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

Increasing tropospheric ozone (O-3) is well-known to decrease leaf photosynthesis under steady-state light through reductions in biochemical capacity. However, the effects of O-3 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 O-3 sensitivities were exposed to elevated O-3. Dynamic photosynthetic CO2 response measurements were conducted to quantify the impact of elevated O-3 and exposure duration on biochemical limitations during photosynthetic induction. We found that elevated O-3 significantly reduced the steady-state light-saturated photosynthetic rate, the maximum rate of carboxylation (V-cmax) and Rubisco content. In addition, elevated O-3 significantly decreased the time constants for slow phases and weighting of the fast phase of the V-cmax induction in poplar clone '546' but not in clone '107'. However, elevated O-3 did not affect the time, it took to reach a given percentage of full V-cmax 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 O-3 concentration and exposure duration. The lack of O-3-induced effects on the activation of Rubisco observed here would simplify the simulation of impacts of O-3 on nonsteady-state photosynthesis in dynamic photosynthetic models.

Automated summary

Elevated O-3 concentration significantly reduces photosynthetic rate, maximum rate of carboxylation, and Rubisco content in hybrid poplar clones. However, it does not affect the time to reach full photosynthetic activation, indicating that Rubisco activity primarily limits photosynthetic induction.