Influence of diurnal variation in mesophyll conductance on modelled (13)C discrimination: results from a field study

Mesophyll conductance to CO(2) (g(m)) limits carbon assimilation and influences carbon isotope discrimination (Δ) under most environmental conditions. Current work is elucidating the environmental regulation of g(m), but the influence of g(m) on model predictions of Δ remains poorly understood. In this study, field measurements of Δ and g(m) were obtained using a tunable diode laser spectroscope coupled to portable photosynthesis systems. These data were used to test the importance of g(m) in predicting Δ using the comprehensive Farquhar model of Δ (Δ(comp)), where g(m) was parameterized using three methods based on: (i) mean g(m); (ii) the relationship between stomatal conductance (g(s)) and g(m); and (iii) the relationship between time of day (TOD) and g(m). Incorporating mean g(m), g(s)-based g(m), and TOD-based g(m) did not consistently improve Δ(comp) predictions of field-grown juniper compared with the simple model of Δ (Δ(simple)) that omits fractionation factors associated with g(m) and decarboxylation. Sensitivity tests suggest that b, the fractionation due to carboxylation, was lower (25‰) than the value commonly used in Δ(comp) (29‰) and Δ(simple) (27‰). These results demonstrate the limits of all tested models in predicting observed juniper Δ, largely due to unexplained offsets between predicted and observed values that were not reconciled in sensitivity tests of variability in g(m), b, or e, the day respiratory fractionation.