Effects of heat and drought stress on post-illumination bursts of volatile organic compounds in isoprene-emitting and non-emitting poplar

Cowpea volatiles induced by beet armyworm or fall armyworm differentially prime maize plants

Exposure to herbivore-induced plant volatiles (HIPVs) is known to enhance the defense responses in plants. This so-called priming effect has only been marginally studied in intercropping systems. We tested whether HIPVs from cowpea, which often serves as an intercrop alongside maize, can prime herbivore-induced volatile emissions in maize. Conventional volatile collection assays and real-time mass spectrometry revealed that maize plants that were exposed to HIPVs from cowpea infested with Spodoptera exigua caterpillars emitted more than control plants when they themselves were subsequently damaged by the same pest. The enhanced emission was only evident on the first day after infestation. Maize plants that were exposed to HIPVs from cowpea infested by S. frugiperda larvae showed no priming effect and released considerably less upon S. frugiperda infestation than upon S. exigua infestation. The latter may be explained by the fact that S. frugiperda is particularly well adapted to feed on maize and is known to suppress maize HIPV emissions. Our results imply that HIPVs from cowpea, depending on the inducing insect herbivore, may strongly prime maize plants. This deserves further investigation, also in other intercropping systems, as it can have important consequences for tritrophic interactions and crop protection.

Effects of Leaf Removal on Free and Glycoconjugate Aromas of Skins and Pulps of Two Italian Red Grapevine Varieties

Leaf removal is a cultural practice mainly aimed at improving cluster zone microclimates and impacting primary and secondary metabolites, such as volatiles. This research aimed to assess the impact of defoliation on free and glycosylated aromas of a neutral ('Nebbiolo') and a semi-aromatic ('Aleatico') red variety. Defoliation was performed at fruit set (BBCH 71) and, for 'Nebbiolo', also at berries touch (BBCH 81) phenological stages. Skins and pulps were separately analyzed by Solid Phase Extraction-Gas Chromatography/Mass Spectrometry. Results showed that the response to defoliation was variety-dependent. For 'Nebbiolo', especially when performed at the berries' touch stage, defoliation had a significant effect on the accumulation of free volatiles and glycosidic precursors. Differently, free and bound 'Aleatico' volatiles were less impacted by defoliation. Interestingly, in both grapevine varieties, defoliation significantly enhanced the accumulation of aroma precursors in grapes' skins, which is of particular relevance for red wine production and their aging potential. Moreover, results could be helpful for the management of grape quality, as defoliation is currently considered as a strategy to address climate change issues.

Age effects of Moso bamboo on leaf isoprene emission characteristics

Isoprene is a highly reactive volatile organic compound that significantly affects atmospheric oxidant capacity, regional air quality, and climate change. Moso bamboo (Phyllostachys edulis), a species widely distributed in tropical and subtropical regions, particularly in China, is a strong isoprene emitter with great potential for carbon sequestration. Carbon sequestration is negatively correlated with culm age; however, the effect of this correlation on isoprene emissions remains unknown. In this study, we investigated the photosynthetic and isoprene emission characteristics of Moso bamboo at different culm ages. The results showed that the age effect on isoprene emission was different from that on photosynthesis; the net photosynthesis rate (Pn) was the highest in young, followed by mature, and then old bamboo, whereas the isoprene emission rate (Iso) was the highest in young, followed by old, and then mature bamboo. Moreover, the percentage of carbon loss as isoprene emission (C-loss) during photosynthesis of old bamboo was 35% higher than that of mature bamboo under standard conditions (leaf temperature: 30°C; light intensity: 1000 µmol m-2 s-1). Therefore, we strongly recommend considering the culm age when establishing an isoprene emission model of Moso bamboo. Additionally, because the Iso and C-loss of old bamboo were higher than those of mature bamboo, we suggest that attention should be paid to the management of bamboo age structure and timely felling of aged bamboo to reduce environmental risk.

