A DYNAMIC-MODEL OF PHOTOSYNTHESIS IN VARYING LIGHT TAKING ACCOUNT OF STOMATAL CONDUCTANCE, C3-CYCLE INTERMEDIATES, PHOTORESPIRATION AND RUBISCO ACTIVATION

A dynamic model of whole leaf C3 photosynthesis is constructed using a modified version of the Farquhar-von Caemmerer approach. The model is designed to provide a physiological basis to understand observations of assimilation in environments with varying photon flux densities, including induction phenomena. The model couples the effect of light activation and dark deactivation of enzymes, stomatal conductance responses, and variations in the pools of carbon cycle intermediates. The dynamic components are viewed on three time scales, the slowest of which (min to h) involves changes in stomatal conductance and the activation state of Rubisco. On a time scale of seconds to a few minutes, adjustments in pools of biochemical components of the photosynthetic pathway occurs. The most rapid time scale corresponds to the equilibration time of intercellular CO2 concentration through gaseous diffusion and is here assumed to occur instantaneously. The model form includes a single pool for reduced intermediates including RuBP, a single pool for components of the glycolate pathway, and a third component corresponding to the activation state of Rubisco. This is coupled to a previously described model for the dynamics of stomatal conductance, giving a final model form consisting of six non-linear ordinary differential equations, of which three control conductance dynamics and three control assimilation. The coupling between these occurs through the variable p(i), the intercellular partial pressure of CO2. Only three of the parameters for the assimilation portion of the model require dynamic data to estimate. The remaining parameters are estimated from steady-state data. The model is calibrated using previously collected data on the tropical understory plant Alocasia macrorrhiza and is shown to have qualitatively similar behaviour to that of experimental measurements using simple changes in PFD, as well as a complex sequence of such changes.