Growth and gas exchange of loblolly pine seedlings as influenced by drought and air pollutants

One-year-old loblolly pine seedlings were exposed to 0₃(≤0.025 or 0.10 μ L L^?1, 4 hr d^?1, 3 d wk^?1) in combination with simulated rain (pH 5.6 or 3.0, 1 hr d^?1, 2 d wk^?1, 0.75 cm hr^?1) for 10 wk. After the 10-wk treatment, the seedlings were submitted to two drought cycles, and water potential, net photosynthesis (Pn), and transpiration (Tr) were measured. Whole-plant fresh weight increment and relative growth rate were significantly increased in seedlings exposed to simulated rain at pH 3.0 compared to pH 5.6. An interaction between 0₃ and simulated rain occurred in height growth. Shoot height elongation was significantly less in seedlings exposed to 0.10 μL L^?1 0₃ + pH 5.6 than in any other pollutant combination after the 10-wk treatment period. There were no significant effects of 0₃ on Pn and Tr prior to the drought cycles; however, after the first drought cycle, Pn was significantly higher in seedlings pre-exposed to 0.10 μL L^?1 0₃ compared to the low 0₃ concentration. The 10-wk treatment with simulated rain at pH 3.0 significantly increased Pn and Tr. The relationship between gas exchange rates and needle water potential during the moisture stress period was affected by preexposure to pollutants. In general, Pn and Tr were more sensitive to decreasing needle water potential in seedlings exposed to pH 3.0 during the first drought cycle and to 0.10 μL L^?1 0₃ during second drought cycle.