Interaction of drought and ozone exposure on isoprene emission from extensively cultivated poplar

Elevated Ozone Reduces the Quality of Tea Leaves but May Improve the Resistance of Tea Plants

Tropospheric ozone (O3) pollution can affect plant nutritional quality and secondary metabolites by altering plant biochemistry and physiology, which may lead to unpredictable effects on crop quality and resistance to pests and diseases. Here, we investigated the effects of O3 (ambient air, Am; ambient air +80 ppb of O3, EO3) on the quality compounds and chemical defenses of a widely cultivated tea variety in China (Camellia sinensis cv. 'Baiye 1 Hao') using open-top chamber (OTC). We found that elevated O3 increased the ratio of total polyphenols to free amino acids while decreasing the value of the catechin quality index, indicating a reduction in leaf quality for green tea. Specifically, elevated O3 reduced concentrations of amino acids and caffeine but shows no impact on the concentrations of total polyphenols in tea leaves. Within individual catechins, elevated O3 increased the concentrations of ester catechins but not non-ester catechins, resulting in a slight increase in total catechins. Moreover, elevated O3 increased the emission of biogenic volatile organic compounds involved in plant defense against herbivores and parasites, including green leaf volatiles, aromatics, and terpenes. Additionally, concentrations of main chemical defenses, represented as condensed tannins and lignin, in tea leaves also increased in response to elevated O3. In conclusion, our results suggest that elevated ground-level O3 may reduce the quality of tea leaves but could potentially enhance the resistance of tea plants to biotic stresses.

A whole ecosystem approach to pear psyllid (Cacopsylla pyri) management in a changing climate

Whole ecosystem-based approaches are becoming increasingly common in pest management within agricultural systems. These strategies consider all trophic levels and abiotic processes within an ecosystem, including interactions between different factors. This review outlines a whole ecosystem approach to the integrated pest management of pear psyllid (Cacopsylla pyri Linnaeus) within pear (Pyrus communis L.) orchards, focusing on potential disruptions as a result of climate change. Pear psyllid is estimated to cost the UK pear industry £5 million per annum and has a significant economic impact on pear production globally. Pesticide resistance is well documented in psyllids, leading to many growers to rely on biological control using natural enemies during the summer months. In addition, multiple insecticides commonly used in pear psyllid control have been withdrawn from the UK and Europe, emphasising the need for alternative control methods. There is growing concern that climate change could alter trophic interactions and phenological events within agroecosystems. For example, warmer temperatures could lead to earlier pear flowering and pest emergence, as well as faster insect development rates and altered activity levels. If climate change impacts pear psyllid differently to natural enemies, then trophic mismatches could occur, impacting pest populations. This review aims to evaluate current strategies used in C. pyri management, discuss trophic interactions within this agroecosystem and highlight potential changes in the top-down and bottom-up control of C. pyri as a result of climate change. This review provides a recommended approach to pear psyllid management, identifies evidence gaps and outlines areas of future research.

Drought mitigates the adverse effects of O3 on plant photosynthesis rather than growth: A global meta-analysis considering plant functional types

Tropospheric ozone (O3 ) is a phytotoxic air pollutant adversely affecting plant growth. High O3 exposures are often concurrent with summer drought. The effects of both stresses on plants are complex, and their interactions are not yet well understood. Here, we investigate whether drought can mitigate the negative effects of O3 on plant physiology and growth based on a meta-analysis. We found that drought mitigated the negative effects of O3 on plant photosynthesis, but the modification of the O3 effect on the whole-plant biomass by drought was not significant. This is explained by a compensatory response of water-deficient plants that leads to increased metabolic costs. Relative to water control condition, reduced water treatment decreased the effects of O3 on photosynthetic traits, and leaf and root biomass in deciduous broadleaf species, while all traits in evergreen coniferous species showed no significant response. This suggested that the mitigating effects of drought on the negative impacts of O3 on the deciduous broadleaf species were more extensive than on the evergreen coniferous ones. Therefore, to avoid over- or underestimations when assessing the impact of O3 on vegetation growth, soil moisture should be considered. These results contribute to a better understanding of terrestrial ecosystem responses under global change.

Influence of Chabazite Zeolite Foliar Applications Used for Olive Fruit Fly Control on Volatile Organic Compound Emission, Photosynthesis, and Quality of Extra Virgin Olive Oil

The olive fruit fly (Bactrocera oleae Rossi) is the most dangerous pest of olive fruits and negatively influences the chemical and sensory quality of the oil produced. Organic farms have few tools against this pest and are constantly looking for effective and sustainable products such as geomaterials, i.e., zeolite. Since a particle film covers the canopy, a study was carried out on the olive tree's responses to zeolite foliar coating. The tested treatments were natural zeolite (NZ), zeolite enriched with ammonium (EZ), and Spintor-Fly® (SF). EZ was associated with higher photosynthetic activity with respect to the other treatments, while no differences were found between SF and NZ. Foliar treatments affect the amount of BVOC produced in both leaves and olives, where 26 and 23 different BVOCs (biogenic volatile organic compounds) were identified but not the type of compounds emitted. Foliar treatment with EZ significantly affected fruit size, and the olive fruit fly more frequently attacked the olives, while treatment with NZ had olives with similar size and attack as those treated with Spintor-Fly®; no difference in oil quantity was detected. Oil produced from olives treated with NZ presented higher values of phenolic content and intensities of bitterness and spiciness than oils from those treated with EZ and SF. According to the results of this study, using zeolite films on an olive tree canopy does not negatively influence plant physiology; it has an impact on BVOC emission and the chemical and sensory characteristics of the oil.

Ozone pollution, water deficit stress and time drive poplar phyllospheric bacterial community structure

Ground-level ozone (O₃) pollution often rise in the summer and coincide with drought stress, which alters the relationships between trees and associated microbial communities in a manner that can have pronounced effects on associated biological activity and ecosystem integrity. Discerning the responses of phyllosphere microbial communities to O₃ and water deficit could highlight the ability of plant-microbe interactions to either exacerbate or mitigate the effects of these stressors. Accordingly, this study was designed as the first report to specifically interrogate the impacts of elevated O₃ and water deficit stress on phyllospheric bacterial community composition and diversity in hybrid poplar saplings. Significant reductions in phyllospheric bacterial alpha diversity indices were observed, with clear evidence of significant time × water deficit stress interactions. The combination of elevated O₃ and water deficit stress shifted in the bacterial community composition over sampling time, resulted in significant increases in the relative abundance of the dominant Gammaproteobacteria phyla together with reductions in Betaproteobacteria. An increased prevalence of Gammaproteobacteria may represent a potential diagnostic dysbiosis-related biosignature associated with poplar disease risk. Significant positive correlations were observed between both Betaproteobacteria abundance and diversity indices and key foliar photosynthetic traits and isoprene emissions, whereas these parameters were negatively correlated with Gammaproteobacteria abundance. These findings suggest that the photosynthesis-related properties in plant leaves are closely related to the makeup of the phyllosphere bacterial community. These data provide novel insight into how plant-associated microbes can help maintain plant health and the stability of the local ecosystem in O₃-polluted and dried environments.

Emission Trade-Off between Isoprene and Other BVOC Components in Pinus massoniana Saplings May Be Regulated by Content of Chlorophylls, Starch and NSCs under Drought Stress

The aim of this work was to study the changes in the BVOCs emission rates and physiological mechanistic response of Pinus massoniana saplings in response to drought stress. Drought stress significantly reduced the emission rates of total BVOCs, monoterpenes, and sesquiterpenes, but had no significant effect on the emission rate of isoprene, which slightly increased under drought stress. A significant negative relationship was observed between the emission rates of total BVOCs, monoterpenes, and sesquiterpenes and the content of chlorophylls, starch, and NSCs, and a positive relationship was observed between the isoprene emission rate and the content of chlorophylls, starch, and NSCs, indicating different control mechanism over the emission of the different components of BVOCs. Under drought stress, the emission trade-off between isoprene and other BVOCs components may be driven by the content of chlorophylls, starch, and NSCs. Considering the inconsistent responses of the different components of BVOCs to drought stress for different plant species, close attention should be paid to the effect of drought stress and global change on plant BVOCs emissions in the future.

