Effects of elevated ozone and carbon dioxide on the dynamic photosynthesis of Fagus crenata seedlings under variable light conditions.

Ozone (O3) is an air pollutant that is toxic to trees. O3 reduces steady-state net photosynthetic rate (A), and the adverse effects of O3 are mitigated under elevated CO2 condition. However, the combined effects of O3 and elevated CO2 on dynamic photosynthesis under variable light conditions have not yet been clarified. In this study, we investigated the effects of O3 and elevated CO2 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 O3 concentration (lower and two times higher than the ambient O3 concentration) and two levels of CO2 concentration (ambient and 700 ppm). Although O3 significantly decreased steady-state A under ambient CO2 concentrations, no significant decrease was observed under elevated CO2 concentrations, indicating the mitigating effect of elevated CO2 on O3-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 O3 and accelerated by elevated CO2. 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 O3 and elevated CO2 accelerated the reduction of A. In contrast to steady-state A, no mitigating effect of elevated CO2 was observed for any parameters related to dynamic photosynthesis. We conclude that the combined effects of O3 and elevated CO2 on A of F. crenata are different under steady-state and variable light conditions, and the O3-induced decrease in leaf A may not be mitigated by elevated CO2 in the field under variable light conditions.

Growth and photosynthetic responses to ozone of Siebold's beech seedlings grown under elevated CO2 and soil nitrogen supply.

Ozone (O3) 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 O3, elevated CO2, and soil nitrogen supply on Siebold's beech seedlings. Seedlings were grown under combinations of two levels of O3 (low and two times ambient O3 concentration), two levels of CO2 (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 CO2 ameliorated O3-induced reductions in photosynthetic activity, whereas the negative effects of O3 on photosynthetic traits were enhanced by soil nitrogen supply. We observed three-factor interactions in photosynthetic traits, with the ameliorative effects of elevated CO2 on O3-induced reductions in the maximum rate of carboxylation being more pronounced under high than under low soil nitrogen conditions in July. In contrast, elevated CO2-induced amelioration of the effects of O3 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. O3 caused a significant increase in S/R under ambient CO2 conditions, whereas no similar effects were observed under elevated CO2 conditions. Collectively, our findings reveal the complex interactive effects of elevated CO2 and soil nitrogen supply on the detrimental effects of O3 on leaf photosynthetic traits, and highlight the importance of taking into consideration differences between the responses of CO2 uptake and growth to these three environmental factors.

Metabolomics Analyses Reveal Metabolites Affected by Plant Growth-Promoting Endophytic Bacteria in Roots of the Halophyte Mesembryanthemum crystallinum.

Mesembryanthemum crystallinum L. (common ice plant) is an edible halophyte. However, if ice plants are used to phytoremediate salinity soil, there are problems of slow initial growth, and a long period before active NaCl uptake occurs under higher salinity conditions. Application of endophytic bacteria may improve the problem, but there remain gaps in our understanding of how endophytic bacteria affect the growth and the biochemical and physiological characteristics of ice plants. The aims of this study were to identify growth-promoting endophytic bacteria from the roots of ice plants and to document the metabolomic response of ice plants after application of selected endophytic bacteria. Two plant growth-promoting endophytic bacteria were selected on the basis of their ability to promote ice plant growth. The two strains putatively identified as Microbacterium spp. and Streptomyces spp. significantly promoted ice plant growth, at 2-times and 2.5-times, respectively, compared with the control and also affected the metabolome of ice plants. The strain of Microbacterium spp. resulted in increased contents of metabolites related to the tricarboxylic acid cycle and photosynthesis. The effects of salt stress were alleviated in ice plants inoculated with the endobacterial strains, compared with uninoculated plants. A deeper understanding of the complex interplay among plant metabolites will be useful for developing microbe-assisted soil phytoremediation strategies, using Mesembryanthemum species.

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 (Cfix). 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 Cfix was reduced in non- and low-fertilized treatments but was unaffected in high-fertilized treatment by O3 fumigation. Our plant growth model could simulate Cfix accurately (<10% error) by considering the impacts of O3 on plant leaf area and photosynthetic capacities, including maximum velocities of carboxylation and electron transport (Vcmax and Jmax, 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 Vcmax and Jmax, φ and θ, or leaf area, caused by 1.5-folds the ambient [O3] fumigation resulted in the following Cfix 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 Cfix 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.

Effects of ozone on the growth and yield of rice (Oryza sativa L.) under different nitrogen fertilization regimes.

