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Tropospheric ozone (O 3 ) is the main phytotoxic air pollutant threatening food security, while ethylenediurea (EDU) can effectively mitigate O 3 -induced crop yield loss. EDU's mode of action, however, remains unclear, and the underlying physiological mechanisms of mitigating O 3 -induced crop yield loss are poorly understood. We cultivated hybrid rice seedlings under two O 3 treatments (NF, nonfiltered ambient air; and NF60, ambient air plus 60 ppb O 3 ) and sprayed foliage with 0 or 450 ppm EDU every ten days and determine photosynthesis-related traits, biomass indicators, and yield components. We found that EDU significantly increased the leaf nitrogen (N) allocation to photosynthesis (N P ) and the grain N accumulation, while the grain N accumulation was positively correlated with N P and root biomass. EDU significantly increased the rice yield mainly by increasing the individual grain weight rather than the number of panicles and grains. While EDU protected from yield loss, the degree of protection was only 31% under NF60 treatment, thus EDU was unable to offer complete protection under high O 3 pollution. These results will be conducive to a better understanding of the EDU protection mechanism and better application of EDU under high O 3 pollution in the future., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

The sensitivity of photosynthesis to temperature has been identified as a key uncertainty for projecting the magnitude of the terrestrial carbon cycle response to future climate change. Although thermal acclimation of photosynthesis under rising temperature has been reported in many tree species, whether tropospheric ozone (O 3 ) affects the acclimation capacity remains unknown. In this study, temperature responses of photosynthesis (light-saturated rate of photosynthesis (A sat ), maximum rates of RuBP carboxylation (V cmax ), and electron transport (J max ) and dark respiration (R dark ) of Populus tremula exposed to ambient O 3 (AO 3 , maximum of 30 ppb) or elevated O 3 (EO 3 , maximum of 110 ppb) and ambient or elevated temperature (ambient +5°C) were investigated in solardomes. We found that the optimum temperature of A sat (T optA ) significantly increased in response to warming. However, the thermal acclimation capacity was reduced by O 3 exposure, as indicated by decreased T optA , and temperature optima of V cmax (T optV ) and J max (T optJ ) under EO 3 . Changes in both stomatal conductance (g s ) and photosynthetic capacity (V cmax and J max ) contributed to the shift of T optA by warming and EO 3 . Neither R dark measured at 25°C ( R dark 25 ) nor the temperature response of R dark was affected by warming, EO 3 , or their combination. The responses of A sat , V cmax , and J max to warming and EO 3 were closely correlated with changes in leaf nitrogen (N) content and N use efficiency. Overall, warming stimulated growth (leaf biomass and tree height), whereas EO 3 reduced growth (leaf and woody biomass). The findings indicate that thermal acclimation of A sat may be overestimated if the impact of O 3 pollution is not taken into account., (© 2021 John Wiley & Sons Ltd.)

Ascorbate in leaf apoplast (ASC apo ) reacts with ozone (O 3 ) and thereby reduces O 3 flux reaching plasmalemma (F pl ). Some studies have shown significant protection of cells from O 3 by ASC apo , while others have questioned its efficacy. Hypothesizing that the protection by ASC apo depends on other variables, we quantified determinants of O 3 detoxification with a model of O 3 transport and reaction in apoplast. The model determines ascorbic acid concentration in apoplast (AA apo ) using measured values of O 3 concentration (c o ), leaf tissue ascorbic acid concentration (AA leaf ), cell wall thickness (L 3 ), apoplastic pH (pH apo ), and stomatal conductance (G sw ). We compared the measured and model-estimated AA apo in leaves of peach (Prunus persica) grown in open-top chambers under non-filtered air (NF) and elevated (EO 3 : NF + 80 ppb) O 3 concentrations. The estimated AA apo in individual leaves agreed well with the measured values (R 2  = .91). Analyses of the simulation results yielded the following findings: (a) The efficacy of O 3 reduction with ASC apo as quantified by fractional reduction (ϕ 3 ) of O 3 flux at the surface of plasmalemma (F pl ) was lowered from 70% in NF to 40% in EO 3 due to the reduction of L 3 . The EO 3 reduced AA apo , but the lower G sw and L 3 in EO 3 increased AA apo resulting in no significant change in AA apo due to EO 3 . ϕ 3 can be calculated with measured values of AA apo and L 3 , and F pl can be estimated with the measurement-based ϕ 3 . (b) When c 0 is increased, F pl increased curvilinearly with the increase of F st : nominal O 3 flux via stomatal diffusion, exhibiting apparent threshold on F st . The deviation of F pl from F st became greater when L 3 , pH apo , and AA leaf were increased. The quantification of ϕ 3 and F pl using leaf traits shall facilitate the understanding of the mechanisms of differential plant sensitivity to O 3 and improve quantification of the O 3 impacts on plants., (© 2020 John Wiley & Sons Ltd.)

