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

The analysis and interpretation of A/C-i curves (net CO2 assimilation rate, A, versus calculated substomatal CO2 concentration, C-i) is dependent upon a number of underlying assumptions. The influence of the C-i value at which the A/C-i curve switches between the Rubisco- and electron transport-limited portions of the curve was examined on A/C-i curve parameter estimates, as well as the effect of mesophyll CO2 conductance (g(m)) values on estimates of the maximum rate of Rubisco-mediated carboxylation (V-cmax). Based on an analysis using 19 woody species from the Pacific Northwest, significant variation occurred in the C-i 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/C-i parameters (e.g. V-cmax) may arise when preliminary estimates of C-i_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/C-i and chlorophyll fluorescence estimates of carboxylation is achieved. The use of the V-cmax parameter to describe accurately the Rubisco activity from the A/C-i curve analysis is also dependent upon the assumption that C-i is approximately equal to chloroplast CO2 concentrations (C-c). If leaf mesophyll conductance is low, C-c will be much lower than C-i and will result in an underestimation of V-cmax from A/C-i curves. A large range of mesophyll conductance (g(m)) 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 V-cmax estimates, considerable variation still existed between species, but the difference in V-cmax between conifer and broadleaf species was reduced from c. 11 mumol m(-2) s(-1) to 4 mumol m(-2) s(-1). For example, A/C-i curve estimates of V-cmax were 31.2+/-6.2 and 42.2+/-4.4 mumol m(-2) s(-1), and A/C-c curve estimates were 41.2+/-7.1 mumol m(-2) s(-1) and 45.0+/-4.8 mumol m(-2) s(-1), for the conifer and broadleaf species, respectively.