Wheat yield responses to stomatal uptake of ozone: Peak vs rising background ozone conditions

Recent decades have seen a changing temporal profile of ground-level ozone (O-3) in Europe. While peaks in O-3 concentrations during summer months have been declining in amplitude, the background concentration has gradually increased as a result of the hemispheric transport of O-3 precursors from other world regions. Ground level O-3 is known to adversely affect O-3-sensitive vegetation, including reducing the yield of O-3-sensitive crops such as common wheat (Triticum aestivum L.). The reduction in wheat yield has been shown to be linearly related to the phytotoxic O-3 dose above a flux threshold of Y (PODY) accumulated over a specific period. In the current study, we tested whether the flux-effect relationships for wheat yield and 1,000-grain weight were affected by the temporal profile of O-3 exposure. A modern wheat cultivar (Skyfall) was exposed to eight different realistic O-3 profiles repeated weekly: four profiles with increasing background O-3 concentrations (ca. 30-60 ppb) including small peaks and four profiles with increasing O-3 peak concentrations (ca. 35-110 ppb). Both wheat yield and 1,000-grain weight declined linearly with increasing PODY. The slope of the flux-effect relationships was not affected significantly by the profile of O-3 exposure. Hence, flux-effect relationships developed for wheat based on exposure to enhanced peak O-3 concentrations are also valid for the changing European O-3 profile with higher background and lower peak concentrations. The current study also shows that the modern wheat cultivar Skyfall is more sensitive to O-3 than European wheat varieties tested for O-3 sensitivity in the 1980s and 1990s.