Effects of Surface Ozone on Stomatal Ozone Uptake Flux in Winter Wheat Under Future Climate Scenarios

To assess stomatal O3 uptake by winter wheat under future climate scenarios, this study analyzed meteorological conditions, atmospheric O3 concentrations, and leaf stomatal conductance based on continuous observations during the winter wheat growing season in the Yangtze River Delta region. The parameterized Jarvis stomatal conductance model was evaluated against field measurements and subsequently applied to estimate the accumulated stomatal O3 uptake (AFst04). A scenario-based framework representing a 1.5 °C warming level and prescribed changes in CO2 and O3 concentrations was then used to assess changes in stomatal O3 uptake under future conditions. The results showed that the mean O3 concentration during the daytime observation period (09:00–17:00) was 49.2 nL·L−1, and the daytime mean O3 concentration exceeded 40 nL·L−1 on 68.5% of the observation days. The stomatal conductance of winter wheat leaves reached its maximum during the flowering stage. Pronounced diurnal variations in stomatal conductance were observed from the jointing to flowering stages, whereas these diurnal variations became less pronounced from the grain-filling to maturity stages. The crop- and region-specific Jarvis model satisfactorily simulated the stomatal conductance of winter wheat leaves, with modeled values showing good consistency with measured values (R2 = 0.85). The observed AFst04 of winter wheat during the study period was 2.39 mmol·m−2. Under the climate-only scenario, in which air temperature was increased by 1.5 °C, CO2 concentration was set to 550 ppm, relative humidity was held constant, and O3 concentration was maintained at the baseline level, the simulated AFst04 decreased to 0.63 mmol·m−2. When the same climate scenario was combined with prescribed 20–25% increases in O3 concentration, the simulated AFst04 increased to 1.24–1.46 mmol·m−2. These findings indicate that, under the scenario conditions examined in this study, warming and elevated CO2 were associated with reduced stomatal O3 uptake by winter wheat through changes in stomatal regulation. However, the prescribed increase in O3 concentration partly offset this reduction, resulting in relatively higher stomatal O3 uptake under the combined climate-O3 scenarios.