Intracellular kynurenine promotes acetaldehyde accumulation, further inducing the apoptosis in soil beneficial fungi Trichoderma guizhouense NJAU4742 under acid stress

In this study, the growth of fungi Trichoderma guizhouense NJAU4742 was significantly inhibited under acid stress, and the genes related to acid stress were identified based on transcriptome analysis. Four genes including tna1, adh2/4, and bna3 were significantly up-regulated. Meanwhile, intracellular hydrogen ions accumulated under acid stress, and ATP synthesis was induced to transport hydrogen ions to maintain hydrogen ion balance. The enhancement of glycolysis pathway was also detected, and a large amount of pyruvic acid from glycolysis was accumulated due to the activity limitation of PDH enzymes. Finally, acetaldehyde accumulated, resulting in the induction of adh2/4. In order to cope with stress caused by acetaldehyde, cells enhanced the synthesis of NAD⁺ by increasing the expression of tna1 and bna3 genes. NAD⁺ effectively improved the antioxidant capacity of cells, but the NAD⁺ supplement pathway mediated by bna3 could also cause the accumulation of kynurenine (KYN), which was an inducer of apoptosis. In addition, KYN had a specific promoting effect on acetaldehyde synthesis by improving the expression of eno2 gene, which led to the extremely high intracellular acetaldehyde in the cell under acidic stress. Our findings provided a route to better understand the response of filamentous fungi under acid stress.

Exogenous stimulation-induced biosynthesis of volatile compounds: Aroma formation of oolong tea at postharvest stage

Volatile organic compounds (VOCs) are produced by plants responding to biotic and abiotic stresses. According to their biosynthetic sources, induced VOCs are divided into three major classes: terpenoids, phenylpropanoid/benzenoid, and fatty acid derivatives. These compounds with specific aroma characteristics importantly contribute to the aroma quality of oolong tea. Shaking and rocking is the crucial procedure for the aroma formation of oolong tea by exerting mechanical damage to fresh tea leaves. Abundant studies have been carried out to investigate the formation mechanisms of VOCs during oolong tea processing in recent years. This review systematically introduces the biosynthesis of VOCs in plants, and the volatile changes due to biotic and abiotic stresses are summarized and expatiated, using oolong tea as an example.

Regeneration of duckweed (Lemna turonifera) involves genetic molecular regulation and cyclohexane release

Plant regeneration is important for vegetative propagation, detoxification and the obtain of transgenic plant. We found that duckweed regeneration could be enhanced by regenerating callus. However, very little is known about the molecular mechanism and the release of volatile organic compounds (VOCs). To gain a global view of genes differently expression profiles in callus and regenerating callus, genetic transcript regulation has been studied. Auxin related genes have been significantly down-regulated in regenerating callus. Cytokinin signal pathway genes have been up-regulated in regenerating callus. This result suggests the modify of auxin and cytokinin balance determines the regenerating callus. Volatile organic compounds release has been analysised by gas chromatography/ mass spectrum during the stage of plant regeneration, and 11 kinds of unique volatile organic compounds in the regenerating callus were increased. Cyclohexane treatment enhanced duckweed regeneration by initiating root. Moreover, Auxin signal pathway genes were down-regulated in callus treated by cyclohexane. All together, these results indicated that cyclohexane released by regenerating callus promoted duckweed regeneration. Our results provide novel mechanistic insights into how regenerating callus promotes regeneration.

Secondary Metabolites Coordinately Protect Grapes from Excessive Light and Sunburn Damage during Development

Sunburn is a physiological disorder that reduces grape quality and vineyard yield. It is the result of excessive sunlight and high temperatures. As climate change continues to increase air temperatures, reports of sunburn damage in vineyards worldwide are becoming more frequent. Grapes produce secondary metabolites (carotenoids, polyphenols and aroma compounds) to counter photooxidative stress and acclimate to higher radiation environments. This study evaluated changes in these compounds in during ripening when grapes were exposed post-flowering (ED) and at véraison (LD), and compared them to a nondefoliated control (ND). ND contained more α-terpineol and violaxanthin, and the defoliated treatments contained more zeaxanthin, β-carotene, C₆ compounds and flavonoids. ED berries adapted better to higher-light environments, displayed larger changes in secondary metabolite concentrations and lower levels of sunburn damage than LD berries did. The composition of berries with increasing sunburn damage was evaluated for the first time. Berries with no damage had the lowest concentrations of flavonoids and oxidized glutathione, and the highest concentrations of chlorophyll and α-terpineol. As damage increased, destruction of photosynthetic pigments, increase in polyphenols and loss of aroma compounds were evidenced. A significant effect of temperature and developmental stage on grape composition was also observed. This study provides a holistic overview of changes in secondary metabolites experienced by grape berries when exposed to excessive light, how these vary along development and how they affect sunburn incidence.

