Leaf water relations determine the trade-off between ozone resistance and stomatal functionality in urban tree species.

Urban trees possess different capacities to mitigate ozone (O 3 ) pollution through stomatal uptake. Stomatal closure protects trees from oxidative damage but limits their growth. To date, it is unclear how plant hydraulic function affect stomatal behaviour and determine O 3 resistance. We assessed gas exchange and hydraulic traits in three subtropical urban tree species, Celtis sinensis, Quercus acutissima, and Q. nuttallii, under nonfiltered ambient air (NF) and elevated O 3 (NF60). NF60 decreased photosynthetic rate (A n ) and stomatal conductance (g s ) only in Q. acutissima and Q. nuttallii. Maintained A n in C. sinensis suggested high O 3 resistance and was attributed to higher leaf capacitance at the full turgor. However, this species exhibited a reduced stomatal sensitivity to vapour pressure deficit and an increased minimal g s under NF60. Such stomatal dysfunction did not decrease intrinsic water use efficiency (WUE) due to a tight coupling of A n and g s . Conversely, Q. acutissima and Q. nuttallii showed maintained stomatal sensitivity and increased WUE, primarily correlated with g s and leaf water relations, including relative water content and osmotic potential at turgor loss point. Our findings highlight a trade-off between O 3 resistance and stomatal functionality, with efficient stomatal control reducing the risk of hydraulic failure under combined stresses.