Hydraulic conductivity, xylem cavitation, and water potential for succulent leaves of Agave deserti and Agave tequilana

Axial hydraulic conductivity (K-h) was measured for fresh, dehydrated, and rehydrated leaves of the Crassulacean acid metabolism (CAM) leaf succulents , Agave deserti and Agave tequilana. Dehydration of leaves at 35 degreesC f or several hours caused K-h to decrease, with a 50% decrease occurring at a leaf water potential of -2.37 MPa for A. deserti and at -1.72 MPa for A. t equilana. When leaves were rehydrated in water, the decrease in Kh was comp letely reversible, indicating that the decrease under dehydrated conditions was caused by xylem cavitation. During extended drought of potted plants, leaves of A. deserti dehydrated less rapidly than those of A. tequilana, re sulting in leaf water potentials of -1.96 MPa and -3.42 MPa, respectively, at 100 d of drought. Also, transpiration decreased 87% at 40 d of drought f or A. deserti compared to 97% for A. tequilana. Based on leaf water potenti als during drought and leaf vulnerability to cavitation, A. deserti and A. tequilana would experience 41% and 80% decreases in K-h, respectively, at 1 00 d of drought. The difference between the two species was primarily cause d by the higher leaf water potentials of A. deserti compared to A. tequilan a, rather than by differences in vulnerability to cavitation. A model of wh ole-plant axial hydraulic conductance for A. deserti indicated that, under well-watered conditions, the leaves had a lower axial hydraulic conductivit y than the roots. After 60 d of drought, however, the root hydraulic conduc tivity was lower than that of the leaves, a phenomenon that would aid in re stricting water loss from the plant to the soil during extended drought.