Complete Parameterization of Photosynthesis Models—An Example for Barley

Complete and robust parameterisation of biophysical CO2 and H2O gas exchange models is an essential requirement for its use as a sub-model of more complex crop models. Here, we address the following crucial points: ① effects of plant and leaf development and leaf insertion height on key model parameters (p), ② re-analysis of the parameters of the temperature response functions, ③ effects of leaf nitrogen content (N a) and growth temperature (T g) on p, and ④ required accuracy of parameter values and time resolution for calculating integrated daily carbon gain (IDC). With regard to these items, we developed an improved version of the LEAFC3-N model that was parameterised completely for barley. In particular, this version of the model accounts for effects of T g on the parameters of the p-N a relationships and on the activation energy parameter ΔH a of the temperature response functions. Based on the derived p-N a relationships, observed gas exchange patterns with respect to leaf senescence, leaf rank, and growth conditions were reproduced by the model fairly well. For calculating integrated daily carbon gain (IDC) with an accuracy of about ±5%, the accuracy of the slope of the V m25-N a and J m25-N a relationships should be of similar order, whereas an inaccuracy up to ±20% can be tolerated for the other parameters. With the given parameterisation, most accurate predictions of IDC will be obtained calculating net photosynthesis rate with high resolution in time (e.g., 1 to 15 min). However, up to a time step of about 1 to 2 h, the bias of IDC will not exceed 5%.