Water transport, xylem cavitation, and environmental influences on plant frequency for various arid-land plants

At a site in the northwestern Sonoran Desert the percent ground cover for the C₃ subshrub Encelia farinosa was eight times higher on more and 20° south-facing slopes than on 20° north-facing slopes. The ground cover of the C₄ bunchgrass Pleuraphis rigida decreased over 50% from 20{dollar} north-facing slopes to the 20° south-facing slope and the CAM leaf succulent Agave deserti had similar ground cover on the two slopes. The key for the relative frequency of E. farinosa was apparently the warmer slopes during the winter, whereas the key for P. rigida was most likely soil water availability. Thus root properties may exert the primary influence on relative plant frequency in this desert ecosystem for which soil temperature and water availability are crucial.; Loss of axial hydraulic conductance as a result of xylem cavitation was examined for roots of the CAM succulents A. deserti and Opuntia ficus-indica using an air-injection and a centrifugal method. Agave deserti had a mean cavitation pressure of −0.93 ± 0.08 MPa compared to −0.70 ± 0.02 MPa for O. ficus-indica , reflecting the greater tolerance of the former species to low water potentials in its native habitat. A model of axial hydraulic flow based upon the cavitation response of these species predicted that water uptake rates are far below the maximum possible, owing to the high root water potentials of these desert succulents.; Hydraulic conductance was measured for fresh, dehydrated and rehydrated leaves of the CAM leaf succulents, A. deserti and A. tequilana. Dehydration of leaves at 35°C for several hours caused the hydraulic conductance to decrease, with 50% loss of hydraulic conductance occurring at a leaf water potential of −2.37 MPa for A. deserti and at −1.72 MPa for A. tequilana. 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, after 100 d of drought. Also, transpiration decreased 87% after 40 d of drought for A. deserti compared to 97% for A. tequilana. A model of whole-plant hydraulic conductance for A. deserti indicated that, under well watered conditions, the leaves were the limiting organ for water movement. After 60 d of drought, however, the roots become limiting for water movement, a phenomenon that would aid in restricting water loss from the plant to the soil during extended drought.