Spatial and species-specific responses of biogenic volatile organic compound (BVOC) emissions to elevated ozone from 2014-2020 in China

China suffered from serious and elevated ozone (O3) pollution during 2014-2020. O3 exposure increased with W126, a biologically based cumulative exposure index, at a rate of 1.738 ppm-hr yr-1. MEGAN3.1 was applied to estimate biogenic volatile organic compound (BVOC) emissions and their response to O3 pollution in China by quantifying species responses to O3 stress. In 2020, China's BVOC emissions were 23.26 Tg when considering the effects of O3 pollution, which was 1.7 % higher than that without O3 stress. Isoprene, monoterpenes, sesquiterpenes, and other VOC emissions changed by -1.0 %, 1.4 %, 15.5 %, and 2.7 %, respectively. The stimulated BVOC emissions were mainly focused on the North China Plain (NCP) and a partial area of the Tibetan Plateau, which increased by >45 %. Changes in monthly emissions differed, with the greatest increase, 181 tons (3.25 %), in August. The seasonal patterns for the impacts of O3 pollution were also distinguished spatially. The elevated O3 exposure caused BVOC emission increases of 104.7 Gg yr-1 during 2014-2020, with isoprene, monoterpenes, sesquiterpenes, and other VOCs contributing -18.6 %, 27.5 %, 40.4 %, and 50.8 %, respectively. The greatest increase in emissions appeared on the NCP and eastern and central China, with annual increases of >100 tons per grid (36 km × 36 km). The interannual variations in BVOC emissions also displayed different seasonal patterns.

Contrasting terpene emissions from canopy and understory vegetation in response to increases in nitrogen deposition and seasonal changes in precipitation

Given global change and shifts in climate are expected to increase BVOC emissions, the quantification of links between environmental conditions, plant physiology, and terpene emission dynamics is required to improve model predictions of ecosystem responses to increasing nitrogen deposition and changes in precipitation regimes. Here, we conducted a two-factor field experiment in sub-tropical forest plots to determine effects of N addition (N), precipitation change (PC), and NP (N and PC combined treatment) on wet and dry season terpene emissions and leaf photosynthetic parameters from canopy and understory species. Changes of β-ocimene and sabinene under PC and NP in the wet season (0.4-5.6-fold change) were the largest contributor to changes in total terpene emissions. In the dry season, the standardized total terpene emission rate was enhanced by 144.9% under N addition and 185.7% under PC for the understory species, while the total terpene emission rate was lower under NP than N addition and PC, indicating that N addition tended to moderate increases in PC-induced understory total terpene emissions. In the wet season, the total terpene emission rate under N and PC was close to ambient conditions for the canopy species, while the total terpene emission rate was enhanced by 54.6% under NP, indicating that N and PC combined treatment had an additive effect on canopy total terpene emissions. Total terpene emission rates increased with rates of net leaf photosynthesis (P_n) and transpiration (Tr) and there was a decoupling between terpene emission rates and P_n under NP, indicating that complex effects between PC and N decreased the regularity of single-factor effects. We recommend that N and PC interaction effects are included in models for the prediction of terpene emissions, particularly from canopy vegetation during the wet season as a major source of forest ecosystem terpene emissions.

Effects of soil drought and nitrogen deposition on BVOC emissions and their O3 and SOA formation for Pinus thunbergii

Soil drought and nitrogen (N) deposition can influence the biogenic volatile organic compound (BVOC) emissions and thereby their ozone (O₃) and secondary organic aerosol (SOA) formation. This study addressed their single and combined effects on BVOC emissions of Pinus thunbergii by laboratory simulation experiments. The results showed that light drought (LD, 50% soil volumetric water content (VWC)) stimulated isoprene, monoterpene, sesquiterpene, and total BVOC emissions, while moderate drought (MD, 30% and 40% VWC) and severe drought (SD, 10% and 20% VWC) inhibited their emissions (except for sesquiterpene in 20% VWC). N deposition decreased other VOC emissions and increased isoprene and sesquiterpene emissions. Total BVOCs and monoterpene were stimulated in low N deposition (LN, 2 g N/(m²·yr)) and inhibited in moderate (MN, 5 g N/(m²·yr)) and high N deposition (HN, 10 g N/(m²·yr)). Under combined treatment of soil drought and N deposition, total BVOC, monoterpene, and other VOC emissions were inhibited, sesquiterpene had no significant change, and isoprene emission was inhibited in MD combined treatment but promoted in SD. The O₃ formation potential (OFP) and SOA formation potential (SOAP) from the changed BVOC emissions were calculated, OFP and SOAP of BVOC emissions and their compositions varied significantly among the treatments. Our study provided theoretical basis for assessing the impact of climate change and atmospheric pollution on BVOC emissions and their contribution to the formation of secondary atmospheric pollution.

Elevated ozone inhibits isoprene emission of a diploid and a triploid genotype of Populus tomentosa by different mechanisms

Ozone (O3) pollution affects plant growth and isoprene (ISO) emission. However, the response mechanism of isoprene emission rate (ISOrate) to elevated O3 (EO3) remains poorly understood. ISOrate was investigated in two genotypes (diploid and triploid) of Chinese white poplar (Populus tomentosa Carr.) exposed to EO3 in an open top chamber system. The triploid genotype had higher photosynthetic rate (A) and stomatal conductance (gs) than the diploid one. EO3 significantly decreased A, gs, and ISOrate of middle and lower leaves in both genotypes. In the diploid genotype, the reduction of ISOrate was caused by a systematic decrease related to ISO synthesis capacity, as indicated by decreased contents of the isoprene precursor dimethylallyl diphosphate and decreased isoprene synthase protein and activity. On the other hand, the negative effect of O3 on ISOrate of the triploid genotype did not result from inhibited ISO synthesis capacity, but from increased ISO oxidative loss within the leaf. Our findings will be useful for breeding poplar genotypes with high yield and lower ISOrate, depending on local atmospheric volatile organic compound/NOx ratio, to cope with both the rising O3 concentrations and increasing biomass demand. They can also inform the incorporation of O3 effects into process-based models of isoprene emission.

Stress-Induced Volatile Emissions and Signalling in Inter-Plant Communication

The sessile plant has developed mechanisms to survive the "rough and tumble" of its natural surroundings, aided by its evolved innate immune system. Precise perception and rapid response to stress stimuli confer a fitness edge to the plant against its competitors, guaranteeing greater chances of survival and productivity. Plants can "eavesdrop" on volatile chemical cues from their stressed neighbours and have adapted to use these airborne signals to prepare for impending danger without having to experience the actual stress themselves. The role of volatile organic compounds (VOCs) in plant-plant communication has gained significant attention over the past decade, particularly with regard to the potential of VOCs to prime non-stressed plants for more robust defence responses to future stress challenges. The ecological relevance of such interactions under various environmental stresses has been much debated, and there is a nascent understanding of the mechanisms involved. This review discusses the significance of VOC-mediated inter-plant interactions under both biotic and abiotic stresses and highlights the potential to manipulate outcomes in agricultural systems for sustainable crop protection via enhanced defence. The need to integrate physiological, biochemical, and molecular approaches in understanding the underlying mechanisms and signalling pathways involved in volatile signalling is emphasised.

Whole-plant compensatory responses of isoprene emission from hybrid poplar seedlings exposed to elevated ozone

It is still unclear whether the responses of isoprene (ISO) emission to elevated O₃ vary with biological organization level (i.e. leaf and whole-plant). To study such responses and the possible reasons explaining their variation, we investigated the effect of O₃ (CF: charcoal-filtered ambient air; E-O₃: non-filtered ambient air enriched with O₃) on ISO emission rate (ISO_rate), net photosynthetic rate (Pn), leaf nitrogen and carbon contents, and leaf growth traits in poplar seedlings (Populus deltoides cv. 55/56 × P. deltoides cv. Imperial) during one growing season. Opposite effects of E-O₃ on Pn were found between upper leaves (positive effect) and lower leaves (negative effect). Compared to CF, E-O₃ significantly decreased leaf mass per area, number of leaves, and leaf biomass, but increased leaf nitrogen content and individual leaf size. In the framework of such compensatory responses, poplar seedlings further increased ISO_rate in upper leaves and decreased ISO_rate in lower leaves, thus preventing significant decrease in the overall whole-plant ISO_rate by E-O₃. The measured whole-plant ISO_rate also showed that the simplistic estimation approaches based on the linear regression between chlorophyll content indicated by soil plant analysis development meter (SPAD value) and leaf-level ISO_rate could not accurately reflect the true response of whole plant to elevated O₃. For more accurate predictions, the potential ISO compensatory response to increasing O₃ concentration should be incorporated into the climate biogeochemical models related to ISO emission.

