Ecophysiology of Tilia americana under ozone stress

Ozone (O3) is an environmental emergency, for its impact on biological systems and non-living materials. In recent decades, the current annual average of O3 levels ranges from 20 to 45 ppb across the globe, which is roughly double the concentration that preceded the Industrial Revolution. Future mixing ratios are expected to raise to even more dangerous levels. Under specific conditions, such as in urban environment, plant systems are affected by this pollutant gas and it is important to understand (i) how these organisms respond to this stress factor, and (ii) which species should be chosen for planting in urban areas, especially in the Mediterranean basin. Generally speaking, the sensitivity/resistance to O3 of trees has only received little attention, so our knowledge of the mechanisms underlying the capacity of these plants to tolerate or avoid oxidative stress is very fragmentary. This study reports on the effects at the physiological and biochemical level of a chronic realistic exposure to O3 (120 ppb of O3 for 5 h days-1 for 45 consecutive days) in a controlled environment facility, in order to assess the variations in the magnitude of responses of the photosynthetic apparatus of Tilia americana saplings. At the end of the exposure, no foliar symptoms were evident, but the pollutant induced relevant biological effects on treated plants in comparison with control individuals: (i) a significant (-41%) reduction in the photosynthetic capacity, starting from 28 days after the beginning of exposure, due to a partial stomatal closure (-28%); (ii) an alteration of non-stomatal photosynthetic parameters derived from CO2 response curve of photosynthetic activity after prolonged exposure (the maximum carboxylation rate of Rubisco and the light-saturated rate of electron transport values became lower than that observed in control plants: -35 and -21%, respectively), (iii) an impairment of the efficiency of photosystem II (PSII), attributable to an increase in minimal fluorescence associated to maximal fluorescence similar to controls that induces a decrease of the Fv/Fm ratio that reached a mean value of 0.75; (iv) a damage to the chlorophyll pigments system, as confirmed by the reduction of the chl a/b ratio, that increased by a 58%, to indicate an evident reduction of light harvest complex of PSII and by the concomitant decrease of the total chl/carotenoids ratio (-12%), showing that there was a need for plants to invest in an enhancement of photoprotective de-excitation pathways mediated by carotenoids. On the basis of the data given here it is confirmed that: (i) biochemical and physiological changes occur during or at the end of O3 treatment, even in the absence of foliar visible effects; (ii) in asymptomatic species, like T. americana, photosynthesis is down-regulated as a response to oxidative stress; (iii) the absence of foliar injury is a confirmation that this species shows a relatively low sensitivity to this contaminant and, for this reason, it can be recommended as a shade tree for streets, parks and lawn planting in urban areas, even under future pollution scenarios