Photosynthetic performance of Quercus ilex L. under long-term ozone exposure probed by carboxylation efficiency, maximum apparent quantum yield and modulated 820 nm reflection

Assessing ozone (O3) effects on photosynthetic machinery is a key issue to understand how the oxidative stress influences vegetation functionality and the ecosystem services provided (Manes et al., 2012). Moreover, data about the RuBisco and photosystem functionality measured during long term experiments could be important to improve the accuracy of ozone flux models to be use for risks assessment (Fares et al., 2012). Trends towards tolerance to O3 have been studied for Mediterranean tree species and contrasting results were obtained over a large number of experiments, probably due to different levels of O3 exposure and growth conditions. Moreover, only few experiment assessed the O3 effects on evergreen species in terms of dynamic response of CO2 assimilation to varying conditions of temperature and photosynthetic photon flux density (Mereu et al., 2011) and, to the best of our knowledge, no data is available about fluorescence of photosystems I (PSI), that plays a key role in response to oxidative stress. In this experiment, we have assessed long-term effects of O3 exposure on Quercus ilex, an evergreen species with great ecological relevance in Mediterranean Basin for its wide natural distribution, as well as widespread use in urban context. Two-years-old seedlings were continuously exposed for 5 months in open-top chambers facility at the Centre for Research on Effects of Pollutants on Ecosystems, in Curno, to three O3 treatments: charcoal filtered air (F, - 41% of ambient air O3), and non-filtered air supplemented with + 35% (NF+) and +71% ppb O3 (NF++) of ambient air O3. Interestingly, it was found that after 40 days of starting experiment, an early response to O3 leads to an increase in leaf dark respiration and photorespiration. Fluorescence parameters were also affected, and in particular the PSI functionality was involved in the first phase of oxidative stress response.