Combined Acute Ozone and Water Stress Alters the Quantitative Relationships between O3 Uptake, Photosynthetic Characteristics and Volatile Emissions in Brassica nigra

Ozone (O₃) entry into plant leaves depends on atmospheric O₃ concentration, exposure time and openness of stomata. O₃ negatively impacts photosynthesis rate (A) and might induce the release of reactive volatile organic compounds (VOCs) that can quench O₃, and thereby partly ameliorate O₃ stress. Water stress reduces stomatal conductance (g_s) and O₃ uptake and can affect VOC release and O₃ quenching by VOC, but the interactive effects of O₃ exposure and water stress, as possibly mediated by VOC, are poorly understood. Well-watered (WW) and water-stressed (WS) Brassica nigra plants were exposed to 250 and 550 ppb O₃ for 1 h, and O₃ uptake rates, photosynthetic characteristics and VOC emissions were measured through 22 h recovery. The highest O₃ uptake was observed in WW plants exposed to 550 ppb O₃ with the greatest reduction and poorest recovery of g_s and A, and elicitation of lipoxygenase (LOX) pathway volatiles 10 min-1.5 h after exposure indicating cellular damage. Ozone uptake was similar in 250 ppb WW and 550 ppb WS plants and, in both treatments, O₃-dependent reduction in photosynthetic characteristics was moderate and fully reversible, and VOC emissions were little affected. Water stress alone did not affect the total amount and composition of VOC emissions. The results indicate that drought ameliorated O₃ stress by reducing O₃ uptake through stomatal closure and the two stresses operated in an antagonistic manner in B. nigra.

Heatwave frequency and seedling death alter stress-specific emissions of volatile organic compounds in Aleppo pine

Biogenic volatile organic compounds (BVOC) play important roles in plant stress responses and can serve as stress indicators. While the impacts of gradual environmental changes on BVOCs have been studied extensively, insights in emission responses to repeated stress and recovery are widely absent. Therefore, we studied the dynamics of shoot gas exchange and BVOC emissions in Pinus halepensis seedlings during an induced moderate drought, two four-day-long heatwaves, and the combination of drought and heatwaves. We found clear stress-specific responses of BVOC emissions. Reductions in acetone emissions with declining soil water content and transpiration stood out as a clear drought indicator. All other measured BVOC emissions responded exponentially to rising temperatures during heat stress (maximum of 43 °C), but monoterpenes and methyl salicylate showed a reduced temperature sensitivity during the second heatwave. We found that these decreases in monoterpene emissions between heatwaves were not reflected by similar declines in their internal storage pools. Because stress intensity was extremely severe, most of the seedlings in the heat-drought treatment died at the end of the second heatwave (dark respiration ceased). Interestingly, BVOC emissions (methanol, monoterpenes, methyl salicylate, and acetaldehyde) differed between dying and surviving seedlings, already well before indications of a reduced vitality became visible in gas exchange dynamics. In summary, we could clearly show that the dynamics of BVOC emissions are sensitive to stress type, stress frequency, and stress severity. Moreover, we found indications that stress-induced seedling mortality was preceded by altered methanol, monoterpene, and acetaldehyde emission dynamics.

