Estimates of potential ozone stomatal uptake in mature trees of Quercus ilex in a Mediterranean climate

This paper is focused on evaluation of the potential ozone stomatal fluxes (FO3) under seasonally varying microclimatic conditions at two levels of the canopy of an evergreen Mediterranean plant species, Holm oak (Quercus ilex L.), and on comparing them with ozone hourly averages, in order to assess what are the main environmental/physiological variables that most affect the definition of critical levels for Mediterranean vegetation. Microclimatic factors such as radiation, temperature and wind velocity greatly affected gas exchange rates and stomatal conductance to water vapour measured at different heights. O3 concentration values highlight a daily cycle with higher nocturnal O3 concentrations above the canopy than below it. Similar O3 trends have been observed by using passive diffusive samplers. As a consequence, potential O3 stomatal fluxes calculated for the upper layer of the canopy are higher than those below the canopy. Moreover, O3 concentration values show an opposite seasonal trend compared to FO3. These opposite trends are clearly due to the stomatal closure in drier months, as a protection against excessive water losses that yield low FO3 values during high O3 concentration. This paper highlights the different contribution of different Holm oak canopy portions to overall O3 uptake impact, attributing important roles to microclimatic conditions and to physiological activity related to stomata opening, which in turn is affected by internal and external effectors (hormones, water availability, hydraulic conductance, etc.). We cannot exclude an O3 effect on stomatal cell guards and on the carbon assimilation process working in the mesophyll cells. Further research needs to be considered to evaluate more clearly the risk of O3 on Mediterranean vegetation taking into consideration microclimatic conditions, plant physiology and possible plant canopy defensive reactions to O3, so as to define an air quality standard to protect the vegetation.