Effects of air pollution on biogenic volatiles and ecological interactions

Effects of urbanization on orchid bee diversity and orchid pollination: From neotropical cloud forests to urban cores

Urbanization is a significant driver of land use change, profoundly impacting biodiversity and ecosystem services worldwide. However, its effects in the tropics, which host some of the planet's highest biodiversity, remain inadequately understood. Orchid bees (Apidae: Euglossini) are key pollinators in Neotropical ecosystems, playing crucial roles in maintaining floral diversity and reproductive success of orchids and other plant families. Yet, little is known about how urbanization influences their diversity and pollination. In this study, we analyzed the diversity and composition or orchid bee communities along an urbanization gradient which extends from the city center to the surrounding cloud forests, which bear high orchid endemism while being highly threatened. Along the same gradient, we further evaluated pollination of a model native orchid, Gongora galeata, which is exclusively pollinated by the bee Euglossa obrima. As expected, increasing urbanization led to a decrease in orchid bee diversity, as well as a clear separation in species composition between urban and non-urban sites and a reduction in G. galeata pollination (i.e. fruit production). However, contrary to our expectations, orchid pollination also decreased with environmental heterogeneity and the abundance of its specific pollinator. Despite urban areas still hosting orchid bee species, our results reveal clear negative effects of urbanization not only on diversity, but also on the ecosystem function of a highly threatened group of bees. This study highlights the importance of considering local factors of urban landscapes for preserving not only biodiversity, but also fundamental ecological processes in cities.

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.

Novel Pseudomonas Species Prevent the Growth of the Phytopathogenic Fungus Aspergillus flavus

In response to the escalating demand for sustainable agricultural methodologies, the utilization of microbial volatile organic compounds (VOCs) as antagonists against phytopathogens has emerged as a viable eco-friendly alternative. Microbial volatiles exhibit rapid diffusion rates, facilitating prompt chemical interactions. Moreover, microorganisms possess the capacity to emit volatiles constitutively, as well as in response to biological interactions and environmental stimuli. In addition to volatile compounds, these bacteria demonstrate the ability to produce soluble metabolites with antifungal properties, such as APE Vf, pyoverdin, and fragin. In this study, we identified two Pseudomonas strains (BJa3 and MCal1) capable of inhibiting the in vitro mycelial growth of the phytopathogenic fungus Aspergillus flavus, which serves as the causal agent of diseases in sugarcane and maize. Utilizing GC/MS analysis, we detected 47 distinct VOCs which were produced by these bacterial strains. Notably, certain volatile compounds, including 1-heptoxydecane and tridecan-2-one, emerged as primary candidates for inhibiting fungal growth. These compounds belong to essential chemical classes previously documented for their antifungal activity, while others represent novel molecules. Furthermore, examination via confocal microscopy unveiled significant morphological alterations, particularly in the cell wall, of mycelia exposed to VOCs emitted by both Pseudomonas species. These findings underscore the potential of the identified BJa3 and MCal1 Pseudomonas strains as promising agents for fungal biocontrol in agricultural crops.

Enhancing knowledge of chemical exposures and fate in honey bee hives: Insights from colony structure and interactions

Honey bees are unintentionally exposed to a wide range of chemicals through various routes in their natural environment, yet research on the cumulative effects of multi-chemical and sublethal exposures on important caste members, including the queen bee and brood, is still in its infancy. The hive's social structure and food-sharing (trophallaxis) practices are important aspects to consider when identifying primary and secondary exposure pathways for residential hive members and possible chemical reservoirs within the colony. Secondary exposures may also occur through chemical transfer (maternal offloading) to the brood and by contact through possible chemical diffusion from wax cells to all hive members. The lack of research on peer-to-peer exposures to contaminants and their metabolites may be in part due to the limitations in sensitive analytical techniques for monitoring chemical fate and dispersion. Combined application of automated honey bee monitoring and modern chemical trace analysis techniques could offer rapid progress in quantifying chemical transfer and accumulation within the hive environment and developing effective mitigation strategies for toxic chemical co-exposures. To enhance the understanding of chemical fate and toxicity within the entire colony, it is crucial to consider both the intricate interactions among hive members and the potential synergistic effects arising from combinations of chemical and their metabolites.

Chemistry, biosynthesis and biology of floral volatiles: roles in pollination and other functions

Covering: 2010 to 2023Floral volatiles are a chemically diverse group of plant metabolites that serve multiple functions. Their composition is shaped by environmental, ecological and evolutionary factors. This review will summarize recent advances in floral scent research from chemical, molecular and ecological perspectives. It will focus on the major chemical classes of floral volatiles, on notable new structures, and on recent discoveries regarding the biosynthesis and the regulation of volatile emission. Special attention will be devoted to the various functions of floral volatiles, not only as attractants for different types of pollinators, but also as defenses of flowers against enemies. We will also summarize recent findings on how floral volatiles are affected by abiotic stressors, such as increased temperatures and drought, and by other organisms, such as herbivores and flower-dwelling microbes. Finally, this review will indicate current research gaps, such as the very limited knowledge of the isomeric pattern of chiral compounds and its importance in interspecific interactions.

Birds differentially prioritize visual and olfactory foraging cues depending on habitat of origin and sex

Animals interpret their environment by combining information from multiple senses. The relative usefulness of different senses may vary between species, habitats and sexes; yet, how multimodal stimuli are integrated and prioritized is unknown for most taxa. We experimentally assessed foraging preferences of great tits (Parus major) to test whether urban and forest individuals prioritize visual and olfactory cues differently during foraging. We trained 13 wild-caught birds to associate multimodal (colour + odour) cues with a food reward and assessed their foraging preferences in a cue-separation test. In this, the birds could choose between the multimodal training cue and its olfactory or visual components. Our results suggest that the birds did not perceive multimodal cues in an integrated way, as their response was not stronger than for unimodal cue components. Urban birds preferred olfactory cues, while forest birds preferred visual cues. Nevertheless, female birds preferred the multimodal cue, while males foraged more randomly with respect to which cue was present. These findings contribute to our understanding of the relative roles of vision and olfaction in bird foraging behaviour. Future work should focus on how habitat- and sex-specific sensory prioritization modifies bird foraging behaviour and foraging success in the context of urban adaptations across populations.

