions were altered with an herbivore stress treatment. During active herbivore feeding, monoterpene and sesquiterpene emissions increased, but SOA mass yields decreased after accounting for absorption effects. SOA mass yields were controlled by sesquiterpene emissions in healthy plants. In contrast, SOA mass yields from stressed plant emissions were controlled by the specific blend of monoterpene emissions. Conservative estimates using a box model approach showed a 1.5- to 2.3-fold aerosol enhancement when the terpene complexity was taken into account. This enhancement was relative to the commonly used model monoterpene, “α-pinene”. These results suggest that simplifying terpene complexity in SOA models could lead to underpredictions in aerosol mass loading.

Plant-derived Secondary Organic Material in the Air and Ecosystems

Biogenic secondary organic aerosol (SOA) and deposited secondary organic material (SOM) are formed by oxidation of volatile organic compounds (VOCs) emitted by plants. Many SOA compounds have much longer chemical lifetimes than the original VOC, and may accumulate on plant surfaces and in soil as SOM because of their low volatility. This suggests that they may have important and presently unrecognized roles in plant adaptation. Using reactive plant terpenoids as a model we propose a three-tier (atmosphere-vegetation-soil) framework to better understand the ecological and evolutionary functions of SOM. In this framework, SOA in the atmosphere is known to affect solar radiation, SOM on the plant surfaces influences the interactive organisms, and wet and dry deposition of SOM on soil affects soil organisms.

Methyl jasmonate-induced emission of biogenic volatiles is biphasic in cucumber: a high-resolution analysis of dose dependence

Methyl jasmonate (MeJA) is a key airborne elicitor activating jasmonate-dependent signaling pathways, including induction of stress-related volatile emissions, but how the magnitude and timing of these emissions scale with MeJA dose is not known. Treatments with exogenous MeJA concentrations ranging from mild (0.2 mM) to lethal (50 mM) were used to investigate quantitative relationships among MeJA dose and the kinetics and magnitude of volatile release in Cucumis sativus by combining high-resolution measurements with a proton-transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS) and GC-MS. The results highlighted biphasic kinetics of elicitation of volatiles. The early phase, peaking in 0.1-1 h after the MeJA treatment, was characterized by emissions of lipoxygenase (LOX) pathway volatiles and methanol. In the subsequent phase, starting in 6-12 h and reaching a maximum in 15-25 h after the treatment, secondary emissions of LOX compounds as well as emissions of monoterpenes and sesquiterpenes were elicited. For both phases, the maximum emission rates and total integrated emissions increased with applied MeJA concentration. Furthermore, the rates of induction and decay, and the duration of emission bursts were positively, and the timing of emission maxima were negatively associated with MeJA dose for LOX compounds and terpenoids, except for the duration of the first LOX burst. These results demonstrate major effects of MeJA dose on the kinetics and magnitude of volatile response, underscoring the importance of biotic stress severity in deciphering the downstream events of biological impacts.

Ozone alters the feeding behavior of the leaf beetle Agelastica coerulea (Coleoptera: Chrysomelidae) into leaves of Japanese white birch (Betula platyphylla var. japonica)

High mixing ratios of ground-level O₃ threaten trophic interactions. In the present study, we conducted laboratory assays, where insect larvae and adults were not directly exposed to O₃, to test the feeding behavior and attraction of the coleopteran leaf beetle Agelastica coerulea to early and late leaves of Japanese white birch (Betula platyphylla var. japonica) treated with ambient or elevated O₃ levels. We found that overwintered adults were not deterred from grazing elevated O₃-treated leaves, but rather preferred them than ambient O₃-treated ones. We also found that the feeding behavior of 2nd instar larvae fed on early or late leaves was not influenced by the O₃ treatment of the leaves when larvae could choose leaves. These observations of the adults and larvae feeding preferences contradict prior observations in the field conditions where the insects avoided leaves in O₃-enriched atmosphere. Since adults preferred elevated O₃-exposed leaves in the present laboratory assays, it is worthy of further investigations whether adults change their grazing preference so as to ensure the leaf palatability as a feeding source for their larvae. Hence, new direction towards detailed ovipositional behavior surveys under field conditions is encouraged.