A meta-analysis on growth, physiological, and biochemical responses of woody species to ground-level ozone highlights the role of plant functional types

Assessing the effects of elevated ozone on physiology, growth, yield and quality of soybean in the past 40 years: A meta-analysis

Soybean (Glycine max) production is seriously threatened by ground-level ozone (O3) pollution. The goal of our study is to summarize the impacts of O3 on physiology, growth, yield, and quality of soybean, as well as root parameters. We performed meta-analysis on the collated 48 peer-reviewed papers published between 1980 and 2019 to quantitatively summarize the response of soybean to elevated O3 concentrations ([O3]). Relative to charcoal-filtered air (CF), elevated [O3] significantly accelerated chlorophyll degradation, enhanced foliar injury, and inhibited growth of soybean, evidenced by great reductions in leaf area (-20.8%), biomass of leaves (-13.8%), shoot (-22.8%), and root (-16.9%). Shoot of soybean was more sensitive to O3 than root in case of biomass. Chronic ozone exposure of about 75.5 ppb posed pronounced decrease in seed yield of soybean (-28.3%). In addition, root environment in pot contributes to higher reduction in shoot biomass and yield of soybean. Negative linear relationships were observed between yield loss and intensity of O3 treatment, AOT40. The larger loss in seed yield was significantly associated with higher reduction in shoot biomass and other yield component. This meta-analysis demonstrates the effects of elevated O3 on soybean were pronounced, suggesting that O3 pollution is still a soaring threat to the productivity of soybean in regions with high ozone levels.

Air pollution monitoring and tree and forest decline in East Asia: A review

Air pollution and atmospheric deposition have adverse effects on tree and forest health. We reviewed studies on tree and forest decline in Northeast and Southeast Asia, Siberia, and the Russian Far East (hereafter referred to as East Asia). This included studies published in domestic journals and languages. We identified information about the locations, causes, periods, and tree species exhibiting decline. Past air pollution was also reviewed. Most East Asian countries show declining trends in SO₂ concentration in recent years, although Mongolia and Russia show increasing trends. Ozone (O₃) concentrations are stable or gradually increasing in the East Asia region, with high maxima. Wet nitrogen (N) deposition was high in China and tropical countries, but low in Russia. The decline of trees and forests primarily occurred in the mid-latitudes of Japan, Korea, China, and Russia. Long-term large N deposition resulted in the N saturation phenomenon in Japan and China, but no clear forest health response was observed. Thereafter, forest decline symptoms, suspected to be caused by O₃, were observed in Japan and China. In East Russia, tree decline occurred around industrial centers in Siberia. Haze events have been increasing in tropical and boreal forests, and particulate matter inhibits photosynthesis. In recent years, chronically high O₃ concentrations, in conjunction with climate change, are likely have adverse effects on tree physiology. The effects of air pollution and related factors on tree decline are summarized. Recently, the effects of air pollution on tree decline have not been apparent under the changing climate, however, monitoring air pollution is indispensable for identifying the cause of tree decline. Further economic growth is projected in Southeast Asia and therefore, the monitoring network should be expanded to tropical and boreal forest zones. Countermeasures such as restoring urban trees and rural forests are important for ensuring future ecosystem services.

Environmental impacts of nitrogen emissions in China and the role of policies in emission reduction

Atmospheric reactive nitrogen (N_r) has been a cause of serious environmental pollution in China. Historically, China used too little N_r in its agriculture to feed its population. However, with the rapid increase in N fertilizer use for food production and fossil fuel consumption for energy supply over the last four decades, increasing gaseous N_r species (e.g. NH₃ and NO_x) have been emitted to the atmosphere and then deposited as wet and dry deposition, with adverse impacts on air, water and soil quality as well as plant biodiversity and human health. This paper reviews the issues associated with this in a holistic way. The emissions, deposition, impacts, actions and regulations for the mitigation of atmospheric N_r are discussed systematically. Both NH₃ and NO_x make major contributions to environmental pollution but especially to the formation of secondary fine particulate matter (PM_2.5), which impacts human health and light scattering (haze). In addition, atmospheric deposition of NH₃ and NO_x causes adverse impacts on terrestrial and aquatic ecosystems due to acidification and eutrophication. Regulations and practices introduced by China that meet the urgent need to reduce N_r emissions are explained and resulting effects on emissions are discussed. Recommendations for improving future N management for achieving 'win-win' outcomes for Chinese agricultural production and food supply, and human and environmental health, are described. This article is part of a discussion meeting issue 'Air quality, past present and future'.

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.

Effects of ethylenediurea (EDU) on regulatory proteins in two maize (Zea mays L.) varieties under high tropospheric ozone phytotoxicity

Rising tropospheric ozone is a major threat to the crops in the present climate change scenario. To investigate the EDU induced changes in proteins, two varieties of maize, the SHM3031 and the PEHM5, (hereafter S and P respectively) were treated with three EDU applications (0= control, 50 and 200 ppm) (hereafter 0= A, 1 and 2 respectively) (SA, S1, S2, PA, P1, P2 cultivar X treatments). Data on the morpho-physiology, enzymatic activity, and protein expression (for the first time) were collected at the vegetative (V, 45 DAG) and flowering (F, 75 DAG) developmental stages. The tropospheric ozone was around 53 ppb enough to cause phytotoxic effects. Protective effects of EDU were recorded in morpho-physiologically and biochemically. SOD, CAT and APX together with GR performed better under EDU protection in SHM3031 variety than PEHM5. The protein expression patterns in SHM3031 at the vegetative stage (28% proteins were increased, 7% were decreased), and at the flowering stage (17% increased, 8% decreased) were found. In PEHM5, a 14% increase and an 18% decrease (vegetative stage) whereas a 16% increase and a 20% decrease (flowering stage) were recorded in protein expression. Some protein functional categories, for instance, photosynthesis, carbon metabolism, energy metabolism, and defense were influenced by EDU. Rubisco expression was increased in SHM3031 whereas differentially expressed in PEHM5. Germin like protein, APX, SOD, and harpin binding proteins have enhanced defense regulatory mechanisms under EDU treatment during prevailing high tropospheric O₃. The present study showed EDU protective roles in C4 plants as proven in C3.

