Modeling volatile isoprenoid emissions--a story with split ends.

Accurate prediction of plant-generated volatile isoprenoid fluxes is necessary for reliable estimation of atmospheric ozone and aerosol formation potentials. In recent years, significant progress has been made in understanding the environmental and physiological controls on isoprenoid emission and in scaling these emissions to canopy and landscape levels. We summarize recent developments and compare different approaches for simulating volatile isoprenoid emission and scaling up to whole forest canopies with complex architecture. We show that the current developments in modeling volatile isoprenoid emissions are "split-ended" with simultaneous but separated efforts in fine-tuning the empirical emission algorithms and in constructing process-based models. In modeling volatile isoprenoid emissions, simplified leaf-level emission algorithms (Guenther algorithms) are highly successful, particularly after scaling these models up to whole regions, where the influences of different ecosystem types, ontogenetic stages, and variations in environmental conditions on emission rates and dynamics partly cancel out. However, recent experimental evidence indicates important environmental effects yet unconsidered and emphasize, the importance of a highly dynamic plant acclimation in space and time. This suggests that current parameterizations are unlikely to hold in a globally changing and dynamic environment. Therefore, long-term predictions using empirical algorithms are not necessarily reliable. We show that process-based models have large potential to capture the influence of changing environmental conditions, in particular if the leaf models are linked with physiologically based whole-plant models. This combination is also promising in considering the possible feedback impacts of emissions on plant physiological status such as mitigation of thermal and oxidative stresses by volatile isoprenoids. It might be further worth while to incorporate main features of these approaches in regional empirically-based emission estimations thereby merging the "split ends".     

Plant VOC emissions: making use of the unavoidable

All plants emit substantial amounts of phytogenic volatile organic compounds (PVOCs), which include alkanes, alkenes, alcohols, aldehydes, eters, esters and carboxylic acids. Defence, communication and/or protection against extreme conditions have been proposed as reasons for these emissions. However, Rosenstiel and colleagues have recently proposed that emission of PVOCs represents a metabolic 'safety valve' by preventing the unnecessary sequestration of phosphates. Additionally, Niinemets and colleagues suggest that the emission rates of some PVOCs are determined by the principal physicochemical characteristics of the emitted compounds, such as their solubility, volatility and diffusivity, rather than by physiological mechanisms, such as their synthesis rates. These two new studies lead to the hypothesis that there is not necessarily a specific role for every PVOC emitted, given that their emission is unavoidable as result of their volatility. However, in some cases, natural selection has worked to take advantage of this volatility.     

The Complexity of Factors Driving Volatile Organic Compound Emissions by Plants

The emissions of volatile organic compounds, VOC, from plants have strong relevance for plant physiology, plant ecology, and atmospheric chemistry. We report here on the current knowledge of the many internal (genetic and biochemical) and external (abiotic - temperature, light, water availability, wind, ozone, and biotic - animal, plant and microorganisms interactions) factors that control emission rates of different VOC by altering their synthesis, vapour pressure or diffusion to the atmosphere. The complex net of these factors, their interactions and the different responses of the different VOC produces the large qualitative and quantitative, spatial and temporal variability of emissions and the frequent deviations from current standard emission models. The need for a co-operative multidisciplinary multiscale research to disentangle this complex and important issue of plant VOC emissions is reminded.     

Genetically engineered maize plants reveal distinct costs and benefits of constitutive volatile emissions in the field

Genetic manipulation of plant volatile emissions is a promising tool to enhance plant defences against herbivores. However, the potential costs associated with the manipulation of specific volatile synthase genes are unknown. Therefore, we investigated the physiological and ecological effects of transforming a maize line with a terpene synthase gene in field and laboratory assays, both above‐ and below ground. The transformation, which resulted in the constitutive emission of (E)‐β‐caryophyllene and α‐humulene, was found to compromise seed germination, plant growth and yield. These physiological costs provide a possible explanation for the inducibility of an (E)‐β‐caryophyllene‐synthase gene in wild and cultivated maize. The overexpression of the terpene synthase gene did not impair plant resistance nor volatile emission. However, constitutive terpenoid emission increased plant apparency to herbivores, including adults and larvae of the above ground pest Spodoptera frugiperda, resulting in an increase in leaf damage. Although terpenoid overproducing lines were also attractive to the specialist root herbivore Diabrotica virgifera virgifera below ground, they did not suffer more root damage in the field, possibly because of the enhanced attraction of entomopathogenic nematodes. Furthermore, fewer adults of the root herbivore Diabrotica undecimpunctata howardii were found to emerge near plants that emitted (E)‐β‐caryophyllene and α‐humulene. Yet, overall, under the given field conditions, the costs of constitutive volatile production overshadowed its benefits. This study highlights the need for a thorough assessment of the physiological and ecological consequences of genetically engineering plant signals in the field to determine the potential of this approach for sustainable pest management strategies.     

