Poster: Photosynthesis

Abs # 343: Virtual leaf: Integrating the light reactions, Calvin cycle and photorespiration in a dynamic model of leaf photosynthesis

Presenter:	Zhu, Xinguang , zhu3@uiuc.edu
Authors	Zhu, Xinguang  (A)   Whitmarsh, John C (B)   Long, Stephen P (A)
Affiliations:	(A): University of Illinois at Urbana Champaign (B): National Institute of Health

The widely used biochemical model of C3 photosynthesis (Farquhar et al. 1980) provides a highly robust basis for predicting steady-state photosynthetic CO₂ uptake rate at different light levels. However, over 90% of the carbon assimilated by leaves of crop canopies and other vegetation in the field is gained by non steady-state photosynthesis as a result of continual fluctuations in light. Modeling dynamic photosynthesis requires a detailed description of the light reactions, Calvin cycle and photorespiratory pathway. The existing dynamic models of photosynthesis omit many of the steps of the light reactions and all or much of the photorespiratory pathway, limiting their value in exploring the effects of genetic manipulation of individual steps. With the rapid increase in computational power and advanced numerical algorithms for solving differential equations, we constructed a more complete dynamic model of photosynthesis. The photosynthetic process was described as 60 coupled differential equations, each representing the rate of concentration change of an intermediate in photosynthesis. Using literature estimates of average amounts and kinetic properties of the enzymes and concentrations of intermediates in healthy C3 leaves, simulations were run for step changes in light level. The simulated transthylakoid pH and CO₂ fixation rates were within 10% of observed changes in C3 leaves. The model successfully simulated the dynamics of photosynthetic CO₂ uptake rate and the Kautsky fluorescence induction curve (FI) observed in intact leaves. The influences of changing kinetics and composition of photosystem II on FI were examined as a first application. This model provides an improved basis for identifying limiting steps in non steady-state photosynthesis.