Abstract: Elevated ozone (O₃) levels pose a threat to tree physiology, with responses varying among genotypes. This study investigated O₃ tolerance in two poplar clones (107 and 546) exposed to elevated O₃ (E-O₃), focusing on photosynthetic performance and stomatal dynamics. Clone 107 demonstrated remarkable resilience, with only minor late-season reductions in light-saturated photosynthesis (A_sat), maximum photosynthetic rate (A_max), and chlorophyll content under E-O₃. However, clone 546 exhibited severe impairment, including dysfunctional photosynthetic light-response curves with a reduction in stomatal limitation and increase in biochemical limitation, indicating shifted constraint dominance. Moreover, there was stomatal paralysis characterized by sluggish kinetics (prolonged t_90gopening, reduced SL_max) and loss of bell-shape-like g_s-PPFD response. Results also revealed collapsed instantaneous water-use efficiency (_iWUE) due to decoupled stomatal conductance (g_s) and CO₂ assimilation. The g_s response remained static under elevated O₃ without corresponding photosynthetic benefit, leading to simultaneous carbon starvation and water loss. E-O₃ also induced chlorophyll degradation and premature senescence in clone 546, suggesting chronic oxidative damage. These findings demonstrate that the sensitivity of clone 546 stems from systemic failure in stomatal regulation and biochemical compensation, while the tolerance of clone 107 reflects maintained photosynthetic stability. The results indicate that SL_max and _iWUE dynamics can be utilized as key diagnostic indicators of O₃ stress in trees and highlight the importance of selecting resistant genotypes such as clone 107 for O₃-polluted regions.

Key words: Air pollution, Ground-level ozone, Urban trees, Stomata, Gas exchange