Observing Ozone Effects on Transpiration, Carbon Assimilation, and Photosynthesis by Perturbing Stomatal Diffusive Resistance

Abstract:

Tropospheric ozone (O₃) is an air pollutant that is harmful to plants and ecosystems, with plants primarily exposed through stomatal uptake. O₃ stomatal uptake has been shown to reduce carbon assimilation due to direct impacts on photosynthesis and by disrupting stomatal control; however, relationships between these two effects are not well understood. Here, we describe a series of laboratory experiments in which we manipulate stomatal diffusive resistance by conducting experiments in both air (80% N₂, 20% O₂) and its analog HelOx (80% He, 20% O₂) under both low (20 ppb) and high (80 ppb) O₃ conditions. This approach allows us to alter stomatal conductance without affecting other aspects of leaf metabolism. Experiments use clipped branches from ozone-exposed sweetgum trees, Liquidambar styraciflua, and continuously monitor O₃ and CO₂ uptake, transpiration, and chlorophyll fluorescence, a proxy for the electron transport rate in photosynthesis. We observe O₃ impacts on leaf-scale transpiration, carbon assimilation, photosynthesis, and stomatal opening/closing on multiple timescales. We also investigate whether O₃ exposure causes carbon assimilation and photosynthesis to decouple.