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 for 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. tequilana. When leaves were rehydrated in water, the decrease in Kh was completely 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, resulting 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 for A. deserti compared to 97% for A. tequilana. Based on leaf water potentials during drought and leaf vulnerability to cavitation, A. deserti and A. tequilana would experience 41% and 80% decreases in K-h, respectively, at 100 d of drought. The difference between the two species was primarily caused by the higher leaf water potentials of A. deserti compared to A. tequilana, rather than by differences in vulnerability to cavitation. A model of whole-plant axial hydraulic conductance for A. deserti indicated that, under well-watered conditions, the leaves had a lower axial hydraulic conductivity than the roots. After 60 d of drought, however, the root hydraulic conductivity was lower than that of the leaves, a phenomenon that would aid in restricting water loss from the plant to the soil during extended drought.