Responses of photosynthesis and long-term water use efficiency to ambient air pollution in urban roadside trees

We conducted on-site studies in Kyoto City, Japan, to evaluate the effect of air pollution by automobile gas exhaust on the leaf photosynthetic functions of four urban roadside tree species. Nitrogen oxides (NO and NO₂) are major air pollutants that are related to automobile gas exhaust. The species-specific response of leaf photosynthesis to air pollution was obtained for single-year data, in which at the high air pollution sites, Rhododendron × pulchrum, Rhaphiolepis indica, and Prunus × yedoensis had a higher restriction of maximum photosynthesis (A_max), while the opposite trend was obtained for Ginkgo biloba. When the data were pooled across the years from 2007 to 2019 in R. pulchrum, the dose-dependent effect of NO and NO₂ on photosynthesis became obvious, in which they decreased A_max and increased the long-term leaf water use efficiency. A spatial variability map for R. pulchrum showed a lower A_max and higher water use efficiency at the heavy traffic areas in Kyoto City, which suggests that R. pulchrum increased tolerance to air pollution and water stress at the expense of the leaf photosynthesis. This study revealed the importance of the evaluation of the species-specific response of photosynthesis to air pollution for the efficient use of urban trees, even in regions with relatively low atmospheric pollution levels such as < 40 ppb of NO or NO₂.