Origin of volatile organic compound emissions from subarctic tundra under global warming

Warming occurs in the Arctic twice as fast as the global average, which in turn leads to a large enhancement in terpenoid emissions from vegetation. Volatile terpenoids are the main class of biogenic volatile organic compounds (VOCs) that play crucial roles in atmospheric chemistry and climate. However, the biochemical mechanisms behind the temperature-dependent increase in VOC emissions from subarctic ecosystems are largely unexplored. Using ¹³ CO₂ -labeling, we studied the origin of VOCs and the carbon (C) allocation under global warming in the soil-plant-atmosphere system of contrasting subarctic heath tundra vegetation communities characterized by dwarf shrubs of the genera Salix or Betula. The projected temperature rise of the subarctic summer by 5°C was realistically simulated in sophisticated climate chambers. VOC emissions strongly depended on the plant species composition of the heath tundra. Warming caused increased VOC emissions and significant changes in the pattern of volatiles toward more reactive hydrocarbons. The ¹³ C was incorporated to varying degrees in different monoterpene and sesquiterpene isomers. We found that de novo monoterpene biosynthesis contributed to 40%-44% (Salix) and 60%-68% (Betula) of total monoterpene emissions under the current climate, and that warming increased the contribution to 50%-58% (Salix) and 87%-95% (Betula). Analyses of above- and belowground ^12/13 C showed shifts of C allocation in the plant-soil systems and negative effects of warming on C sequestration by lowering net ecosystem exchange of CO₂ and increasing C loss as VOCs. This comprehensive analysis provides the scientific basis for mechanistically understanding the processes controlling terpenoid emissions, required for modeling VOC emissions from terrestrial ecosystems and predicting the future chemistry of the arctic atmosphere. By changing the chemical composition and loads of VOCs into the atmosphere, the current data indicate that global warming in the Arctic may have implications for regional and global climate and for the delicate tundra ecosystems.

Heat Waves Change Plant Carbon Allocation Among Primary and Secondary Metabolism Altering CO2 Assimilation, Respiration, and VOC Emissions

Processes controlling plant carbon allocation among primary and secondary metabolism, i.e., carbon assimilation, respiration, and VOC synthesis are still poorly constrained, particularly regarding their response to stress. To investigate these processes, we simulated a 10-day 38°C heat wave, analysing real-time carbon allocation into primary and secondary metabolism in the Mediterranean shrub Halimium halimifolium L. We traced position-specific ¹³C-labeled pyruvate into daytime VOC and CO₂ emissions and during light-dark transition. Net CO₂ assimilation strongly declined under heat, due to three-fold higher respiration rates. Interestingly, day respiration also increased two-fold. Decarboxylation of the C1-atom of pyruvate was the main process driving daytime CO₂ release, whereas the C2-moiety was not decarboxylated in the TCA cycle. Heat induced high emissions of methanol, methyl acetate, acetaldehyde as well as mono- and sesquiterpenes, particularly during the first two days. After 10-days of heat a substantial proportion of ¹³C-labeled pyruvate was allocated into de novo synthesis of VOCs. Thus, during extreme heat waves high respiratory losses and reduced assimilation can shift plants into a negative carbon balance. Still, plants enhanced their investment into de novo VOC synthesis despite associated metabolic CO₂ losses. We conclude that heat stress re-directed the proportional flux of key metabolites into pathways of VOC biosynthesis most likely at the expense of reactions of plant primary metabolism, which might highlight their importance for stress protection.

Isoprene: New insights into the control of emission and mediation of stress tolerance by gene expression

Isoprene is a volatile compound produced in large amounts by some, but not all, plants by the enzyme isoprene synthase. Plants emit vast quantities of isoprene, with a net global output of 600 Tg per year, and typical emission rates from individual plants around 2% of net carbon assimilation. There is significant debate about whether global climate change resulting from increasing CO2 in the atmosphere will increase or decrease global isoprene emission in the future. We show evidence supporting predictions of increased isoprene emission in the future, but the effects could vary depending on the environment under consideration. For many years, isoprene was believed to have immediate, physical effects on plants such as changing membrane properties or quenching reactive oxygen species. Although observations sometimes supported these hypotheses, the effects were not always observed, and the reasons for the variability were not apparent. Although there may be some physical effects, recent studies show that isoprene has significant effects on gene expression, the proteome, and the metabolome of both emitting and nonemitting species. Consistent results are seen across species and specific treatment protocols. This review summarizes recent findings on the role and control of isoprene emission from plants.

