Improved method for measuring the apparent CO2 photocompensation point resolves the impact of multiple internal conductances to CO2 to net gas exchange
There is growing interest in accurate and comparable measurements of the CO2 photocompensation point (Γ*), a vital parameter to model leaf photosynthesis. The Γ* is measured as the common intercept of several CO2 response curves, but this method may incorrectly estimate Γ* by using linear fits to extrapolate curvilinear responses and single conductances to convert intercellular photocompensation points (Ci*) to chloroplastic Γ*. To determine the magnitude and minimize the impact of these artifacts on Γ* determinations we used a combination of meta-analysis, modeling, and original measurements to develop a framework to accurately determine Ci*. Our modeling indicated that the impact of using linear fits could be minimized based on the measurement CO2 range. We also propose a novel method of analyzing common intercept measurements using slope-intercept regression. Our modeling indicated that slope-intercept regression is a robust analytical tool that can help determine if a measurement is biased due to multiple internal conductances to CO2. Application of slope-intercept regression to Nicotiana tabacum and Glycine max revealed that multiple conductances likely have little impact to Ci* measurements in these species. These findings present a robust and easy to apply protocol to help resolve key questions concerning CO2 conductance through leaves.