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