Effects of ozone on stomatal ozone uptake in leaves of Fagus crenata seedlings grown under different CO₂ concentrations

Ozone is an atmospheric pollutant that adversely affects woody species, causing impaired stomatal control known as stomatal sluggishness. Stomatal sluggishness may affect stomatal ozone uptake because ozone enters the leaf through stomata. On the other hand, the assumed future elevated CO2 may mitigate the effects of ozone on stomata. In this study, the effects of ozone-induced stomatal sluggishness on stomatal ozone uptake in leaves of Fagus crenata seedlings were investigated under different CO2 concentrations. Seedlings of F. crenata were grown under the combinations of two levels of ozone (low and two times ambient ozone concentration) and two levels of CO2 (ambient and 700 ppm) for three growing seasons (2018-2020). In July of the third growing season, we determined leaf gas exchange traits and estimated stomatal ozone uptake using coupled photosynthesis and stomatal conductance (g(s)) models. Ozone significantly increased the g(s) while there was no significant change in photosynthetic capacities, indicating stomatal sluggishness. Ozone also altered the relationship between g(s) and net photosynthetic rate, with a significant increase in the slope of the Ball-Berry-Leuning g(s) model. Sensitivity analysis of stomatal ozone uptake revealed that ozone-induced stomatal sluggishness enhances cumulative ozone uptake. Although elevated CO2 induced stomatal closure and decreased stomatal ozone uptake by 30-40%, there was no mitigative effects of elevated CO2 on ozone-induced stomatal sluggishness. Our results suggest the importance of integrating ozone-induced change in stomatal control into model for estimating stomatal ozone uptake and the consequent effects of ozone on woody species.