Mesophyll Conductance: Methods of Estimation and Potential Use to Improve Water Use Efficiency.

Reliable estimation of mesophyll conductance (g_m) thus biochemical parameters for C₃-potosynthesis model is a challenging task but a pre-requisite to accurately assess the plant primary productivity. Estimation of gm even becomes increasingly important due to its role to improve water use efficiency which could potentially be independent of the stomatal conductance (g_s) or the maximum rate of carboxylation. Despite the recent development of various methods, precise gm estimation is still a daunting challenge. Largely the discrepancies among the methods of gm estimations are found due to the prediction of theoretical CO₂ concentration in the chloroplasts (C_c) which is based on gas exchange (GE) or chlorophyll fluorescence (CF) measurement. Due to the inherent limitations of various methods, use of two or more independent gm estimation methods sharing few common assumptions have been recommended. Here we attempt to evaluate various gm estimation methods using experimental data on soybean grown at a range of phosphorus nutrition under ambient and elevated CO₂ concentrations. Results showed a common pattern of gm and biochemical parameters response to the treatments despite the significant impact of methods on gm and carboxylation efficiency. Moreover, various g_m estimation methods did not affect the partitioning of stomatal and non-stomatal limitation to photosynthesis. We highlight the details of a fairly unexplored but simple gm estimation method which is based on the oxygen sensitivity of Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) enzyme and its potential application to improve water use efficiency in crops.