Mapping pollen allergenicity from urban trees in Valencia: A tool for green infrastructure planning.

Urban trees provide many benefits to citizens but also have associated disservices such as pollen allergenicity. Pollen allergies affect 40% of the European population, a problem that will be exacerbated with climate change by lengthening the pollen season. The allergenic characteristics of the urban trees and urban parks of the city of Valencia (Spain) have been studied. The Value of Potential Allergenicity (VPA) was calculated for all species. The most abundant allergenic trees with a very high VPA were the cypresses, followed by Platanus x hispanica and species of genera Morus, Acer and Fraxinus, with a high VPA. On the contrary, Citrus x aurantium, Melia azedarach, Washingtonia spp., Brachychiton spp. and Jacaranda mimosifolia were among the most abundant low allergenic trees. VPA was mapped for the city and a hot spot analysis was applied to identify areas of clustering of high and low VPA values. This geostatistical analysis provides a comprehensive representation of the VPA patterns which is very useful for urban green infrastructure planning. The Index of Urban Green Zone Allergenicity (IUGZA) was calculated for the main parks of the city. The subtropical and tropical flora component included many entomophilous species and the lowest share of high and very high allergenic trees in comparison with the Mediterranean and Temperate components. Overall, a diversification of tree species avoiding clusters of high VPA trees, and the prioritization of species with low VPA are good strategies to minimize allergy-related impacts of urban trees on human health.

Fungal community composition predicts forest carbon storage at a continental scale.

Forest soils harbor hyper-diverse microbial communities which fundamentally regulate carbon and nutrient cycling across the globe. Directly testing hypotheses on how microbiome diversity is linked to forest carbon storage has been difficult, due to a lack of paired data on microbiome diversity and in situ observations of forest carbon accumulation and storage. Here, we investigated the relationship between soil microbiomes and forest carbon across 238 forest inventory plots spanning 15 European countries. We show that the composition and diversity of fungal, but not bacterial, species is tightly coupled to both forest biotic conditions and a seven-fold variation in tree growth rates and biomass carbon stocks when controlling for the effects of dominant tree type, climate, and other environmental factors. This linkage is particularly strong for symbiotic endophytic and ectomycorrhizal fungi known to directly facilitate tree growth. Since tree growth rates in this system are closely and positively correlated with belowground soil carbon stocks, we conclude that fungal composition is a strong predictor of overall forest carbon storage across the European continent.

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.

Lichen Biodiversity and Near-Infrared Metabolomic Fingerprint as Diagnostic and Prognostic Complementary Tools for Biomonitoring: A Case Study in the Eastern Iberian Peninsula.

In the 1990s, a sampling network for the biomonitoring of forests using epiphytic lichen diversity was established in the eastern Iberian Peninsula. This area registered air pollution impacts by winds from the Andorra thermal power plant, as well as from photo-oxidants and nitrogen depositions from local and long-distance transport. In 1997, an assessment of the state of lichen communities was carried out by calculating the Index of Atmospheric Purity. In addition, visible symptoms of morphological injury were recorded in nine macrolichens pre-selected by the speed of symptom evolution and their wide distribution in the territory. The thermal power plant has been closed and inactive since 2020. During 2022, almost 25 years later, seven stations of this previously established biomonitoring were revaluated. To compare the results obtained in 1997 and 2022, the same methodology was used, and data from air quality stations were included. We tested if, by integrating innovative methodologies (NIRS) into biomonitoring tools, it is possible to render an integrated response. The results displayed a general decrease in biodiversity in several of the sampling plots and a generalised increase in damage symptoms in the target lichen species studied in 1997, which seem to be the consequence of a multifactorial response.

Trends in urban air pollution over the last two decades: A global perspective.