To investigate whether nitrogen (N) load affects the ozone (O 3 ) stomatal flux-effect relationship for birch biomass, three-year old birch saplings were exposed to seven different O 3 profiles (24 h mean of 35-66 ppb) and four different N loads (10, 30, 50 and 70 kg ha -1  yr -1 ) in precision-controlled hemispherical glasshouses (solardomes) in 2012 and 2013. Stomatal conductance (g s ) under optimal growth conditions was stimulated by enhanced N supply but was not significantly affected by enhanced O 3 exposure. Birch root, woody (stem + branches) and total biomass (root + woody) were not affected by the Phytotoxic Ozone Dose (POD 1 SPEC) after two seasons of O 3 exposure, and enhanced N supply stimulated biomass production independent of POD 1 SPEC (i.e. there were no POD 1 SPEC × N interactions). There was a strong linear relationship between the stem cross-sectional area and tree biomass at the end of the experiment, which was not affected by O 3 exposure or N load. Enhanced N supply stimulated the stem cross-sectional area at the end of season 2, but not at the end of season 1, which suggests a time lag before tree biomass responded to enhanced N supply. There was no significant effect of POD 1 SPEC on stem cross-sectional area after either the first or second growing season of the experiment. Contrasting results reported in the literature on the interactive impacts of O 3 and N load on tree physiology and growth are likely due to species-specific responses, different duration of the experiments and/or a limitation of the number of O 3 and N levels tested., (Copyright © 2019 Elsevier B.V. All rights reserved.)

We investigated the effects of elevated ozone (O 3 ) 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 O 3 , and the allocation of N to different leaf components: photosynthetic apparatus, cell walls, and others under elevated O 3 level. Our results showed that elevated O 3 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 O 3 , but the more sensitive clone showed greater reduction of N area and LMA due to O 3 . We also found positive relationships between N area and photosynthetic parameters, e.g. light-saturated photosynthetic rate (A sat ). Furthermore, elevated O 3 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 O 3 damages at the expense of photosynthetic N. The change of N allocation in plant leaves in response to elevated O 3 could have an impact on ecological processes, e.g. leaf litter decomposition., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Finite mesophyll conductance (g m ) reduces the rate of CO 2 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 (O 3 ) affects g m . We quantified the relative importance of three different types of limitations of photosynthesis in poplar trees exposed to elevated O 3 : decreases in stomatal conductance, g m and biochemical photosynthetic capacity. The O 3 -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 O 3 on stomatal conductance. Of the O 3 -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 O 3 exposure. Our findings suggest that declines in g m may play a key role in limiting photosynthesis of plants exposed to elevated O 3 , an effect hitherto overlooked., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Apoplastic ascorbate (ASC apo ) is an important contributor to the detoxification of ozone (O 3 ). The objective of the study is to explore whether ASC apo is stimulated by elevated O 3 concentrations. The detoxification of O 3 by ASC apo was quantified in tobacco (Nicotiana L), soybean (Glycine max (L.) Merr.) and poplar (Populus L), which were exposed to charcoal-filtered air (CF) and elevated O 3 treatments (E-O 3 ). ASC apo in the three species were significantly increased by E-O 3 compared with the values in the filtered treatment. For all three species, E-O 3 significantly increased the malondialdehyde (MDA) content and decreased light-saturated rate of photosynthesis (A sat ), suggesting that high O 3 has induced injury/damage to plants. E-O 3 significantly increased redox state in the apoplast (redox state apo ) for all species, whereas no effect on the apoplastic dehydroascorbate (DHA apo ) was observed. In leaf tissues, E-O 3 significantly enhanced reduced-ascorbate (ASC) and total ascorbate (ASC+DHA) in soybean and poplar, but significantly reduced these in tobacco, indicating different antioxidative capacity to the high O 3 levels among the three species. Total antioxidant capacity in the apoplast (TAC apo ) was significantly increased by E-O 3 in tobacco and poplar, but leaf tissue TAC was significantly enhanced only in tobacco. Leaf tissue superoxide anion (O 2 •- ) in poplar and hydrogen peroxide (H 2 O 2 ) in tobacco and soybean were significantly increased by E-O 3 . The diurnal variation of ASC apo , with maximum values occurring in the late morning and lower values experienced in the afternoon, appeared to play an important role in the harmful effects of O 3 on tobacco, soybean and poplar., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Wheat is a major staple food and its sensitivity to the gas pollutant ozone (O 3 ) depends on the cultivar. However, few chamber-less studies assessed current ambient O 3 effects on a large number of wheat cultivars. In this study, we used ethylenediurea (EDU), an O 3 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 O 3 over an hourly concentration threshold of 40 ppb) over the growing season at Beijing suburb, China. EDU significantly increased light-saturated photosynthesis rate (A sat ), 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 O 3 ) 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 A sat , chlorophyll, and carotenoid. The cultivar-specific sensitivity to O 3 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 O 3 among cultivars. Generally, the results indicate that present O 3 concentration is threatening wheat production in Northern China, highlighting the urgent need for policy-making actions to protect this critical staple food.

