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Ozone (O 3 ) is an air pollutant that is toxic to trees. O 3 reduces steady-state net photosynthetic rate (A), and the adverse effects of O 3 are mitigated under elevated CO 2 condition. However, the combined effects of O 3 and elevated CO 2 on dynamic photosynthesis under variable light conditions have not yet been clarified. In this study, we investigated the effects of O 3 and elevated CO 2 on dynamic photosynthesis in the leaves of Fagus crenata seedlings under variable light conditions. The seedlings were grown under four gas treatments comprising two levels of O 3 concentration (lower and two times higher than the ambient O 3 concentration) and two levels of CO 2 concentration (ambient and 700 ppm). Although O 3 significantly decreased steady-state A under ambient CO 2 concentrations, no significant decrease was observed under elevated CO 2 concentrations, indicating the mitigating effect of elevated CO 2 on O 3 -induced adverse effects on steady-state A. During photosynthetic induction, the response of A to the change in photosynthetic photon flux density (PPFD) from 50 (low light) to 1000 μmol m -2  s -1 (high light) showed that the increase in A was slowed by O 3 and accelerated by elevated CO 2 . Under fluctuating light condition of repeating low light for 4 min and high light for 1 min, A at end of each high light period gradually decreased in all treatments, and O 3 and elevated CO 2 accelerated the reduction of A. In contrast to steady-state A, no mitigating effect of elevated CO 2 was observed for any parameters related to dynamic photosynthesis. We conclude that the combined effects of O 3 and elevated CO 2 on A of F. crenata are different under steady-state and variable light conditions, and the O 3 -induced decrease in leaf A may not be mitigated by elevated CO 2 in the field under variable light conditions., 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 © 2023 Elsevier B.V. All rights reserved.)

Ozone (O 3 ) is a phytotoxic air pollutant, the adverse effects of which on growth and photosynthesis are modified by other environmental factors. In this study, we examined the combined effects of O 3 , elevated CO 2 , and soil nitrogen supply on Siebold's beech seedlings. Seedlings were grown under combinations of two levels of O 3 (low and two times ambient O 3 concentration), two levels of CO 2 (ambient and 700 ppm), and three levels of soil nitrogen supply (0, 50, and 100 kg N ha -1 year -1 ) during two growing seasons (2019 and 2020), with leaf photosynthetic traits being determined during the second season. We found that elevated CO 2 ameliorated O 3 -induced reductions in photosynthetic activity, whereas the negative effects of O 3 on photosynthetic traits were enhanced by soil nitrogen supply. We observed three-factor interactions in photosynthetic traits, with the ameliorative effects of elevated CO 2 on O 3 -induced reductions in the maximum rate of carboxylation being more pronounced under high than under low soil nitrogen conditions in July. In contrast, elevated CO 2 -induced amelioration of the effects of O 3 on stomatal function-related traits was more pronounced under low soil nitrogen conditions. Although we observed several two- or three-factor interactions of gas and soil treatments with respect to leaf photosynthetic traits, the shoot to root dry mass (S/R) ratio was the only parameter for which a significant interaction was detected among seedling growth parameters. O 3 caused a significant increase in S/R under ambient CO 2 conditions, whereas no similar effects were observed under elevated CO 2 conditions. Collectively, our findings reveal the complex interactive effects of elevated CO 2 and soil nitrogen supply on the detrimental effects of O 3 on leaf photosynthetic traits, and highlight the importance of taking into consideration differences between the responses of CO 2 uptake and growth to these three environmental factors., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Ozone (O 3 ) 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 O 3 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 O 3 -induced change in the whole-plant carbon fixation amount (C fix ). Fagus crenata seedlings were exposed to three O 3 levels (charcoal-filtered air or 1.0- or 1.5-folds ambient [O 3 ]) 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 O 3 fumigation. Our plant growth model could simulate C fix accurately (<10% error) by considering the impacts of O 3 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 [O 3 ] 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 O 3 on C fix of F. crenata seedlings grown under various fertilization levels. Further, the impacts of O 3 and soil nutrient on these photosynthetic capacities and plant leaf area should be considered to predict O 3 -induced changes in carbon fixation by forest tree species using the process-based plant growth model., 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 © 2020 Elsevier B.V. All rights reserved.)

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 CO 2 concentration (C i ) (A/C i curve) assuming that mesophyll conductance for CO 2 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 CO 2 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.

