We present a comment about "Ozone risk assessment for plants: central role of metabolism-dependent changes in reducing power" by Dizengremel, Le Thiec, Bagard, and Jolivet. As tools for summarizing plant O(3) sensitivity in simple indices, Dizengremel et al. suggest: reducing power, as antioxidant regeneration through the Halliwell/Asada cycle requires NADPH from the photosynthetic light reaction; Rubisco/PEPc ratio, as an index of the energy balance between anabolic and catabolic reactions; and water-use efficiency as a time-integrated approximation of the carbon gain to stomatal O(3) uptake ratio. The scientific background is solid, and simple enough (although expensive) to be translated into modelling and routine use. In the last decade, several approaches have been developed, mostly by using photosynthesis as a metric of defence. All these approaches should be experimentally tested in different and realistic conditions, before the results are transferred to the field and used in effective O(3) flux modelling and assessment.