A/Ci curve analysis across a range of woody plant species: Influence of regression analysis parameters and mesophyll conductance

Abstract

The analysis and interpretation of A/Ci curves (net CO 2 assimilation rate, A, versus calculated substomatal CO2 concentration, Ci) is dependent upon a number of underlying assumptions. The influence of the Ci value at which the A/C i curve switches between the Rubisco-and electron transport-limited portions of the curve was examined on A/Ci curve parameter estimates, as well as the effect of mesophyll CO2 conductance (gm) values on estimates of the maximum rate of Rubisco-mediated carboxylation (Vcmax). Based on an analysis using 19 woody species from the Pacific Northwest, significant variation occurred in the Ci value where the Rubisco- and electron transport-limited portions of the curve intersect (C i_t), ranging from 20 Pa to 152 Pa and averaging c. 71 Pa and 37 Pa for conifer and broadleaf species, respectively. Significant effects on estimated A/Ci parameters (e.g. Vcmax) may arise when preliminary estimates of Ci_t, necessary for the multiple regression analyses, are set either too high or too low. However, when the appropriate threshold is used, a significant relationship between A/Ci and chlorophyll fluorescence estimates of carboxylation is achieved. The use of the Vcmax parameter to describe accurately the Rubisco activity from the A/Ci curve analysis is also dependent upon the assumption that C i is approximately equal to chloroplast CO2 concentrations (Cc). If leaf mesophyll conductance is low, Cc will be much lower than Ci and will result in an underestimation of V cmax from A/Ci curves. A large range of mesophyll conductance (gm) values was observed across the 19 species (0.005±0.002 to 0.189±0.011 mol m-2 s-1 for Tsuga heterophylla and Quercus garryana, respectively) and, on average, g m was 1.9 times lower for the conifer species (0.058±0.017 mol m-2 s-1 for conifers versus 0.112±0.020 mol m -2 s-1 for broadleaves). When this mesophyll limitation was accounted for in Vcmax estimates, considerable variation still existed between species, but the difference in Vcmax between conifer and broadleaf species was reduced from c. 11 μmol m-2 s -1 to 4 μmol m-2 s-1. For example, A/C i curve estimates of Vcmax were 31.2±6.2 and 42.2±4.4 μmol m-2 s-1, and A/Cc curve estimates were 41.2±7.1 μmol m-2 s-1 and 45.0±4.8 μmol m-2 s-1, for the conifer and broadleaf species, respectively.