Isotopic and Water Relation Responses to Ozone and Water Stress in Seedlings of Three Oak Species with Different Adaptation Strategies

The impact of global changes on forest ecosystem processes is based on the species-specific responses of trees to the combined effect of multiple stressors and the capacity of each species to acclimate and cope with the environment modification. Combined environmental constraints can severely affect plant and ecological processes involved in plant functionality. This study provides novel insights into the impact of a simultaneous pairing of abiotic stresses (i.e., water and ozone (O3) stress) on the responses of oak species. Water stress (using 40 and 100% of soil water content at field capacity—WS and WW treatments, respectively) and O3 exposure (1.0, 1.2, and 1.4 times the ambient concentration—AA, 1.2AA, and 1.4AA, respectively) were carried out on Quercus robur L., Quercus ilex L., and Quercus pubescens Willd. seedlings, to study physiological traits (1. isotope signature [δ13C, δ18O and δ15N], 2. water relation [leaf water potential, leaf water content], 3. leaf gas exchange [light-saturated net photosynthesis, Asat, and stomatal conductance, gs]) for adaptation strategies in a Free-Air Controlled Exposure (FACE) experiment. Ozone decreased Asat in Q. robur and Q. pubescens while water stress decreased it in all three oak species. Ozone did not affect δ13C, whereas δ18O was influenced by O3 especially in Q. robur. This may reflect a reduction of gs with the concomitant reduction in photosynthetic capacity. However, the effect of elevated O3 on leaf gas exchange as indicated by the combined analysis of stable isotopes was much lower than that of water stress. Water stress was detectable by δ13C and by δ18O in all three oak species, while δ15N did not define plant response to stress conditions in any species. The δ13C signal was correlated to leaf water content (LWC) in Q. robur and Q. ilex, showing isohydric and anisohydric strategy, respectively, at increasing stress intensity (low value of LWC). No interactive effect of water stress and O3 exposure on the isotopic responses was found, suggesting no cross-protection on seasonal carbon assimilation independently on the species adaptation strategy.

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

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

Toward an impact assessment of ozone on plant carbon fixation using a process-based plant growth model: A case study of Fagus crenata grown under different soil nutrient levels

Ozone (O₃) in the troposphere, an air pollutant with phytotoxicity, is considered as a driver of global warming, because it reduces plant carbon fixation. Recently, a process-based plant growth model has been used in evaluating the O₃ impacts on plants (Schauberger et al., 2019). To make the evaluation more rigorous, we developed a plant growth model and clarified the key factors driving O₃-induced change in the whole-plant carbon fixation amount (C_fix). Fagus crenata seedlings were exposed to three O₃ levels (charcoal-filtered air or 1.0- or 1.5-folds ambient [O₃]) with three soil fertilization levels (non-, low-, or high-fertilized), i.e., a total of nine treatments. The C_fix was reduced in non- and low-fertilized treatments but was unaffected in high-fertilized treatment by O₃ fumigation. Our plant growth model could simulate C_fix accurately (<10% error) by considering the impacts of O₃ on plant leaf area and photosynthetic capacities, including maximum velocities of carboxylation and electron transport (V_cmax and J_max, respectively), and the initial slope and convexity of the curve of the electron transport velocity response to photosynthetic photon flux density (φ and θ, respectively). Furthermore, the model revealed that changes in V_cmax and J_max, φ and θ, or leaf area, caused by 1.5-folds the ambient [O₃] fumigation resulted in the following C_fix changes: -1.6, -5.8, or -16.4% in non-fertilized seedlings, -4.1, -4.4, or -9.3% in low-fertilized seedlings, and -4.6, -7.6, or +5.8% in high-fertilized seedlings. Therefore, photosynthetic capacities (particularly φ and θ) and leaf area are important factors influencing the impact of O₃ on C_fix of F. crenata seedlings grown under various fertilization levels. Further, the impacts of O₃ and soil nutrient on these photosynthetic capacities and plant leaf area should be considered to predict O₃-induced changes in carbon fixation by forest tree species using the process-based plant growth model.

Interactive effects of air pollutants and atmospheric moisture stress on aspen growth and photosynthesis along an urban-rural gradient

Atmospheric pollution could significantly alter tree growth independently and synergistically with meteorological conditions. North China offers a natural experiment for studying how plant growth responds to air pollution under different meteorological conditions, where rapid economic growth has led to severe air pollution and climate changes increase drought stress. Using a single aspen clone (Populus euramericana Neva.) as a 'phytometer', we conducted three experiments to monitor aspen leaf photosynthesis and stem growth during in situ exposure to atmospheric pollutants along the urban-rural gradient around Beijing. We used stepwise model selection to select the best multiple linear model, and we used binned regression to estimate the effects of air pollutants, atmospheric moisture stress and their interactions on aspen leaf photosynthesis and growth. Our results indicated that ozone (O₃) and vapor pressure deficit (VPD) inhibited leaf photosynthesis and stem growth. The interactive effect of O₃ and VPD resulted in a synergistic response: as the concentration of O₃ increased, the negative impact of VPD on leaf photosynthesis and stem growth became more severe. We also found that nitrogen (N) deposition had a positive effect on stem growth, which may have been caused by an increase in canopy N uptake, although this hypothesis needs to be confirmed by further studies. The positive impact of aerosol loading may be due to diffuse radiation fertilization effects. Given the decline in aerosols and N deposition amidst increases in O₃ concentration and drought risk, the negative effects of atmospheric pollution on tree growth may be aggravated in North China. In addition, the interaction between O₃ and VPD may lead to a further reduction in ecosystem productivity.