Ground-level ozone (O3), fine particles (PM2.5), and nitrogen dioxide (NO2) are the most harmful urban air pollutants regarding human health effects. Here, we aimed at assessing trends in concurrent exposure of global urban population to O3, PM2.5, and NO2 between 2000 and 2019. PM2.5, NO2, and O3 mean concentrations and summertime mean of the daily maximum 8-h values (O3 MDA8) were analyzed (Mann-Kendall test) using data from a global reanalysis, covering 13,160 urban areas, and a ground-based monitoring network (Tropospheric Ozone Assessment Report), collating surface O3 observations at nearly 10,000 stations worldwide. At global scale, PM2.5 exposures declined slightly from 2000 to 2019 (on average, - 0.2 % year-1), with 65 % of cities showing rising levels. Improvements were observed in the Eastern US, Europe, Southeast China, and Japan, while the Middle East, sub-Saharan Africa, and South Asia experienced increases. The annual NO2 mean concentrations increased globally at 71 % of cities (on average, +0.4 % year-1), with improvements in North America and Europe, and increases in exposures in sub-Saharan Africa, Middle East, and South Asia regions, in line with socioeconomic development. Global exposure of urban population to O3 increased (on average, +0.8 % year-1 at 89 % of stations), due to lower O3 titration by NO. The summertime O3 MDA8 rose at 74 % of cities worldwide (on average, +0.6 % year-1), while a decline was observed in North America, Northern Europe, and Southeast China, due to the reduction in precursor emissions. The highest O3 MDA8 increases (>3 % year-1) occurred in Equatorial Africa, South Korea, and India. To reach air quality standards and mitigate outdoor air pollution effects, actions are urgently needed at all governance levels. More air quality monitors should be installed in cities, particularly in Africa, for improving risk and exposure assessments, concurrently with implementation of effective emission control policies that will consider regional socioeconomic imbalances.

Ozone pollution threatens the production of major staple crops in East Asia.

East Asia is a hotspot of surface ozone (O3) pollution, which hinders crop growth and reduces yields. Here, we assess the relative yield loss in rice, wheat and maize due to O3 by combining O3 elevation experiments across Asia and air monitoring at about 3,000 locations in China, Japan and Korea. China shows the highest relative yield loss at 33%, 23% and 9% for wheat, rice and maize, respectively. The relative yield loss is much greater in hybrid than inbred rice, being close to that for wheat. Total O3-induced annual loss of crop production is estimated at US$63 billion. The large impact of O3 on crop production urges us to take mitigation action for O3 emission control and adaptive agronomic measures against the rising surface O3 levels across East Asia.

Urban population exposure to air pollution in Europe over the last decades.

BACKGROUND: The paper presents an overview of air quality in the 27 member countries of the European Union (EU) and the United Kingdom (previous EU-28), from 2000 to 2017. We reviewed the progress made towards meeting the air quality standards established by the EU Ambient Air Quality Directives (European Council Directive 2008/50/EC) and the World Health Organization (WHO) Air Quality Guidelines by estimating the trends (Mann-Kendal test) in national emissions of main air pollutants, urban population exposure to air pollution, and in mortality related to exposure to ambient fine particles (PM2.5) and tropospheric ozone (O3). RESULTS: Despite significant reductions of emissions (e.g., sulfur oxides: ~ 80%, nitrogen oxides: ~ 46%, non-methane volatile organic compounds: ~ 44%, particulate matters with a diameter lower than 2.5 µm and 10 µm: ~ 30%), the EU-28 urban population was exposed to PM2.5 and O3 levels widely exceeding the WHO limit values for the protection of human health. Between 2000 and 2017, the annual PM2.5-related number of deaths decreased (- 4.85 per 106 inhabitants) in line with a reduction of PM2.5 levels observed at urban air quality monitoring stations. The rising O3 levels became a major public health issue in the EU-28 cities where the annual O3-related number of premature deaths increased (+ 0.55 deaths per 106 inhabitants). CONCLUSIONS: To achieve the objectives of the Ambient Air Quality Directives and mitigate air pollution impacts, actions need to be urgently taken at all governance levels. In this context, greening and re-naturing cities and the implementation of fresh air corridors can help meet air quality standards, but also answer to social needs, as recently highlighted by the COVID-19 lockdowns.

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.

Yield and economic losses of winter wheat and rice due to ozone in the Yangtze River Delta during 2014-2019.

Ground-level ozone (O3) is the main phytotoxic air pollutant causing crop yield reduction in China. As the main grain producing area in China, the Yangtze River Delta (YRD) is facing serious O3 pollution. This study analyzed the hourly ground-level O3 observation data of 158 stations from 2014 to 2019 in YRD, and grain production data of 193 districts and counties. The exposure-response relationships based on AOT40 (accumulated hourly O3 concentration above 40 ppb) was used to estimate the yield loss and economic loss of two food crops (winter wheat and rice). This study used spatial interpolation and calculated the specific data values of each district and county in order to improve the assessment reliability. For years 2014-2019, averaged O3 concentration during the 75 days growing period of rice and wheat were 33.1-50.6 ppb and 32.2-48.0 ppb, AOT40 value were 5.2-12.0 ppm h and 4.6-9.4 ppm h, and the averaged relative yield losses were 4.9%-11.4% and 9.4%-19.3%, respectively. The trend of O3 in the YRD in a six-year period peaked in 2016 and 2017 for rice and winter wheat, respectively. During 2014-2017, the average estimated yield loss of rice was 2445 Mt. accounting for about 9.1% of the actual production, and the average estimated economic loss was about 1037 million USD; for winter wheat, it was 2025 Mt, 20.4% and 736 million USD, respectively. These results urge governments to provide effective policies and measures to control O3 pollution.

