A Laboratory Study Investigating Ozone Effects on Transpiration, Carbon Assimilation, and Photosynthesis by Perturbing Stomatal Diffusive Resistance

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 through direct impacts on photosynthesis and by disrupting stomatal control; however, relationships between these two effects are not well understood. Here, I describe a series of laboratory experiments in which I manipulated stomatal diffusive resistance by conducting experiments in both air (80% N2, 20% O2) and its analog HelOx (80% He, 20% O2) under both low and high O3 conditions. The approach allowed me to alter stomatal conductance without affecting other aspects of leaf metabolism and the within-chamber O3 lifetime. Experiments used cuttings from ozone-exposed sweetgum trees, Liquidambar styraciflua, continuously monitoring O3 and CO2 uptake, transpiration, leaf temperature, and chlorophyll fluorescence, a proxy for photosynthetic electron transport. My methodology allowed observation of the dynamics of O3 uptake, which I find to be stomatally limited, with no evidence for an additional internal rate-limiting resistance. My results suggest isolated elevated O3 exposures do not affect transpiration, carbon assimilation, or their coupling.