Interactive effects of ozone exposure and nitrogen addition on tree root traits and biomass allocation pattern: An experimental case study and a literature meta-analysis

Ground-level ozone (O₃) pollution often co-occurs with anthropogenic nitrogen (N) deposition. Many studies have explored how O₃ and soil N affect aboveground structure and function of trees, but it remains unclear how belowground processes change over a spectrum of N addition and O₃ concentrations levels. Here, we explored the interactive impact of O₃ (five levels) and soil N (four levels) on fine and coarse root biomass and biomass allocation pattern in poplar clone 107 (Populus euramericana cv. '74/76'). We then evaluated the modifying effects of N on the responses of tree root biomass to O₃ via a synthesis of published literature. Elevated O₃ inhibited while N addition stimulated root biomass, with more pronounced effects on fine roots than on coarse root. The root:shoot (R:S) ratio was markedly decreased by N addition but remained unaffected by O₃. No interactive effects between O₃ and N were observed on root biomass and R:S ratio. The slope of log-log linear relationship between shoot and root biomass (i.e. scaling exponent) was increased by N, but not significantly affected by O₃. The analysis of published literature further revealed that the O₃-induced reduction in tree root biomass was not modified by soil N. The results suggest that higher N addition levels enhance faster allocation of shoot biomass while shoot biomass scales isometrically with root biomass across multiple O₃ levels. N addition does not markedly alter the sensitivity of root biomass of trees to O₃. These findings highlight that the biomass allocation exhibits a differential response to environmentally realistic levels of O₃ and N, and provide an important perspective for understanding and predicting net primary productivity and carbon dynamics in O₃-polluted and N-enriched environments.

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.

Climate change and carbon sink: a bibliometric analysis

In recent years, climate change and carbon sinks have been widely studied by the academic community, and relevant research results have emerged in abundance. In this paper, a scientometric analysis of 747 academic works published between 1991 and 2018 related to climate change and carbon sinks is presented to characterize the intellectual landscape by identifying and revealing the basic characteristics, research power, intellectual base, research topic evolution, and research hotspots in this field. The results show that ① the number of publications in this field has increased rapidly and the field has become increasingly interdisciplinary; ② the most productive authors and institutions in this subject area are in the USA, China, Canada, Australia, and European countries, and the cooperation between these researchers is closer than other researchers in the field; ③ 11 of the 747 papers analyzed in this study have played a key role in the evolution of the field; and ④ in this paper, we divide research hotspots into three decade-long phases (1991-1999, 2000-2010, and 2011-present). Drought problems have attracted more and more attention from scholars. In the end, given the current trend of the studies, we conclude a list of research potentials of climate change and carbon sinks in the future. This paper presents an in-depth analysis of climate change and carbon sink research to better understand the global trends and directions that have emerged in this field over the past 28 years, which can also provide reference for future research in this field.

The ozone sensitivity of five poplar clones is not related to stomatal conductance, constitutive antioxidant levels and morphology of leaves

Ground-level ozone (O₃) is an important phytotoxic air pollutant in China. In order to compare the sensitivity of common poplar clones to O₃ in China and explore the possible mechanism, five poplar clones, clone DQ (Populus cathayana), clone 84 K (P. alba × P. glandulosa), clone WQ156 (P. deltoids × P. cathayana), clone 546 (P. deltoides cv. '55/56' × P. deltoides cv. 'Imperial') and clone 107 (P. euramericana cv. '74/76') were exposed to four O₃ treatments. According to the date of the initial visible O₃ symptom and the slopes of O₃ exposure-response relationships with the relative light-saturated rate of CO₂ assimilation, we found that clone DQ and clone 546 were the most sensitive to O₃, clone 84 K and clone WQ156 were the less sensitive, and clone 107 was the most tolerant, which could provide a basis to select O₃ tolerant clones for poplar planting at areas with serious O₃ pollution. Elevated O₃ significantly reduced photosynthetic parameters, total phenols content, potential antioxidant capacity, leaf mass per area and biomass of five poplar clones, and there were significant interactions between O₃ and clones for most photosynthetic parameters. Elevated O₃ also significantly increased malondialdehyde content and total ascorbate content. The responses of total antioxidant capacity for poplar clones to elevated O₃ were different, as indicated by the increase for clone 107 and reduction for other clones under elevated O₃ treatment. Our results on the sensitivity of different poplar clones to O₃ are not related to leaf stomatal conductance, leaf constitutive antioxidant levels or leaf morphology of plant grown in clean air. The possible reason is little difference in leaf traits among clones within close species, suggesting that more properties of plants should be considered for exploring the sensitivity mechanism of close species, such as mesophyll conductance, antioxidant enzyme activity and apoplastic antioxidants.

Effects of ozone on maize (Zea mays L.) photosynthetic physiology, biomass and yield components based on exposure- and flux-response relationships

Since the Industrial Revolution, the global ambient O3 concentration has more than doubled. Negative impact of O3 on some common crops such as wheat and soybeans has been widely recognized, but there is relatively little information about maize, the typical C4 plant and third most important crop worldwide. To partly compensate this knowledge gap, the maize cultivar (Zhengdan 958, ZD958) with maximum planting area in China was exposed to a range of chronic ozone (O3) exposures in open top chambers (OTCs). The O3 effects on this highly important crop were estimated in relation to two O3 metrics, AOT40 (accumulated hourly O3 concentration over a threshold of 40 ppb during daylight hours) and POD6 (Phytotoxic O3 Dose above a threshold flux of 6 nmol O3 m-2 s-1 during a specified period). We found that (1) the reduced light-saturated net photosynthetic rate (Asat) mainly caused by non-stomatal limitations across heading and grain filling stages, but the stomatal limitations at the former stage were stronger than those at the latter stage; (2) impact of O3 on water use efficiency (WUE) of maize was significantly dependent on developmental stage; (3) yield loss induced by O3 was mainly due to a reduction in kernels weight rather than in the number of kernels; (4) the performance of AOT40 and POD6 was similar, according to their determination coefficients (R2); (5) the order of O3 sensitivity among different parameters was photosynthetic parameters > biomass parameters > yield-related parameters; (6) Responses of Asat to O3 between heading and gran filling stages were significantly different based on AOT40 metric, but not POD6. The proposed O3 metrics-response relationships will be valuable for O3 risk assessment in Asia and also for crop productivity models including the influence of O3.

