Growth and photosynthetic response of Fagus crenata seedlings to ozone and/or elevated carbon dioxide

We investigated the effects of ozone (O-3) and/or elevated CO2 concentration ([CO2]) on growth and photosynthetic traits of Fagus crenata seedlings. Two-year-old seedlings were grown in four experimental treatments comprising two O-3 treatments (charcoal-filtered air and 100 nmol mol(-1) O-3; 6 h/day, 3 days/week) in combination with two CO2 treatments (350 and 700 mu mol mol(-1)) for 18 weeks in environmental control growth chambers. The four treatments were designated as control, elevated O-3, elevated CO2, and elevated CO2 + O-3. Dry matter growth of the seedlings was greater in elevated CO2 + O-3 than in elevated CO2. In elevated CO2 + O-3, a marked increase of second-flush leaves, considered a compensative response to O-3, was observed. The net photosynthetic rate of first-flush leaves in elevated CO2 + O-3 increased earlier and was maintained for a longer period of time than that in elevated CO2. Because emergence of second-flush leaves of F. crenata is greatly affected by the amount of assimilation products of first-flush leaves in current year, we consider that an early increase in the net photosynthetic rate of first-flush leaves contributed to the marked increase in second-flush leaf emergence under elevated CO2 + O-3. These results imply that we must account for changes in compensative capacity with respect to not only morphological traits but also phenological traits and physiological functions such as photosynthesis when evaluating effects of O-3 on F. crenata under elevated [CO2].