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

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 observed under elevated CO₂ concentrations, indicating the mitigating effect of elevated CO₂ on O₃-induced adverse effects 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 μmol 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.