Growth form rather than phylogenetic relationship predicts broad volatile emission patterns in the Brassicaceae

Volatile organic compounds (VOCs) released from plants are known to mediate indirect defense against herbivores and trigger intra- and interplant signaling. While systemic defense response can be mediated both via volatile and vascular signals, it is not clear whether common ancestry and/or plant growth forms influence the choice of either mode in planta. We hypothesize that larger woody plants with a complex anatomy should rely more on volatile-mediated signaling, apparently to circumvent vascular restrictions that slow down the communication over a large distance. On the other hand, in smaller herbaceous plants faster systemic response can be achieved via vascular signaling. To investigate whether plant VOCs emission is related to plant phylogeny or growth form, we studied the composition of herbivory-induced plant volatiles in 13 Brassicaceae species representing all four evolutionary lineages, because this family is characterized by both a well-resolved phylogeny and highly diverse growth forms. Our results revealed that woody species consistently emitted a more complex blend of volatiles than herbaceous species. However, phylogenetic relatedness of the species did not explain the observed volatile emission patterns. This emphasizes the influence of growth form, rather than phylogenetic relationships on the variation in plant volatile emissions. Our findings suggest that woody, perennial plant species emit diverse VOCs, likely because these compounds comprise a more efficient mode of defense response in these large, anatomically complex plants.     

Drought effects on volatile organic compound emissions from Scots pine stems

Tree stems have been identified as sources of volatile organic compounds (VOCs) that play important roles in tree defence and atmospheric chemistry. Yet, we lack understanding on the magnitude and environmental drivers of stem VOC emissions in various forest ecosystems. Due to the increasing importance of extreme drought, we studied drought effects on the VOC emissions from mature Scots pine (Pinus sylvestris L.) stems. We measured monoterpenes, acetone, acetaldehyde and methanol emissions with custom-made stem chambers, online PTR-MS and adsorbent sampling in a drought-prone forest over the hot-dry summer of 2018 and compared the emission rates and dynamics between trees in naturally dry conditions and under long-term irrigation (drought release). The pine stems were significant monoterpene sources. The stem monoterpene emissions potentially originated from resin, based on their similar monoterpene spectra. The emission dynamics of all VOCs followed temperature at a daily scale, but monoterpene and acetaldehyde emission rates decreased nonlinearly with drought over the summer. Despite the dry conditions, large peaks of monoterpene, acetaldehyde and acetone emissions occurred in late summer potentially due to abiotic or biotic stressors. Our results highlight the potential importance of stem emissions in the ecosystem VOC budget, encouraging further studies in diverse environments.     

Terpene emission in tissue culture

Tissue cultured plants’ vessel headspace is subject to changes during subculture, and the analysis of its variation offers a non-destructive approach for monitoring plant physiology. Among the volatile organic compounds (VOCs) that can be released by plants and be potentially recovered in the airspace of plant tissue cultures, terpenes are very important since they can offer a snapshot of the physiological status of the plant under in vitro cultivation. Terpenes are synthesized from carbon directly shunted from the photosynthetic carbon fixation cycle and their emission is under genetic and environmental control. The experiments described in this paper propose the evaluation of the plant terpene profile in the culture’s headspace as an early indicator of plant stress through the characterization of plant terpene production. Monitoring of terpene emission as a plant response to mechanical stress such as plant wounding showed an increased isoprene and monoterpene emission rates in the first hour after cutting. The comparison of headspace composition of cultures of two fruit rootstocks, Colt and GF677, showed the former having higher emissions of isoprene, α-pinene and limonene than the latter. A decreasing emission trend was observed during subculture, apparently as a result of culture aging. The in vitro headspace analysis of different myrtle (Myrtus communis L.) clones showed a specific and distinctive terpene emission profile. VOC monitoring of in vitro culture headspace is discussed as a non-destructive approach useful for its relation with major physiological activities of culture and for the determination of the potential production of terpenes.     

