Ozone and drought stress: Interactive effects on gas exchange in Aleppo pine (Pinus halepensis Mill.)

Two-year-old Aleppo pine seedlings were exposed over twenty months in open-top chambers to either above-ambient ozone (O3) concentrations or charcoal-filtered air. Subsequently, water was applied following plant requirements in the well watered treatment (WW), while it was withheld for 11 days in the water-stressed seedlings (WS). Then, the seedlings of both water treatments were irrigated to field capacity. Four seedlings per water treatment were used within each chamber. The daily variation in gas exchange rates of Aleppo pine seedlings from the different treatments were studied during and after water stress. Ozone exposure and a moderate water shortage induced similar reductions in the gas exchange rates. An antagonistic response modulated by the severity of water stress followed the combined exposure to both O3 and water stress; a partial recovery from water stress was observed two days after rewatering to field capacity, while the seedlings that were not exposed to O3 showed a faster recovery.