Effects of elevated ozone and carbon dioxide on the dynamic photosynthesis of Fagus crenata seedlings under variable light conditions

Ozone (O3) is an air pollutant that is toxic to trees. O3 reduces steady-state net photosynthetic rate (A), and the adverse effects of O3 are mitigated under elevated CO2 condition. However, the combined effects of O3 and elevated CO2 on dynamic photosynthesis under variable light conditions have not yet been clarified. In this study, we investigated the effects of O3 and elevated CO2 on dynamic photosynthesis in the leaves of Fagus crenata seedlings under variable light conditions. The seedlings were grown under four gas treatments comprising two levels of O3 concentration (lower and two times higher than the ambient O3 concentration) and two levels of CO2 concentration (ambient and 700 ppm). Although O3 significantly decreased steady-state A under ambient CO2 concentrations, no significant decrease was ob-served under elevated CO2 concentrations, indicating the mitigating effect of elevated CO2 on O3-induced adverse ef-fects on steady-state A. During photosynthetic induction, the response of A to the change in photosynthetic photon flux density (PPFD) from 50 (low light) to 1000 & mu;mol m-2 s-1 (high light) showed that the increase in A was slowed by O3 and accelerated by elevated CO2. Under fluctuating light condition of repeating low light for 4 min and high light for 1 min, A at end of each high light period gradually decreased in all treatments, and O3 and elevated CO2 accelerated the reduction of A. In contrast to steady-state A, no mitigating effect of elevated CO2 was observed for any parameters related to dynamic photosynthesis. We conclude that the combined effects of O3 and elevated CO2 on A of F. crenata are different under steady-state and variable light conditions, and the O3-induced decrease in leaf A may not be mitigated by elevated CO2 in the field under variable light conditions.