Improving the estimation of mesophyll conductance to CO2: on the role of electron transport rate correction and respiration

Mesophyll conductance (gm) can markedly limit photosynthetic CO2 assimilation and is required to estimate the parameters of the Farquhar–von Caemmerer–Berry (FvCB) model properly. The variable J (electron transport rate) is the most frequently used method for estimating gm, and the correct determination of J is one of its requirements. Recent evidence has shown that calibrating J can lead to some errors in estimating gm, but to what extent the parameterization of the FvCB model is affected by calibrations is not well known. In addition to determining the FvCB parameters, variants of the J calibration method were tested to address whether varying CO2 or light levels, possible alternative electron sinks, or contrasting leaf structural properties might play a role in determining differences in αβ, the product of the leaf absorptance (α) and the photosystem II optical cross-section (β). It was shown that differences in αβ were mainly attributed to the use of A/Ci or A/PPFD curves to calibrate J. The different αβ values greatly influenced gm, leading to a high number of unrealistic values in addition to affecting the estimates of the FvCB model parameters. A new approach was devised to retrieve leaf respiration in the light from combined A/Ci and A/Cc curves and a framework to understand the high variation in observed gm values. Overall, a background is provided to decrease the noise in gm, facilitating data reporting and allowing better retrieval of the information presented in A/Ci and A/Cc curves.