Received; March 25, 2009. Accepted; June 6, 2009. Abstract 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 artifi- cially 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 (gsmax) multiplied by modifications of photo- synthetically active radiation incident on the leaf (PARl), 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/gsmax and P/Pmax ( Pmax is the maximum value of P) of a flag leaf measured in PARl＝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 Pmax multiplied by modifications of PARl 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 PARl were also modeled. By applying the logarithmic profile of [O3] above the canopy, and modeling the roughness length for [O3] (z0O3) with the wind speed, the [O3]canopy was reproduced successfully. To model PARl, the inclina- tion factor of a leaf ( Fl) 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). Fl 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. Key words: Elevated O3, FACE, Photosynthesis, Stomatal conductance, Wheat. Modeling the stomatal conductance and photosynthesis of a flag leaf of wheat under elevated O 3 concentration Hiroki OUE * , Zhaozhong FENG ** , Jing P ANG ** , Akira MIYATA *** , Masayoshi MANO *** , Kazuhiko K OBAYASHI ** , and Jianguo ZHU **** * Faculty of Agriculture, Ehime University, 3–5–7 Tarumi, Matsuyama, 790–8566, Japan ** Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1–1–1 Yayoi, Bunkyo-ku, Tokyo, 113–8657, Japan *** National Institute for Agro-Environmental Science, 3–1–3 Kannondai, Tsukuba, 305–8604, Japan **** Institute of Soil Science, Chinese Academy of Sciences, East Beijing Road 71, Nanjing 210008, P. R.China 1. Introduction The tropospheric O3 concentration is increasing, especially in East Asia (Akimoto, 2003; Naja and Akimoto, 2004; Dentener et al ., 2005). O3 has been well known to damage plant tissues, hinder photosynthesis and reduce crop yield (Fiscus et al., 2005). Wheat has been ranked as one of the most sensitive crops to O3 (Mills et al ., 2003). To predict the O3 impact and thus the yield loss in crops correctly, stomatal Full Paper J. Agric. Meteorol. (農業気象) 65 (3): 239-248, 2009 239