Amplified ozone pollution in cities during the COVID-19 lockdown.

The effect of lockdown due to coronavirus disease (COVID-19) pandemic on air pollution in four Southern European cities (Nice, Rome, Valencia and Turin) and Wuhan (China) was quantified, with a focus on ozone (O3). Compared to the same period in 2017-2019, the daily O3 mean concentrations increased at urban stations by 24% in Nice, 14% in Rome, 27% in Turin, 2.4% in Valencia and 36% in Wuhan during the lockdown in 2020. This increase in O3 concentrations is mainly explained by an unprecedented reduction in NOx emissions leading to a lower O3 titration by NO. Strong reductions in NO2 mean concentrations were observed in all European cities, ~53% at urban stations, comparable to Wuhan (57%), and ~65% at traffic stations. NO declined even further, ~63% at urban stations and ~78% at traffic stations in Europe. Reductions in PM2.5 and PM10 at urban stations were overall much smaller both in magnitude and relative change in Europe (~8%) than in Wuhan (~42%). The PM reductions due to limiting transportation and fuel combustion in institutional and commercial buildings were partly offset by increases of PM emissions from the activities at home in some of the cities. The NOx concentrations during the lockdown were on average 49% lower than those at weekends of the previous years in all cities. The lockdown effect on O3 production was ~10% higher than the weekend effect in Southern Europe and 38% higher in Wuhan, while for PM the lockdown had the same effect as weekends in Southern Europe (~6% of difference). This study highlights the challenge of reducing the formation of secondary pollutants such as O3 even with strict measures to control primary pollutant emissions. These results are relevant for designing abatement policies of urban pollution.

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 (Pnleaf) and ISO (ISOleaf) responses, plant-level ISO (ISOplant) responses to O3, N addition and moderate drought were more marked after long exposure (September) than short exposure duration (July). EO3 significantly decreased ISOleaf and Pnleaf, while WR and N50 significantly increased them. Although O3 and water interacted significantly to affect Pnleaf over the exposure duration, neither N50 nor WR mitigated the negative effects of EO3 on ISOleaf. When ISO was scaled up to the plant level, the WR-induced increase in ISOleaf under EO3 was offset by a reduction in total leaf area. By contrast, effects of EO3 on ISOplant 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.

Yield and economic losses in maize caused by ambient ozone in the North China Plain (2014-2017).

Maize is the second most important crop per harvested area in the world. The North China Plain (NCP) is a highly populated and relevant agricultural region in China, experiencing some of the highest ozone (O3) concentrations worldwide. It produces ~24% of the total maize production of China in years 2014-2017. For these years, we used observational O3 data in combination with geostatistic methods to estimate county-level production and economic losses due to O3 in the NCP. AOT40 (accumulated ozone exposure over an hourly threshold of 40 ppb) values during the maize growing season (90 days before maturity) progressively increased in the four consecutive years: 13.7 ppm h, 15.4 ppm h, 16.9 ppm h and 22.7 ppm h. Mean relative yield losses were 8.2% in 2014, 9.2% in 2015, 10.4% in 2016 and 13.4% in 2017. These yield losses, derived from exposure-response functions, resulted in crop production losses of 530.3 × 104 t, 617.8 × 104 t, 713.8 × 104 t, and 953.4 × 104 t, as well as economic losses of 2343 million USD, 2672 million USD, 1887 million USD, and 2404 million USD from 2014 to 2017. The NCP is a key area in China for monitoring the effectiveness of the clean air action policies aiming at reducing emissions of air pollutants. Despite these measures, O3 concentrations have increased in NCP, and reduction of this pollutant are challenging. We suggest an increase in the number of rural air quality stations for better characterizing O3 trends in cropland areas, as well as the application of different mitigation measures. They may involve more stringent air quality regulations and changes in crops, breeding tolerant cultivars and a crop management taking into account O3 pollution.

Assessment of O3-induced yield and economic losses for wheat in the North China Plain from 2014 to 2017, China.