The Systems Architecture of Molecular Memory in Poplar after Abiotic Stress

Throughout the temperate zones, plants face combined drought and heat spells in increasing frequency and intensity. Here, we compared periodic (intermittent, i.e., high-frequency) versus chronic (continuous, i.e., high-intensity) drought-heat stress scenarios in gray poplar (Populus× canescens) plants for phenotypic and transcriptomic effects during stress and after recovery. Photosynthetic productivity after stress recovery exceeded the performance of poplar trees without stress experience. We analyzed the molecular basis of this stress-related memory phenotype and investigated gene expression responses across five major tree compartments including organs and wood tissues. For each of these tissue samples, transcriptomic changes induced by the two stress scenarios were highly similar during the stress phase but strikingly divergent after recovery. Characteristic molecular response patterns were found across tissues but involved different genes in each tissue. Only a small fraction of genes showed similar stress and recovery expression profiles across all tissues, including type 2C protein phosphatases, the LATE EMBRYOGENESIS ABUNDANT PROTEIN4-5 genes, and homologs of the Arabidopsis (Arabidopsis thaliana) transcription factor HOMEOBOX7. Analysis of the predicted transcription factor regulatory networks for these genes suggested that a complex interplay of common and tissue-specific components contributes to the coordination of post-recovery responses to stress in woody plants.

Alternative Carbon Sources for Isoprene Emission

Isoprene and other plastidial isoprenoids are produced primarily from recently assimilated photosynthates via the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway. However, when environmental conditions limit photosynthesis, a fraction of carbon for MEP pathway can come from extrachloroplastic sources. The flow of extrachloroplastic carbon depends on the species and on leaf developmental and environmental conditions. The exchange of common phosphorylated intermediates between the MEP pathway and other metabolic pathways can occur via plastidic phosphate translocators. C1 and C2 carbon intermediates can contribute to chloroplastic metabolism, including photosynthesis and isoprenoid synthesis. Integration of these metabolic processes provide an example of metabolic flexibility, and results in the synthesis of primary metabolites for plant growth and secondary metabolites for plant defense, allowing effective use of environmental resources under multiple stresses.

Green leaf volatile-burst in Arabidopsis is governed by galactolipid oxygenation by a lipoxygenase that is under control of calcium ion

Plants form green leaf volatiles (GLVs) almost instantly after tissue disruption caused by damages, such as herbivore damage. This rapid formation of GLVs, namely GLV-burst, is an essential factor for the plants' GLV-dependent direct and indirect defenses. However, mechanism of GLV-burst remains unknown. We observed that the formation of monogalactosyldiacylglycerol hydroperoxides (MGDG-OOHs) by Arabidopsis lipoxygenase 2 (AtLOX2) governs GLV-burst in Arabidopsis. Addition of a Ca²⁺ selective chelating reagent, BAPTA, during tissue disruption effectively suppressed the formation of MGDG-OOHs as well as GLV-burst. This suppression was relieved by the addition of Ca²⁺. Therefore, we propose that Ca²⁺-dependent activation of AtLOX2 facilitates GLV-burst formation as observed in leukotriene formation, which is regulated by Ca²⁺-dependent activation of LOXs in animal cells.

Volatilomics: a non-invasive technique for screening plant phenotypic traits

BACKGROUND

Climate change represents a grand challenge for agricultural productivity. Understanding complex plant traits such as stress tolerance, disease resistance or crop yield is thus essential for breeding and the development of sustainable agriculture strategies. When screening for the most robust plant phenotypes, fast, high-throughput phenotyping represents the means of choice.

RESULTS

We have developed a plant phenotyping platform to measure the emission of volatile organic compounds (VOCs), photosynthetic gas exchange and transpiration under ambient, or abiotic and biotic stress conditions. These parameters are highly suitable markers to non-invasively and dynamically study plant growth and plant stress status, making them perfect test variables for long-term, online plant monitoring. Here we introduce the new phenotyping platform, termed VOC-SCREEN, and present results of a first case study with three barley cultivars, demonstrating that the plant's volatilome can be successfully applied to discriminate different barley varieties.