Effects of elevated ozone on the emission of volatile isoprenoids from flowers and leaves of rose (Rosa sp.) varieties

Tropospheric ozone (O₃) affects isoprenoid emissions, and floral emissions in particular, which may result in potential impacts on the interactions of plants with other organisms. The effects of ozone (O₃) on isoprenoid emissions have been investigated for many years, while knowledge on O₃ effects on floral emissions is still scarce and the relevant mechanism has not been clarified so far. We investigated the effects of O₃ on floral and foliar isoprenoid emissions (mainly isoprene, monoterpenes and sesquiterpenes) and their synthase substrates from three rose varieties (CH, Rosa chinensis Jacq. var. chinensis; SA, R. hybrida 'Saiun'; MO, R. hybrida 'Monica Bellucci') at different exposure durations. Results indicated that the O₃-induced stimulation after short-term exposure (35 days after the beginning of O₃ exposure) was significant only for sesquiterpene emissions from flowers, while long-term O₃ exposure (90 days after the beginning of O₃ exposure) significantly decreased both foliar and floral monoterpene and sesquiterpene emissions. In addition, the observed decline of emissions under long-term O₃ exposure resulted from the limitation of synthase substrates, and the responses of emissions and substrates varied among varieties, with the greatest variation in the O₃-sensitive variety. These findings provide important insights on plant isoprenoid emissions and species selection for landscaping, especially in areas with high O₃ concentration.

Review on plant terpenoid emissions worldwide and in China

Biogenic volatile organic compounds (BVOCs), particularly terpenoids, can significantly drive the formation of ozone (O₃) and secondary organic aerosols (SOA) in the atmosphere, as well as directly or indirectly affect global climate change. Understanding their emission mechanisms and the current progress in emission measurements and estimations are essential for the accurate determination of emission characteristics, as well as for evaluating their roles in atmospheric chemistry and climate change. This review summarizes the mechanisms of terpenoid synthesis and release, biotic and abiotic factors affecting their emissions, development of emission observation techniques, and emission estimations from hundreds of published papers. We provide a review of the main observations and estimations in China, which contributes a significant proportion to the total global BVOC emissions. The review suggests the need for further research on the comprehensive effects of environmental factors on terpenoid emissions, especially soil moisture and nitrogen content, which should be quantified in emission models to improve the accuracy of estimation. In China, it is necessary to conduct more accurate measurements for local plants in different regions using the dynamic enclosure technique to establish an accurate local emission rate database for dominant tree species. This will help improve the accuracy of both national and global emission inventories. This review provides a comprehensive understanding of terpenoid emissions as well as prospects for detailed research to accurately describe terpenoid emission characteristics worldwide and in China.

Diurnal variation in BVOC emission and CO2 gas exchange from above- and belowground parts of two coniferous species and their responses to elevated O3

Increased tropospheric ozone (O₃) concentrations in boreal forests affect the emission of biogenic volatile organic compounds (BVOCs), which play crucial roles in biosphere-atmosphere feedbacks. Although it has been well documented that BVOC emissions are altered in response to elevated O₃, consequent effects on the carbon budget have been largely unexplored. Here, we studied the effects of elevated O₃ (80 nmol mol^-1) on diurnal variation of BVOC emissions and gas exchange of CO₂ from above- and belowground parts of Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) and further investigated effects on the carbon budget. In spring, elevated O₃ decreased BVOC emissions and net photosynthesis rate (Pn) from above-ground parts of both species. As BVOC emissions have a causal relationship with dormancy recovery, O₃-induced decreases in BVOC emissions indicated the inhibition of dormancy recovery. Contrary to the spring results, in summer BVOC emissions from aboveground parts were increased in response to elevated O₃ in both species. Decreases in Pn indicated O₃ stress. O₃-induced monoterpene emissions from aboveground were the main volatile defense response. Elevated O₃ had little effect on BVOC emissions from belowground parts of either species in spring or summer. In spring, elevated O₃ decreased the proportion of carbon emitted as BVOCs relative to that assimilated by photosynthesis (the proportion of BVOC-C loss) at the soil-plant system levels in both species. In summer, elevated O₃ resulted in a net CO₂-C loss at the soil-plant system level of Scots pine. During this process, O₃-induced BVOC-C loss can represent a significant fraction of carbon exchange between the atmosphere and Scots pine. In Norway spruce, the effects of O₃ were less pronounced. The current results highlight the need for prediction of BVOC emissions and their contributions to the carbon budget in boreal forests under O₃ stress.

Date palm responses to a chronic, realistic ozone exposure in a FACE experiment

Date palms are highly economically important species in hot arid regions, which may suffer ozone (O₃) pollution equivalently to heat and water stress. However, little is known about date palm sensitivity to O₃. Therefore, to identify their resistance mechanisms against elevated O₃, physiological parameters (leaf gas exchange, chlorophyll fluorescence and leaf pigments) and biomass growth responses to realistic O₃ exposure were tested in an isoprene-emitting date palm (Phoenix dactylifera L. cv. Nabut Saif) by a Free-Air Controlled Exposure (FACE) facility with three levels of O₃ (ambient [AA, 45 ppb as 24-h average], 1.5 x AA and 2 x AA). We found a reduction of photosynthesis only at 2 x AA although some foliar traits known as early indicators of O₃ stress responded already at 1.5 x AA, such as increased dark respiration, reduced leaf pigment content, reduced maximum quantum yield of PSII, inactivation of the oxygen evolving complex of PSII and reduced performance index PI_TOT. As a result, O₃ did not affect most of the growth parameters although significant declines of root biomass occurred only at 2 x AA. The major mechanism in date palm for reducing the severity of O₃ impacts was a restriction of stomatal O₃ uptake due to low stomatal conductance and O₃-induced stomatal closure. In addition, an increased respiration in elevated O₃ may indicate an enhanced capacity of catabolizing metabolites for detoxification and repair. Interestingly, date palm produced low amounts of monoterpenes, whose emission was stimulated in 2 x AA, although isoprene emission declined at both 1.5 and 2 x AA. Our results warrant more research on a biological significance of terpenoids in plant resistance against O₃ stress.

Stomatal response drives between-species difference in predicted leaf water-use efficiency under elevated ozone

Ozone-induced changes in the relationship between photosynthesis (A_n) and stomatal conductance (g_s) vary among species, leading to inconsistent water use efficiency (WUE) responses to elevated ozone (O₃). Thus, few vegetation models can accurately simulate the effects of O₃ on WUE. Here, we conducted an experiment exposing two differently O₃-sensitive species (Cotinus coggygria and Magnolia denudata) to five O₃ concentrations and investigated the impact of O₃ exposure on predicted WUE using a coupled A_n-g_s model. We found that increases in stomatal O₃ uptake caused linear reductions in the maximum rates of Rubisco carboxylation (V_cmax) and electron transport (J_max) in both species. In addition, a negative linear correlation between O₃-induced changes in the minimal g_s of the stomatal model (g₀) derived from the theory of optimal stomatal behavior and light-saturated photosynthesis was found in the O₃-sensitive M. denudata. When the O₃ dose-based responses of V_cmax and J_max were included in a coupled A_n-g_s model, simulated A_n under elevated O₃ were in good agreement with observations in both species. For M. denudata, incorporating the O₃ response of g₀ into the coupled model further improved the accuracy of the simulated g_s and WUE. In conclusion, the modified V_cmax, J_max and g₀ method presented here provides a foundation for improving the prediction for O₃-induced changes in A_n, g_s and WUE.

