The response of spring wheat (Triticum aestivum L.) to ozone at higher elevations

Abstract

summaryThe flux of O3 was determined in open‐top chambers (OTC) used to investigate its effect on spring wheat (Triticum aestivum L., cv. Albis) in 1989 and 1990. The experimental site was located at 900 m above sea level at Zimmerwald, near Bern (Switzerland). The aims were to evaluate the use of OTCs for O3 flux measurements under field conditions, to assess the role of stomata in controlling the O3 fluxes, and to establish a quantitative relationship between radiation‐weighted O3 concentrations and O3 flux. Measurements were carried out from full expansion of flag leaves until the onset of senescence. Ozone flux was determined by mass balance using the concentrations of O3 measured at the inlet and outlet of the OTC. The CO2 exchange rate was corrected for soil‐borne COO and used as a reference. Measurements of temperature, photosynthetically active radiation (PAR), saturated water vapour pressure deficit (SVPD), and boundary layer conductance were used to describe the microclimate inside OTCs.In the warmer microclimate in 1989, the plant canopy was characterized by a smaller leaf area index (LAI) than in 1990, while the fluxes of O3 and CO2 during daytime were generally larger in 1989. The diurnal patterns of fluxes of O2 and CO2, in OTCs supplied with unfiltered air were similar. It is estimated that O3 absorption via the stomata contributed 50–70% of its total flux. Identical relationships between leaf conductance for O2, measured by porometry and leaf conductance calculated from O3 flux were found in both years, but measured leaf conductance during daytime was generally smaller in 1990 than in 1989. The results indicate that stomatal conductance largely controlled O3 flux, and that the canopy structure has an influence on the overall conductance of the canopy.Different linear functions were obtained for the relationship between radiation‐weighted O3 concentration and O3 flux, using data from OTCs supplied with either charcoal‐filtered air, unfiltered air or unfiltered air enriched with O. (two levels). These relationships form the basis for the calculation of mean O3 fluxes which can be used as an exposure index in the exposure‐response analysis.