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

The conductance of transpiring leaves to liquid water (K-leaf) was measured across a range of steady-state leaf water potentials (Psi(leaf)). Manipulating the transpiration rate in excised leaves enabled us to vary Psi(leaf) 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 K-leaf 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 K-leaf while Psi(leaf) remained above -1.2 MPa, although desiccation of leaves beyond this point resulted in a rapid decline in K-leaf. In all other species measured, declining Psi(leaf) led to a proportional decrease in K-leaf, such that midday Psi(leaf) of unstressed plants in the field was sufficient to depress K-leaf by an average of 37%. It was found that maximum K-leaf was strongly correlated with maximum CO2 assimilation rate, while K-leaf = 0 occurred at a Psi(leaf) slightly less negative than at leaf turgor loss. A strong linear correlation across species between Psi(leaf) at turgor loss and Psi(leaf) at K-leaf = 0 raises the possibility that declining K-leaf 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 K-leaf approaches zero.