Ozone Amplifies Water Loss from Mature Trees in the Short Term But Decreases It in the Long Term

We measured whole-tree transpiration of mature Fagus sylvatica and Picea abies trees exposed to ambient and twice-ambient O3 regimes (1xO3 and 2xO3 free-air fumigation). After eight years, mean daily total transpiration did not vary with the O3 regime over the 31 days of our study, even though individual daily values increased with increasing daily O3 peaks in both species. Although the environmental parameters were similar at 1xO3 and 2xO3, the main factors affecting daily transpiration were vapour pressure deficit in 2xO3 spruce and O3 peaks in beech. For a mechanistic explanation, we measured O3-induced sluggish stomatal responses to variable light (sunflecks) by means of leaf-level gas exchange measurements only in the species where O3 was a significant factor for transpiration, i.e., beech. Stomata were always slower in closing than in opening. The 2xO3 stomata were slower in opening and mostly in closing than 1xO3 stomata, so that O3 uptake and water loss were amplified before a steady state was reached. Such delay in the stomatal reaction suggests caution when assessing stomatal conductance under O3 pollution, because recording gas exchange at the time photosynthesis reached an equilibrium resulted in a significant overestimation of stomatal conductance when stomata were closing (ab. 90% at 1xO3 and 250% at 2xO3). Sun and shade leaves showed similar sluggish responses, thus suggesting that sluggishness may occur within the entire crown. The fact that total transpiration was similar at 1xO3 and 2xO3, however, suggests that the higher water loss due to stomatal sluggishness was offset by lower steady-state stomatal conductance at 2xO3. In conclusion, O3 exposure amplified short-term water loss from mature beech trees by slowing stomatal dynamics, while decreased long-term water loss because of lower steady-state stomatal conductance. Over the short term of this experiment, the two responses offset each other and no effect on total transpiration was observed.

Economic losses due to ozone impacts on human health, forest productivity and crop yield across China

China's economic growth has significantly increased emissions of tropospheric ozone (O₃) precursors, resulting in increased regional O₃ pollution. We analyzed data from >1400 monitoring stations and estimated the exposure of population and vegetation (crops and forests) to O₃ pollution across China in 2015. Based on WHO metrics for human health protection, the current O₃ level leads to +0.9% premature mortality (59,844 additional cases a year) with 96% of populated areas showing O₃-induced premature death. For vegetation, O₃ reduces annual forest tree biomass growth by 11-13% and yield of rice and wheat by 8% and 6%, respectively, relative to conditions below the respective AOT40 critical levels (CL). These CLs are exceeded over 98%, 75% and 83% of the areas of forests, rice and wheat, respectively. Using O₃ exposure-response functions, we evaluated the costs of O₃-induced losses in rice (7.5 billion US$), wheat (11.1 billion US$) and forest production (52.2 billion US$) and SOMO35-based morbidity for respiratory diseases (690.9 billion US$) and non-accidental mortality (7.5 billion US$), i.e. a total O₃-related cost representing 7% of the China Gross Domestic Product in 2015.

Ozone exposure- and flux-yield response relationships for maize

A stomatal ozone (O₃) flux-response relationship for relative yield of maize was established by parameterizing a Jarvis stomatal conductance model. For the function (f_VPD) describing the limitation of stomatal conductance by vapor pressure deficit (VPD, kPa), cumulative VPD during daylight hours was superior to hourly VPD. The latter function is proposed as a methodological improvement of this multiplicative model when stomatal conductance peaks during the morning and it is reduced later as it is the case of maize in this experiment. The model agreed relatively well with the measured stomatal conductance (R² = 0.63). Based on the comparison of R² values of the response functions, POD₆ (Phytotoxic Ozone Dose over an hourly threshold 6 nmol m^-2 s^-1) and AOT40 (accumulated hourly O₃ concentrations over a threshold of 40 ppb) performed similarly. The critical levels based on POD₆ and AOT40 for 5% reduction in maize yield were 1.17 mmol m^-2 PLA and 8.70 ppm h, respectively. In comparison with other important crops, the ranking of sensitivity of maize strongly differed depending on the O₃ metric used, AOT40 or POD₆. The newly proposed response functions are relevant for O₃ risk assessment for this crop in Asia.

Altered stomatal dynamics of two Euramerican poplar genotypes submitted to successive ozone exposure and water deficit

The impact of ozone (O₃) pollution events on the plant drought response needs special attention because spring O₃ episodes are often followed by summer drought. By causing stomatal sluggishness, O₃ could affect the stomatal dynamic during a subsequent drought event. In this context, we studied the impact of O₃ exposure and water deficit (in the presence or in the absence of O₃ episode) on the stomatal closure/opening mechanisms relative to irradiance or vapour pressure deficit (VPD) variation. Two genotypes of Populus nigra x deltoides were exposed to various treatments for 21 days. Saplings were exposed to 80 ppb/day O₃ for 13 days, and then to moderate drought for 7 days. The curves of the stomatal response to irradiance and VPD changes were determined after 13 days of O₃ exposure, and after 21 days in the case of subsequent water deficit, and then fitted using a sigmoidal model. The main responses under O₃ exposure were stomatal closure and sluggishness, but the two genotypes showed contrasting responses. During stomatal closure induced by a change in irradiance, closure was slower for both genotypes. Nonetheless, the genotypes differed in stomatal opening under light. Carpaccio stomata opened more slowly than control stomata, whereas Robusta stomata tended to open faster. These effects could be of particular interest, as stomatal impairment was still present after O₃ exposure and could result from imperfect recovery. Under water deficit alone, we observed slower stomatal closure in response to VPD and irradiance, but faster stomatal opening in response to irradiance, more marked in Carpaccio. Under the combined treatment, most of the parameters showed antagonistic responses. Our results highlight that it is important to take genotype-specific responses and interactive stress cross-talk into account to improve the prediction of stomatal conductance in response to various environmental modifications.

