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O3 Effects on Photosynthesis and Stomatal Conductance in WheatThis thesis presents new process-based models to predict the response of vegetation to interactive effects of concurrently changing environmental variables. The combination of new process-based models with the biochemical mechanistic model equations of photosynthesis and simple canopy models, allow the prediction of responses of isoprene emission rates, wheat leaf CO2 assimilation, and wheat productivity to various scenarios of climate and atmospheric change, consistent with changes predicted by the "business as usual" scenario, IS92a. Isoprene, a biogenic hydrocarbon emitted by many tree species, plays a key role in atmospheric chemistry and is a major precursor to phytotoxic ozone. As isoprene emission is highly temperature sensitive, the findings of the most recent research into isoprene synthesis and emission were used to construct a process-based model, to simulate the effects of environmental change on rates of isoprene emission from leaves. This model was subsequently scaled up to the canopy level, using a simple sunlit/shaded canopy model, and leaf energy budget equations. A second new process-based model, based on published data, was constructed to simulate the effects of acute ozone exposure on wheat leaf photosynthesis, and was also subsequently scaled up to the canopy level. In addition, the model of acute ozone effects was adapted to predict the effects of chronic ozone exposure on wheat photosynthesis, and scaled-up to predict the interactive effects of elevated [CO2] and [O3] on wheat productivity. Predictions are presented and discussed. The research illustrates the need for process-based models to predict the interactive effects of concurrently changing environmental factors on vegetation, in order to quantify the feedback effects under future atmospheric and climate conditions. |
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