Roles of plant volatiles in defence against microbial pathogens and microbial exploitation of volatiles

Plants emit a large variety of volatile organic compounds during infection by pathogenic microbes, including terpenes, aromatics, nitrogen‐containing compounds, and fatty acid derivatives, as well as the volatile plant hormones, methyl jasmonate, and methyl salicylate. Given the general antimicrobial activity of plant volatiles and the timing of emission following infection, these compounds have often been assumed to function in defence against pathogens without much solid evidence. In this review, we critically evaluate current knowledge on the toxicity of volatiles to fungi, bacteria, and viruses and their role in plant resistance as well as how they act to induce systemic resistance in uninfected parts of the plant and in neighbouring plants. We also discuss how microbes can detoxify plant volatiles and exploit them as nutrients, attractants for insect vectors, and inducers of volatile emissions, which stimulate immune responses that make plants more susceptible to infection. Although much more is known about plant volatile–herbivore interactions, knowledge of volatile–microbe interactions is growing and it may eventually be possible to harness plant volatiles to reduce disease in agriculture and forestry. Future research in this field can be facilitated by making use of the analytical and molecular tools generated by the prolific research on plant–herbivore interactions.     

Variability and stability of tea weevil‐induced volatile emissions from tea plants with different weevil densities,photoperiod and infestation duration

Abstract After herbivore attack, many plants emit herbivore‐induced plant volatiles (HIPVs). HIPVs can attract carnivores and/or repel herbivores, thereby mediating tritrophic plant–herbivore–carnivore interactions. HIPVs act as chemical information between organisms; hence, their variability and stability are vital. In the present study, variations in the volatile emissions, from the tea plant Camellia sinensis (O. Ktze) damaged by the tea weevil Myllocerinus aurolineatus (Voss) (Coleoptera: Curculionidae), with weevil densities, photoperiod and infestation duration, were investigated. The volatiles induced by high‐density weevils were more abundant in composition and amount than those induced by low‐density weevils, whether at noon, night or after weevil removal. The induced volatile emissions were similar on the second and third day after infestation, and the emissions of the major induced compounds displayed diurnal cycles. Linalool, (E,E)‐α‐farnesene, and benzyl nitrile were emitted mainly at noon, whereas 1,3,8‐p‐menthatriene and (E)‐β‐ocimene were maximally emitted at night. Given the different emission dynamics, significant differences were found between noon‐ and night‐induced volatiles. In summary, tea plants damaged by different weevil densities emitted a relatively stable signal at a particular time. This stability could be attributed to the similarities under the two densities of the main induced volatile compounds, their relative ratios and the emission dynamics of the induced volatiles.     

Leaf rust induced volatile organic compounds signalling in willow during the infection

Plants are known to emit volatile organic compounds (VOC) in response to various biotic or abiotic stresses. Although the VOC emission in the case of insect attacks is well described, there is only little known about the impact of pathogens on plant emission. In the present study, we used a willow-leaf rust system to describe the effects of a biotrophic fungal infection on the VOC emission pattern of willow leaves. We detected that isoprene emissions from rust-infected leaves decreased threefold compared to control. The total monoterpene emissions did not change although a stress-signalling compound (Z)-β-ocimene showed an increase in infected plants on several days. The infection also increased the emission of sesquiterpenes and lipoxygenase products (LOX) by factors of 175-fold and 10-fold, respectively. The volatile emission signals showed two clear peaks during the experiment. At 6, 7 and 12 days post-infection (dpi), the relative volatile emission signal increased to about sixfold compared to uninfected plants. These time points are directly connected to rust infection since at 6 dpi the first rust pustules appeared on the leaves and at 12 dpi necrosis had developed around several pustules. We present correlations between LOX and sesquiterpene emission signals, which suggest at least two different steps in eliciting the volatile emission.     

