Contrasting photosynthetic responses to ambient air pollution between the urban shrub Rhododendron × pulchrum and urban tall tree Ginkgo biloba in Kyoto city: stomatal and leaf mesophyll morpho-anatomies are key traits

Key message: Rhododendron × pulchrum avoided air pollution by adjusting its stomatal density, while Ginkgo biloba tolerated air pollution via a small stomatal density and high mesophyll thickness. Abstract: Information on the physiological mechanisms underlying species-specific photosynthetic responses to ambient air pollution is essential for enhancing the multiple services provided by urban trees. We conducted an on-site investigation of Rhododendron × pulchrum and Ginkgo biloba, which are the most common shrub and tall roadside tree used in Japan, in order to clarify their photosynthetic responses at sites with different air pollution levels in Kyoto city, Japan. The shrub tree R. × pulchrum and tall tree G. biloba exhibited contrasting responses to air pollution mainly from automobile exhaust gas. R. × pulchrum had a lower photosynthetic rate and stomatal conductance at high-pollution sites than at low-pollution sites, while no reductions were observed at high-pollution sites for G. biloba. The stomatal density of R. × pulchrum negatively correlated with atmospheric nitrogen oxides (NO and NO₂) concentrations. R. × pulchrum avoided the effects of air pollution by reducing stomatal density at high-pollution sites at the expense of reducing CO₂ uptake, while G. biloba appeared to have the ability to tolerate high air pollution levels by reducing the pollution load per mesophyll cell surface area with a low stomatal density and large mesophyll thickness. In conclusion, R. × pulchrum and G. biloba both acclimate to urban environments through an avoidance or tolerance strategy for air pollution by regulating stomatal and/or mesophyll morphologies.