Response of isoprene emission from poplar saplings to ozone pollution and nitrogen deposition depends on leaf position along the vertical canopy profile

We investigated isoprene (ISO) emission and gas exchange in leaves from different positions along the vertical canopy profile of poplar saplings (Populus euramericana cv. '74/76'). For a growing season, plants were subjected to four N treatments, control (NC, no N addition), low N (LN, 50 kg N ha^-1year^-1), middle N (MN, 100 kg N ha^-1year^-1), high N (HN, 200 kg N ha^-1year^-1) and three O₃ treatments (CF, charcoal-filtered ambient air; NF, non-filtered ambient air; NF + O₃, NF + 40 ppb O₃). Our results showed the effects of O₃ and/or N on standardized ISO rate (ISO_rate) and photosynthetic parameters differed along with the leaf position, with larger negative effects of O₃ and positive effects of N on ISO_rate and photosynthetic parameters in the older leaves. Expanded young leaves were insensitive to both treatments even at very high O₃ concentration (67 ppb as 10-h average) and HN treatment. Significant O₃ × N interactions were only found in middle and lower leaves, where ISO_rate declined by O₃ just when N was limited (NC and LN). With increasing light-saturated photosynthesis and chlorophyll content, ISO_rate was reduced in the upper leaves but on the contrary increased in middle and lower leaves. The responses of ISO_rate to AOT40 (accumulated exposure to hourly O₃ concentrations > 40 ppb) and POD_Y (accumulative stomatal uptake of O₃ > Y nmol O₃ m^-2 PLA s^-1) were not significant in upper leaves, but ISO_rate significantly decreased with increasing AOT40 or POD_Y under limited N supply in middle leaves but at all N levels in lower leaves. Overall, ISO_rate changed along the vertical canopy profile in response to combined O₃ and N exposure, a behavior that should be incorporated into multi-layer canopy models. Our results are relevant for modelling regional isoprene emissions under current and future O₃ pollution and N deposition scenarios.

Ozone exposure, nitrogen addition and moderate drought dynamically interact to affect isoprene emission in poplar

Ozone (O₃) pollution can induce changes in plant growth and metabolism, and in turn, affects isoprene emission (ISO), but the extent of these effects may be modified by co-occurring soil water and nitrogen (N) availability. To date, however, much less is known about the combined effects of two of these factors on isoprene emission from plants. We investigated for the first time the combined effects of O₃ exposure (CF, charcoal-filtered air; EO₃, non-filtered air plus 40 ppb of O₃), N addition (N0, no additional N; N50, 50 kg ha^-1 year^-1 of N) and moderate drought (WW, well-watered; WR, 40% of WW irrigation) on photosynthetic carbon assimilation and ISO emission in hybrid poplar at both leaf- and plant-level over time. Consistent with leaf-level photosynthesis (Pn_leaf) and ISO (ISO_leaf) responses, plant-level ISO (ISO_plant) responses to O₃, N addition and moderate drought were more marked after long exposure (September) than short exposure duration (July). EO₃ significantly decreased ISO_leaf and Pn_leaf, while WR and N50 significantly increased them. Although O₃ and water interacted significantly to affect Pn_leaf over the exposure duration, neither N50 nor WR mitigated the negative effects of EO₃ on ISO_leaf. When ISO was scaled up to the plant level, the WR-induced increase in ISO_leaf under EO₃ was offset by a reduction in total leaf area. By contrast, effects of EO₃ on ISO_plant were not changed by N addition. Our results highlight that the dynamic effects on ISO emission change over the exposure duration depending on involved co-occurring factors and evaluation scales.

Modified regional biogenic VOC emissions with actual ozone stress and integrated land cover information: A case study in Yangtze River Delta, China

The biogenic volatile organic compounds (BVOCs) emissions are influenced by ambient ozone (O₃) concentrations and vegetation cover. In most studies, however, the interaction between O₃ and plants has not been considered and there are uncertainties in land cover input and emission factors (EFs) in BVOCs emission estimation, particularly at the regional scale. In this study, an O₃ exposure-isoprene (ISOP) response function was developed using meta-analysis, and the EFs of ISOP and land cover inputs were updated by integrating local measurement and investigation data in the Yangtze River Delta (YRD) region. Five different cases were developed to explore the impacts of O₃ and input variables on the BVOCs emissions using the Model of Emissions of Gases and Aerosols from Nature (MEGAN). The impacts of those variables on O₃ simulation were further examined with air quality modeling. We found that the ISOP emissions were restrained in the city cluster along the Yangtze River during the growing season due to their negative feedback to O₃ exposure for deciduous broadleaf forests. The estimation of BVOCs emissions strongly depended on EFs, and the global EFs underestimated the ISOP emissions in July by 37%, mostly in southern YRD. Different land cover datasets with various fractions and spatial distributions of plant function types resulted in a variation of 200-400 Gg in ISOP emissions in July across YRD. Air quality modeling indicated that BVOCs contributed 10%, 12%, and 11% to the 1-h mean, the maximum daily 1-h average, and the maximum daily 8-h average O₃ concentrations, respectively, for July across the YRD region. Due to the NOx restriction, the spatial distribution of BVOCs emissions was inconsistent with that of their contribution to O₃ formation. The O₃ simulation was more sensitive to the changed BVOCs emissions in the area with relatively large contribution of BVOCs to O₃ formation.

Environmental and physiological controls on diurnal and seasonal patterns of biogenic volatile organic compound emissions from five dominant woody species under field conditions

Biogenic volatile organic compounds (BVOCs) play essential roles in tropospheric chemistry, on both regional and global scales. The emissions of large quantities of species-specific BVOC depend not only on environmental (temperature, T; photosynthetically active radiation, PAR), but also physiological parameters (i.e. net photosynthetic rate, P_n; transpiration rate, T_r; stomatal conductance, g_s and intercellular CO₂ concentration, C_i). Here, isoprene, monoterpene and sesquiterpene emissions were determined from five dominant mature woody tree species in northern China, which are two evergreen conifers (Pinus tabuliformis and Platycladus orientalis) and three broad-leaved deciduous trees (Quercus variabilis, Populus tomentosa and Robinia pseudoacacia). A dynamic enclosure technique combined with GC-MS was used to sample BVOCs and analyse their fractional composition at daily and annual scales. The diurnal data showed that both isoprene and monoterpene emissions increased with increasing temperature, and reached their maximum emission rates in the peak of growing season for both coniferous and broad-leaved species. The emissions of individual compound within the monoterpenes and sesquiterpenes were statistically correlated with each other for all species. Furthermore, some oxygenated monoterpene emissions were highly correlated to sesquiterpenes in all tree species. Linking BVOC emissions to environmental and leaf physiological parameters exhibited that monoterpene emissions were linearly and positively correlated to the variation of T, PAR, P_n and T_r, while their relationship to g_s and C_i is more complex. Collectively, these findings provided important information for improving current model estimations in terms of the linkage between BVOC emissions and plant physiological traits. The data presented in this study can be used to update emission capacity used in models, as this is the first time of reporting BVOC emissions from five dominant species in this region. The whole-year measurement of leaf-level BVOCs can also advance our understanding of seasonal variation in BVOC emissions.

Water stress rather than N addition mitigates impacts of elevated O3 on foliar chemical profiles in poplar saplings

Tropospheric ozone (O₃) pollution can alter tree chemical profiles, and in turn, affect forest ecosystem function. However, the magnitude of these effects may be modified by variations in soil water and nutrient availability, which makes it difficult to predict the impacts of O₃ in reality. Here we assessed the effects of elevated O₃ alone, and in combination with soil water deficit and N addition, on the phytochemical composition of hybrid poplar (Populus deltoides cv. '55/56' × P. deltoides cv. 'Imperial'). Potted trees were grown in open-top chambers (OTCs) under either charcoal-filtered air or elevated O₃ (non-filtered air +40 ppb of O₃), and trees within each OTC were grown with four combinations of water (well-watered or water deficit) and nitrogen (with or without N addition) levels. We found that elevated O₃ alone stimulated the accumulation of foliar nitrogen, soluble sugar, and lignin while inhibiting the accumulation of starch, but had limited impacts on condensed tannins and salicinoids in poplar saplings. Graphical vector analysis revealed that these changes in concentrations of nitrogen, starch and lignin were due largely to altered metabolic processes, while increased soluble sugar concentration related mainly to decreased leaf biomass in most cases. The effects of O₃ on poplar foliar chemical profiles depended on soil water, but not soil N, availability. Specifically, O₃-mediated changes in carbohydrates and lignin were mitigated by decreased soil water content. Taken together, these results suggested that nitrogen acquisition, carbohydrates mobilization and lignification play a role in poplar tolerance to O₃. Moreover, the impacts of elevated O₃ on phytochemistry of poplar leaves can be context-dependent, with potential consequences for ecosystem processes under future global change scenarios. Our results highlight the needs to consider multi-factors environments to optimize the management of plantations under changing environments.