Volatile emissions from thawing permafrost soils are influenced by meltwater drainage conditions

Vast amounts of carbon are bound in both active layer and permafrost soils in the Arctic. As a consequence of climate warming, the depth of the active layer is increasing in size and permafrost soils are thawing. We hypothesize that pulses of biogenic volatile organic compounds are released from the near‐surface active layer during spring, and during late summer season from thawing permafrost, while the subsequent biogeochemical processes occurring in thawed soils also lead to emissions. Biogenic volatile organic compounds are reactive gases that have both negative and positive climate forcing impacts when introduced to the Arctic atmosphere, and the knowledge of their emission magnitude and pattern is necessary to construct reliable climate models. However, it is unclear how different ecosystems and environmental factors such as drainage conditions upon permafrost thaw affect the emission and compound composition. Here we show that incubations of frozen B horizon of the active layer and permafrost soils collected from a High Arctic heath and fen release a range of biogenic volatile organic compounds upon thaw and during subsequent incubation experiments at temperatures of 10°C and 20°C. Meltwater drainage in the fen soils increased emission rates nine times, while having no effect in the drier heath soils. Emissions generally increased with temperature, and emission profiles for the fen soils were dominated by benzenoids and alkanes, while benzenoids, ketones, and alcohols dominated in heath soils. Our results emphasize that future changes affecting the drainage conditions of the Arctic tundra will have a large influence on volatile emissions from thawing permafrost soils – particularly in wetland/fen areas.     

Flooding induced emissions of volatile signalling compounds in three tree species with differing waterlogging tolerance

To gain insight into variations in waterlogging responsiveness, net assimilation rate, stomatal conductance, emissions of isoprene and marker compounds of anoxic metabolism ethanol and acetaldehyde, and stress marker compounds nitric oxide (NO), volatile products of lipoxygenase (LOX) pathway and methanol were studied in seedlings of temperate deciduous tree species Alnus glutinosa, Populus tremula and Quercus rubra (from highest to lowest waterlogging tolerance) throughout sustained root zone waterlogging of up to three weeks. In all species, waterlogging initially resulted in reductions in net assimilation and stomatal conductance and enhanced emissions of ethanol, acetaldehyde, NO, LOX products and methanol, followed by full or partial recovery depending on process and species. Strong negative correlations between g_s and internal NO concentration and NO flux, valid within and across species, were observed throughout the experiment. Isoprene emission capacity was not related to waterlogging tolerance. Less waterlogging tolerant species had greater reduction and smaller acclimation capacity in foliage physiological potentials, and larger emission bursts of volatile stress marker compounds. These data collectively provide encouraging evidence that emissions of volatile organics and NO can be used as quantitative measures of stress tolerance and acclimation kinetics in temperate trees.     

Regulation of Floral Terpenoid Emission and Biosynthesis in Sweet Basil (<Emphasis Type="Italic">Ocimum basilicum</Emphasis>)

Past studies have focused on the composition of essential oil of Ocimum basilicum leaves, but data on composition and regulation of its aerial emissions, especially floral volatile emissions, are scarce. We studied the chemical profile, within-flower spatial distribution (sepals, petals, pistils with stamina, and pedicels), diurnal emission kinetics and effects of exogenous methyl jasmonate (MeJA) application on the emission of floral volatiles by dynamic headspace collection, and identification using gas chromatography-mass spectrometry (GC–MS) and proton-transfer reaction mass spectrometry. We observed more abundant floral emissions from flowers compared with leaves. Sepals were the main emitters of floral volatiles among the flower parts studied. The emissions of lipoxygenase compounds and monoterpenoids, but not sesquiterpene emissions, displayed a diurnal variation driven by light. Response to exogenous MeJA treatment of flowers consisted of a rapid stress response and a longer-term acclimation response. The initial response was associated with enhanced emissions of fatty acid derivatives, monoterpenoids, and sesquiterpenoids without variation of the composition of individual compounds. The longer-term response was associated with enhanced monoterpenoid and sesquiterpenoid emissions with profound changes in the emission spectrum. According to correlated patterns of terpenoid emission changes upon stress, highlighted by a hierarchical cluster analysis, candidate terpenoid synthases responsible for observed diversity and complexity of released terpenoid blends were postulated. We conclude that flower volatile emissions differ quantitatively and qualitatively from leaf emissions, and overall contribute importantly to O. basilicum flavor, especially under stress conditions.     

Secondary air pollutants and forests--new perspectives

Air pollution has been known to affect forests for over a century, and many of the mechanisms of pollutant deposition and effects have been established, at least for forest trees. Changes in air quality as a result of emission controls in Europe and North America, or as a result of rapid industrialisation in southern and eastern Asia, have highlighted new pollution problems. This paper, by reference to recent publications, highlights two areas where more research is required: the interactions of photochemical oxidants with biogenic emissions of volatile organic compounds, and their impact on ecological signalling; and the role of atmospheric particles in changing the leaf surface environments in forests.     