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.

The effects of elevated ozone on the accumulation and allocation of poplar biomass depend strongly on water and nitrogen availability

Ozone (O₃) pollution can alter carbon allocation and reduce tree growth - both above and below ground, but the extent of these effects depends on the variation in soil water and nutrient availability. Here we present the accumulation and allocation of biomass in poplar clone 546 (Populus deltoides cv. '55/56' × P. deltoides cv. 'Imperial') for one growing season at two O₃ concentrations (charcoal-filtered air [CF] and non-filtered air + 40 ppb of O₃ [E-O₃]), two watering regimes (well-watered [WW] and reduced watering at 40% of WW irrigation [RW]) and two soil nitrogen addition treatments (no addition [N0] and the addition of 50 kg N ha^-1 year^-1 [N50]). We found that the deleterious effects of E-O₃ depended on the supply of water and nitrogen. Specifically, when the supplies of water and/or N (WW and/or N50) were abundant, E-O₃ significantly reduced whole plant biomass by >15% but had no significant effect on biomass when these supplies were limited (RW and N0). A significant reduction of biomass by E-O₃ occurred earlier in fine roots than in other plant organs, indicating greater sensitivity of fine root to E-O₃. These results suggest that rising O₃ concentrations may not ubiquitously lead to a large reduction in plant biomass since plant growth is often jointly constrained by water and nutrients.

Effect of Long-Term vs. Short-Term Ambient Ozone Exposure on Radial Stem Growth, Sap Flux and Xylem Morphology of O3-Sensitive Poplar Trees

High ozone (O3) pollution impairs the carbon and water balance of trees, which is of special interest in planted forests. However, the effect of long-term O3 exposure on tree growth and water use, little remains known. In this study, we analysed the relationships of intra-annual stem growth pattern, seasonal sap flow dynamics and xylem morphology to assess the effect of long term O3 exposure of mature O3-sensitive hybrid poplars (‘Oxford’ clone). Rooted cuttings were planted in autumn 2007 and drip irrigated with 2 liters of water as ambient O3 treatment, or 450 ppm ethylenediurea (N-[2-(2-oxo-1-imidazolidinyl)ethyl]-N0-phenylurea, abbreviated as EDU) solution as O3 protection treatment over all growing seasons. During 2013, point dendrometers and heat pulses were installed to monitor radial growth, stem water relations and sap flow. Ambient O3 did not affect growth rates, even if the seasonal culmination point was 20 days earlier on average than that recorded in the O3 protected trees. Under ambient O3, trees showed reduced seasonal sap flow, however, the lower water use was due to a decrease of Huber value (decrease of leaf area for sapwood unit) rather than to a change in xylem morphology or due to a direct effect of sluggish stomatal responses on transpiration. Under high evaporative demand and ambient O3 concentrations, trees showed a high use of internal stem water resources modulated by stomatal sluggishness, thus predisposing them to be more sensitive water deficit during summer. The results of this study help untangle the compensatory mechanisms involved in the acclimation processes of forest species to long-term O3 exposure in a context of global change.

Air pollution reduction in China: Recent success but great challenge for the future

China's rapid economic growth has caused severe air pollution, raising serious concerns about the growing evidence of its negative health, environmental, and economic impacts. Consequently, the Chinese government has implemented a number of policies and measures to reduce air pollution. Relying on published information over the last three decades in China, we analyzed trends in air pollutant emissions (SO₂ and NO_x) and concentrations of particulate matter (PM) and ozone (O₃). During the past decade, SO₂ and NO_x emissions had declined throughout China and concentrations of PM_2.5 and PM₁₀ had considerably decreased in most cities, but average reported 90th MDA8 O₃, M7, and AOT40 O₃ for 31 capital cities showed an increasing trend between 2013 and 2017. Despite progress in air pollution reduction and an increasing number of "clear sky" days, PM concentrations throughout China remain higher than the World Health Organization guidelines, and urban smog and haze remain a major threat to human health and the environment. Thus far, significant emission reductions have occurred largely through robust administrative power, especially when emission reductions were tied to the performance evaluations and promotion of government officials. Similar to most already-industrialized nations, China is now shifting away from SO₂-dominated to NO_x- and O₃-dominated air pollution. Existing technologies and improved operations of existing control equipment appear unlikely to achieve sufficient reductions in NO_x and O₃ pollution. Considering the complex relationship between O₃, NO_x, VOCs, weather, and socio-economic changes in China, it is necessary to increase research on impacts of increasing ozone on plants and to adopt novel technologies and implemented to further reduce air pollution to levels that will protect human health and the environment.

Impacts of air pollution on human and ecosystem health, and implications for the National Emission Ceilings Directive: Insights from Italy

Across the 28 EU member states there were nearly half a million premature deaths in 2015 as a result of exposure to PM_2.5, O₃ and NO₂. To set the target for air quality levels and avoid negative impacts for human and ecosystems health, the National Emission Ceilings Directive (NECD, 2016/2284/EU) sets objectives for emission reduction for SO₂, NOx, NMVOCs, NH₃ and PM_2.5 for each Member State as percentages of reduction to be reached in 2020 and 2030 compared to the emission levels into 2005. One of the innovations of NECD is Article 9, that mentions the issue of "monitoring air pollution impacts" on ecosystems. We provide a clear picture of what is available in term of monitoring network for air pollution impacts on Italian ecosystems, summarizing what has been done to control air pollution and its effects on different ecosystems in Italy. We provide an overview of the impacts of air pollution on health of the Italian population and evaluate opportunities and implementation of Article 9 in the Italian context, as a case study beneficial for all Member States. The results showed that SO₄^2- deposition strongly decreased in all monitoring sites in Italy over the period 1999-2017, while NO₃^- and NH₄⁺ decreased more slightly. As a consequence, most of the acid-sensitive sites which underwent acidification in the 1980s partially recovered. The O₃ concentration at forest sites showed a decreasing trend. Consequently, AOT40 (the metric identified to protect vegetation from ozone pollution) showed a decrease, even if values were still above the limit for forest protection (5000 ppb h^-1), while PODy (flux-based metric under discussion as new European legislative standard for forest protection) showed an increase. National scale studies pointed out that PM10 and NO2 induced about 58,000 premature deaths (year 2005), due to cardiovascular and respiratory diseases. The network identified for Italy contains a good number of monitoring sites (6 for terrestrial ecosystem monitoring, 4 for water bodies monitoring and 11 for ozone impact monitoring) distributed over the territory and will produce a high number of monitored parameters for the implementation of the NECD.