Concurrent anthropogenic air pollutants enhance recruitment of a specialist parasitoid

Air pollutants-such as nitrogen oxides, emitted in diesel exhaust, and ozone (O3)-disrupt interactions between plants, the insect herbivore pests that feed upon them and natural enemies of those herbivores (e.g. parasitoids). Using eight field-based rings that emit regulated quantities of diesel exhaust and O3, we investigated how both pollutants, individually and in combination, altered the attraction and parasitism rate of a specialist parasitoid (Diaeretiella rapae) on aphid-infested and un-infested Brassica napus plants. Individual effects of O3 decreased D. rapae abundance and emergence by 37% and 55%, respectively, compared with ambient (control) conditions. When O3 and diesel exhaust were emitted concomitantly, D. rapae abundance and emergence increased by 79% and 181%, respectively, relative to control conditions. This attraction response occurred regardless of whether plants were infested with aphids and was associated with an increase in the concentration of aliphatic glucosinolates, especially gluconapin (3-butenyl-glucosinolate), within B. napus leaves. Plant defensive responses and their ability to attract natural aphid enemies may be beneficially impacted by pollution exposure. These results demonstrate the importance of incorporating multiple air pollutants when considering the effects of air pollution on plant-insect interactions.

Molecular Cloning and Functional Analysis of DXS and FPS Genes from Zanthoxylum bungeanum Maxim

Zanthoxylum bungeanum Maxim. (Z. bungeanum) has attracted attention for its rich aroma. The aroma of Z. bungeanum is mainly volatile terpenes synthesized by plant terpene metabolic pathways. However, there is little information on Z. bungeanum terpene metabolic gene. In this study, the coding sequence of 1-deoxy-D-xylulose-5-phosphate synthase (DXS) and farnesyl pyrophosphate synthase (FPS) were cloned from Z. bungeanum cv. 'Fengxiandahongpao.' ZbDXS and ZbFPS genes from Z. bungeanum with CDS lengths of 2172 bp and 1029 bp, respectively. The bioinformatics results showed that Z. bungeanum was closely related to citrus, and it was deduced that ZbFPS were hydrophilic proteins without the transmembrane domain. Subcellular localization prediction indicated that ZbDXS was most likely to be located in chloroplasts, and ZbFPS was most likely to be in mitochondria. Meanwhile, the 3D protein structure showed that ZbDXS and ZbFPS were mainly composed of α-helices, which were folded into a single domain. In vitro enzyme activity experiments showed that purified proteins ZbDXS and ZbFPS had the functions of DXS enzyme and FPS enzyme. Transient expression of ZbDXS and ZbFPS in tobacco significantly increased tobacco's terpene content. Moreover, ZbDXS and ZbFPS were expressed in different tissues of Z. bungeanum, and the relative expression of the two genes was the highest in fruits. Therefore, this suggests that ZbDXS and ZbFPS are positively related to terpene synthesis. This study could provide reference genes for improving Z. bungeanum breeding as well as for the Rutaceae research.

Transcriptome analysis of the growth-promoting effect of volatile organic compounds produced by Microbacterium aurantiacum GX14001 on tobacco (Nicotiana benthamiana)

BACKGROUND

Plant growth-promoting rhizobacteria (PGPR) release volatile organic compounds (VOCs), which promote plant growth.

RESULTS

A potential PGPR strain GX14001 was isolated from marine samples, and the VOCs produced by GX14001 significantly promoted tobacco (Nicotiana benthamiana) growth in a plate experiment. Based on 16S rRNA sequence alignment and physiological and biochemical characterization, GX14001 was identified as Microbacterium aurantiacum. Comparative transcriptome analysis was conducted between GX14001 VOCs-treated tobacco and the control; it was found that 1286 genes were upregulated and 1088 genes were downregulated. Gene ontology (GO) analysis showed that upregulated genes were involved in three biological processes: polysaccharide metabolic, polysaccharide catabolic and carbohydrate metabolic. The downregulated genes were involved in six biological processes, namely cell redox homeostasis, cellular homeostasis, carbohydrate metabolic process, homeostatic process, obsolete electron transport, and regulation of biological quality. Kyoto encyclopedia of genes and genomes (KEGG) pathway analysis showed that 190 upregulated differentially expressed genes were mainly involved in plant hormone signal transduction, phenylpropyl biosynthesis, plant-pathogen interaction, and flavonoid biosynthesis. The 148 downregulated differentially expressed genes were mainly involved in plant hormone signal transduction and the metabolism of ascorbic, aldehyde, and pyruvate acids. Further analysis revealed that many genes were differentially expressed in the metabolic pathways of plant hormone signals, which were speculated to be the main reason why GX14001 VOCs promoted tobacco growth. To further study its regulatory mechanism, we found that GX14001 promoted plant growth through auxin, salicylic acid, and gibberellin in Arabidopsis mutant experiments.

CONCLUSION

The VOCs produced by Microbacterium aurantiacum GX14001 may promote the growth of tobacco through the auxin, salicylic acid and gibberellin pathways.

Anthropogenic air pollutants reduce insect-mediated pollination services

Common air pollutants, such as nitrogen oxides (NO_x), emitted in diesel exhaust, and ozone (O₃), have been implicated in the decline of pollinating insects. Reductionist laboratory assays, focused upon interactions between a narrow range of flowering plant and pollinator species, in combination with atmospheric chemistry models, indicate that such pollutants can chemically alter floral odors, disrupting the cues that foraging insects use to find and pollinate flowers. However, odor environments in nature are highly complex and pollination services are commonly provided by suites of insect species, each exhibiting different sensitivities to different floral odors. Therefore, the potential impacts of pollution-induced foraging disruption on both insect ecology, and the pollination services that insects provide, are currently unknown. We conducted in-situ field studies to investigate whether such pollutants could reduce pollinator foraging and as a result the pollination ecosystem service that those insects provide. Using free-air fumigation, we show that elevating diesel exhaust and O₃, individually and in combination, to levels lower than is considered safe under current air quality standards, significantly reduced counts of locally-occurring wild and managed insect pollinators by 62-70% and their flower visits by 83-90%. These reductions were driven by changes in specific pollinator groups, including bees, flies, moths and butterflies, and coincided with significant reductions (14-31%) in three different metrics of pollination and yield of a self-fertile test plant. Quantifying such effects provides new insights into the impacts of human-induced air pollution on the natural ecosystem services upon which we depend.

Tropospheric Ozone Alters the Chemical Signal Emitted by an Emblematic Plant of the Mediterranean Region: The True Lavender (Lavandula angustifolia Mill.)

