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

Tropospheric ozone (O-3) pollution frequently overlaps with drought episodes but the combined effects are not yet understood. We investigated the physiological and biomass responses of an O-3 sensitive hybrid poplar clone ('546') under three O-3 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 O-3. Impairment of photosynthesis by O-3 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 O-3 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 O-3 were less in RW than in WW for total biomass per plant. A stomatal O-3 flux-based dose-response relationship was parameterized considering water stress effects, which explained biomass losses much better than a concentration-based approach. The O-3 critical level of Phytotoxic Ozone Dose over a threshold of 7 nmol O-3.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 O-3 levels in most parts of China threaten poplar growth and that interaction with water availability is a key factor for O-3 risk assessment. (C) 2017 Elsevier Ltd. All rights reserved.