In aims to estimate stomatal O₃ flux and predict the yield loss of wheat in eastern China, the focus was on developing a stomatal conductance (gs) model and a photosynthesis (P) model of a wheat flag leaf. To date, relationships between the relative yield of European spring wheat and the stomatal O₃ flux of the wheat flag leaf have been presented by Pleijel et al. (2000), UNECE (2004) and many researchers with high correlations on the basis of the open-top chamber experiments.
The gs and P of flag leaves and other winter wheat leaves were measured, both in the ambient field and FACE (Free Air Concentration Enrichment) field, whereby the O₃ concentration ([O₃]) was artificially elevated, on 6 days from before heading to before harvest, 2008. The gs model was developed by a multiplicative approach with a maximum value of gs (gs_max) multiplied by modifications of photosynthetically active radiation incident on the leaf (PAR_l), accumulated exposure of O₃ over a threshold concentration of 40 ppb for daylight hours (AOT40) at the canopy top, phenology, VPD and the time of day. A relationship between gs/gs_max and P/P_max (P_max is the maximum value of P) of a flag leaf measured in PAR_l=2000 μmol m^-2 s^-1 was represented by a linear regression line with a slope of 1.0 and intercept of 0.0. Therefore, the P model was developed by the multiplicative approach with P_max multiplied by modifications of PAR_l for P and the same other parameters as the gs model. The models reproduced the gs and P of a flag leaf and even those of the other leaves successfully.
For the practical use of these models, [O₃] at the canopy top ([O₃]_canopy) and PAR_l were also modeled. By applying the logarithmic profile of [O₃] above the canopy, and modeling the roughness length for [O₃] (z_0O3) with the wind speed, the [O₃]_canopy was reproduced successfully. To model PAR_l, the inclination factor of a leaf (F_l) was decided so that the modeled vertical profile of shortwave radiation within the canopy could fit the measurements using the vertical two streams shortwave radiation model (e.g. Oue, 2003a; 2003b). F_l at z=0.6-0.8 m, where most flag leaves exist, was found to be from 0.4 to 0.6 for a wheat canopy of LAI=5.