Among air pollutants, tropospheric ozone (O3) is one of the most stressful for organisms due to its strong oxidative potential. For instance, high ozone concentration ([O3]) has the potential to affect (i) the emission of volatile organic compounds (VOCs) by plants and (ii) the lifetime of these VOCs in the atmosphere, and consequently disturb crucial signals in the interactions between plants and other organisms. However, despite the determinant role of VOCs emitted by flowers for pollinator attraction, a very limited number of studies have investigated the impact of O3 on floral VOCs. In this study, we investigated the effect of high [O3] episodes on the VOCs emitted by a flowering Mediterranean plant: the true lavender (Lavandula angustifolia Mill., Lamiaceae). To do so, in controlled conditions, we exposed (i) the entire plant to high but realistic [O3] (200 ppb for 5 h) and (ii) only the VOCs emitted by lavender to increasing [O3] (0, 40, 80, 120, and 200 ppb). We sampled VOCs of lavender in both conditions and analyzed them by Gas Chromatography-Mass Spectrometry in order to qualify and quantify the flowering lavender’s emissions and the reaction of VOCs with O3 in the atmosphere. Our results showed that exposure to high [O3] during a short period (5 h) did not affect the emission of VOCs by flowering lavender. Incidentally, we also showed that the chemical signal varied in quantities and proportions over the day. Moreover, we showed that after their emission by the plant, composition of the VOCs changed quantitatively and qualitatively in an atmosphere containing [O3] naturally observed nowadays. Quantities of several of the major terpenes emitted by lavender decreased drastically during O3 exposure, whereas concentrations of some VOCs increased, such as carbonyls and carboxylic acids, which are probably reaction products of terpenes with O3. Exposure to high [O3] thus directly affected the proportions of VOCs in the atmosphere. Because pollinators generally use a blend of VOCs in particular proportions as a signal to localize flowers, the numerous pollinators of lavender may experience difficulty in recognizing specific floral odors during frequent and moderate [O3] episodes in the Mediterranean region.

Ozone Mitigates the Adverse Effects of Diesel Exhaust Pollutants on Ground-Active Invertebrates in Wheat

There is growing evidence to demonstrate that air pollution is affecting invertebrates both directly (e.g., causing physiological stress responses) and indirectly (e.g., via changes in host plant chemistry and/or by disruption of communication by volatile odours). Many of the studies to-date have focused upon winged insects and disruption of in-flight foraging. Therefore, in this study we investigated how the community composition of predominantly ground-dwelling invertebrates in fields of winter wheat are affected by two of the most ubiquitous lower tropospheric air pollutants, diesel exhaust emissions (including nitrogen oxides–NOx) and ozone (O3), both individually and in combination, over 2 years. Pitfall traps, located within the rings of a Free-Air Diesel and Ozone Enrichment (FADOE) facility, were used to sample invertebrates. The facility consisted of eight 8 m-diameter rings, which allowed elevation of the pollutants above ambient levels (ca 49–60 ppb NOx and 35–39 ppb O3) but within levels currently defined as safe for the environment by the Environmental Protection Agency. The invertebrates collected were taxonomically identified and characterised by diet specialisation, mobility and functional group. Taxonomic richness and Shannon’s diversity index were calculated. Even under the relatively low levels of air pollution produced, there were adverse impacts on invertebrate community composition, with greater declines in the abundance and taxonomic richness of invertebrates in the diesel exhaust treatment compared with O3 treatment. In the combined treatment, pollutant levels were lower, most likely because NOx and O3 react with one another, and consequently a lesser negative effect was observed on invertebrate abundance and taxonomic richness. Specialist-feeding and winged invertebrate species appeared to be more sensitive to the impacts of the pollutants, responding more negatively to air pollution treatments than generalist feeders and wingless species, respectively. Therefore, these results suggest a more severe pollution-mediated decline in specialist- compared with generalist-feeding invertebrates, and in more mobile (winged) individuals. Understanding how invertebrate communities respond to air pollutants alone and in combination will facilitate predictions of how terrestrial environments respond to changes in anthropogenic emissions, especially as we shift away from fossil fuel dependence and therefore manipulate the interactions between these two common pollutants.

Limited influence of experimentally induced predation risk on granivory in a tropical forest

Seed predation by rodents can strongly influence plant recruitment and establishment. The extent to which predation risk indirectly alters plant survival in tropical forests via impacts on granivory is unclear, making it difficult to assess the cascading impacts of widespread predator loss on tree recruitment and species composition. Experimental field studies that manipulate predation risk can help address these knowledge gaps and reveal whether antipredator responses among small mammals influence plant survival. We used camera traps and seed predation experiments to test the effects of perceived predation risk (via predator urine gel) on foraging behaviour of and seed removal by murid rodents in an unlogged and unhunted rainforest in Malaysian Borneo. We also explored the influence of seed traits (e.g., seed size) on removal by granivores and assessed whether granivore preferences for particular species were affected by predator urine. Murid visits to seed plots were positively related to overall seed removal, but were not affected by predator scent. Granivory was the lowest for the largest-seeded (>6 g) plant in our study, but was not influenced by predation risk. Predator urine significantly affected removal of one seed taxon (Dimoocarpus, ∼0.8 g), suggesting that removal by granivores may be affected by predation risk for some seed species but not others. This could have implications for plant species composition but may not affect the overall level of granivory.

The Effects of Ozone on Visual Attraction Traits of Erodium paularense (Geraniaceae) Flowers: Modelled Perception by Insect Pollinators

Ozone (O₃) effects on the visual attraction traits (color, perception and area) of petals are described for Erodium paularense, an endangered plant species. Plants were exposed to three O₃ treatments: charcoal-filtered air (CFA), ambient (NFA) and ambient + 40 nL L^-1 O₃ (FU+) in open-top chambers. Changes in color were measured by spectral reflectance, from which the anthocyanin reflectance index (ARI) was calculated. Petal spectral reflectance was mapped onto color spaces of bees, flies and butterflies for studying color changes as perceived by different pollinator guilds. Ozone-induced increases in petal reflectance and a rise in ARI under NFA were observed. Ambient O₃ levels also induced a partial change in the color perception of flies, with the number of petals seen as blue increasing to 53% compared to only 24% in CFA. Butterflies also showed the ability to partially perceive petal color changes, differentiating some CFA petals from NFA and FU+ petals through changes in the excitation of the UV photoreceptor. Importantly, O₃ reduced petal area by 19.8 and 25% in NFA and FU+ relative to CFA, respectively. In sensitive species O₃ may affect visual attraction traits important for pollination, and spectral reflectance is proposed as a novel method for studying O₃ effects on flower color.