Effects of ozone and ammonium sulfate on cauliflower: Emphasis on the interaction between plants and insect herbivores

Ammonium sulfate [(NH₄)₂SO₄] deposition and elevated ozone (O₃) concentrations may negatively affect plants and trophic interactions. This study aimed to evaluate for the first time the interactive effects of high (NH₄)₂SO₄ load and elevated O₃ levels on cauliflower (Brassica oleracea L.) under field conditions. Cauliflower seedlings were treated with 0 (AS0) or 50 (AS50) kg ha^-1 (NH₄)₂SO₄ and exposed to ambient (AOZ, ≈20 ppb) or elevated (EOZ, ≈55 ppb) O₃ for about one month, in a Free Air O₃ Concentration Enrichment (FACE) system. The oligophagous diamondback moth (Plutella xylostella Linnaeus, 1758) showed a clear preference towards the seedlings treated with AS50, which intensively grazed. Plant-herbivore interactions were driven by (NH₄)₂SO₄ availability, rather than O₃, via increased nitrogen content in the leaves. Further laboratory bioassays were followed to confirm the validity of these observations using polyphagous Eri silkmoth larvae (Samia ricini) as a biological model in a standardized experimental setup. Choice assays, where larvae could select leaves among leaf samples from the different experimental conditions, and no-choice assays, where larvae could graze leaves from just one experimental condition, were conducted. In the choice assay, the larvae preferred AS50-treated leaves, in agreement with the field observations with diamondback moth. In the no-choice assay, larval body mass growth was inhibited when fed with leaves treated with EOZ and/or AS50. Larvae fed with AS50-treated leaves displayed increased mortality. These observations coincide with higher NO₃ and Zn content in AS50-treated leaves. This study shows that plant-herbivore interactions can be driven by (NH₄)₂SO₄ availability, independently of O₃, and suggests that high N deposition may have severe health implications in animals consuming such plant tissues. Key message: Plant-herbivore interactions are driven by high (NH₄)₂SO₄ availability, independently of O₃.

Investigation of ozone deposition to vegetation under warm and dry conditions near the Eastern Mediterranean coast

Dry deposition of ozone (O₃) to vegetation is an important removal pathway for tropospheric O₃, while O₃ uptake through plant stomata negatively affects vegetation and leads to climate change. Both processes are controlled by vegetation characteristics and ambient conditions via complex mechanisms. Recent studies have revealed that these processes can be fundamentally impacted by coastal effects, and by dry and warm conditions in ways that have not been fully characterized, largely due to lack of measurements under such conditions. Hence, we hypothesized that measuring dry deposition of O₃ to vegetation along a sharp spatial climate gradient, and at different distances from the coast, can offer new insights into the characterization of these effects on O₃ deposition to vegetation and stomatal uptake, providing important information for afforestation management and for climate and air-quality model improvement. To address these hypotheses, several measurement campaigns were performed at different sites, including pine, oak, and mixed Mediterranean forests, at distances of 20-59 km from the Eastern Mediterranean coast, under semiarid, Mediterranean and humid Mediterranean climate conditions. The eddy covariance technique was used to quantify vertical O₃ flux (F_tot) and its partitioning to stomatal flux (F_st) and non-stomatal flux (F_ns). Whereas F_st tended to peak around noon under humid Mediterranean and Mediterranean conditions in summer, it was strongly limited by drought under semiarid conditions from spring to early winter, with minimum average F_st/F_tot of 8-11% during the summer. F_ns in the area was predominantly controlled by relative humidity (RH), whereas increasing F_ns with RH for RH < 70% indicated enhancement of F_ns by aerosols, via surface wetness stimulation. At night, efficient turbulence due to sea and land breezes, together with increased RH, resulted in strong enhancement of F_tot. Extreme dry surface events, some induced by dry intrusion from the upper troposphere, resulted in positive F_ns events.

Elevated ozone reduced leaf nitrogen allocation to photosynthesis in poplar

We investigated the effects of elevated ozone (O₃) concentration on leaf nitrogen (N), a key determinant of plant photosynthesis, with two clones of poplar grown in open-top chambers. We focus on the difference between mass-based leaf N concentration (N_mass) and area-based one (N_area) in their responses to elevated O₃, and the allocation of N to different leaf components: photosynthetic apparatus, cell walls, and others under elevated O₃ level. Our results showed that elevated O₃ significantly increased N_mass, but reduced N_area and leaf mass per area (LMA). The two clones showed no difference in N_mass response to O₃, but the more sensitive clone showed greater reduction of N_area and LMA due to O₃. We also found positive relationships between N_area and photosynthetic parameters, e.g. light-saturated photosynthetic rate (A_sat). Furthermore, elevated O₃ significantly reduced photosynthetic N-use efficiency (PNUE) and leaf N allocation to photosynthetic components, while increasing N allocation to cell walls and other components. We concluded that plants invested more N in cell walls and other components to resist O₃ damages at the expense of photosynthetic N. The change of N allocation in plant leaves in response to elevated O₃ could have an impact on ecological processes, e.g. leaf litter decomposition.

