Fitting photosynthetic carbon dioxide and irradiance response curves for C4 leaves

Gas exchange measurements provide crucial insights into the complex mechanisms of photosynthesis. Responses of CO2 assimilation rate (A) to intercellular CO2 partial pressure (Ci) and irradiance (I) link gas exchange measurements to the underlying photosynthetic biochemistry of a leaf. The unique biochemistry and leaf anatomy which distinguish C4 photosynthesis make it necessary to apply models and fitting routines which appropriately parameterize and incorporate these characteristics. Here we provide updates to the C4 photosynthesis model by improving the parameterization of cyclic electron flow in C4 photosynthesis using experimentally derived values from Setaria viridis. We additionally describe two fitting routines for assessing C4 photosynthesis based on the updated model. Fitting of a CO2 response curve (A/Ci) provides estimates of maximum phosphoenolpyruvate carboxylase activity (Vpmax) and maximum Rubisco activity (Vcmax), and calculates the linear electron transport rate (J) needed to sustain the measured CO2 assimilation rate (A). Fitting of an irradiance response curve (A/I) provides estimates for the maximum linear electron transport rate (Jmax), day respiration rate (Rd), the quantum yield (ϕCO2), and light compensation point (Γlight). Values of the above output parameters are provided both at the measurement temperature and at 25 °C for ease of comparative reporting. The fitting tool has been designed in Microsoft Excel to minimize barriers to entry and enable simplicity of fitting while simultaneously catering to individuals with diverse expertise and experience in C4 gas exchange modelling.