The grain quality of spring wheat (Triticum aestivum L.) in relation to elevated ozone uptake and carbon dioxide exposure

Ozone (O3) and carbon dioxide (CO2) effects on the quality of spring wheat (Triticum aestivum L.) were evaluated. The dataset originated from 13 European open-top chamber experiments, including three countries, 10 years and four cultivars. The O3 exposure significantly reduced the protein yield, even though the grain protein concentration (GPC) was increased and tended to increase the Zéleny value and the Hagberg falling number. These variables reached values representing enhanced grain maturity after elevated O3 exposure, possibly explained by O3 induced early senescence. The CO2 elevation was demonstrated to increase the protein yield but reduce the GPC and the Zéleny value. Both O3 and CO2 were demonstrated to affect wheat grain yield (GY) but by different means; O3 reduced GY (P < 0.001) mainly by limiting the 1000-grain weight, while CO2 enhanced GY (P < 0.01) mainly by increasing the number of grains produced per unit ground area. Two earlier used O3 indices, the accumulated stomatal uptake of O3 above a flux threshold of 6 nmol m−2 s−1 (AFst6), and the accumulated O3 dose above a concentration threshold of 40 nmol mol−1 (AOT40), were employed in parallel in the derivation of dose–response relationships with all investigated variables (GY, number of grains per unit area, 1000-grain weight, protein yield, GPC, Zéleny value, wet gluten, dry gluten, starch concentration, Hagberg falling number, specific weight and water quotient). The uptake-based index (AFst6) tended to better explain variation in the response variables and yielded dose–response regressions of a higher statistical quality than did the external-dose index (AOT40). Based on an independent subset of the present data, the validities of (1) a negative linear relationship between GPC and GY and (2) a positive but decelerating relationship between protein yield and GY including both O3 and CO2 treatments were tested and confirmed.