On the induction of volatile organic compound emissions by plants as consequence of wounding or fluctuations of light and temperature

Among the volatile organic compounds (VOCs) emitted by plants, some are characteristic of stress conditions, but their biosynthesis and the metabolic and environmental control over the emission are still unclear. We performed experiments to clarify whether (1) the emission following wounding can occur at distance from the wounding site, from VOC pools subjected to metabolic signals; and (2) the emission of biogenic VOCs generated by membrane damage (e.g. consequent to wounding or ozone exposure) can also be induced by exposure to high light and high temperature, recurrent in nature. In Phragmites australis, leaf cutting caused large and rapid bursts of acetaldehyde both at the cutting site and on parts of the cut leaf distant from the cutting site. This emission was preceded by a transient stomatal opening and did not occur in conditions preventing stomatal opening. This suggests the presence of a large pool of leaf acetaldehyde whose release is under stomatal control. VOCs other than isoprene, particularly acetaldehyde and (E)-2-hexenal, one of the C-6 compounds formed by the denaturation of membrane lipids, were released by leaves exposed to high temperature and high light. The high-temperature treatment (45 degrees C) also caused a rapid stimulation and then a decay of isoprene emission in Phragmites leaves. Isoprene recovered to the original emission level after suspending the high-temperature treatment, suggesting a temporary deficit of photosynthetically formed substrate under high temperature. Emission of C-6 compounds was slowly induced by high temperature, and remained high, indicating that membrane denaturation occurs also after suspending the high-temperature treatment. Conversely, the emission of C-6 compounds was limited to the high-light episode in Phragmites. This suggests that a membrane denaturation may also occur in conditions that do not damage other important plant processes such as the photochemistry of photosynthesis of photoinhibition-insensitive plants. In the photoinhibition-sensitive Arabidopsis thaliana mutant NPQ1, a large but transient emission of (E)-2-hexenal was also observed a few minutes after the high-light treatment, indicating extensive damage to the membranes. However, (E)-2-hexenal emission was not observed in Arabidopsis plants fumigated with isoprene during the high-light treatment. This confirms that isoprene can effectively protect cellular membranes from denaturation. Our study indicates that large, though often transient, VOC emissions by plants occur in nature. In particular, we demonstrate that VOCs can be released by much larger tissues than those wounded and that even fluctuations of light and temperature regularly observed in nature can induce their emissions. This knowledge adds information that is useful for the parameterization of the emissions and for the estimate of biogenic VOC load in the atmosphere.     

Seasonal variation in methane emission from a temperate Phragmites-dominated marsh: effect of growth stage and plant-mediated transport

The eddy covariance technique was employed with a tunable diode laser spectrometer to quantify methane flux from a prairie marsh dominated by Phragmites australis in north-central Nebraska, USA. The observations spanned the entire growing season (April to October) and a wide range of weather conditions, allowing a quantitative assessment of the physical and biological controls of methane emission in this ecosystem. Diel patterns in methane emission varied markedly depending on plant growth stage. Prior to plant emergence above water, the rate of methane emission from the marsh was fairly constant throughout the day. After emergence above water, there was a gradual increase in methane emission after sunrise with a peak in late afternoon. Significant changes in diel patterns were observed after tillering. Then, the diel pattern was characterized by a mid- to late-morning peak and a 2-to 4-fold increase in methane emissions from night to daytime. In early stages of plant growth, molecular diffusion through dead/live plants and the standing water column seemed to be the primary transport mechanism. After tillering, a transition occurred in the transport mechanism from a molecular diffusion to a convective throughflow, which is a rapid and active gas transport driven by pressure differences. The role of convective throughflow became less important as the plants senesced. Integrated methane emission over the six-month measurement period (April–October) was about 64 g CH₄ m^–2. On an annual basis, we estimate the annual methane emission from this ecosystem to be ≈ 80 g CH₄ m^–2 and that about 80% of the total methane emission occurred between late April and late October.     

