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, Hymenocleasalsola Torr. and A. Gray and Ambrosia dumosa (A. Gray) Payne,were collected along a climatic gradient, germinated in a greenhouseand the plants tested for their vulnerability to cavitationby the air-pressure method. Differences among populations wereevident in A. dumosa, but not in H. salsola. Greenhouse treatmentssimulating regimes in temperature and relative humidity encounteredin different desert environments did not cause appreciable changesin vulnerability to cavitation. It is suggested that a homeostaticmechanism may have helped in maintaining a constant water potentialdrop in the xylem with little need for an adjustment in theresistance to cavitation. Different plant organs had differentvulnerabilities to cavitation, with roots being the most andwoody stems the least susceptible. Young green twigs were intermediate.A simulation model confirmed that low water potentials are mostlikely to cause runaway cavitation in the roots, not in theother organs. It is hypothesized that green twigs are adaptedto the favourable water conditions of the growing season, whilewoody stems are adapted to endure prolonged periods of droughtstress. Key words: Cavitation, xylem embolism, hydraulic conductance, Hymenoclea salsola, Ambrosia dumosa