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

Abstract

Mesophyll conductance (g m ) 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 g m, 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 g m , 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 CO 2 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/C i or A/PPFD curves to calibrate J. The different αβ values greatly influenced g m , 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/C i and A/C c curves and a framework to understand the high variation in observed g m values. Overall, a background is provided to decrease the noise in g m , facilitating data reporting and allowing better retrieval of the information presented in A/C i and A/C c curves.