Tropospheric ozone (O3) is a pollutant of widespread concern in the world and especially in China for its negative effects on agricultural crops. For the first time, yield and economic losses of wheat between 2014 and 2017 were estimated for the North China Plain (NCP) using observational hourly O3 data from 312 monitoring stations and exposure-response functions based on AOT40 index (accumulated hourly O3 concentration above 40 ppb) from a Chinese study. AOT40 values from 2014 to 2017 during the wheat growing seasons (75-days, 44 before and 30 after mid-anthesis) ranged from 3.1 to 14.9 ppm h, 4.9-17.5 ppm h, 7.3-17.6 ppm h, and 0.5-18.6 ppm h, respectively. The highest AOT40 values were observed in the Beijing-Tianjin-Hebei region. The values of relative yield losses from 2014 to 2017 were in the ranges of 6.4-30.5%, 10.0-35.8%, 14.9-34.1%, and 21.6-38.2%, respectively. The total wheat production losses in NCP for 2014-2017 accounted for 18.5%, 22.7%, 26.2% and 30.8% in the whole production, while the economic losses amounted to 6,292 million USD, 8,524 million USD, 10,068 million USD, and 12,404 million USD, respectively. The important impact of O3 in this area, which is of global importance, should be considered when assessing wheat yield production. Our results also show an increasing trend in AOT40, relative yield loss, total crop production loss and economic loss in the four consecutive years.

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.

A quantitative assessment of hormetic responses of plants to ozone.

Evaluations of ozone effects on vegetation across the globe over the last seven decades have mostly incorporated exposure levels that were multi-fold the preindustrial concentrations. As such, global risk assessments and derivation of critical levels for protecting plants and food supplies were based on extrapolation from high to low exposure levels. These were developed in an era when it was thought that stress biology is framed around a linear dose-response. However, it has recently emerged that stress biology commonly displays non-linear, hormetic processes. The current biological understanding highlights that the strategy of extrapolating from high to low exposure levels may lead to biased estimates. Here, we analyzed a diverse sample of published empirical data of approximately 500 stimulatory, hormetic-like dose-responses induced by ozone in plants. The median value of the maximum stimulatory responses induced by elevated ozone was 124%, and commonly <150%, of the background response (control), independently of species and response variable. The maximum stimulatory response to ozone was similar among types of response variables and major plant species. It was also similar among clades, between herbaceous and woody plants, between deciduous and evergreen trees, and between annual and perennial herbaceous plants. There were modest differences in the stimulatory response between genera and between families which may reflect different experimental designs and conditions among studies. The responses varied significantly upon type of exposure system, with open-top chambers (OTCs) underestimating the maximum stimulatory response compared to free-air ozone-concentration enrichment (FACE) systems. These findings suggest that plants show a generalized hormetic stimulation by ozone which is constrained within certain limits of biological plasticity, being highly generalizable, evolutionarily based, and maintained over ecological scales. They further highlight that non-linear responses should be taken into account when assessing the ozone effects on plants.

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 (fVPD) 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.

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 (Pn) 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 Pn, total biomass and isoprene emission. Relationships of light-saturated photosynthesis rates (Asat), 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.

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.

Intraspecific variation in sensitivity of winter wheat (Triticum aestivum L.) to ambient ozone in northern China as assessed by ethylenediurea (EDU).

Wheat is a major staple food and its sensitivity to the gas pollutant ozone (O3) depends on the cultivar. However, few chamber-less studies assessed current ambient O3 effects on a large number of wheat cultivars. In this study, we used ethylenediurea (EDU), an O3 protectant whose protection mechanisms are still unclear, to test photosynthetic pigments, gas exchange, antioxidants, and yield of 15 cultivars exposed to 17.4 ppm h AOT40 (accumulated O3 over an hourly concentration threshold of 40 ppb) over the growing season at Beijing suburb, China. EDU significantly increased light-saturated photosynthesis rate (Asat), photosynthetic pigments (i.e., chlorophyll and carotenoid), and total antioxidant capacity, while reduced malondialdehyde and reduced ascorbate contents. In comparison with EDU-treated plants (control), plants treated with water (no protection from ambient O3) significantly decreased yield, weight of 1000 grains, and harvest index by 20.3%, 15.1%, and 14.2%, respectively, across all cultivars. There was a significant interaction between EDU and cultivars in all tested variables with exception of Asat, chlorophyll, and carotenoid. The cultivar-specific sensitivity to O3 was ranked from highly sensitive (> 25% change) to less sensitive (< 10% change) by comparing the difference of the average grain yield of plants applied with and without EDU. Neither stomatal conductance nor antioxidant capacity contributed to the different response of the cultivars to EDU, suggesting that another mechanism contributes to the large variation in response to O3 among cultivars. Generally, the results indicate that present O3 concentration is threatening wheat production in Northern China, highlighting the urgent need for policy-making actions to protect this critical staple food.