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

Tong, Lei; Xiao, Hang; Qian, Feizhong; Huang, Zhongwen; Feng, Jiayong; Wang, Xiaoke

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

2016

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).

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