In Practice

Fitting photosynthetic carbon dioxide response curves for C₃ leaves

This page provides information on a curve fitting utility for analyzing A/Ci curves as described in Sharkey et al. (2007). The published paper has information on the way the utility works.

Below you can download an Excel file that will fit the following parameters: V_cmax, J, TPU, R_d, and g_m (maximum carboxylation rate allowed by rubisco, rate of photosynthetic electron transport [based on NADPH requirement], triose phosphate use, day respiration, and mesophyll conductance). The method requires at least five data pairs of A and c_i (CO₂ in parts per million by volume, mole fraction) or C_i (CO₂ partial pressure in Pa). The utility requires users to indicate the presumed limiting factor. The CO₂ partial pressure at the sites of carboxylation is calculated, and values for the five parameters listed above are estimated.

It is not possible to lock the cells in the functional version and if they are inadvertently changed, the utility might not work properly. We recommend downloading a new copy each time you use it. Updates and improvements will also be posted so downloading a fresh copy will ensure the most up-to-date version.

Using the calculator

Download the Excel file. Instructions are provided at the top of the page. The file runs macros so if your operating system is set to block macros you will have to change that to allow them. If you have never used Solver, you may need to invoke Solver from the Tools Menu, Add-ins, check Solver. Excel will ask if you want to install it, you may need the original installation disks to install it. Once Solver is installed on your computer you will not need to install it again.

When you click on “Solve”, Solver will be run. The values of V_cmax, J, TPU, g_m, and R_d that minimize differences between predicted and observed photosynthesis rates are given and used to generate the lines on the graph. If one or another point is far from the line you could consider running the analysis without that point by setting the “Limitation” to 0. The graph data can also be updated by varying the R_d and g_m values.

If you are having any problems using the calculator please see our Help / FAQ page.

Click Here to download the Excel file and start calculating >>
(to download the file, please right-click on the link and select "Save Target As...")

(Last updated: 15th October 2007, version 1.1)