The Combined Effect of Elevated O3 Levels and TYLCV Infection Increases the Fitness of Bemisia tabaci Mediterranean on Tomato Plants

Global change and biotic stress, such as tropospheric contamination and virus infection, can individually modify the quality of host plants, thereby altering the palatability of the plant for herbivorous insects. The bottom-up effects of elevated O3 and tomato yellow leaf curl virus (TYLCV) infection on tomato plants and the associated performance of Bemisia tabaci Mediterranean (MED) were determined in open-top chambers. Elevated O3 decreased eight amino acid levels and increased the salicylic acid (SA) and jasmonic acid (JA) content and the gene expression of pathogenesis-related protein (PR1) and proteinase inhibitor (PI1) in both wild-type (CM) and JA defense-deficient tomato genotype (spr2). TYLCV infection and the combination of elevated O3 and TYLCV infection increased eight amino acids levels, SA content and PR1 expression, and decreased JA content and PI1 expression in both tomato genotypes. In uninfected tomato, elevated O3 increased developmental time and decreased fecundity by 6.1 and 18.8% in the CM, respectively, and by 6.8 and 18.9% in the spr2, respectively. In TYLCV-infected tomato, elevated O3 decreased developmental time and increased fecundity by 4.6 and 14.2%, respectively, in the CM and by 4.3 and 16.8%, respectively, in the spr2. These results showed that the interactive effects of elevated O3 and TYLCV infection partially increased the amino acid content and weakened the JA-dependent defense, resulting in increased population fitness of MED on tomato plants. This study suggests that whiteflies would be more successful at TYLCV-infected plants than at uninfected plants in elevated O3 levels.

Toward stomatal-flux based forest protection against ozone: The MOTTLES approach

European standards for the protection of forests from ozone (O₃) are based on atmospheric exposure (AOT40) that is not always representative of O₃ effects since it is not a proxy of gas uptake through stomata (stomatal flux). MOTTLES "MOnitoring ozone injury for seTTing new critical LEvelS" is a LIFE project aimed at establishing a permanent network of forest sites based on active O₃ monitoring at remote areas at high and medium risk of O₃ injury, in order to define new standards based on stomatal flux, i.e. POD_Y (Phytotoxic Ozone Dose above a threshold Y of uptake). Based on the first year of data collected at MOTTLES sites, we describe the MOTTLES monitoring station, together with protocols and metric calculation methods. AOT40 and POD_Y, computed with different methods, are then compared and correlated with forest-health indicators (radial growth, crown defoliation, visible foliar O₃ injury). For the year 2017, the average AOT40 calculated according to the European Directive was even 5 times (on average 1.7 times) the European legislative standard for the protection of forests. When the metrics were calculated according to the European protocols (EU Directive 2008/50/EC or Modelling and Mapping Manual LTRAP Convention), the values were well correlated to those obtained on the basis of the real duration of the growing season (i.e. MOTTLES method) and were thus representative of the actual exposure/flux. AOT40 showed opposite direction relative to PODY. Visible foliar O₃ injury appeared as the best forest-health indicator for O₃ under field conditions and was more frequently detected at forest edge than inside the forest. The present work may help the set-up of further long-term forest monitoring sites dedicated to O₃ assessment in forests, especially because flux-based assessments are recommended as part of monitoring air pollution impacts on ecosystems in the revised EU National Emissions Ceilings Directive.

Influence of physiological and environmental factors on the diurnal variation in emissions of biogenic volatile compounds from Pinus tabuliformis

Biological volatile organic compounds (BVOCs) have a large influence on atmospheric environmental quality, climate change and the carbon cycle. This study assesses the composition and diurnal variation in emission rates of BVOCs from Pinus tabuliformis, using an enclosure technique. Environmental parameters (temperature and light intensity) and physiological parameters (net photosynthetic rate, P_n; stomatal conductance, g_s; intercellular CO₂ concentration, C_i; and transpiration rate, T_r) that may affect emission behavior were continuously monitored. The 10 most abundant compound groups emitted by P. tabuliformis were classified by gas chromatography-mass spectrometry. The dominant monoterpenoid compounds emitted were α-pinene, β-myrcene, α-farnesene and limonene. The diurnal emission rate of BVOCs changed with temperature and light intensity, with dynamic analysis of BVOCs emissions revealing that their emission rates were more affected by temperature than light. The variation in monoterpene emission rates was consistent with estimates of P_n, g_s and T_r. Basal emission rates (at 30 °C,) of the main BVOCs ranged from 0.006 to 0.273 μg  ^-1/(hr g), while the basal ER standardization coefficients ranged from 0.049 to 0.144 °C^-1. Overall, these results provide a detailed reference for the effective selection and configuration of tree species to effectively prevent and control atmospheric pollution.

Isoprene is more affected by climate drivers than monoterpenes: A meta-analytic review on plant isoprenoid emissions

Isoprene and monoterpenes (MTs) are among the most abundant and reactive volatile organic compounds produced by plants (biogenic volatile organic compounds). We conducted a meta-analysis to quantify the mean effect of environmental factors associated to climate change (warming, drought, elevated CO₂ , and O₃ ) on the emission of isoprene and MTs. Results indicated that all single factors except warming inhibited isoprene emission. When subsets of data collected in experiments run under similar change of a given environmental factor were compared, isoprene and photosynthesis responded negatively to elevated O₃ (-8% and -10%, respectively) and drought (-15% and -42%), and in opposite ways to elevated CO₂ (-23% and +55%) and warming (+53% and -23%, respectively). Effects on MTs emission were usually not significant, with the exceptions of a significant stimulation caused by warming (+39%) and by elevated O₃ (limited to O₃ -insensitive plants, and evergreen species with storage organs). Our results clearly highlight individual effects of environmental factors on isoprene and MT emissions, and an overall uncoupling between these secondary metabolites produced by the same methylerythritol 4-phosphate pathway. Future results from manipulative experiments and long-term observations may help untangling the interactive effects of these factors and filling gaps featured in the current meta-analysis.

Trends and inter-relationships of ground-level ozone metrics and forest health in Lithuania

Lithuania is representative of maritime to continental climate, no water limitation, and moderate ground-level ozone (O₃) pollution. We investigated the trends of meteorological variables and O₃ and how these environmental conditions associate with tree health from 2001 onward. Ozone metrics for forest protection, based on Accumulated O₃ exposure Over a Threshold of X ppb (AOTX) or on Phytotoxic O₃ Dose over a Y threshold (PODY), were modeled at nine ICP-Forests plots over the time period 2001-2014. Tree-response indicators, i.e. crown defoliation and visible foliar O₃ injury, were assessed during annual field surveys carried out at each ICP-Forests plot over the time period 2007-2017. Mann-Kendall and Sen statistical tests were applied to estimate changes over time of meteorological variables, response indicators and O₃ metrics. Finally, the O₃ metrics were correlated (Spearman test) to the response indicators over the common period 2007-2014. Over this time period, trend analyses revealed an increasingly hotter (+0.27 °C decade^-1, on average) and drier climate (rainfall, -48 mm decade^-1). A reduction was found for O₃ annual mean (-0.28 ppb decade^-1, on average) and AOT40 (-2540 ppb·h decade^-1, on average) whereas an increase was found for POD0 (+0.40 mmol m^-2 decade^-1, on average). Visible foliar O₃ injury increased (+0.17% decade^-1), while an improvement of the crown conditions (-5.0% decade^-1) was observed. AOT40 was significantly associated with crown defoliation while PODY and soil water content were correlated with visible foliar O₃ injury. As visible foliar O₃ injury was negligible in all the studied species, the results suggest that moderate O₃ pollution (approximately 30 ppb as annual average) does not induce biologically significant effects on this forest vegetation under the current conditions, however the overall O₃ risk (POD0) is expected to increase in the future under a hotter and drier climate.