To examine whether the sensitivity of growth and yield of rice (Oryza sativa L.) to ozone (O3) varies under different nitrogen (N) fertilization conditions, rice cultivar 'Koshihikari' was exposed to O3 under either standard N (SN) fertilization or no N (NN) fertilization. The rice plants were subjected to three gas treatments (charcoal-filtered air (CF) and O3 at 1.0 (1.0×O3) and 1.5 (1.5×O3) times the ambient concentration) in combination with two conditions of N fertilization. At five time points throughout the growth period, plant samples were collected to measure the leaf area and dry mass of each plant organ. At the final harvest, yield, yield components, and harvest index were measured. There was a significant interactive effect of O3 and N on leaf, stem, root, and whole-plant dry mass at the final harvest. The dry mass of each plant organ and the whole-plant dry mass of rice plants grown in 1.5×O3 were significantly lower than those in the plants grown in CF and 1.0×O3 under SN, whereas there were no significant differences in the dry mass among the three gas treatments under NN. Brown rice yield was significantly reduced by the exposure to O3 under SN, but not under NN. Relative yield loss rate based on the AOT40 (accumulated exposure over a threshold of 40 nmol mol-1) was pronounced under SN, whereas relative yield was almost unchanged at different AOT40 levels under NN. We concluded that the sensitivity of growth and yield of rice to O3 is dependent on N levels in the soil; the exposure to ambient levels of O3 has a negative effect on rice under SN, but not under NN.

Ozone changes the linear relationship between photosynthesis and stomatal conductance and decreases water use efficiency in rice.

Ozone is an important air pollutant that affects growth, transpiration, and water use efficiency (WUE) in plants. Integrated models of photosynthesis (An) and stomatal conductance (Gs) (An-Gs) are useful tools to consistently assess the impacts of ozone on plant growth, transpiration, and WUE. However, there is no information on how to incorporate the influence of ozone into An-Gs integrated models for crops. We focused on the Ball-Woodrow-Berry (BWB) relationship, which is a key equation in An-Gs integrated models, and aimed to address the following questions: (i) how does ozone change the BWB relationship for crops?; (ii) are there any difference in the changes in the BWB relationship among cultivars?, and (iii) how do the changes in the BWB relationship increase or decrease WUE for crops? We grew four rice cultivars in a field under ambient or Free-Air Concentration Enrichment (FACE) of ozone in China and measured An and Gs using a portable photosynthesis analyzer. We simulated WUE in individual leaves during the ripening period under different BWB relationships. The results showed that ozone significantly changed the BWB relationship only for the most sensitive cultivar, which showed an increase in the intercept of the BWB relationship under FACE conditions. These results imply that changes in the BWB relationship are related to the ozone sensitivity of the cultivar. Simulations of an An-Gs integrated model showed that increases in the intercept of the BWB relationship from 0.01 to 0.1 mol(H2O) m-2 s-1 indicated decreases in WUE by 22%. Since a reduction in WUE indicates increases in water demand per unit of crop growth, air pollution from ozone could be a critical issue in regions where agricultural water is limited, such as in rainfed paddy fields.

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 (Vcmax) and/or maximum electron transport rate (Jmax) estimated from response of net photosynthetic rate (A) to intercellular CO2 concentration (Ci) (A/Ci curve) assuming that mesophyll conductance for CO2 diffusion (gm) is infinite. Although it is known that Ci-based Vcmax and Jmax are potentially influenced by gm, its contribution to ozone responses in Ci-based Vcmax and Jmax is still unclear. In the present study, therefore, we analysed photosynthetic processes including gm 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 gm, and analysed A/Ci curve and A/Cc curve (Cc: chloroplast CO2 concentration). We also determined the Rubisco and chlorophyll contents in leaves. In September, ozone significantly decreased Ci-based Vcmax. At the same time, ozone decreased gm, whereas there was no significant effect of ozone on Cc-based Vcmax or the contents of Rubisco and chlorophyll in leaves. These results suggest that ozone-induced reduction in Ci-based Vcmax is a result of the decrease in gm rather than in carboxylation capacity. The decrease in gm by elevated ozone was offset by an increase in Ci, and Cc did not differ depending on ozone treatment. Since Cc-based Vcmax was also similar, A was not changed by elevated ozone. We conclude that gm is an important factor for reduction in Ci-based Vcmax of Siebold's beech under elevated ozone.

Photosynthetic responses to ozone of upper and lower canopy leaves of Fagus crenata Blume seedlings grown under different soil nutrient conditions.

We aimed to clarify the effects of ozone (O3) 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 (Vcmax) to cumulative stomatal O3 uptake (ΣFst) and reduction rate of Vcmax per unit ΣFst as an index of detoxification capacity for O3. 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 O3 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 O3-induced reduction in Vcmax in September. However, nutrient supply did not significantly increase ΣFst and reduce the detoxification capacity for O3. On the other hand, the degree of O3-induced reduction in Vcmax of upper canopy leaves was higher as compared with that of lower canopy leaves in August due to the higher ΣFst. However, the reduction rate of Vcmax per unit ΣFst in lower canopy leaves was higher than that in upper canopy leaves, indicating lower detoxification capacity for O3 in lower canopy leaves. Reduction rate of Vcmax per unit ΣFst 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 CO2 assimilation is likely to be associated with detoxification capacity for O3 in upper and lower canopy leaves of F. crenata seedlings grown under different soil nutrient conditions.

Modeling of stomatal conductance to estimate stomatal ozone uptake by Fagus crenata, Quercus serrata, Quercus mongolica var. crispula and Betula platyphylla.

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.

Modeling stomatal conductance and ozone uptake of Fagus crenata grown under different nitrogen loads.

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.