Emission mechanism of floral scent in Petunia axillaris

The mechanism of floral scent emission was studied in Petunia axillaris, a plant with a diurnal rhythm of scent output. The emission rate of each volatile compound oscillated in synchrony with its endogenous concentration, so that the intensity of the floral scent appeared to be determined by the endogenous concentrations. The composition of major volatiles in the flower tissue and the flower headspace showed characteristic differences. A negative correlation was found between the boiling points of the volatile compounds and the ratio of their emitted and endogenous concentrations, indicating that the composition of the floral scent depends directly on the endogenous composition of the volatile compounds. We conclude that in P. axillaris, the physiological regulation of floral scent emission operates not in the vaporization process but in the control of the endogenous concentrations of volatiles through biosynthesis and metabolic conversion.     

Release of lipoxygenase products and monoterpenes by tomato plants as an indicator of Botrytis cinerea-induced stress

Changes in emission of volatile organic compounds (VOCs) from tomato induced by the fungus Botrytis cinerea were studied in plants inoculated by spraying with suspensions containing B. cinerea spores. VOC emissions were analysed using on-line gas chromatography–mass spectrometry, with a time resolution of about 1 h, for up to 2 days after spraying. Four phases were delimited according to the starting point and the applied day/night rhythm of the experiments. These phases were used to demonstrate changes in VOC flux caused by B. cinerea infestation. Tomato plants inoculated with B. cinerea emitted a different number and amount of VOCs after inoculation compared to control plants that had been sprayed with a suspension without B. cinerea spores. The changes in emissions were dependent on time after inoculation as well as on the severity of infection. The predominant VOCs emitted after inoculation were volatile products from the lipoxygenase pathway (LOX products). The increased emission of LOX products proved to be a strong indicator of a stress response, indicating that VOC emissions can be used to detect plant stress at an early stage. Besides emission of LOX products, there were also increases in monoterpene emissions. However, neither increased emission of LOX products nor of monoterpenes is specific for B. cinerea attack. The emission of LOX products is also induced by other stresses, and increased emission of monoterpenes seems to be the result of mechanical damage induced by secondary stress impacts on leaves.     

Practical approaches to plant volatile analysis

Plants emit volatile organic compounds (VOCs) that play important roles in their interaction with the environment and have a major impact on atmospheric chemistry. The development of static and dynamic techniques for headspace collection of volatiles in combination with gas chromatography-mass spectrometry analysis has significantly improved our understanding of the biosynthesis and ecology of plant VOCs. Advances in automated analysis of VOCs have allowed the monitoring of fast changes in VOC emissions and facilitated in vivo studies of VOC biosynthesis. This review presents an overview of methods for the analysis of plant VOCs, including their advantages and disadvantages, with a focus on the latest technical developments. It provides guidance on how to select appropriate instrumentation and protocols for biochemical, physiological and ecologically relevant applications. These include headspace analyses of plant VOCs emitted by the whole organism, organs or enzymes as well as advanced on-line analysis methods for simultaneous measurements of VOC emissions with other physiological parameters.     

Minor effects of two elicitors of insect and pathogen resistance on volatile emissions and parasitism of Spodoptera frugiperda in Mexican maize fields

Synthetic elicitors can be used to induce resistance in plants against pathogens and arthropod herbivores. Such compounds may also change the emission of herbivore-induced plant volatiles, which serve as important cues for parasitic wasps to locate their hosts. Therefore, the use of elicitors in the field may affect biological control of insect pests. To test this, we treated maize seedlings growing in a subtropical field in Mexico with methyl jasmonate (MeJA), an elicitor of defense responses against many insects, and benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester (BTH), an elicitor of resistance against certain pathogens. Volatile emission, herbivore infestation, pathogen infection, and plant performance (growth and grain yield) of treated and untreated maize plants were measured. Application of BTH slightly reduced volatile emission in maize, while MeJA increased the emission compared to control treatments. Despite the apparent changes in volatile emissions, the elicitor application did not consistently affect infestation by Spodoptera frugiperda larvae, the main insect pest found on the maize seedlings, and had only marginal effects on parasitism rates. Similarly, there were no treatment effects on infestation by other herbivores and pathogens. Results for the six replications that stretched over one summer and one winter season were highly variable, with parasitism rates and the species composition of the parasitoids differing significantly between seasons. This variability, as well as the severe biotic and abiotic stresses on young seedlings might explain why we measured only slight effects of elicitor application on pest incidence and biological control in this specific field study. Indeed, an additional field experiment under milder and more standardized conditions revealed that BTH induced significant resistance against Bipolaris maydis, a major pathogen in the experimental maize fields. Similar affects can be expected for herbivory and parasitism rates.