Plant age at the time of ozone exposure affects flowering patterns, biotic interactions and reproduction of wild mustard

Exposure of plants to environmental stressors can modify their metabolism, interactions with other organisms and reproductive success. Tropospheric ozone is a source of plant stress. We investigated how an acute exposure to ozone at different times of plant development affects reproductive performance, as well as the flowering patterns and the interactions with pollinators and herbivores, of wild mustard plants. The number of open flowers was higher on plants exposed to ozone at earlier ages than on the respective controls, while plants exposed at later ages showed a tendency for decreased number of open flowers. The changes in the number of flowers provided a good explanation for the ozone-induced effects on reproductive performance and on pollinator visitation. Ozone exposure at earlier ages also led to either earlier or extended flowering periods. Moreover, ozone tended to increase herbivore abundance, with responses depending on herbivore taxa and the plant age at the time of ozone exposure. These results suggest that the effects of ozone exposure depend on the developmental stage of the plant, affecting the flowering patterns in different directions, with consequences for pollination and reproduction of annual crops and wild species.

Effects of air pollution exposure on social behavior: a synthesis and call for research

BACKGROUND

There is a growing literature from both epidemiologic and experimental animal studies suggesting that exposure to air pollution can lead to neurodevelopmental and neuropsychiatric disorders. Here, we suggest that effects of air pollutant exposure on the brain may be even broader, with the potential to affect social decision-making in general.

METHODS

We discuss how the neurobiological substrates of social behavior are vulnerable to air pollution, then briefly present studies that examine the effects of air pollutant exposure on social behavior-related outcomes.

RESULTS

Few experimental studies have investigated the effects of air pollution on social behavior and those that have focus on standard laboratory tests in rodent model systems. Nonetheless, there is sufficient evidence to support a critical need for more research.

CONCLUSION

For future research, we suggest a comparative approach that utilizes diverse model systems to probe the effects of air pollution on a wider range of social behaviors, brain regions, and neurochemical pathways.

The Urban River Syndrome: Achieving Sustainability Against a Backdrop of Accelerating Change

Human activities have been affecting rivers and other natural systems for millennia. Anthropogenic changes to rivers over the last few centuries led to the accelerating state of decline of coastal and estuarine regions globally. Urban rivers are parts of larger catchment ecosystems, which in turn form parts of wider nested, interconnected systems. Accurate modelling of urban rivers may not be possible because of the complex multisystem interactions operating concurrently and over different spatial and temporal scales. This paper overviews urban river syndrome, the accelerating deterioration of urban river ecology, and outlines growing conservation challenges of river restoration projects. This paper also reviews the river Thames, which is a typical urban river that suffers from growing anthropogenic effects and thus represents all urban rivers of similar type. A particular emphasis is made on ecosystem adaptation, widespread extinctions and the proliferation of non-native species in the urban Thames. This research emphasizes the need for a holistic systems approach to urban river restoration.

Ozone Pollution Alters Olfaction and Behavior of Pollinators

Concentration of air pollutants, particularly ozone (O₃), has dramatically increased since pre-industrial times in the troposphere. Due to the strong oxidative potential of O₃, negative effects on both emission and lifetime in the atmosphere of plant volatile organic compounds (VOCs) have already been highlighted. VOCs alteration by O₃ may potentially affect the attraction of pollinators that rely on these chemical signals. Surprisingly, direct effects of O₃ on the olfaction and the behavioral response of pollinators have not been investigated so far. We developed a comprehensive experiment under controlled conditions to assess O₃ physiological and behavioral effects on two pollinator species, differing in their ecological traits. Using several realistic concentrations of O₃ and various exposure times, we investigated the odor antennal detection and the attraction to VOCs present in the floral scents of their associated plants. Our results showed, in both species, a clear effect of exposure to high O₃ concentrations on the ability to detect and react to the floral VOCs. These effects depend on the VOC tested and its concentration, and the O₃ exposure (concentration and duration) on the pollinator species. Pollination systems may, therefore, be impaired in different ways by increased levels of O₃, the effects of which will likely depend on whether the exposure is chronic or, as in this study, punctual, likely causing some pollination systems to be more vulnerable than others. While several studies have already shown the negative impact of O₃ on VOCs emission and lifetime in the atmosphere, this study reveals, for the first time, that this impact alters the pollinator detection and behavior. These findings highlight the urgent need to consider air pollution when evaluating threats to pollinators.

Ozone disrupts the communication between plants and insects in urban and suburban areas: an updated insight on plant volatiles

Plant-insect interactions are basic components of biodiversity conservation. To attain the international Sustainable Development Goals (SDGs), the interactions in urban and in suburban systems should be better understood to maintain the health of green infrastructure. The role of ground-level ozone (O₃) as an environmental stress disrupting interaction webs is presented. Ozone mixing ratios in suburbs are usually higher than in the center of cities and may reduce photosynthetic productivity at a relatively higher degree. Consequently, carbon-based defense capacities of plants may be suppressed by elevated O₃ more in the suburbs. However, contrary to this expectation, grazing damages by leaf beetles have been severe in some urban centers in comparison with the suburbs. To explain differences in grazing damages between urban areas and suburbs, the disruption of atmospheric communication signals by elevated O₃ via changes in plant-regulated biogenic volatile organic compounds and long-chain fatty acids are considered. The ecological roles of plant volatiles and the effects of O₃ from both a chemical and a biological perspective are presented. Ozone-disrupted plant volatiles should be considered to explain herbivory phenomena in urban and suburban systems.

SUPPLEMENTARY INFORMATION

The online version of this article contains supplementary material available at (10.1007/s11676-020-01287-4) to authorized users.

Pollination in the Anthropocene: a Moth Can Learn Ozone-Altered Floral Blends

Insect pollination is essential to many unmanaged and agricultural systems and as such is a key element in food production. However, floral scents that pollinating insects rely on to locate host plants may be altered by atmospheric oxidants, such as ozone, potentially making these cues less attractive or unrecognizable to foraging insects and decreasing pollinator efficacy. We demonstrate that levels of tropospheric ozone commonly found in many rural areas are sufficient to disrupt the innate attraction of the tobacco hawkmoth Manduca sexta to the odor of one of its preferred flowers, Nicotiana alata. However, we further find that visual navigation together with associative learning can offset this disruption. Foraging moths that initially find an ozone-altered floral scent unattractive can target an artificial flower using visual cues and associate the ozone-altered floral blend with a nectar reward. The ability to learn ozone-altered floral odors may enable pollinators to maintain communication with their co-evolutionary partners and reduce the negative impacts that anthropogenically elevated oxidants may have on plant-pollinator systems.

