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.