Mesophyll conductance limitation of photosynthesis in poplar under elevated ozone

Finite mesophyll conductance (g_m) reduces the rate of CO₂ diffusion from the leaf intercellular space to the chloroplast and constitutes a major limitation of photosynthesis in trees. While it is well established that g_m is decreased by stressors such as drought and high temperature, few studies have investigated if the phytotoxic air pollutant ozone (O₃) affects g_m. We quantified the relative importance of three different types of limitations of photosynthesis in poplar trees exposed to elevated O₃: decreases in stomatal conductance, g_m and biochemical photosynthetic capacity. The O₃-induced reductions in light-saturated net photosynthesis were linked to significant declines in g_m and biochemical photosynthetic capacity (in particular carboxylation). There was no significant effect of O₃ on stomatal conductance. Of the O₃-induced limitations on photosynthesis, g_m limitation was by far the most important (-16%) while biochemical limitation (-8%) was rather small. Both limitations grew in magnitude over the study period and varied in response to leaf-specific O₃ exposure. Our findings suggest that declines in g_m may play a key role in limiting photosynthesis of plants exposed to elevated O₃, an effect hitherto overlooked.

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.

Effects of elevated ground-level ozone on paddy soil bacterial community and assembly mechanisms across four years

It is well known that elevated ground-level ozone (eO₃) poses a threat to the ecosystem. Little knowledge about the underground variables, especially on soil microorganisms, however, has been revealed. Such knowledge will tremendously help to advance our understanding of the correlation between ecosystems and climate change, as well as our ability to predict future trajectory. For this purpose, we have collected soil DNA samples (eO₃ vs. Ambient, each having 36 samples) over four years. Our results have verified the temporal responses and the underlying assembly mechanisms of the paddy bacterial community to eO₃. Contrary to the widespread consensus, it was found that eO₃ stimulated bacterial alpha diversities. The higher complexity and the centralization of the co-occurrence network of the bacterial community suggested that this stimulation was due to a microbial survival strategy in response to the limited resources, which led to the instability of the community. Furthermore, the observed slower temporal turnover of the bacterial community composition in response to eO₃ was due to the decreased deterministic processes derived from plants, which implied that eO₃ disrupted the coordination between soil microorganisms and rice crop. All above phenomena provided novel insights into the adverse influences of eO₃ on the soil microbial community. If O₃ concentration increases continuously, the adverse effects will be aggravated and harm the related ecological functions.

Interactive effects of changing stratospheric ozone and climate on tropospheric composition and air quality, and the consequences for human and ecosystem health

The composition of the air we breathe is determined by emissions, weather, and photochemical transformations induced by solar UV radiation. Photochemical reactions of many emitted chemical compounds can generate important (secondary) pollutants including ground-level ozone (O₃) and some particulate matter, known to be detrimental to human health and ecosystems. Poor air quality is the major environmental cause of premature deaths globally, and even a small decrease in air quality can translate into a large increase in the number of deaths. In many regions of the globe, changes in emissions of pollutants have caused significant changes in air quality. Short-term variability in the weather as well as long-term climatic trends can affect ground-level pollution through several mechanisms. These include large-scale changes in the transport of O₃ from the stratosphere to the troposphere, winds, clouds, and patterns of precipitation. Long-term trends in UV radiation, particularly related to the depletion and recovery of stratospheric ozone, are also expected to result in changes in air quality as well as the self-cleaning capacity of the global atmosphere. The increased use of substitutes for ozone-depleting substances, in response to the Montreal Protocol, does not currently pose a significant risk to the environment. This includes both the direct emissions of substitutes during use and their atmospheric degradation products (e.g. trifluoroacetic acid, TFA).

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.

Measurement-based investigation of ozone deposition to vegetation under the effects of coastal and photochemical air pollution in the Eastern Mediterranean

Dry deposition of ozone (O₃) to vegetation is an important pathway for its removal from the troposphere, and it can lead to adverse effects in plants and changes in climate. However, our mechanistic understanding of O₃ dry deposition is insufficient to adequately account for it in global and regional models, primarily because this process is highly complicated by feedback mechanisms and sensitivity to specific characteristics of vegetative environment and atmospheric dynamics and composition. We hypothesized that measuring dry deposition of O₃ to vegetation near the Eastern Mediterranean (EM) coast, where large variations in meteorological conditions and photochemical air pollution frequently occur, would enable identifying the mechanisms controlling O₃ deposition to vegetation. Moreover, we have only limited knowledge of O₃ deposition to vegetation occurring near a coastline, under air pollution, or in the EM. This study investigated O₃ deposition to mixed Mediterranean vegetation between the summers of 2015 and 2017, 3.6 km away from the EM coast, using the eddy covariance technique to quantify vertical flux of O₃ and its partitioning to stomatal and non-stomatal flux, concurrent with nitrogen oxide (NO_x), sulfur dioxide and carbon monoxide. Surprisingly, nighttime O₃-deposition velocity (V_d) was smaller than daytime V_d by only ~20-37% on average for all measurement periods, primarily related to moderate nighttime atmospheric stability due to proximity to the seashore. We provide evidence for the role of sea-salt aerosols in enhancing O₃ deposition via surface-wetness buildup at low relative humidity near the coast, and for daytime enhancement of O₃ deposition by the combined effects of biogenic volatile organic compound emission and surface-wetness buildup. We further show that NO_x emitted from elevated emission sources can reduce O₃ deposition, and even lead to a positive O₃ flux, demonstrating the importance of adequately taking into account the impact of air pollution on O₃ deposition to vegetation.

Enzyme activity modification in adult beetles (Agelastica coerulea) inhabiting birch trees in an ozone-enriched atmosphere

Tropospheric ozone (O₃) is a naturally occurring gas in the atmosphere. However, the concentration of O₃ increased in the twentieth century. Although the effects of O₃ on vegetation have been extensively studied since the 1950s, limited information exists regarding the effects of O₃ on insect herbivores. In particular, evidence is lacking regarding the effects of O₃ on the biology of insect herbivores. Agelastica coerulea Baly (1874) is a coleopteran species that grazes on Betulaceae plants. In this study, to investigate the effects of O₃ on A. coerulea biology for the first time, female adult insects were collected from Japanese white birch trees grown in a Free Air Controlled Exposure System (FACE) in Sapporo, Japan. These beetles inhabited trees exposed either to ambient or to elevated O₃ for 23 days. After collection, the enzyme activities in the beetles were measured. Elevated O₃ led to a greater total antioxidant activity and lower α- and β-esterase activities, a phenomenon that may suggest an increased resistance of the beetles to stress. Our results are further discussed with regard to biological and toxicological aspects. Collectively, our findings indicate that total antioxidants and α- and β-esterase activities can serve as effective O₃ biomarker systems in this beetle species. This adaptive response of the beetle, which was induced by moderate O₃ exposure, should be further tested across generations and for its protection against greater exposure.

