Modelling photosynthesis irradiance curves

The modeling and simulation of photosynthesis/irradiance responses is poorly developed in comparison to photosynthesis/CO2 concentration responses. Often the strategy adopted is to use the non-mechanistic rectangular hyperbola construction of Thornley and Johnston. It would be better if a model existed that had mechanistic foundations. It is relatively easy to build a model electron transport through photosystem I, and some years ago we described such a model for FPSI irradiance curves. This model was composed of parameters that reperesented the rate constant for P700 excitation and oxidation per unit irradiance, the rate constant for P700+ reduction, the size of high potential donor pool per unit P700, and the equilibrium constant between the reduced high potential pool and P700+. It has been experimentally shown that FPSI and the relative rate of PSI electron transport is usually closely related the corresponding parameter for PSII or CO2 fixation. It therefore seemed likely that a PSI based model could be adapted for use in simulating or fitting PSII and CO2 irradiance responses. Adapting the PSI model for PSII is simple and allows measured PSII irradiance responses to be fitted and, within certain limits, to be simulated. Even CO2 fixation/irradiance responses, which in principle can have complicated relationships to photosynthetic electron transport, can often be accurately fitted by a simple variant of the basic PSI electron transport model. The integration of these models with the well established biochemically based models of C3 photosynthesis will hopefully yield an improved means for modeling and simulating photosynthesis.