Ozone uptake modelling and flux-response relationships - an assessment of ozone-induced yield loss in spring wheat

Measurements of stomatal conductance on field grown spring wheat ( Triticum aestivum L.) from two experiments conducted in southwest Sweden were combined to validate and adjust the Jarvis type of multiplicative stomatal conductance model presented by Emberson et al. (Environ. Pollut. 109 (2000) 403). The adjusted model (Ostad model) and the Emberson model are based on the boundary line technique. The aging of the flag leaf became important for stomatal conductance at about 500 degrees days after anthesis, on average 30 days after anthesis. Elevated ozone concentrations were assumed to influence the stomatal conductance in relation to the effect on the leaf life span. During the hours after noon the stomata tended to close to an extent that could not be explained by the combined effects of leaf temperature, leaf-to-air vapour pressure difference (VPD LA ) or solar radiation. For these reasons factors describing the reduction of stomatal conductance caused by ozone and time of day were introduced in the calibration of the Ostad stomatal conductance model. VPD LA induced closure of stomata at ≈1.5 kPa. In elevated carbon dioxide concentration (680 μmol mol −1 ) the stomatal conductance was reduced by approximately 60%. Test with the data from Ostad showed that the Ostad multiplicative model had an r 2 -value of 0.59 for the relationship between calculated and observed conductance. The Ostad as well as the Emberson models were used to estimate the cumulated uptake of ozone (CUO) by the wheat flag leaves. The relationship between CUO based on the Ostad model cumulated from anthesis to harvest, with a threshold for the uptake rate of 5 nmol m −2 s −1 and relative yield loss, resulted in a higher r 2 -value (0.90) than any other CUO model or relationships based on the accumulated ozone exposure over 40 nmol mol −1 (AOT40). The corresponding relationships between relative yield and CUO based on the Emberson model and with AOT40 were however also statistically significant.