Elevated ozone decreases the activity of Rubisco in poplar but not its activation under fluctuating light.

Increasing tropospheric ozone (O3) is well-known to decrease leaf photosynthesis under steady-state light through reductions in biochemical capacity. However, the effects of O3 on photosynthetic induction and its biochemical limitations in response to fluctuating light remain unclear, despite the rapid fluctuations of light intensity occurring under field conditions. In this study, two hybrid poplar clones with different O3 sensitivities were exposed to elevated O3. Dynamic photosynthetic CO2 response measurements were conducted to quantify the impact of elevated O3 and exposure duration on biochemical limitations during photosynthetic induction. We found that elevated O3 significantly reduced the steady-state light-saturated photosynthetic rate, the maximum rate of carboxylation (Vcmax) and Rubisco content. In addition, elevated O3 significantly decreased the time constants for slow phases and weighting of the fast phase of the Vcmax induction in poplar clone '546' but not in clone '107'. However, elevated O3 did not affect the time, it took to reach a given percentage of full Vcmax activation or photosynthetic induction in either clone. Overall, photosynthetic induction was primarily limited by the activity of Rubisco rather than the regeneration of ribulose-1,5-biphosphate regardless of O3 concentration and exposure duration. The lack of O3-induced effects on the activation of Rubisco observed here would simplify the simulation of impacts of O3 on nonsteady-state photosynthesis in dynamic photosynthetic models.