Water stress mitigates the negative effects of ozone on photosynthesis and biomass in poplar plants.

Tropospheric ozone (O3 ) pollution frequently overlaps with drought episodes but the combined effects are not yet understood. We investigated the physiological and biomass responses of an O3 sensitive hybrid poplar clone ('546') under three O3 levels (charcoal-filtered ambient air, non-filtered ambient air (NF), and NF plus 40 ppb) and two watering regimes (well-watered (WW) and reduced watering (RW), i.e. 40% irrigation) for one growing season. Water stress increased chlorophyll and carotenoid contents, protecting leaves from pigment degradation by O3 . Impairment of photosynthesis by O3 was also reduced by stomatal closure due to water stress, which preserved light-saturated CO2 assimilation rate, and the maximum carboxylation efficiency. Water stress increased water use efficiency of the leaves while O3 decreased it, showing significant interactions. Effects were more evident in older leaves than in younger leaves. Water stress reduced biomass production, but the negative effects of O3 were less in RW than in WW for total biomass per plant. A stomatal O3 flux-based dose-response relationship was parameterized considering water stress effects, which explained biomass losses much better than a concentration-based approach. The O3 critical level of Phytotoxic Ozone Dose over a threshold of 7 nmol O3 .m-2 .s-1 (POD7 ) for a 4% biomass loss in this poplar clone under different water regimes was 4.1 mmol m-2 . Our results suggest that current O3 levels in most parts of China threaten poplar growth and that interaction with water availability is a key factor for O3 risk assessment.