Three different methods for measuring xylem cavitation and embolism: a comparison

Three different methods for measuring xylem embolism due to water cavitation were compared—the acoustic method, the hydraulic method and the anatomical method. Young plants of Ceratonia siliqua L. were water stressed for 9, 16 and 23 d. Xylem cavitation was detected by counting ultrasound (100–300 kHz) acoustic emissions (AE) from 1-year-old twigs (acoustic method). Xylem embolism was detected by measuring the loss of hydraulic conductivity of twigs of the same age (hydraulic method). The blockage of single xylem conduits was detected by perfusing Safranin into the xylem of 1-year-old twigs of stressed plants and measuring the number and the diameters of non-conducting xylem conduits, under the microscope (anatomical method). It was noted that: (a) the number of AE and the loss of conductivity increased with the water stress applied; (b) a linear relation seemed to exist between the number of AE and the loss of conductivity, suggesting that the AE counted could be only (or mainly) produced in the xylem conduits; (c) the vulnerability of the xylem conduits to embolism was a direct function of their diameter; and (d) the measured loss of conductivity was of the same order of magnitude as the theoretical one. The three methods gave fairly similar results. Nonetheless, they are not alternative to one another in that: (a) the acoustic method allows continuous recordings to be made but does not provide information about the actual damage suffered by plants; (b) the hydraulic method is very informative but destructive; and (c) the anatomical method is very useful both in phytogcographical and in genetic improvement studies.