PS 15-191 - Elevated ozone exposure impacts water-use efficiency of white pine seedlings through changes in stomatal conductance

Abstract: Tropospheric ozone (O₃) is a phytotoxic air pollutant formed through reactions involving nitrogen oxides and volatile organic compounds. Ecosystems often face high O₃ episodes, and many ecosystems have experienced increasing phytotoxic ozone dose through stomatal uptake. O₃ uptake can reduce photosynthesis and biomass of major forest tree species threatening a critical carbon sink. Furthermore, O₃ uptake can reduce stomatal conductance, but studies have also reported stomatal sluggishness which decreases stomatal sensitivity to environmental changes such as soil moisture. Resolving the impact of O₃ on stomatal function is critical as it has direct implications for the role of terrestrial vegetation in surface-atmosphere exchanges of water vapor. Here, we tested the impact of O₃ exposure on net photosynthesis (A_net), stomatal conductance (G_s,w), and intrinsic water-use efficiency (iWUE) of eastern white pine (Pinus strobus) seedlings during well-watered and water stressed conditions.
Seedlings were grown in growth chambers and water was withheld from the water stressed seedlings until a substantial decline in A_net and G_s,w before O₃ exposure. Both well-watered and water stressed seedlings were exposed to 100 – 200 ppbv O₃ over four days. Gas exchange measurements were collected before and after O₃ exposure for four groups of seven seedlings: control, O₃ exposure, water stressed, and water stressed with O₃ exposure. A 42% mean reduction in G_s,w in well-watered seedlings treated with O₃ resulted in a 26% mean increase in iWUE. Treating water stressed seedlings with O₃ resulted in a lower decline in G_s,w (p < 0.05) and they were able to maintain their light saturated photosynthetic capacity compared to seedlings experiencing water stress alone.
White pine individuals have been reported to range in photosynthetic suppression in response to ozone from susceptible to resistant. Our results suggest that O₃ exposure can result in stomatal sluggishness in white pine seedlings without reductions in net photosynthesis during water stress. Exposure to elevated O₃ during well-watered conditions can reduce stomatal conductance without reductions in net photosynthesis suggesting an enhanced role of antioxidants in enhancing water-use efficiency in white pine seedlings.