Early and late adjustments of the photosynthetic traits and stomatal density in Q uercus ilex L . grown in an ozone‐enriched environment

Quercus ilex L. seedlings were exposed in open‐top chambers for one growing season to three levels of ozone (O 3 ): charcoal filtered air, non‐filtered air supplemented with +30% or +74% ambient air O 3 . Key functional parameters related to photosynthetic performance and stomatal density were measured to evaluate the response mechanisms of Q. ilex to chronic O 3 exposure, clarifying how ecophysiological traits are modulated during the season in an ozone‐enriched environment. Dark respiration showed an early response to O 3 exposure, increasing approximately 45% relative to charcoal‐filtered air in both O 3 enriched treatments. However, at the end of the growing season, maximum rate of assimilation (A max ) and stomatal conductance (g s ) showed a decline (−13% and −36%, for A max and g s , respectively) only in plants under higher O 3 levels. Photosystem I functionality supported the capacity of Q. ilex to cope with oxidative stress by adjusting the energy flow partitioning inside the photosystems. The response to O 3 was also characterised by increased stomatal density in both O 3 enriched treatments relative to controls. Our results suggest that in order to improve the reliability of metrics for O 3 risk assessment, the seasonal changes in the response of g s and photosynthetic machinery to O 3 stress should be considered.