A field-based quantitative analysis of sublethal effects of air pollution on pollinators

While the impact of air pollution on human health is well studied, mechanistic impacts of air pollution on wild systems, including those providing essential ecosystem services, are largely unknown, but directly impact our health and well-being. India is the world's largest fruit producer, second most populous country, and contains 9 of the world's 10 most polluted cities. Here, we sampled Giant Asian honey bees, Apis dorsata, at locations with varying air pollution levels in Bangalore, India. We observed significant correlations between increased respirable suspended particulate matter (RSPM) deposition and changes in bee survival, flower visitation, heart rate, hemocyte levels, and expression of genes related to lipid metabolism, stress, and immunity. Lab-reared Drosophila melanogaster exposed to these same sites also exhibited similar molecular and physiological differences. Our study offers a quantitative analysis on the current impacts of air pollution on insects, and indicates the urgency for more nonhuman studies to accurately assess the effects of pollution on our natural world.

The role of sensory drive in floral evolution

Sensory drive theory posits that the evolution of communication signals is shaped by the sensory systems of receivers and the habitat conditions under which signals are received. It has inspired an enormous body of research, advancing our understanding of signal evolution and speciation in animals. In plants, the extreme diversification of floral signals has fascinated biologists for over a century. While processes involved in sensory drive probably play out in plant-pollinator communication, the theory has not been formally synthesized in this context. However, it has untapped potential to explain mechanisms underlying variation in pollinator preferences across populations, and how environmental conditions impact floral signal transmission and perception. Here I develop a framework of sensory drive for plant-pollinator interactions, identifying similarities and differences from its original conception. I then summarize studies that shed light on how the primary processes of sensory drive - habitat transmission, perceptual tuning, and signal matching - apply to the evolution of floral color and scent. Throughout, I propose research avenues and approaches to assess how sensory drive shapes floral diversity. This framework will be important for explaining patterns of extant floral diversity and examining how altered signaling conditions under global change will impact the evolutionary trajectory of floral traits.

Ozone affects plant, insect, and soil microbial communities: A threat to terrestrial ecosystems and biodiversity

Elevated tropospheric ozone concentrations induce adverse effects in plants. We reviewed how ozone affects (i) the composition and diversity of plant communities by affecting key physiological traits; (ii) foliar chemistry and the emission of volatiles, thereby affecting plant-plant competition, plant-insect interactions, and the composition of insect communities; and (iii) plant-soil-microbe interactions and the composition of soil communities by disrupting plant litterfall and altering root exudation, soil enzymatic activities, decomposition, and nutrient cycling. The community composition of soil microbes is consequently changed, and alpha diversity is often reduced. The effects depend on the environment and vary across space and time. We suggest that Atlantic islands in the Northern Hemisphere, the Mediterranean Basin, equatorial Africa, Ethiopia, the Indian coastline, the Himalayan region, southern Asia, and Japan have high endemic richness at high ozone risk by 2100.

Plant-mediated effects of ozone on herbivores depend on exposure duration and temperature

Abiotic stress by elevated tropospheric ozone and temperature can alter plants’ metabolism, growth, and nutritional value and modify the life cycle of their herbivores. We investigated how the duration of exposure of Sinapis arvensis plants to high ozone and temperature levels affect the life cycle of the large cabbage white, Pieris brassicae. Plants were exposed to ozone-clean (control) or ozone-enriched conditions (120 ppb) for either 1 or 5 days and were afterwards kept in a greenhouse with variable temperature conditions. When given the choice, P. brassicae butterflies laid 49% fewer eggs on ozone-exposed than on control plants when the exposure lasted for 5 days, but showed no preference when exposure lasted for 1 day. The caterpillars took longer to hatch on ozone-exposed plants and at lower ambient temperatures. The ozone treatment had a positive effect on the survival of the eggs. Ozone decreased the growth of caterpillars reared at higher temperatures on plants exposed for 5 days, but not on plants exposed for 1 day. Overall, longer exposure of the plants to ozone and higher temperatures affected the life cycle of the herbivore more strongly. With global warming, the indirect impacts of ozone on herbivores are likely to become more common.

Exploiting Plant Volatile Organic Compounds (VOCs) in Agriculture to Improve Sustainable Defense Strategies and Productivity of Crops

There is an urgent need for new sustainable solutions to support agriculture in facing current environmental challenges. In particular, intensification of productivity and food security needs require sustainable exploitation of natural resources and metabolites. Here, we bring the attention to the agronomic potential of volatile organic compounds (VOCs) emitted from leaves, as a natural and eco-friendly solution to defend plants from stresses and to enhance crop production. To date, application of VOCs is often limited to fight herbivores. Here we argue that potential applications of VOCs are much wider, as they can also protect from pathogens and environmental stresses. VOCs prime plant's defense mechanisms for an enhanced resistance/tolerance to the upcoming stress, quench reactive oxygen species (ROS), have potent antimicrobial as well as allelopathic effects, and might be important in regulating plant growth, development, and senescence through interactions with plant hormones. Current limits and drawbacks that may hamper the use of VOCs in open field are analyzed, and solutions for a better exploitation of VOCs in future sustainable agriculture are envisioned.

Pheromones-based sexual selection in a rapidly changing world

Insects utilise chemical cues for a range of different purposes and the complexity and degree of specificity of these signals is arguably unparalleled in the animal kingdom. Chemical signals are particularly important for insect reproduction and the selective pressures driving their evolution and maintenance have been the subject of previous reviews. However, the world in which chemical cues evolved and are maintained is changing at an unprecedented rate. How (or indeed whether) chemical signals used in sexual selection will respond is largely unknown. Here, we explore how recent increases in urbanisation and associated anthropogenic impacts may affect how chemical signals are produced and perceived. We focus on four anthropomorphic influences which have the potential to interact with pheromone-mediated sexual selection processes; climatic temperature shifts, exposure to chemical pollutants, the presence of artificial light at night and nutrient availability. Our aim is to provide a broad overview of key areas where the rapidly changing environment of the future might specifically affect pheromones utilised in sexual selection.

