DEPENDENCY OF C(I) C(A) AND LEAF TRANSPIRATION EFFICIENCY ON THE VAPOR-PRESSURE DEFICIT/

The leaf transpiration efficiency (A/E, where A is the assimilation ra te and E the transpiration rate) is widely used to evaluate plant resp onses to the environment, yet little attention has been paid to its re lationship with vapour pressure deficit (D), the driving force for E. The proposed model is based on the increasingly recognised linear rela tionship between the ratio of intercellular to ambient CO2 partial pre ssures (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 c(i)/c(a) is constant. A/E predicted by the model agreed with that measured for the C-3 Encelia farinosa and the C-4 Pleuraphis rig ida, common species in the north-western Sonoran Desert, based on gas exchange measured in the field and in environmental chambers. The depe ndency of c(i)/c(a) and A/E on D was additionally evaluated using publ ished data for five other C-3 species and two other C-4 species. Gener ally, c(i)/c(a) was more sensitive to changes in D for the C-4 species than the C-3 species. The predictions for A/E by the model were also compared with predictions using a constant c(i)/c(a), either a general c(i)/c(a) (0.7 for C-3 and 0.3 for C-4) or a species-dependent mean c (i)/c(a). Overall, the proposed model performed best for both the C-3 and C-4 species; using the general c(i)/c(a) always resulted in an ove r-prediction of A/E.