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