Volatile-Mediated Interactions between Cabbage Plants in the Field and the Impact of Ozone Pollution

Plants constitutively release volatile organic compounds (VOCs), but qualitatively and quantitatively alter their emission of VOCs in response to biotic and abiotic stresses. The blend of VOCs emitted reflects the physiological status of the plant. Plants may be exposed to the VOC blend emitted by their near neighbors and gain information that allows them to adjust their own defenses. These plant-plant interactions may potentially be exploited to protect crops from pests, but they can be disturbed by abiotic factors making the process sensitive to environmental perturbation. Despite numerous studies describing plant-plant interactions, relatively few have been conducted with agriculturally significant cultivated plant varieties under field conditions. Here we studied plant-plant interactions in a conspecific association of Brassica oleracea var. capitata (cabbage) and show that undamaged plants exposed to neighbors damaged by the herbivore Pieris brassicae are primed for stronger volatile emissions upon subsequent herbivore attack. We conducted a field study in an ozone free-air concentration enrichment (FACE) facility with ambient and elevated ozone levels and found that elevated tropospheric ozone significantly alters the priming of VOCs in receiver plants. We conclude that plant-plant interactions may prime defensive attributes of receiver plants under field conditions, but are impaired by ozone pollution. Therefore, when planning the manipulation of plant-plant interactions for agricultural purposes, the potential effects of atmospheric pollutants should be considered.

Current and future ozone risks to global terrestrial biodiversity and ecosystem processes

Risks associated with exposure of individual plant species to ozone (O3) are well documented, but implications for terrestrial biodiversity and ecosystem processes have received insufficient attention. This is an important gap because feedbacks to the atmosphere may change as future O3 levels increase or decrease, depending on air quality and climate policies. Global simulation of O3 using the Community Earth System Model (CESM) revealed that in 2000, about 40% of the Global 200 terrestrial ecoregions (ER) were exposed to O3 above thresholds for ecological risks, with highest exposures in North America and Southern Europe, where there is field evidence of adverse effects of O3, and in central Asia. Experimental studies show that O3 can adversely affect the growth and flowering of plants and alter species composition and richness, although some communities can be resilient. Additional effects include changes in water flux regulation, pollination efficiency, and plant pathogen development. Recent research is unraveling a range of effects belowground, including changes in soil invertebrates, plant litter quantity and quality, decomposition, and nutrient cycling and carbon pools. Changes are likely slow and may take decades to become detectable. CESM simulations for 2050 show that O3 exposure under emission scenario RCP8.5 increases in all major biomes and that policies represented in scenario RCP4.5 do not lead to a general reduction in O3 risks; rather, 50% of ERs still show an increase in exposure. Although a conceptual model is lacking to extrapolate documented effects to ERs with limited or no local information, and there is uncertainty about interactions with nitrogen input and climate change, the analysis suggests that in many ERs, O3 risks will persist for biodiversity at different trophic levels, and for a range of ecosystem processes and feedbacks, which deserves more attention when assessing ecological implications of future atmospheric pollution and climate change.

Will climate change affect insect pheromonal communication?

Understanding how climate change will affect species interactions is a challenge for all branches of ecology. We have only limited understanding of how increasing temperature and atmospheric CO₂ and O₃ levels will affect pheromone-mediated communication among insects. Based on the existing literature, we suggest that the entire process of pheromonal communication, from production to behavioural response, is likely to be impacted by increases in temperature and modifications to atmospheric CO₂ and O₃ levels. We argue that insect species relying on long-range chemical signals will be most impacted, because these signals will likely suffer from longer exposure to oxidative gases during dispersal. We provide future directions for research programmes investigating the consequences of climate change on insect pheromonal communication.

Atmospheric transformation of plant volatiles disrupts host plant finding

Plant-emitted volatile organic compounds (VOCs) play important roles in plant-insect interactions. Atmospheric pollutants such as ozone (O3) can react with VOCs and affect the dynamics and fidelity of these interactions. However, the effects of atmospheric degradation of plant VOCs on plant-insect interactions remains understudied. We used a system comprising Brassica oleracea subsp. capitata (cabbage) and the specialist herbivore Plutella xylostella to test whether O3-triggered VOC degradation disturbs larval host orientation, and to investigate the underlying mechanisms. Larvae oriented towards both constitutive and larva-induced cabbage VOC blends, the latter being the more attractive. Such behaviour was, however, dramatically reduced in O3-polluted environments. Mechanistically, O3 rapidly degraded VOCs with the magnitude of degradation increasing with O3 levels. Furthermore, we used Teflon filters to collect VOCs and their reaction products, which were used as odour sources in behavioural tests. Larvae avoided filters exposed to O3-transformed VOCs and spent less time searching on them compared to filters exposed to original VOCs, which suggests that some degradation products may have repellent properties. Our study clearly demonstrates that oxidizing pollutants in the atmosphere can interfere with insect host location, and highlights the need to address their broader impacts when evaluating the ecological significance of VOC-mediated interactions.

Ozone Differentially Affects Perception of Plant Volatiles in Western Honey Bees

Floral scents play a key role in mediating plant-pollinator interactions. Volatile organic compounds (VOCs) emitted by flowers are used by flower visitors as olfactory cues to locate flowers, both from a distance and at close range. More recently it has been demonstrated that reactive molecules such as ozone can modify or degrade VOCs, and this may impair the communication between plants and their pollinators. However, it is not known whether such reactive molecules also may affect the olfactory system of pollinators, and thus not only influence signal transmission but perception of the signal. In this study, we used electroantennographic measurements to determine the effect of increased levels of ozone on antennal responses in western honey bees (Apis mellifera L.). Linalool and 2-phenylethanol, both known to be involved in location of flowers by the bees, and (Z)-3-hexenyl acetate, a widespread green leaf volatile also detected by bees, were used. The results showed that ozone affected antennal responses to the different substances differently. Ozone decreased antennal responses to (Z)-3-hexenyl acetate, whereas responses to linalool and 2-phenylethanol were not influenced by ozone. Overall, the study does not provide evidence that pollination by honey bees is impaired by damage in the olfactory system of the bees caused by increased levels of ozone, at least when linalool and 2-phenylethanol are the attractive signals. However, the results also suggest that ozone can change the overall perception of an odor blend. This might have negative effects in pollination systems and other organismic interactions mediated by specific ratios of compounds.

