Differential Effects of Ozone Exposure on Carbon Assimilation and Stomatal Conductance

Humans are indirectly increasing concentrations of surface ozone through industrial processes. Ozone is known to have negative impacts on plants, including reductions in crop yields, plant growth, and visible leaf injury. Ozone directly influences photosynthesis via two mechanisms: 1) the oxidation of cellular components (i.e., influencing leaf internal biochemistry and transport) and 2) altering stomatal functioning, ultimately changing conductance. Carbon exchange at the leaf level is governed by both conductance and carboxylation processes, but water exchange depends primarily on the size of the stomatal aperture. Thus, the possibility exists that ozone exposure will differentially affect plant-mediated carbon and water fluxes. Further, these differential effects of ozone are not explicitly expressed in most modeling efforts. We investigated how ozone changes both stomatal conductance and carbon assimilation using controlled open-top chamber experiments and then incorporated our experimental findings into modified Farquhar and Ball-Berry based photosynthesis and stomatal conductance models. In experiments, we observed carbon assimilation and conductance decreases in response to ozone. However, the decrease in carbon assimilation was larger than the decrease in conductance to water vapor, thereby changing the relationship between carbon gain and water loss at the leaf level. In addition, the relationship between photosynthesis and transpiration weakened significantly after 12 weeks of ozone exposure, suggesting a decoupling of photosynthesis and stomatal conductance. We used this information to modify biochemical parameters in the Farquhar model and the Ball-Berry coefficient to determine whether these models are able to simulate plant performance under ozone exposure.