Fitting photosynthetic carbon dioxide and irradiance response curves for C ₄ leaves

Gas exchange measurements provide crucial insights into the complex mechanisms of photosynthesis. Responses of CO ₂ assimilation rate ( A ) to intercellular CO ₂ partial pressure ( C _i ) and irradiance ( I ) link gas exchange measurements to the underlying photosynthetic biochemistry of a leaf. The unique biochemistry and leaf anatomy which distinguish C ₄ photosynthesis make it necessary to apply models and fitting routines which appropriately parameterise and incorporate these characteristics. Here we provide updates to the C ₄ photosynthesis model by improving the parameterisation of cyclic electron flow in C ₄ photosynthesis using experimentally derived values from S. viridis. We additionally describe two fitting routines for assessing C ₄ photosynthesis based on the updated model. Fitting of a CO ₂ response curve ( A/C _i ) provides estimates of maximum PEP carboxylase activity ( V _pmax ) and maximum Rubisco activity ( V _cmax ), and calculates the electron transport rate ( J ) needed to sustain the measured CO ₂ assimilation rate ( A ). Fitting of an irradiance response curve ( A/I ) provides estimates for the maximum electron transport rate ( J _max ), day respiration rate ( R _d ), the quantum yield ( ϕ _CO2 ) and light compensation point ( Γ _light ). Values of the above output parameters are provided at both the measurement temperature and at 25 °C for ease of comparative reporting. The fitting tool has been designed in Microsoft Excel to minimise barriers to entry and enable simplicity of fitting while simultaneously catering to individuals with diverse expertise and experience in C ₄ gas exchange modelling.