We aimed to clarify the effects of ozone (O 3 ) on photosynthetic ability of upper and lower canopy leaves of Fagus crenata Blume seedlings grown under different soil nutrient conditions. To accomplish this objective, we analyzed the response of photosynthetic parameters such as maximum carboxylation rate (V cmax ) to cumulative stomatal O 3 uptake (ΣF st ) and reduction rate of V cmax per unit ΣF st as an index of detoxification capacity for O 3 . The seedlings of Fagus crenata were grown for two growing seasons (2014-2015) in nine treatments comprised of a combination of three levels of gas treatments (charcoal-filtered air or 1.0- or 1.5-times ambient O 3 concentration) and three levels of soil nutrient treatments (non-fertilized or a supply of relatively low or high concentrations of compound fertilizer). The nutrient supply significantly increased the degree of O 3 -induced reduction in V cmax in September. However, nutrient supply did not significantly increase ΣF st and reduce the detoxification capacity for O 3 . On the other hand, the degree of O 3 -induced reduction in V cmax of upper canopy leaves was higher as compared with that of lower canopy leaves in August due to the higher ΣF st . However, the reduction rate of V cmax per unit ΣF st in lower canopy leaves was higher than that in upper canopy leaves, indicating lower detoxification capacity for O 3 in lower canopy leaves. Reduction rate of V cmax per unit ΣF st over the threshold, which is assumed to be proportional to gross photosynthetic rate, was similar between upper and lower canopy leaves. Therefore, capacity of photosynthetic CO 2 assimilation is likely to be associated with detoxification capacity for O 3 in upper and lower canopy leaves of F. crenata seedlings grown under different soil nutrient conditions., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

To construct stomatal conductance models and estimate stomatal O3 uptake for Fagus crenata, Quercus serrata, Quercus mongolica var. crispula and Betula platyphylla, stomatal conductance (gs) was measured in seedlings of the four tree species. Better estimates of gs were made by incorporating the acute effects of O3 on gs into the models and the models could explain 34-52% of the variability in gs. Although the O3 concentration was relatively high in spring from April to May, COU of F. crenata, Q. serrata and Q. mongolica var. crispula were relatively low and the ratios of COU in spring to total COU in one year were 16.8% in all tree species because of low gs limited mainly by leaf pre-maturation and/or low temperature. The COU of B. platyphylla were relatively high mainly because of rapid leaf maturation and lower optimal temperature for stomatal opening., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

A multiplicative stomatal conductance model was constructed to estimate stomatal O3 uptake of Fagus crenata exposed to O3 under different N loads to the soil. Our stomatal conductance model included environmental functions such as the stomatal responses of F. crenata to diurnal changes, chronic O3 stress (AOT0), acute O3 stress (O3 concentration), and nitrogen load to soil. The model could explain 62% of the variability in stomatal conductance. We suggest therefore that stomatal closure induced by O3 and N load-induced soil acidification must be taken into account in developing a stomatal conductance model for estimating stomatal O3 uptake for future risk assessment of O3 impact on Japanese forest tree species such as F. crenata., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Proanthocyanidins (PAs) or condensed tannins are a major group of phenolic compounds in the leaves of birch trees and many other woody and herbaceous plants. These compounds constitute a significant allocation of carbon in leaves and are involved in plant responses to environmental stress factors, such as pathogens or herbivores. In some plants, PA concentrations are affected by atmospheric carbon dioxide (CO(2)) and ozone (O(3)) levels that may influence, for example, species fitness, community structure, or ecosystem nutrient cycling. Therefore, a study on the quantitative response of PAs to elevated concentrations of carbon dioxide (CO(2)) and ozone (O(3)) was undertaken in seedlings of Betula platyphylla, Betula ermanii, and Fagus crenata. Seedlings were exposed to ambient or elevated O(3) and CO(2) levels during two growing seasons in the Kanto district in Japan. Ten open-top chambers were used for five different treatments with two replicates: filtered air (FA), ambient air (ambient O(3), 43 ppb; ambient CO(2), 377 ppm), elevated O(3) (1.5 x ambient O(3), 66 ppb), elevated CO(2) (1.5 x ambient CO(2), 544 ppm), and elevated O(3) and CO(2) combined. In addition, seedlings growing in natural conditions outside of chambers were studied. Leaf samples were analyzed for total PA concentrations by butanol-HCl assay and for polymeric PA concentrations by normal-phase high-performance liquid chromatography. Total PA concentrations in leaves of all species were similarly affected by different treatments. They were significantly higher in seedlings treated with elevated CO(2) and O(3) combined, and in seedlings growing outside chambers compared with the FA controls. F. crenata contained only traces of polymeric PAs, but significant species x treatment interaction was observed in the polymeric PA concentrations in B. ermanii and B. platyphylla. In B. platyphylla, leaves treated with elevated CO(2) + O(3) differed significantly from all other treatments. It was suggested that the strongest effect of elevated CO(2) and O(3) combined on leaf PA contents resulted from the additive effect of these environmental factors on phenolic biosynthesis.