CONCLUSION

Volatilomics is a promising technique to non-invasively screen for plant phenotypic traits.

Diel rhythms in the volatile emission of apple and grape foliage

This study investigated the diel emission of volatile organic compounds (VOCs) from intact apple (Malus x domestica Borkh., cv. Golden Delicious) and grape (Vitis vinifera L., cv. Pinot Noir) foliage. Volatiles were monitored continuously for 48 h by proton transfer reaction - time of flight - mass spectrometry (PTR-ToF-MS). In addition, volatiles were collected by closed-loop-stripping-analysis (CLSA) and characterized by gas chromatography-mass spectrometry (GC-MS) after 1 h and again 24 and 48 h later. Fourteen and ten volatiles were characterized by GC-MS in apple and grape, respectively. The majority of these were terpenes, followed by green leaf volatiles, and aromatic compounds. The PTR-ToF-MS identified 10 additional compounds and established their diel emission rhythms. The most abundant volatiles displaying a diel rhythm included methanol and dimethyl sulfide in both plants, acetone in grape, and mono-, homo- and sesquiterpenes in apple. The majority of volatiles were released from both plants during the photophase; whereas methanol, CO₂, methyl-butenol and benzeneacetaldehyde were released at significantly higher levels during the scotophase. Acetaldehyde, ethanol, and some green leaf volatiles showed distinct emission bursts in both plants following the daily light switch-off. These new results obtained with a combined analytical approach broaden our understanding of the rhythms of constitutive volatile release from two important horticultural crops. In particular, diel emission of sulfur and nitrogen-containing volatiles are reported here for the first time in these two crops.

Monoterpenes Support Systemic Acquired Resistance within and between Plants

This study investigates the role of volatile organic compounds in systemic acquired resistance (SAR), a salicylic acid (SA)-associated, broad-spectrum immune response in systemic, healthy tissues of locally infected plants. Gas chromatography coupled to mass spectrometry analyses of SAR-related emissions of wild-type and non-SAR-signal-producing mutant plants associated SAR with monoterpene emissions. Headspace exposure of Arabidopsis thaliana to a mixture of the bicyclic monoterpenes α-pinene and β-pinene induced defense, accumulation of reactive oxygen species, and expression of SA- and SAR-related genes, including the SAR regulatory AZELAIC ACID INDUCED1 (AZI1) gene and three of its paralogs. Pinene-induced resistance was dependent on SA biosynthesis and signaling and on AZI1 Arabidopsis geranylgeranyl reductase1 mutants with reduced monoterpene biosynthesis were SAR-defective but mounted normal local resistance and methyl salicylate-induced defense responses, suggesting that monoterpenes act in parallel with SA The volatile emissions from SAR signal-emitting plants induced defense in neighboring plants, and this was associated with the presence of α-pinene, β-pinene, and camphene in the emissions of the "sender" plants. Our data suggest that monoterpenes, particularly pinenes, promote SAR, acting through ROS and AZI1, and likely function as infochemicals in plant-to-plant signaling, thus allowing defense signal propagation between neighboring plants.

De novo post-illumination monoterpene burst in Quercus ilex (holm oak)

