Investigating the Impact of Ozone Dry Deposition on Saguaro Cacti (Carnegiea Gigantea) in Saguaro National Park, Arizona

Surface level ozone pollution can negatively impact vegetation health through ozone dry deposition, often occurring through the uptake of ozone through stomata. Plants that photosynthesise through C3 and C4 pathways open their stomata during the daytime, when surface level ozone concentrations peak. In contrast, plants that photosynthesise through crassulacean acid metabolism (CAM) pathways, such as the saguaro cacti (Carnegiea gigantea), uniquely open their stomata during the nighttime and close them during the daytime. Previous research has not yet investigated how CAM photosynthesis modifies ozone uptake, leading us to explore the specific impacts surface level ozone pollution has on CAM plants.

In this work, we combine ozone concentration data from ozone sensors on EPA air quality monitors, eddy covariance fluxes, and monthly saguaro cacti stomatal conductance data to explore the influence of CAM photosynthesis on ozone dry deposition. We observed an inverse relationship between ozone concentrations and stomatal conductance. This relationship is consistent with the hypothesis that plants with CAM pathways likely intake less surface level ozone compared to their C3 and C4 pathway plants counterparts. We further use these data points to estimate dry deposition velocity and stomatal ozone deposition fluxes to saguaro cacti. Our findings indicate that because saguaro cacti open their stomata at night, their ozone uptake is reduced compared to if the stomata were open during the daytime, in which ozone concentrations peak. Thus, CAM photosynthetic pathways may be protective against ozone and other photochemical pollutant damage.