Functional responses of two Mediterranean pine species in an ozone Free-Air Controlled Exposure (FACE) experiment

Effects of the phytotoxic and widespread ozone (O3) pollution may be species specific, but knowledge on Mediterranean conifer responses to long-term realistic exposure is still limited. We examined responses regarding to photosynthesis, needle biochemical stress markers and carbon and nitrogen (N) isotopes of two Mediterranean pine species (Pinus halepensis Mill. and Pinus pinea L.). Seedlings were grown in a Free-Air Controlled Exposure experiment with three levels of O3 (ambient air, AA [38.7 p.p.b. as daily average]; 1.5 × AA and 2.0 × AA) during the growing season (May-October 2019). In P. halepensis, O3 caused a significant decrease in the photosynthetic rate, which was mainly due to a reduction of both stomatal and mesophyll diffusion conductance to CO2. Isotopic analyses indicated a cumulative or memory effect of O3 exposure on this species, as the negative effects were highlighted only in the late growing season in association with a reduced biochemical defense capacity. On the other hand, there was no clear effect of O3 on photosynthesis in P. pinea. However, this species showed enhanced N allocation to leaves to compensate for reduced photosynthetic N- use efficiency. We conclude that functional responses to O3 are different between the two species determining that P. halepensis with thin needles was relatively sensitive to O3, while P. pinea with thicker needles was more resistant due to a potentially low O3 load per unit mass of mesophyll cells, which may affect species-specific resilience in O3-polluted Mediterranean pine forests.