Mesophyll conductance to CO2 in leaves of Siebold's beech (Fagus crenata) seedlings under elevated ozone

Ozone is an air pollutant that negatively affects photosynthesis in woody plants. Previous studies suggested that ozone-induced reduction in photosynthetic rates is mainly attributable to a decrease of maximum carboxylation rate (V_cmax) and/or maximum electron transport rate (J_max) estimated from response of net photosynthetic rate (A) to intercellular CO₂ concentration (C_i) (A/C_i curve) assuming that mesophyll conductance for CO₂ diffusion (g_m) is infinite. Although it is known that C_i-based V_cmax and J_max are potentially influenced by g_m, its contribution to ozone responses in C_i-based V_cmax and J_max is still unclear. In the present study, therefore, we analysed photosynthetic processes including g_m in leaves of Siebold's beech (Fagus crenata) seedlings grown under three levels of ozone (charcoal-filtered air or ozone at 1.0- or 1.5-times ambient concentration) for two growing seasons in 2016-2017. Leaf gas exchange and chlorophyll fluorescence were simultaneously measured in July and September of the second growing season. We determined the A, stomatal conductance to water vapor and g_m, and analysed A/C_i curve and A/C_c curve (C_c: chloroplast CO₂ concentration). We also determined the Rubisco and chlorophyll contents in leaves. In September, ozone significantly decreased C_i-based V_cmax. At the same time, ozone decreased g_m, whereas there was no significant effect of ozone on C_c-based V_cmax or the contents of Rubisco and chlorophyll in leaves. These results suggest that ozone-induced reduction in C_i-based V_cmax is a result of the decrease in g_m rather than in carboxylation capacity. The decrease in g_m by elevated ozone was offset by an increase in C_i, and C_c did not differ depending on ozone treatment. Since C_c-based V_cmax was also similar, A was not changed by elevated ozone. We conclude that g_m is an important factor for reduction in C_i-based V_cmax of Siebold's beech under elevated ozone.

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.

Ozone risk assessment is affected by nutrient availability: Evidence from a simulation experiment under free air controlled exposure (FACE)

Assessing ozone (O₃) risk to vegetation is crucial for informing policy making. Soil nitrogen (N) and phosphorus (P) availability could change stomatal conductance which is the main driver of O₃ uptake into a leaf. In addition, the availability of N and P could influence photosynthesis and growth. We thus postulated that the sensitivity of plants to O₃ may be changed by the levels of N and P in the soil. In this study, a sensitive poplar clone (Oxford) was subject to two N levels (N0, 0 kg N ha^-1; N80, 80 kg N ha^-1), three P levels (P0, 0 kg P ha^-1; P40, 40 kg P ha^-1; P80, 80 kg P ha^-1) and three levels of O₃ exposure (ambient concentration, AA; 1.5 × AA; 2.0 × AA) for a whole growing season in an O₃ free air controlled exposure (FACE) facility. Flux-based (POD_{0 to 6}) and exposure-based (W126 and AOT40) dose-response relationships were fitted and critical levels (CLs) were estimated for a 5% decrease of total annual biomass. It was found that N and P availability modified the dose-response relationships of biomass responses to O₃. Overall, the N supply decreased the O₃ CLs i.e. increased the sensitivity of poplar to O₃. Phosphorus alleviated the O₃-caused biomass loss and increased the CL. However, such mitigation effects of P were found only in low N and not in high N conditions. In each nutritional treatment, similar performance was found between flux-based and exposure-based indices. However, the flux-based approach was superior, as compared to exposure indices, to explain the biomass reduction when all nutritional treatments were pooled together. The best O₃ metric for risk assessments was POD₄, with 4.6 mmol m^-2 POD₄ as a suitable CL for Oxford poplars grown under various soil N and P conditions.

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.

Tropospheric ozone assessment report: Global ozone metrics for climate change, human health, and crop/ecosystem research

Assessment of spatial and temporal variation in the impacts of ozone on human health, vegetation, and climate requires appropriate metrics. A key component of the Tropospheric Ozone Assessment Report (TOAR) is the consistent calculation of these metrics at thousands of monitoring sites globally. Investigating temporal trends in these metrics required that the same statistical methods be applied across these ozone monitoring sites. The nonparametric Mann-Kendall test (for significant trends) and the Theil-Sen estimator (for estimating the magnitude of trend) were selected to provide robust methods across all sites. This paper provides the scientific underpinnings necessary to better understand the implications of and rationale for selecting a specific TOAR metric for assessing spatial and temporal variation in ozone for a particular impact. The rationale and underlying research evidence that influence the derivation of specific metrics are given. The form of 25 metrics (4 for model-measurement comparison, 5 for characterization of ozone in the free troposphere, 11 for human health impacts, and 5 for vegetation impacts) are described. Finally, this study categorizes health and vegetation exposure metrics based on the extent to which they are determined only by the highest hourly ozone levels, or by a wider range of values. The magnitude of the metrics is influenced by both the distribution of hourly average ozone concentrations at a site location, and the extent to which a particular metric is determined by relatively low, moderate, and high hourly ozone levels. Hence, for the same ozone time series, changes in the distribution of ozone concentrations can result in different changes in the magnitude and direction of trends for different metrics. Thus, dissimilar conclusions about the effect of changes in the drivers of ozone variability (e.g., precursor emissions) on health and vegetation exposure can result from the selection of different metrics.