Photosynthesis and carbon balance

The requirements for modelling photosynthesis and related processes within the framework of a Functional-Structural Plant Model (FSPM; cf. Vos et al. this volume) are discussed. A combined local gas (carbon dioxide, water vapour) and radiant energy exchange model (GREM) is presented, that was specified to be embedded into an FSPM. The model accounts for the effect of organ nitrogen content (N) on gas exchange expressing certain key model parameters as functions of N. This approach enables the model also to account for the observed effects of growth conditions and organ development on gas exchange, since such effects could to a large part be ascribed to concurrent changes in N. The GREM was parameterized for leaf blades of barley (Hordeum vulgare L.) plants. The combined FSPM-GREM system was successfully applied in simulation studies providing reliable predictions of (1) diurnal time courses of carbon dioxide and water vapour exchange of leaf blades and (2) overall carbon balance and dry-mass accumulation of barley plants during ontogenesis.