Models of Photosynthesis

A BRIEF HISTORY Our model of photosynthesis (8) published some 20 years ago in Planta has had an impact and seen application that far exceeded our expectations. Per- haps it is useful to reflect on what this model did and why we published it. It is important to note that our model is not a complete model of photosynthesis. It makes no attempt to treat all of the steps in this important process; rather, it was a synthesis, a sim- plified view of the already (in 1980) overwhelming knowledge of the contributing mechanisms. In the years preceding our model a great body of work had accumulated describing the responses of CO 2 exchange by leaves to a wide range of environ- mental conditions (temperature, CO2 concentration, light intensity, humidity, and oxygen concentration). These responses were quite reproducible, but diffi- cult to explain. Pieces began to fall into place that informed our ignorance. Perhaps the pivotal event was the finding by George Bowes and Bill Ogren that O2 was a competitive inhibitor of CO2 fixation by Rubisco and an alternative substrate leading to a side reaction that fueled photorespiration. Others added findings that integrated photorespiration into photo- synthetic carbon metabolism. This synthesis pro- vided a plausible explanation of the manifold inter- actions between O2 and CO2 on the photosynthesis of leaves. In our model we linked equations describing Rubisco kinetics with others on the stoichiometry of the photosynthetic carbon reduction cycle and the photorespiratory carbon oxidation cycle, particularly on their energetic (electron transport and ATP syn- thesis) requirements. Building on the pioneering modeling of Hall, Tenhunen, Peisker, Laisk, and oth- ers, we then drew together biochemical and organelle level observations of the temperature dependencies of these phenomena, and combined them with an empirical equation for the dependence of "potential" electron transport rate on absorbed irradiance. Our model attempted to match generalized observations of the photosynthetic gas exchange of leaves with