The Effects of Dual Ozone and Drought Stresses on the Photosynthetic Properties of Acer rubrum and A. pictum

Abstract

Ozone (O3) pollution is accompanied by drought stress, especially at high temperatures. Tree species in cities often face dual stresses from O3 and drought. In this study, Acer rubrum ‘Autumn Blaze’ and A. pictum were used as test plants in open-top-chambers (OTCs) to investigate the trees most tolerant to increasing O3 and drought stresses in urban gardens. The results showed that the dual stresses induced a change in A. rubrum’s leaf coloration from green to red. The leaf representation in A. rubrum was more variable than that of A. pictum. The leaf pigment content affected the plant leaf color difference, and the Chl and Car contents of both species were negatively correlated with L*. Under the dual stresses of O3 and drought, the changes in the net photosynthetic rate (Pn) and transpiration rate (Tr) were less variable in A. rubrum than A. pictum. The stomatal conductance (Gs) was more sensitive to higher O3 stress, the effect of which was enhanced by moderate drought (MD) conditions on Gs. The Tr decreased more significantly under drought stress, which mitigated the effect of O3 stress on the stomatal limit value (Ls). A. rubrum displayed differential color changes, resulting in greater structural heterogeneity within the garden landscape. The saplings adjusted their photosynthetic parameters under the dual stresses, whereas the dual stresses played an antagonistic role in protecting A. rubrum, suggesting that A. rubrum can resist O3 and drought. Our study suggests that A. rubrum is an alternative tree species for inclusion in urban gardens exposed to increasing O3 and drought stresses.