Emissions from human industrial processes are indirectly increasing concentrations of surface ozone (O₃), a pollutant known to have negative impacts on plants, including reductions in crop yields, plant growth, and visible leaf injury.  Ozone directly influences photosynthesis via two mechanisms: 1) the oxidation of cellular components (i.e., influencing leaf internal biochemistry and transport) and 2) altering stomatal functioning, ultimately changing conductance and transpiration.  Carbon exchange at the leaf level is governed by both conductance and carboxylation processes, but water exchange depends primarily on the size of the stomatal aperture.  Previous experimental work supports that ozone exposure differentially affects plant-mediated carbon and water fluxes.  After 12 weeks of O₃ exposure, the normally tight relationship between photosynthesis and transpiration weakened significantly, suggesting a decoupling of photosynthesis and stomatal conductance.  However, these differential effects of O₃ are not explicitly expressed in most modeling efforts.  We simulated O₃ damage to plant physiology using Farquhar and Ball-Berry based photosynthesis and stomatal conductance models to determine the parameterization that best simulated experimental response data.  We then incorporated the optimal parameterizations into the Community Land Model at the National Center for Atmospheric Research to determine global responses to a fixed O₃ value.  
Results/Conclusions

Most models that incorporate O₃ damage to plant physiology decrease photosynthesis after model calculations (post-hoc modifications).  Our results demonstrate that the Farquhar photosynthesis code, used in most regional and global models, best simulates O₃ damage to photosynthesis when V_cmax, which estimates the efficiency of carboxylating enzymes, is modified during calculations rather than post-hoc modifications.  Because the Farquhar photosynthesis values are directly used to model Ball-Berry stomatal conductance, O₃-induced decreases to photosynthesis also decrease stomatal conductance.  However, both V_cmax and post-hoc modifications to photosynthesis simulate stomatal conductance values that are much lower than observed conductance values.  Conductance responses to O₃ are best simulated when photosynthesis is unchanged and the Ball-Berry code incorporates a direct modification to stomatal conductance based on experimental data.  These results suggest that regional and global models currently incorporating O₃ damage to plants potentially generate significant error in estimates of transpiration through over-predicting decreases in stomatal conductance.  Therefore, O₃-induced changes in plant release of water, arguably the most important greenhouse gas, are currently poorly described.