Declining hydraulic efficiency as transpiring leaves desiccate: two types of response *

The conductance of transpiring leaves to liquid water (Kleaf) was measured across a range of steady-state leaf water potentials (Ψleaf). Manipulating the transpiration rate in excised leaves enabled us to vary Ψleaf in the range −0.1 MPa to less than −1.5 MPa while using a flowmeter to monitor the transpiration stream. Employing this technique to measure how desiccation affects Kleaf in 19 species, including lycophytes, ferns, gymnosperms and angiosperms, we found two characteristic responses. Three of the six angiosperm species sampled maintained a steady maximum Kleaf while Ψleaf remained above −1.2 MPa, although desiccation of leaves beyond this point resulted in a rapid decline in Kleaf. In all other species measured, declining Ψleaf led to a proportional decrease in Kleaf, such that midday Ψleaf of unstressed plants in the field was sufficient to depress Kleaf by an average of 37%. It was found that maximum Kleaf was strongly correlated with maximum CO2 assimilation rate, while Kleaf = 0 occurred at a Ψleaf slightly less negative than at leaf turgor loss. A strong linear correlation across species between Ψleaf at turgor loss and Ψleaf at Kleaf = 0 raises the possibility that declining Kleaf was related to declining cell turgor in the leaf prior to the onset of vein cavitation. The vulnerability of leaves rehydrating after desiccation was compared with vulnerability of leaves during steady-state evaporation, and differences between methods suggest that in many cases vein cavitation occurs only as Kleaf approaches zero