Ozone exposure affects leaf wettability and tree water balance.

•  Relatively little is known about the influences of growing-season background ozone (O3 ) concentrations on leaf cuticles and foliar water loss. •  Using fumigation chambers, leaf wettability and foliar water loss were studied in two poplar species, Populus nigra and P. euramericana, and a conifer, Pseudotsuga menziesii, under three O3 regimes; control (approx. 1 ppbv O3 ), urban O3 exposure (13-41 ppbv O3 ), and montane O3 exposure (30-45 ppbv O3 ). •  Urban O3 exposure delayed a decrease in droplet contact angles over time in Populus leaves by 2-4 wk, and decreased droplet contact angles of P. menziesii foliage. Ozone exposure increased foliar water loss and minimal conductance to water vapour for P. euramericana, but not P. nigra and P. menziesii. Both Populus species had lower photosynthetic biomass in O3 treatments, due to production of fewer new leaves, premature leaf abscission and decreased leaf size (P. euramericana only). Leaf abscission was preceded by foliar injury symptoms characteristic of O3 exposure. •  Results suggest that exposure to [O3 ] common during the growing season can increase water loss in Populus saplings, but this effect might be offset by decreased foliar biomass. Importantly, responses were highly species specific in a given O3 treatment.