Early and late adjustments of the photosynthetic traits and stomatal density in Quercus 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 gs, 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 gs and photosynthetic machinery to O-3 stress should be considered.