Effects of carbon dioxide and ozone on wheat crop yield and grain quality

Abstract: Atmospheric concentrations of carbon dioxide (CO2) and ozone (O3) have steadily increased since the industrial revolution. CO2 and O3 directly affect plant physiology, CO2 being an essential substrate for photosynthesis, while O3 is an oxidative agent causing damage to plant tissues. Due to strong concerns for future food security, effects on crop production are of particular interest. Wheat is a major food crop globally, being the second most important energy source. Accordingly, the overall aim of this thesis was to explore the general effects of elevated CO2 and O3 on wheat crops. CO2 and O3 impacts on wheat crops were systematically reviewed, using meta-analysis to estimate average effects, and deriving response functions to assess the effect size in relation to the concentration of either CO2 or O3. The underlying effects of O3 on grain nutrients was further explored in three experimental studies. Wheat yield increased by 25% on average under elevated CO2, but there was no further yield stimulation above 600 ppm. Elevated CO2 decreased grain protein concentration by 8% on average, but the effect was overestimated in pot grown plants. There was also a CO2-induced reduction in concentration of other grain nutrients, where CO2 effects on Fe and S were strongly correlated to the effects on protein but showed no relationship with grain yield stimulation. O3-induced reductions in wheat grain yield was shown to be mainly due to a decrease in grain mass, while grain number was only reduced to a small extent. Grain starch concentration was significantly reduced under O3 exposure, making starch yield the wheat yield variable most strongly affected by O3. O3 enhanced concentrations but strongly reduced the yield of important wheat grain nutrients such as protein, P, Mg, K, Ca, Zn and Mg. Both concentration and yield of Cd were reduced by O3. A comparison among our most important staple crops showed that O3 promoted a larger protein yield loss in soybean compared to rice and wheat. O3 reduced harvest index (HI) for most nutrient elements, but also for Cd, while the total element pool in aboveground biomass was unaffected (except for P). Consequently, the O3-induced reduction in grain element yield can be explained by lower remobilization rates rather than reduced uptake. There was a strong correlation of element HI when comparing sites and cultivars, indicating that it is primarily element specific and not strongly dependent on growing environment and genotypic differences. An experiment testing the interaction between O3, heat and drought stress showed that O3 effects on light saturated photosynthesis, grain mass and several grain nutrient concentrations were reduced under drought. Grain concentrations of protein, Ca and Zn were closely linked to grain yield regardless of O3, heat and drought stress. The significant impacts on wheat yield and grain quality suggest that there is a need to incorporate the influence of both CO2 and O3 in assessments of current and future global food security, but also account for the modifying effect of soil moisture.