Abstract: Aims The combined effects of ozone (O3) pollution and nitrogen addition on plant photosynthesis are still controversial, which may be related to factors such as measurement period, time, and leaf position. This study mainly explored the differences in the response of photosynthetic parameters of poplar to O3 and nitrogen addition at different growth stages, diurnal variation, and leaf positions, in order to comprehensively understand the combined effects of O3 pollution and nitrogen addition on plant carbon sequestration. Methods The cuttings of hybrid poplar clone 107 (Populus euramericana cv. ‘74/76’) were exposed to six open top chambers. Two O3 concentration treatments were set up (NF, non-filtered ambient air; NF45, NF+45 nmol·mol-1 O3), with four nitrogen addition treatments nested under each O3 treatment (N0, no N added; N50, N0 + 50 kg N·hm-2·a-1; N100, N0 + 100 kg N·hm-2·a-1; N200, N0 + 200 kg N·hm-2·a-1). The photosynthetic parameters of poplar at different growth stages, daily times, and leaf positions were measured. Important Findings When averaged across four nitrogen treatments, NF45 treatment significantly reduced the entire growth season photosynthetic rate (Pn), stomatal conductance (gs), and chlorophyll content (SPAD) by 44.2%, 18.2%, and 24.7% compared with NF treatment, respectively. However, O3 increased the intercellular carbon dioxide concentration (Ci) by 9.0%, indicating that the reduction of Pn by O3 was mainly limited by non-stomatal factors. The results showed that NF45 treatment had a greater reduction in leaf Pn and SPAD at the middle and late stages of growth than at the early stages. The response of Pn and SPAD to O3 varied among different leaf positions. O3 significantly reduced Pn and SPAD at the lower leaf position, while O3 increased Pn and SPAD at the topmost leaf position, indicating a compensatory effect of plants in response to O3 stress. O3 caused varying degrees of decrease in Pn of poplar at different times within a day, but there was no statistically significant interaction between O3 and time on the photosynthetic parameters. The nitrogen addition signifi-cantly increased the leaf Pn and SPAD of poplar throughout the entire growing season, with no significant effect on gs, but significantly reduced Ci. There was no significant difference in the promotion of photosynthesis by nitrogen addition at different growth stages and leaf positions. In addition, no significant interaction effect was observed between O3 and nitrogen addition on all photosynthetic parameters, indicating that nitrogen addition did not alleviate the negative impact of O3 on poplar photosynthesis.

Key words: Ozone, Nitrogen addition, Growth period, Diurnal dynamics, Leaf position, Poplar