Vulnerability to cavitation in populations of two desert species, Hymenoclea salsola and Ambrosia dumosa, from different climatic regions

Seeds from different populations of two desert species, Hymenoclea salsola Torr. and A. Gray and Ambrosia dumosa (A. Gray) Payne, were collected along a climatic gradient, germinated in a greenhouse and the plants tested for their vulnerability to cavitation by the air-pressure method. Differences among populations were evident in A. dumosa, but not in H. salsola. Greenhouse treatments simulating regimes in temperature and relative humidity encountered in different desert environments did not cause appreciable changes in vulnerability to cavitation. It is suggested that a homeostatic mechanism may have helped in maintaining a constant water potential drop in the xylem with little need for an adjustment in the resistance to cavitation. Different plant organs had different vulnerabilities to cavitation, with roots being the most and woody stems the least susceptible. Young green twigs were intermediate. A simulation model confirmed that low water potentials are most likely to cause runaway cavitation in the roots, not in the other organs. It is hypothesized that green twigs are adapted to the favourable water conditions of the growing season, while woody stems are adapted to endure prolonged periods of drought stress.