Deciphering the main determinants of O3 tolerance in Euramerican poplar genotypes

Tropospheric ozone (O₃) is the main secondary pollutant and considered to be the most damaging for growth and productivity. O₃ is well known to induce oxidative stress and Reactive Oxygen Species accumulation in leaf tissues. Several mechanisms have been suggested to enable trees to cope with such stress; however, their relative contribution to O₃ tolerance is still unclear. Here, ten Euramerican poplar genotypes (Populus deltoides × nigra) were investigated regarding their response to 120 ppb of O₃ for 3 weeks in order to determine main mechanisms and identify the key traits and strategies linked to a better tolerance to O₃-induced oxidative stress. Results showed that ascorbate peroxidase and ascorbate regeneration through monodehydroascorbate reductase are the main determinants of O₃ tolerance in Euramerican poplar, in protecting photosynthesis capacity from oxidative stress and therefore, maintaining growth and productivity. Besides, stomatal closure was harmful in sensitive genotypes, suggesting that avoiding strategy can be further deleterious under chronic ozone. Finally, O₃-induced early senescence appeared essential when up scaling leaf-level mechanistic response to whole-plant productivity, in fine-tuning resource reallocation and photosynthesis area.

Molecular response of poplar to single and combined ozone and drought

High concentration in ground-level ozone (O₃) and water deficit affect forest ecosystems service. Previously we found intercellular CO₂ concentration and isoprene emission were affected by the combination of O₃ and drought, but the molecular mechanisms controlling these phenotypes are still open questions. In this study, we investigated the stomatal conductance (g_s) and transcriptome changes in an O₃-sensitive hybrid poplar exposed to two O₃ levels [charcoal-filtered ambient air (CF) and non-filtered ambient air plus 40 ppb (NF40)] and two water conditions [well-watered (W) and moderate drought (D)]. NF40 reduced the g_s more under D than W. We identified the differentially expressed genes (DEGs) from pairwise comparisons and found the poplar's molecular response to drought was counteracted by elevated O₃. From nine clusters obtained through K-means clustering, 12 core transcription factors were identified. DEGs involved in isoprene biosynthesis and phytohormones signal pathways indicate the molecular response and stomatal closure of poplar under O₃ and/or drought might be through MEP/DOXP and ABA-dependent pathways. In addition, 102 Helitrons capturing DEGs were involved in response to O₃ and/or drought and related with ABA-dependent pathway. This integrated analysis provides multi-dimensional insights to understand the molecular response to the combination of O₃ and drought.

Current ambient and elevated ozone effects on poplar: A global meta-analysis and response relationships

The effects of current and future elevated O₃ concentrations (e[O₃]) were investigated by a meta-analysis for poplar, a widely distributed genus in the Northern Hemisphere with global economic importance. Current [O₃] has significantly reduced CO₂ assimilation rate (P_n) by 33% and total biomass by 4% in comparison with low O₃ level (charcoal-filtered air, CF). Relative to CF, an increase in future [O₃] would further enhance the reduction in total biomass by 24%, plant height by 17% and plant leaf area by 19%. Isoprene emissions could decline by 34% under e[O₃], with feedback implications in reducing the formation of secondary air pollutants including O₃. Reduced stomatal conductance and lower foliar area might increase runoff and freshwater availability in O₃ polluted areas. Higher cumulated O₃ exposure over a threshold of 40 ppb (AOT40) induced larger reductions in P_n, total biomass and isoprene emission. Relationships of light-saturated photosynthesis rates (A_sat), total biomass and chlorophyll content with AOT40 using a global dataset are provided. These relationships are expected to improve O₃ risk assessment and also to support the inclusion of the effect of O₃ in models addressing plantation productivity and carbon sink capacity.

Increase of apoplastic ascorbate induced by ozone is insufficient to remove the negative effects in tobacco, soybean and poplar

Apoplastic ascorbate (ASCapo) is an important contributor to the detoxification of ozone (O3). The objective of the study is to explore whether ASCapo is stimulated by elevated O3 concentrations. The detoxification of O3 by ASCapo was quantified in tobacco (Nicotiana L), soybean (Glycine max (L.) Merr.) and poplar (Populus L), which were exposed to charcoal-filtered air (CF) and elevated O3 treatments (E-O3). ASCapo in the three species were significantly increased by E-O3 compared with the values in the filtered treatment. For all three species, E-O3 significantly increased the malondialdehyde (MDA) content and decreased light-saturated rate of photosynthesis (Asat), suggesting that high O3 has induced injury/damage to plants. E-O3 significantly increased redox state in the apoplast (redox stateapo) for all species, whereas no effect on the apoplastic dehydroascorbate (DHAapo) was observed. In leaf tissues, E-O3 significantly enhanced reduced-ascorbate (ASC) and total ascorbate (ASC+DHA) in soybean and poplar, but significantly reduced these in tobacco, indicating different antioxidative capacity to the high O3 levels among the three species. Total antioxidant capacity in the apoplast (TACapo) was significantly increased by E-O3 in tobacco and poplar, but leaf tissue TAC was significantly enhanced only in tobacco. Leaf tissue superoxide anion (O2•-) in poplar and hydrogen peroxide (H2O2) in tobacco and soybean were significantly increased by E-O3. The diurnal variation of ASCapo, with maximum values occurring in the late morning and lower values experienced in the afternoon, appeared to play an important role in the harmful effects of O3 on tobacco, soybean and poplar.

Predicting the effect of ozone on vegetation via linear non-threshold (LNT), threshold and hormetic dose-response models

The nature of the dose-response relationship in the low dose zone and how this concept may be used by regulatory agencies for science-based policy guidance and risk assessment practices are addressed here by using the effects of surface ozone (O₃) on plants as a key example for dynamic ecosystems sustainability. This paper evaluates the current use of the linear non-threshold (LNT) dose-response model for O₃. The LNT model has been typically applied in limited field studies which measured damage from high exposures, and used to estimate responses to lower concentrations. This risk assessment strategy ignores the possibility of biological acclimation to low doses of stressor agents. The upregulation of adaptive responses by low O₃ concentrations typically yields pleiotropic responses, with some induced endpoints displaying hormetic-like biphasic dose-response relationships. Such observations recognize the need for risk assessment flexibility depending upon the endpoints measured, background responses, as well as possible dose-time compensatory responses. Regulatory modeling strategies would be significantly improved by the adoption of the hormetic dose response as a formal/routine risk assessment option based on its substantial support within the literature, capacity to describe the entire dose-response continuum, documented explanatory dose-dependent mechanisms, and flexibility to default to a threshold feature when background responses preclude application of biphasic dose responses.

CAPSULE

The processes of ozone hazard and risk assessment can be enhanced by incorporating hormesis into their principles and practices.

Antioxidative responses of three oak species under ozone and water stress conditions

Plants are frequently exposed to adverse environmental conditions such as drought and ozone (O₃). Under these conditions, plants can survive due to their ability to adjust their metabolism. The aim of the present study was to compare the detoxification mechanisms of three oak species showing different O₃ sensitivity and water use strategy. Two-year-old seedlings of Quercus ilex, Q. pubescens and Q. robur were grown under the combination of three levels of O₃ (1.0, 1.2 and 1.4 times the ambient O₃ concentration) and three levels of water availability (on average 100, 80 and 42% of field capacity i.e. well-watered, moderate drought and severe drought, respectively) in an O₃ Free Air Controlled Exposure facility. Ozone and drought induced the accumulation of reactive oxygen species (ROS) and this phenomenon was species-specific. Sometimes, ROS accumulation was not associated with membrane injury suggesting that several antioxidative defence mechanisms inhibited or alleviated the oxidative damage. Both O₃ and drought increased total carotenoids that were able to prevent the peroxidation action by free radicals in Q. ilex, as confirmed by unchanged malondialdehyde by-product values. The concomitant decrease of total flavonoids may be related to the consumption of these compounds by the cell to inhibit the accumulation of hydrogen peroxide. Unchanged total phenols confirmed that Q. ilex has a superior ability to counteract oxidative conditions. Similar responses were found in Q. pubescens, although the negative impact of both factors was less efficiently faced than in the sympatric Q. ilex. In Q. robur, high O₃ concentrations and severe drought induced a partial rearrangement of the phenylpropanoid pathways. These antioxidative mechanisms were not able to protect the cell structure (as confirmed by ROS accumulation) suggesting that Q. robur showed a lower degree of tolerance than the other two species.

