Theoretical reconsiderations when estimating the mesophyll conductance to CO2 diffusion in leaves of C3 plants by analysis of combined gas exchange and chlorophyll fluorescence measurements

Existing methods to estimate the mesophyll conductance to CO2 diffusion (gm) are often based on combined gas exchange and chlorophyll fluorescence measurements. However, estimations of average gm by these methods are often unreliable either because the range of usable data is too narrow or because the estimations are very sensitive to measurement errors. We describe three method variants to estimate gm, for which a wider range of data are usable. They use curve-fitting techniques, which minimise the sum of squared model deviations from the data for A (CO2 assimilation rate) or for J (linear electron transport rate). Like the existing approaches, they are all based on common physiological principles assuming that electron transport limits A. The proposed variants were far less sensitive than the existing approaches to 'measurement noise' either created randomly in the generated data set or inevitably existing in real data sets. Yet, the estimates of gm from the three variants differed by approximately 15%. Moreover, for each variant, a stoichiometric uncertainty in linear electron transport-limited photosynthesis can cause another 15% difference. Any estimation of gm using gas exchange and chlorophyll fluorescence measurements should be considered with caution, especially when gm is hig