Towards understanding oscillations: a mathematical model of the biochemistry of photosynthesis

A general model of electron transport, carbon assimilation, starch and sucrose synthesis was built on the basis of two partial models. Individual reactions were described by their $\Delta$ G'$_0$, V$_m$ and Km values for substrates and products. The system of 33 differential equations was solved on a personal computer programmed in TurboPASCAL 3.0. The rate of cytosolic fructose bisphosphatase (FBPase) is modelled to be dependent on the concentration of fructose 2,6-bisphosphate (F2,6BP). The synthesis of the latter is activated by inorganic phosphate and inactivated by triose phosphates. The quantum efficiency of PSII is depressed at high proton charge in thylakoids and at high redox states of the electron transport chain. One of the aims of the model was to check whether these regulatory systems could cause oscillations in photosynthesis. Transients calculated from a low to high photon flux density and from a low to high CO2 concentration revealed an overshoot but no oscillations. Therefore, it has not been sufficiently proved whether cytosolic FBPase and PSII activity control oscillations in photosynthesis. The phosphate-limited photosynthesis is stable in cases where UDPglucose pyrophosphorylase and ADPglucose pyrophosphorylase have greater affinity for ATP (UTP) than CO2 assimilation. In a phosphate-limited state high $\Delta$pH is not generated, as electron transport becomes limited by the low concentration of 1,3-diphosphoglycerate.