Climate Change Effects on Secondary Compounds of Forest Trees in the Northern Hemisphere

Plant secondary compounds (PSCs), also called secondary metabolites, have high chemical and structural diversity and appear as non-volatile or volatile compounds. These compounds may have evolved to have specific physiological and ecological functions in the adaptation of plants to their growth environment. PSCs are produced by several metabolic pathways and many PSCs are specific for a few plant genera or families. In forest ecosystems, full-grown trees constitute the majority of plant biomass and are thus capable of producing significant amounts of PSCs. We summarize older literature and review recent progress in understanding the effects of abiotic and biotic factors on PSC production of forest trees and PSC behavior in forest ecosystems. The roles of different PSCs under stress and their important role in protecting plants against abiotic and biotic factors are also discussed. There was strong evidence that major climate change factors, CO2 and warming, have contradictory effects on the main PSC groups. CO2 increases phenolic compounds in foliage, but limits terpenoids in foliage and emissions. Warming decreases phenolic compounds in foliage but increases terpenoids in foliage and emissions. Other abiotic stresses have more variable effects. PSCs may help trees to adapt to a changing climate and to pressure from current and invasive pests and pathogens. Indirect adaptation comes via the effects of PSCs on soil chemistry and nutrient cycling, the formation of cloud condensation nuclei from tree volatiles and by CO2 sequestration into PSCs in the wood of living and dead forest trees.

Emission of volatile organic compounds from plants shows a biphasic pattern within an hormetic context

Biogenic volatile organic compounds (BVOCs) are released to the atmosphere from vegetation. BVOCs aid in maintaining ecosystem sustainability via a series of functions, however, VOCs can alter tropospheric photochemistry and negatively affect biological organisms at high concentrations. Due to their critical role in ecosystem and environmental sustainability, BVOCs receive particular attention by global change biologists. To understand how plant VOC emissions affect stress responses within a dose-response context, dose responses should be evaluated. This commentary collectively documents hormetic-like responses of plant-emitted VOCs to external stimuli. Hormesis is a generalizable biphasic dose response phenomenon where the response to low doses acts in an opposite way at high doses. These collective findings suggest that ecological implications of low-level stress that may alter BVOC emissions should be considered in future studies. This commentary promotes new insights into the interface between biological systems and environmental change that influence several parts of the globe, and provide a base for advancing hazard assessment testing strategies and protocols to provide decision makers with adequate data for generating environmental standards.

Effects of elevated ozone concentration and nitrogen addition on ammonia stomatal compensation point in a poplar clone

The stomatal compensation point of ammonia (χ_s) is a key factor controlling plant-atmosphere NH₃ exchange, which is dependent on the nitrogen (N) supply and varies among plant species. However, knowledge gaps remain concerning the effects of elevated atmospheric N deposition and ozone (O₃) on χ_s for forest species, resulting in large uncertainties in the parameterizations of NH₃ incorporated into atmospheric chemistry and transport models (CTMs). Here, we present leaf-scale measurements of χ_s for hybrid poplar clone '546' (Populusdeltoides cv. 55/56 x P. deltoides cv. Imperial) growing in two N treatments (N0, no N added; N50, 50 kg N ha^-1 yr^-1 urea fertilizer added) and two O₃ treatments (CF, charcoal-filtered air; E-O₃, non-filtered air plus 40 ppb) for 105 days. Our results showed that χ_s was significantly reduced by E-O₃ (41%) and elevated N (19%). The interaction of N and O₃ was significant, and N can mitigate the negative effects of O₃ on χ_s. Elevated O₃ significantly reduced the light-saturated photosynthetic rate (A_sat) and chlorophyll (Chl) content and significantly increased intercellular CO₂ concentrations (Ci), but had no significant effect on stomatal conductance (g_s). By contrast, elevated N did not significantly affect all measured photosynthetic parameters. Overall, χ_s was significantly and positively correlated with A_sat, g_s and Chl, whereas a significant and negative relationship was observed between χ_s and Ci. Our results suggest that O₃-induced changes in A_sat, Ci and Chl may affect χ_s. Our findings provide a scientific basis for optimizing parameterizations of χ_s in CTMs in response to environmental change factors (i.e., elevated N deposition and/or O₃) in the future.

Nationwide ground-level ozone measurements in China suggest serious risks to forests

We processed hourly ozone (O₃) concentrations collected in 2015 and in 2016 by a network of 1497 stations across China, with the main aim of assessing the risk that present ambient O₃ exposure is posing to Chinese forests. Our results indicate that the values of the metrics AOT40 (the accumulated hourly O₃ concentrations above 40 ppb during daylight hours) recommended as European Union standard, and W126 (the sum of weighted hourly concentrations from 8:00 to 20:00) recommended as USA standard for forest protection, exceeded the critical levels (5 ppm h across 6 months for AOT40 and 7-21 ppm h over 3 months for W126) on average by 5.1 and 1.2 times, respectively. N100 showed on average 65 annual exceedances of 100 ppb as hourly value. The 12-h and 24-h averages showed a small difference, suggesting high concentrations also at night. Risk was higher for the northern temperate climate than for the southern tropical and sub-tropical climates, and overall for the northern regions than for the southern regions. Higher risk occurred in the non-urban areas than in the urban areas in northern, south-west and north-west China, whereas risk was higher at urban areas in eastern and southern China. The overall results of this first nationwide assessment suggest a significant risk for forests over the entire China and warrant for urgent measures for controlling O₃ precursor emissions and establishing standards of protection.

Five-year volume growth of European beech does not respond to ozone pollution in Italy

A unique database of stand volume growth, estimated as periodic annual volume increment (in m³ ha^-1 per year over the period 2001-2005) from 728 European beech (Fagus sylvatica L.) sites distributed across Italy, was used to assess the effects of ambient ozone (O₃), expressed as annual average (M24), accumulated exposure above a 40 ppb hourly threshold (AOT40), and total stomatal ozone flux (POD0). Growth data were from the National forest inventory of Italy, while climate data and ozone concentrations were computed by the WRF and CHIMERE models, respectively. Results show that the growth increased with increasing solar radiation and air temperature and decreased with increasing number of cold days, while effects of soil water content and O₃ were not significant. In contrast, the literature results suggest that European beech is sensitive to both drought and O₃. Ozone levels resulted to be very high (48 ppb M24, 51,200 ppb h AOT40, 21.08 mmol m^-2 POD0, on average) and thus able to potentially affect European beech growth. We hypothesize that the high-frequency signals of soil water and O₃ got lost when averaged over 5 years and recommended finer time-resolution investigations and inclusion of other factors of variability, e.g., thinning, tree age, and size.

Ozone risk assessment in three oak species as affected by soil water availability

To derive ozone (O₃) dose-response relationships for three European oak species (Quercus ilex, Quercus pubescens, and Quercus robur) under a range of soil water availability, an experiment was carried out with 2-year-old potted seedlings exposed to three levels of water availability in the soil and three levels of O₃ pollution for one growing season in an ozone free-air controlled exposure (FACE) facility. Total biomass losses were estimated relative to a hypothetical clean air at the pre-industrial age, i.e., at 10 ppb as daily average (M24). A stomatal conductance model was parameterized with inputs from the three species for calculating the stomatal O₃ flux. Exposure-based (M24, W126, and AOT40) and flux-based (phytotoxic O₃ dose (POD)_0-3) dose-response relationships were estimated and critical levels (CL) were calculated for a 5% decline of total biomass. Results show that water availability can significantly affect O₃ risk assessment. In fact, dose-response relationships calculated per individual species at each water availability level resulted in very different CLs and best metrics. In a simplified approach where species were aggregated on the basis of their O₃ sensitivity, the best metric was POD_0.5, with a CL of 6.8 mmol m^-2 for the less O₃-sensitive species Q. ilex and Q. pubescens and of 3.5 mmol m^-2 for the more O₃-sensitive species Q. robur. The performance of POD₀, however, was very similar to that of POD_0.5, and thus a CL of 6.9 mmol m^-2 POD₀ and 3.6 mmol m^-2 POD₀ for the less and more O₃-sensitive oak species may be also recommended. These CLs can be applied to oak ecosystems at variable water availability in the soil. We conclude that POD_y is able to reconcile the effects of O₃ and soil water availability on species-specific oak productivity.

