Daytime and Phenological Characteristics of O3 and CO2 Fluxes of Winter Wheat Canopy Under Short-Term O3 Exposure

To analyze the daytime and phenological variations of canopy O3 and CO2 uptake of winter wheat, the canopy fluxes of wheat plants were measured using a chamber system with four different O3 levels (0, 40, 80, and 120 nmol mol−1) being applied. During the daytime (7:30–18:00 hours), canopy fluxes usually peaked around noon in early growing stages, while a generally decreasing trend from morning to afternoon was observed in the later stages. O3 and CO2 fluxes were positively and negatively correlated with O3 concentration, respectively. Significant differences were observed in O3 fluxes but CO2 fluxes among O3 treatments. Photosynthetically active radiation (PAR) and vapor pressure deficit (VPD) could affect canopy gas uptake in opposite ways. On the phenological timescale, both O3 and CO2 fluxes followed the variation of leaf area index (LAI) with the maximum occurring simultaneously at the booting stage. The daytime mean fluxes varied from −10.6 to −17.2 nmol m−2 s−1 for O3 and from −5.9 to −19.6 μmol m−2 s−1 for CO2. Quantitatively important O3 deposition (−3.1∼−11.6 nmol m−2 s−1) was also observed at night with the ratios being about 40∼70 % relative to the daytime O3 fluxes for most measuring days, which indicates a significant contribution from non-stomatal components to canopy O3 removal. This study confirms that environmental variables and plant phenology are important factors in regulating canopy O3 and CO2 uptake. O3 exposure (≤120 nmol mol−1) could not significantly affect the CO2 uptake of wheat canopy in a short time (ca. 10 min).