Ozone exposure consistently increases δ13C in wheat grain

Tropospheric ozone (O 3 ) is a regional air pollutant, formed by solar radiation from nitrogen oxides and volatile organic compounds. It is known to impair crop yields. The mechanisms of O 3 damage to plants are linked to gas exchange and carbon metabolism. The carbon isotopic signature in plant tissues represented by δ 13 C offers a time-integrating approach to assess the performance of plant gas exchange. We combined wheat grain δ 13 C data from seven O 3 experiments performed in four countries (Switzerland, Spain, Sweden, United Kingdom). For one experiment δ 13 C data for stems were available. There was a significant positive relationship between grain δ 13 C and O 3 exposure (R 2 =0.37). Using a relative scale to account for variation in the δ 13 C level among experiments, a stronger linear relationship was obtained (R 2 =0.77). Furthermore, the relative yield loss from O 3 was negatively linked to the relative effect on δ 13 C (R 2 =0.72). Stems were more depleted in 13 C than grain but also showed a significant, less steep, positive δ 13 C relationship with O 3 exposure. The most important conclusion from the positive relationship between δ 13 C and O 3 exposure is that the O 3 effect on stomatal conductance dominates over the impairment of CO 2 fixation by Rubisco. However, also discrimination associated with redistribution of carbohydrates from non-reproductive plant parts to grains can contribute to the O 3 effect on δ 13 C. Based on the unified pattern of δ 13 C response over a range of experiments performed in different sites, we conclude that the mechanisms of O 3 damage in wheat with respect to gas exchange are highly consistent. • Ozone consistently increased δ 13 C of wheat grain • The ozone effect on stomatal conductance was larger than that on carboxylation • Grain yield loss was correlated with grain δ 13 C at harvest • δ 13 C was higher in grain compared to stems • grain δ 15 N was not affected by ozone