Dependency of c_I/c_a and Leaf Transpiration Efficiency on the Vapour Pressure Deficit

Abstract

The leaf transpiration efficiency (A/E, where A is the assimilation rate and E the transpiration rate) is widely used to evaluate plant responses to the environment, yet little attention has been paid to its relationship with vapour pressure deficit (D), the driving force for E. The proposed model is based on the increasingly recognised linear relationship between the ratio of intercellular to ambient CO₂ partial pressures (c_I/c_a) and D. Unlike previous models for A/E, the proposed model does not assume that the leaf and air temperatures are the same or that ci/ca is constant. A/E predicted by the model agreed with that measured for the C₃ Encelia farinosa and the C₄ Pleuraphis rigida, common species in the north-westem Sonoran Desert, based on gas exchange measured in the field and in environmental chambers. The dependency of c_I/c_a and A/E on D was additionally evaluated using published data for five other C₃ species and two other C₄ species. Generally, ci/ca was more sensitive to changes in D for the C₄ species than the C₃ species. The predictions for A/E by the model were also compared with predictions using a constant ci/ca, either a general c_I/c_a (0.7 for C₃ and 0.3 for C₄) or a species-dependent mean c_I/c_a. Overall, the proposed model performed best for both the C₃ and C₄ species; using the general c_I/c_a always resulted in an over-prediction of A/E.