Comparison of the A-C_c curve fitting methods in determining maximum ribulose 1·5-bisphosphate carboxylase/oxygenase carboxylation rate, potential light saturated electron transport rate and leaf dark respiration

A review of the literature revealed that a variety of methods are currently used for fitting net assimilation of CO₂-chloroplastic CO ₂ concentration (A-C_c) curves, resulting in considerable differences in estimating the A-C_c parameters [including maximum ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco) carboxylation rate (V_cmax), potential light saturated electron transport rate (J_max), leaf dark respiration in the light (R_d), mesophyll conductance (g_m) and triose-phosphate utilization (TPU)]. In this paper, we examined the impacts of fitting methods on the estimations of V _cmax, J_max, TPU, R_d and g_m using grid search and non-linear fitting techniques. Our results suggested that the fitting methods significantly affected the predictions of Rubisco-limited (A_c), ribulose 1,5-bisphosphate-limited (A_j) and TPU-limited (A_p) curves and leaf photosynthesis velocities because of the inconsistent estimate of V_cmax, J_max, TPU, R _d and g_m, but they barely influenced the J_max : V_cmax, V_cmax : R_d and J_max : TPU ratio. In terms of fitting accuracy, simplicity of fitting procedures and sample size requirement, we recommend to combine grid search and non-linear techniques to directly and simultaneously fit V_cmax, J_max, TPU, R_d and g_m with the whole A-C_c curve in contrast to the conventional method, which fits V_cmax, R_d or g_m first and then solves for V_cmax, J_max and/or TPU with V_cmax, R_d and/or g_m held as constants.