Ozone exposure consistently increases δ13C in wheat grain

Abstract

Tropospheric ozone (O₃) 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 O₃ damage to plants are linked to gas exchange and carbon metabolism. The carbon isotopic signature in plant tissues represented by δ¹³C offers a time-integrating approach to assess the performance of plant gas exchange. We combined wheat grain δ¹³C data from seven O₃ experiments performed in four countries (Switzerland, Spain, Sweden, United Kingdom). For one experiment δ¹³C data for stems were available. There was a significant positive relationship between grain δ¹³C and O₃ exposure (R²=0.37). Using a relative scale to account for variation in the δ¹³C level among experiments, a stronger linear relationship was obtained (R²=0.77). Furthermore, the relative yield loss from O₃ was negatively linked to the relative effect on δ¹³C (R²=0.72). Stems were more depleted in ¹³C than grain but also showed a significant, less steep, positive δ¹³C relationship with O₃ exposure. The most important conclusion from the positive relationship between δ¹³C and O₃ exposure is that the O₃ effect on stomatal conductance dominates over the impairment of CO₂ fixation by Rubisco. However, also discrimination associated with redistribution of carbohydrates from non-reproductive plant parts to grains can contribute to the O₃ effect on δ¹³C. Based on the unified pattern of δ¹³C response over a range of experiments performed in different sites, we conclude that the mechanisms of O₃ damage in wheat with respect to gas exchange are highly consistent.