The sensitivity of Chinese soybean cultivars to ambient ozone (O 3 ) in the field is unknown, although soybean is a major staple food in China. Using ethylenediurea (EDU) as an O 3 protectant, we tested the gas exchange, pigments, antioxidants and biomass of 19 cultivars exposed to 28ppm·hr AOT40 (accumulated O 3 over an hourly concentration threshold of 40ppb) over the growing season at a field site in China. By comparing the average biomass with and without EDU, we estimated the cultivar-specific sensitivity to O 3 and ranked the cultivars from very tolerant (<10% change) to highly sensitive (>45% change), which helps in choosing the best-suited cultivars for local cultivation. Higher lipid peroxidation and activity of the ascorbate peroxidase enzyme were major responses to O 3 damage, which eventually translated into lower biomass production. The constitutional level of total ascorbate in the leaves was the most important parameter explaining O 3 sensitivity among these cultivars. Surprisingly, the role of stomatal conductance was insignificant. These results will guide future breeding efforts towards more O 3 -tolerant cultivars in China, while strategies for implementing control measures of regional O 3 pollution are being implemented. Overall, these results suggest that present ambient O 3 pollution is a serious concern for soybean in China, which highlights the urgent need for policy-making actions to protect this critical staple food., (Copyright © 2017. Published by Elsevier B.V.)

To evaluate the ozone (O 3 ) sensitivity among peach tree (Prunus persica) cultivars widely planted in Beijing region and explore the possible eco-physiological response mechanisms, thirteen cultivars of peach seedlings were exposed to either charcoal-filtered air or elevated O 3 (E-O 3 , non-filtered ambient air plus 60 ppb) for one growing season in open-top chambers. Leaf structure, stomatal structure, gas exchange and chlorophyll a fluorescence, photosynthetic pigments, antioxidant defense system and lipid peroxidation were measured in three replicated chambers. Results showed that E-O 3 significantly reduced abaxial epidemis thickness, but no effects on the thicknesses of adaxial epidemis, palisade parenchyma and spongy parenchyma. Stomatal area, density and conductance were not significantly affected by E-O 3 . E-O 3 significantly accelerated leaf senescence, as indicated by increased lipid peroxidation and more declines in light-saturated photosynthetic rate and pigments contents. The reduced ascorbate content (ASC) was decreased but antioxidant enzyme activity (CAT, APX and SOD) and total antioxidant capacity (TAC) were significantly increased by E-O 3 among cultivars. The cultivars with visible symptoms also had more reductions in net photosynthetic rate than those without visible symptoms. Ozone sensitivity among cultivars was strongly linked to leaf mass per area (LMA), antioxidant enzymes activity e.g. SOD, APX rather than stomatal parameters (stomatal area, density and conductance) and ASC. Results could provide a theoretical basis for selecting and breeding the ozone-resistant cultivars of peach trees grown in high O 3 -polluted regions., (Copyright © 2017 Elsevier Ltd. All rights reserved.)