Drought mitigates the adverse effects of O3 on plant photosynthesis rather than growth: A global meta-analysis considering plant functional types.

Tropospheric ozone (O3 ) is a phytotoxic air pollutant adversely affecting plant growth. High O3 exposures are often concurrent with summer drought. The effects of both stresses on plants are complex, and their interactions are not yet well understood. Here, we investigate whether drought can mitigate the negative effects of O3 on plant physiology and growth based on a meta-analysis. We found that drought mitigated the negative effects of O3 on plant photosynthesis, but the modification of the O3 effect on the whole-plant biomass by drought was not significant. This is explained by a compensatory response of water-deficient plants that leads to increased metabolic costs. Relative to water control condition, reduced water treatment decreased the effects of O3 on photosynthetic traits, and leaf and root biomass in deciduous broadleaf species, while all traits in evergreen coniferous species showed no significant response. This suggested that the mitigating effects of drought on the negative impacts of O3 on the deciduous broadleaf species were more extensive than on the evergreen coniferous ones. Therefore, to avoid over- or underestimations when assessing the impact of O3 on vegetation growth, soil moisture should be considered. These results contribute to a better understanding of terrestrial ecosystem responses under global change.