Quantum chemical study on the stability of honeybee queen pheromone against atmospheric factors

The managed honeybee, Apis mellifera, has been experienced a puzzling event, termed as colony collapse disorder (CCD), in which worker bees abruptly disappear from their hives. Potential factors include parasites, pesticides, malnutrition, and environmental stresses. However, so far, no definitive relationship has been established between specific causal factors and CCD events. Here we theoretically test whether atmospheric environment could disturb the chemical communication between the queen and their workers in a colony. A quantum chemistry method has been used to investigate for the stability of the component of A. mellifera queen mandibular pheromone (QMP), (E)-9-keto-2-decenoic acid (9-ODA), against atmospheric water and free radicals. The results show that 9-ODA is less likely to react with water due to the high barrier heights (~36.5 kcal · mol(-1)) and very low reaction rates. However, it can easily react with triplet oxygen and hydroxyl radicals because of low or negative energy barriers. Thus, the atmospheric free radicals may disturb the chemical communication between the queen and their daughters in a colony. Our pilot study provides new insight for the cause of CCD, which has been reported throughout the world.

Language of plants: Where is the word?

Plants emit biogenic volatile organic compounds (BVOCs) causing transcriptomic, metabolomic and behavioral responses in receiver organisms. Volatiles involved in such responses are often called "plant language". Arthropods having sensitive chemoreceptors can recognize language released by plants. Insect herbivores, pollinators and natural enemies respond to composition of volatiles from plants with specialized receptors responding to different types of compounds. In contrast, the mechanism of how plants "hear" volatiles has remained obscured. In a plant-plant communication, several individually emitted compounds are known to prime defense response in receiver plants with a specific manner according to the chemical structure of each volatile compound. Further, composition and ratio of volatile compounds in the plant-released plume is important in plant-insect and plant-plant interactions mediated by plant volatiles. Studies on volatile-mediated plant-plant signaling indicate that the signaling distances are rather short, usually not longer than one meter. Volatile communication from plants to insects such as pollinators could be across distances of hundreds of meters. As many of the herbivore induced VOCs have rather short atmospheric life times, we suggest that in long-distant communications with plant volatiles, reaction products in the original emitted compounds may have additional information value of the distance to emission source together with the original plant-emitted compounds.

Ozone degrades floral scent and reduces pollinator attraction to flowers

In this work we analyzed the degradation of floral scent volatiles from Brassica nigra by reaction with ozone along a distance gradient and the consequences for pollinator attraction. For this purpose we used a reaction system comprising three reaction tubes in which we conducted measurements of floral volatiles using a proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS) and GC-MS. We also tested the effects of floral scent degradation on the responses of the generalist pollinator Bombus terrestris. The chemical analyses revealed that supplementing air with ozone led to an increasing reduction in the concentrations of floral volatiles in air with distance from the volatile source. The results revealed different reactivities with ozone for different floral scent constituents, which emphasized that ozone exposure not only degrades floral scents, but also changes the ratios of compounds in a scent blend. Behavioural tests revealed that floral scent was reduced in its attractiveness to pollinators after it had been exposed to 120 ppb O3 over a 4.5 m distance. The combined results of chemical analyses and behavioural responses of pollinators strongly suggest that high ozone concentrations have significant negative impacts on pollination by reducing the distance over which floral olfactory signals can be detected by pollinators.

The Effects of Diesel Exhaust Pollution on Floral Volatiles and the Consequences for Honey Bee Olfaction

There is growing evidence of a substantial decline in pollinators within Europe and North America, most likely caused by multiple factors such as diseases, poor nutrition, habitat loss, insecticides, and environmental pollution. Diesel exhaust could be a contributing factor to this decline, since we found that diesel exhaust rapidly degrades floral volatiles, which honey bees require for flower recognition. In this study, we exposed eight of the most common floral volatiles to diesel exhaust in order to investigate whether it can affect volatile mediated plant-pollinator interaction. Exposure to diesel exhaust altered the blend of common flower volatiles significantly: myrcene was considerably reduced, β-ocimene became undetectable, and β-caryophyllene was transformed into its cis-isomer isocaryophyllene. Proboscis extension response (PER) assays showed that the alterations of the blend reduced the ability of honey bees to recognize it. The chemically reactive nitrogen oxides fraction of diesel exhaust gas was identified as capable of causing degradation of floral volatiles.

Plant volatiles in polluted atmospheres: stress responses and signal degradation

Plants emit a plethora of volatile organic compounds, which provide detailed information on the physiological condition of emitters. Volatiles induced by herbivore feeding are among the best studied plant responses to stress and may constitute an informative message to the surrounding community and further function in plant defence processes. However, under natural conditions, plants are potentially exposed to multiple concurrent stresses with complex effects on the volatile emissions. Atmospheric pollutants are an important facet of the abiotic environment and can impinge on a plant's volatile-mediated defences in multiple ways at multiple temporal scales. They can exert changes in volatile emissions through oxidative stress, as is the case with ozone pollution. The pollutants, in particular, ozone, nitrogen oxides and hydroxyl radicals, also react with volatiles in the atmosphere. These reactions result in volatile breakdown products, which may themselves be perceived by community members as informative signals. In this review, we demonstrate the complex interplay among stresses, emitted signals, and modification in signal strength and composition by the atmosphere, collectively determining the responses of the biotic community to elicited signals.

Multisensory integration of colors and scents: insights from bees and flowers

Karl von Frisch's studies of bees' color vision and chemical senses opened a window into the perceptual world of a species other than our own. A century of subsequent research on bees' visual and olfactory systems has developed along two productive but independent trajectories, leaving the questions of how and why bees use these two senses in concert largely unexplored. Given current interest in multimodal communication and recently discovered interplay between olfaction and vision in humans and Drosophila, understanding multisensory integration in bees is an opportunity to advance knowledge across fields. Using a classic ethological framework, we formulate proximate and ultimate perspectives on bees' use of multisensory stimuli. We discuss interactions between scent and color in the context of bee cognition and perception, focusing on mechanistic and functional approaches, and we highlight opportunities to further explore the development and evolution of multisensory integration. We argue that although the visual and olfactory worlds of bees are perhaps the best-studied of any non-human species, research focusing on the interactions between these two sensory modalities is vitally needed.

Catabolism of volatile organic compounds influences plant survival

Plants emit a diverse array of phytogenic volatile organic compounds (VOCs). The production and emission of VOCs has been an important area of research for decades. However, recent research has revealed the importance of VOC catabolism by plants and VOC degradation in the atmosphere for plant growth and survival. Specifically, VOC catabolism and degradation have implications for plant C balance, tolerance to environmental stress, plant signaling, and plant-atmosphere interactions. Here we review recent advances in our understanding of VOC catabolism and degradation, propose experiments for investigating VOC catabolism, and suggest ways to incorporate catabolism into VOC emission models. Improving our knowledge of VOC catabolism and degradation is crucial for understanding plant metabolism and predicting plant survival in polluted environments.

