Ozone effects on wheat in relation to CO2: modelling short-term and long-term responses of leaf photosynthesis and leaf duration

A combined stomatal-photosynthesis model was extended to simulate the effec ts of ozone exposure on leaf photosynthesis and leaf duration in relation t o CO2. We assume that ozone has a short-term and a long-term effect on the Rubisco-limited rate of photosynthesis, A(c). Elevated CO2 counteracts ozon e damage via stomatal closure. Ozone is detoxified at uptake rates below a threshold value above which A(c) decreases linearly with the rate of ozone uptake. Reduction in A(c) is transient and depends on leaf age. Leaf durati on decreases depending on accumulated ozone uptake. This approach is introd uced into the mechanistic crop simulation model AFRCWHEAT2. The derived mod el, AFRCWHEAT2-O3, is used to test the capability of these assumptions to e xplain responses at the plant and crop level. Simulations of short-term and long-term responses of leaf photosynthesis, l eaf duration and plant and crop growth to ozone exposure in response to CO2 are analysed and compared with experimental data derived from the literatu re. The model successfully reproduced published responses of leaf photosynt hesis, leaf duration, radiation use efficiency and final biomass of wheat t o elevated ozone and CO2. However, simulations were unsatisfactory for cumu lative radiation interception which had some impact on the accuracy of pred ictions of final biomass. There were responses of leaf-area index to CO2 an d ozone as a result of effects on tillering which were not accounted for in the present model. We suggest that some model assumptions need to be teste d, or analysed further to improve the mechanistic understanding of the comb ined effects of changes in ozone and CO2 concentrations on leaf photosynthe sis and senescence. We conclude that research is particularly needed to imp rove the understanding of leaf-area dynamics in response to ozone exposure and elevated CO2.