High doses of ethylenediurea (EDU) as soil drenches did not increase leaf N content or cause phytotoxicity in willow grown in fertile soil

Ground-level ozone (O₃) levels are nowadays elevated in wide regions of the Earth, causing significant effects on plants that finally lead to suppressed productivity and yield losses. Ethylenediurea (EDU) is a chemical compound which is widely used in research projects as phytoprotectant against O₃ injury. The EDU mode of action remains still unclear, while there are indications that EDU may contribute to plants with nitrogen (N) when the soil is poor in N and the plants have relatively small leaf area. To reveal whether the N content of EDU acts as a fertilizer to plants when the soil is not poor in N and the plants have relatively large total plant leaf area, willow plants (Salix sachalinensis Fr. Schm) were exposed to low ambient O₃ levels and treated ten times (9-day interval) with 200mL soil drench containing 0, 800 or 1600mg EDU L^-1. Fertilizer was added to a nutrient-poor soil, and the plants had an average plant leaf area of 9.1m² at the beginning of EDU treatments. Indications for EDU-induced hormesis in maximum electron transport rate (J_max) and ratio of intercellular to ambient CO₂ concentration (C_i:C_a) were observed at the end of the experiment. No other EDU-induced effects on leaf greenness and N content, maximum quantum yield of photosystem II (F_v/F_m), gas exchange, growth and matter production suggest that EDU did not act as N fertilizer and did not cause toxicity under these experimental conditions.

Differential regulation of volatile emission from Eucalyptus globulus leaves upon single and combined ozone and wounding treatments through recovery and relationships with ozone uptake

Both ozone and wounding constitute two key abiotic stress factors, but their interactive effects on plant constitutive and stress-elicited volatile (VOC) emissions are poorly understood. Furthermore, the information on time-dependent modifications in VOC release during recovery from a combined stress is very limited. We studied the modifications in photosynthetic characteristics and constitutive and stress-induced volatile emissions in response to single and combined applications of acute ozone (4, 5, and 6 ppm) and wounding treatments through recovery (0.5-75 h) in a constitutive isoprene and mono- and sesquiterpene emitter Eucalyptus globulus. Overall, the photosynthetic characteristics were surprisingly resistant to all ozone and wounding treatments. Constitutive isoprene emissions were strongly upregulated by ozone and combined ozone and wounding treatments and remained high through recovery phase, but wounding applied alone reduced isoprene emission. All stress treatments enhanced emissions of lipoxygenase pathway volatiles (LOX), mono- and sesquiterpenes, saturated aldehydes (C7-C10), benzenoids, and geranylgeranyl diphosphate (GGDP) pathway volatiles. Once elicited, GGDP volatile, saturated aldehyde and benzenoid emissions remained high through the recovery period. In contrast, LOX emissions, and total mono- and sesquiterpene emissions decreased through recovery period. However, secondary rises in total sesquiterpene emissions at 75 h and in total monoterpenes at 25-50 h were observed. Overall, acute ozone and wounding treatments synergistically altered gas exchange characteristics and stress volatile emissions. Through the treatments and recovery period, stomatal ozone uptake rate and volatile emission rates were poorly correlated, reflecting possible ozone-scavenging effect of volatiles and thus, reduction of effective ozone dose and elicitation of induced defense by the acute ozone concentrations applied. These results underscore the important role of interactive stresses on both constitutive and induced volatile emission responses.

Transcriptomic analysis of Pak Choi under acute ozone exposure revealed regulatory mechanism against ozone stress

BACKGROUND

Ground-level ozone (O3) is one of the major air pollutants, which cause oxidative injury to plants. The physiological and biochemical mechanisms underlying the responses of plants to O3 stress have been well investigated. However, there are limited reports about the molecular basis of plant responses to O3. In this study, a comparative transcriptomic analysis of Pak Choi (Brassica campestris ssp. chinensis) exposed to different O3 concentrations was conducted for the first time.

RESULTS

Seedlings of Pak Choi with five leaves were exposed to non-filtered air (NF, 31 ppb) or elevated O3 (E-O3, 252 ppb) for 2 days (8 h per day, from 9:00-17:00). Compared with plants in the NF, a total of 675 differentially expressed genes (DEGs) were identified in plants under E-O3, including 219 DEGs with decreased expressions and 456 DEGs with increased expressions. Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses revealed that O3 stress invoked multiple cellular defense pathways to mitigate the impaired cellular integrity and metabolism, including 'glutathione metabolism', 'phenylpropanoid biosynthesis', 'sulfur metabolism', 'glucosinolate biosynthesis', 'cutin, suberine and wax biosynthesis' and others. Transcription factors potentially involved in this cellular regulation were also found, such as AP2-ERF, WRKY, JAZ, MYB etc. Based on the RNA-Seq data and previous studies, a working model was proposed integrating O3 caused reactive oxygen burst, oxidation-reduction regulation, jasmonic acid and downstream functional genes for the regulation of cellular homeostasis after acute O3 stress.

CONCLUSION

The present results provide a valuable insight into the molecular responses of Pak Choi to acute O3 stress and the specific DEGs revealed in this study could be used for further functional identification of key allelic genes determining the O3 sensitivity of Pak Choi.

No significant interactions between nitrogen stimulation and ozone inhibition of isoprene emission in Cathay poplar

Isoprene emission from plants subject to a combination of ozone (O₃) and nitrogen (N) has never been investigated. Cathay poplar (Populus cathayana) saplings were exposed to O₃ (CF, charcoal-filtered air, NF, non-filtered ambient air and E-O₃, non-filtered air +40ppb) and N treatments (N0, 0kgNha^-1year^-1, N50, 50kgNha^-1year^-1 and N100, 100kgNha^-1year^-1) for 96days. Increasing O₃ exposure decreased isoprene emission (11.5% in NF and 57.9% in E-O₃), as well as light-saturated photosynthetic rate (A_sat) and chlorophyll content, while N load increased isoprene emission (19.6% in N50 and 33.4% in N100) as well as A_sat and chlorophyll content. Although O₃ and N interacted significantly in A_sat, N did not mitigate the negative effects of O₃ on isoprene emission, i.e. the combined effects were additive and did not interact. These results warrant more research on the combined effects of co-existing global change factors on future isoprene emission and atmospheric chemical processes.

Effect of elevated CO2 and O3 on phytohormone-mediated plant resistance to vector insects and insect-borne plant viruses

Climatic variations are becoming important limiting factors for agriculture productivity, as they not only directly affect the plant net primary productivity but can also modulate the outbreak of plant diseases and pests. Elevated CO₂ and O₃ are two important climatic factors that have been widely studied before. Elevated CO₂ or O₃ alters the host plant physiology and affects the vector insects and plant viruses via bottom-up effects of the host plants. Many studies have shown that elevated CO₂ or O₃ decreases the plant nitrogen content, which modulates the characteristics of vector insects. Recent evidence also reveals that hormone-dependent signaling pathways play a critical role in regulating the response of insects and plant viruses to elevated CO₂ or O₃. In the current review, we describe how elevated CO₂ or O₃ affects the vector insects and plant viruses by altering the SA and JA signaling pathways. We also discuss how changes in the feeding behavior of vector insects or the occurrence of plant viruses affects the interactions between vector insects and plant viruses under elevated CO₂ or O₃. We suggest that new insights into the upstream network that regulates hormone signaling and top-down effects of natural enemies would provide a comprehensive understanding of the complex interactions taking place under elevated CO₂ or O₃.