Where do herbivore-induced plant volatiles go?

Herbivore induced plant volatiles (HIPVs) are specific volatile organic compounds (VOC) that a plant produces in response to herbivory. Some HIPVs are only produced after damage, while others are also produced by intact plants, but in lower quantities. Among the known functions of HIPVs are within plant volatile signaling to activate systemic plant defenses, the priming and activation of defenses in neighboring plants and the attraction of natural enemies of herbivores. When released into the atmosphere a plant's control over the produced compounds ends. However, many of the HIPVs are highly reactive with atmospheric oxidants and their atmospheric life times could be relatively short, often only a few minutes. We summarise the potential ecological and atmospheric processes that involve the reaction products of HIPVs in their gaseous, liquid and solid secondary organic aerosol (SOA) forms, both in the atmosphere and after deposition on plant surfaces. A potential negative feedback loop, based on the reactions forming SOA from HIPVs and the associated stimulation of sun screening cloud formation is presented. This hypothesis is based on recent field surveys in the geographical areas facing the greatest degree of global warming and insect outbreaks. Furthermore, we discuss how these processes could benefit the individual plant or conspecifics that originally released the HIPVs into the atmosphere. Further ecological studies should aim to elucidate the possible reasons for biosynthesis of short-lived volatile compounds to have evolved as a response to external biotic damage to plants.

Patterns of genetic and reproductive traits differentiation in Mainland vs. Corsican populations of bumblebees

Populations on islands often exhibit lower levels of genetic variation and ecomorphological divergence compared to their mainland relatives. While phenotypic differentiation in characters, such as size or shape among insular organisms, has been well studied, insular differentiation in quantitative reproductive traits involved in chemical communication has received very little attention to date. Here, we investigated the impact of insularity on two syntopic bumblebee species pairs: one including species that are phylogenetically related (Bombus terrestris and B. lucorum), and the other including species that interact ecologically (B. terrestris and its specific nest inquiline B. vestalis). For each bumblebee species, we characterized the patterns of variation and differentiation of insular (Corsican) vs. mainland (European) populations (i) with four genes (nuclear and mitochondrial, 3781 bp) and (ii) in the chemical composition of male marking secretions (MMS), a key trait for mate attraction in bumblebees, by gas chromatography-mass spectrometry (GC-MS). Our results provide evidence for genetic differentiation in Corsican bumblebees and show that, contrary to theoretical expectations, island populations of bumblebees exhibit levels of genetic variation similar to the mainland populations. Likewise, our comparative chemical analyses of MMS indicate that Corsican populations of bumblebees are significantly differentiated from the mainland yet they hold comparative levels of within-population MMS variability compared to the mainland. Therefore, insularity has led Corsican populations to diverge both genetically and chemically from their mainland relatives, presumably through genetic drift, but without a decrease of genetic diversity in island populations. We hypothesize that MMS divergence in Corsican bumblebees was driven by a persistent lack of gene flow with mainland populations and reinforced by the preference of Corsican females for sympatric (Corsican) MMS. The impoverished Corsican bumblebee fauna has not led to relaxation of stabilizing selection on MMS but to consistent differentiation chemical reproductive traits on the island.

Plant volatile organic compounds (VOCs) in ozone (O3) polluted atmospheres: the ecological effects

Tropospheric ozone (O3) is an important secondary air pollutant formed as a result of photochemical reactions between primary pollutants, such as nitrogen oxides (NOx), and volatile organic compounds (VOCs). O3 concentrations in the lower atmosphere (troposphere) are predicted to continue increasing as a result of anthropogenic activity, which will impact strongly on wild and cultivated plants. O3 affects photosynthesis and induces the development of visible foliar injuries, which are the result of genetically controlled programmed cell death. It also activates many plant defense responses, including the emission of phytogenic VOCs. Plant emitted VOCs play a role in many eco-physiological functions. Besides protecting the plant from abiotic stresses (high temperatures and oxidative stress) and biotic stressors (competing plants, micro- and macroorganisms), they drive multitrophic interactions between plants, herbivores and their natural enemies e.g., predators and parasitoids as well as interactions between plants (plant-to-plant communication). In addition, VOCs have an important role in atmospheric chemistry. They are O3 precursors, but at the same time are readily oxidized by O3, thus resulting in a series of new compounds that include secondary organic aerosols (SOAs). Here, we review the effects of O3 on plants and their VOC emissions. We also review the state of current knowledge on the effects of ozone on ecological interactions based on VOC signaling, and propose further research directions.

Variation in natural plant products and the attraction of bodyguards involved in indirect plant defenseThe present review is one in the special series of reviews on animal–plant interactions

Plants can respond to feeding or egg deposition by herbivorous arthropods by changing the volatile blend that they emit. These herbivore-induced plant volatiles (HIPVs) can attract carnivorous natural enemies of the herbivores, such as parasitoids and predators, a phenomenon that is called indirect plant defense. The volatile blends of infested plants can be very complex, sometimes consisting of hundreds of compounds. Most HIPVs can be classified as terpenoids (e.g., (E)-β-ocimene, (E,E)-α-farnesene, (E)-4,8-dimethyl-1,3,7-nonatriene), green leaf volatiles (e.g., hexanal, (Z)-3-hexen-1-ol, (Z)-3-hexenyl acetate), phenylpropanoids (e.g., methyl salicylate, indole), and sulphur- or nitrogen-containing compounds (e.g., isothiocyanates or nitriles, respectively). One highly intriguing question has been which volatiles out of the complex blend are the most important ones for the carnivorous natural enemies to locate "suitable host plants. Here, we review the methods and techniques that have been used to elucidate the carnivore-attracting compounds. Electrophysiological methods such as electroantennography have been used with parasitoids to elucidate which compounds can be perceived by the antennae. Different types of elicitors and inhibitors have widely been applied to manipulate plant volatile blends. Furthermore, transgenic plants that were genetically modified in specific steps in one of the signal transduction pathways or biosynthetic routes have been used to find steps in HIPV emission crucial for indirect plant defense. Furthermore, we provide an overview on biotic and abiotic factors that influence the emission of HIPVs and how this can affect the interactions between members of different trophic levels. Consequently, we review the progress that has been made in this exciting research field during the past 30 years since the first studies on HIPVs emerged and we highlight important issues to be addressed in the future.