Explicit proof for de novo origin of a rare post-illumination monoterpene burst and its consistency under low O ₂ , shows interaction of photorespiration, photosynthesis, and isoprenoid biosynthesis during light-dark transitions. Quercus ilex L (holm oak) constitutively emits foliar monoterpenes in an isoprene-like fashion via the methyl erythritol phosphate (MEP) pathway located in chloroplasts. Isoprene-emitting plants are known to exhibit post-illumination isoprene burst, a transient emission of isoprene in darkness. An analogous post-illumination monoterpene burst (PiMB) had remained elusive and is reported here for the first time in Q. ilex. Using ¹³CO₂ labelling, we show that PiMB is made from freshly fixed carbon. PiMB is rare at ambient (20%) O₂, absent at high (50%) O₂, and becomes consistent in leaves exposed to low (2%) O₂. PiMB is stronger and occurs earlier at higher temperatures. We also show that primary and secondary post-illumination CO ₂ bursts (PiCO₂B) are sensitive to O₂ in Q. ilex. The primary photorespiratory PiCO₂B is absent under both ambient and low O₂, but is induced under high (>50%) O₂, while the secondary PiCO₂B (of unknown origin) is absent under ambient, but present at low and high O₂. We propose that post-illumination recycling of photorespired CO₂ competes with the MEP pathway for photosynthetic carbon and energy, making PiMB rare under ambient O₂ and absent at high O₂. PiMB becomes consistent when photorespiration is suppressed in Q. ilex.

Emission of Volatile Compounds from Apple Plants Infested with Pandemis heparana Larvae, Antennal Response of Conspecific Adults, and Preliminary Field Trial

This study investigated the volatile emission from apple (Malus x domestica Borkh., cv. Golden Delicious) foliage that was either intact, mechanically-damaged, or exposed to larval feeding by Pandemis heparana (Denis and Schiffermüller) (Lepidoptera: Tortricidae). Volatiles were collected by closed-loop-stripping-analysis and characterized by gas chromatography-mass spectrometry in three time periods: after 1 h and again 24 and 48 h later. Volatiles for all treatments also were monitored continuously over a 72-h period by the use of proton transfer reaction - time of flight-mass spectrometry (PTR-ToF-MS). In addition, the volatile samples were analyzed by gas chromatography-electroantennographic detection (GC-EAD) using male and female antennae of P. heparana. Twelve compounds were detected from intact foliage compared with 23 from mechanically-damaged, and 30 from P. heparana-infested foliage. Interestingly, six compounds were released only by P. heparana-infested foliage. The emission dynamics of many compounds measured by PTR-ToF-MS showed striking differences according to the timing of herbivory and the circadian cycle. For example, the emission of green leaf volatiles began shortly after the start of herbivory, and increased over time independently from the light-dark cycle. Conversely, the emission of terpenes and aromatic compounds showed a several-hour delay in response to herbivory, and followed a diurnal rhythm. Methanol was the only identified volatile showing a nocturnal rhythm. Consistent GC-EAD responses were found for sixteen compounds, including five aromatic ones. A field trial in Sweden demonstrated that benzyl alcohol, 2-phenylethanol, phenylacetonitrile, and indole lures placed in traps were not attractive to Pandemis spp. adults, but 2-phenylethanol and phenylacetonitrile when used in combination with acetic acid were attractive to both sexes.

Sesquiterpene emissions from Alternaria alternata and Fusarium oxysporum: Effects of age, nutrient availability, and co-cultivation

Alternaria alternata is one of the most studied fungi to date because of its impact on human life – from plant pathogenicity to allergenicity. However, its sesquiterpene emissions have not been systematically explored. Alternaria regularly co-occurs with Fusarium fungi, which are common plant pathogens, on withering plants. We analyzed the diversity and determined the absolute quantities of volatile organic compounds (VOCs) in the headspace above mycelial cultures of A. alternata and Fusarium oxysporum under different conditions (nutrient rich and poor, single cultures and co-cultivation) and at different mycelial ages. Using stir bar sorptive extraction and gas chromatography–mass spectrometry, we observed A. alternata to strongly emit sesquiterpenes, particularly during the early growth stages, while emissions from F. oxysporum consistently remained comparatively low. The emission profile characterizing A. alternata comprised over 20 sesquiterpenes with few effects from nutrient quality and age on the overall emission profile. Co-cultivation with F. oxysporum resulted in reduced amounts of VOCs emitted from A. alternata although its profile remained similar. Both fungi showed distinct emission profiles, rendering them suitable biomarkers for growth-detection of their phylotype in ambient air. The study highlights the importance of thorough and quantitative evaluations of fungal emissions of volatile infochemicals such as sesquiterpenes.