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.
Ozone O 3 is an air pollutant that negatively affect carbon budget in woody plants. In the present study, we aimed to clarify the effects of ozone on soil respiration rate of Siebold's beech seedlings Fagus crenata grown under different soil-nutrient conditions. Seedlings were grown under three levels of O 3 fumigation charcoal-filtered air or O 3 at 1.0 or 1.5 times ambient concentration in combination with three levels of nutrient supplies non-, low-or high-fertilised for two growing seasons. We determined soil respiration rate in July, August, September, and October of the second growing season. The seedlings were harvested to determine the dry mass in October. Significant effect of O 3 on soil respiration rate was not observed in all measurements. There was a significant interaction between O 3 and nutrient supply for whole-root dry mass. The dry mass in non-fertilised and low-fertilised treatments was reduced by O 3 , whereas O 3 did not affect dry mass in the high-fertilised treatment. On the other hand, neither significant effects of O 3 , nor a significant interaction between O 3 and nutrient supply for the biomass allocations were observed. Coefficient of positive correlation in the relation of soil respiration rate with dry mass of fine-root across the all treatments was higher than that in the relation of soil respiration rate with coarse-root and whole-root dry mass. These results indicate that no significant effect of O 3 on soil respiration was mainly attributable to no response of fine root dry mass to elevated O 3 . Soil nutrient supply decreased soil respiration rate in August. Our results emphasize the importance of fine root in the response of soil respiration to elevated O 3 . To clarify the response of soil respiration to elevated O 3 , future researches on the effect of O 3 on fine root dynamics including turnover and indirect effect on soil microbial respiration are needed.
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