Theoretical reconsiderations when estimating the mesophyll conductance to CO(2) diffusion in leaves of C(3) plants by analysis of combined gas exchange and chlorophyll fluorescence measurements.

Existing methods to estimate the mesophyll conductance to CO(2) diffusion (g(m)) are often based on combined gas exchange and chlorophyll fluorescence measurements. However, estimations of average g(m) 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 g(m), 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 (CO(2) 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 g(m) 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 g(m) using gas exchange and chlorophyll fluorescence measurements should be considered with caution, especially when g(m) is high.