PSII photochemistry is the primary target of oxidative stress imposed by ozone in Tilia americana

Trees are essential in the urban environment not only because of their aesthetic and social values, but also for their effects on air quality. Data of the present experiment show some of the integrated mechanisms that may confer sensitivity/tolerance in Tilia americana saplings exposed to ozone treatment (120 ppb, 5 h day−1 for 45 consecutive days) in order to improve the management of green spaces responding to oxidative stress. At the end of exposure, plants do not exhibit any foliar symptoms. Profiles related to membrane injury, photosynthetic gas exchange, chlorophyll a fluorescence, pigment content and enzymes/metabolites linked to the synthesis of phenylpropanoids show a vulnerability in terms of: (i) change in the ionic and solute reactions of the membrane cells (maximum value of +34%, 28 days after the beginning of fumigation, compared to controls); (ii) inhibition of the carbon assimilation (−41%), mostly as a consequence of non-stomatal limitation of assimilation rate; (iii) progressive inhibition of the Calvin cycle, as demonstrated by decreases in Rubisco carboxylation efficiency and regeneration capacity (−35 and −21%, respectively, at the end of exposure), quantum yield of electron transfer at PSII and in the fraction of energy passively dissipated as heat and fluorescence (−34% and −31%); (iv) senescence process (decline in demand for reducing power and energy at the end of treatment); (v) damage to the chlorophyll pigment system. However, the activation of xanthophylls cycle and phenylpropanoid metabolism, that can be considered a strategy in plants in order to regulate light absorbed energy and to scavenge reactive oxygen species does not preserve PSII photochemistry from impairment at the end of exposure.