Journal of Experimental Botany, Vol. 46, No. 290, pp. 1177-1183, September 1995 Journal of Experimental Botany Vulnerability of xylem to embolism in relation to leaf water potential and stomatal conductance in Fagus sylvatica f. purpurea and Populus balsamifera U. Hacke and J.J. Sauter 1 Botanisches Institut der Christian-Albrechts-Universita't zu Kiel, OlshausenstraBe 40, D-24098 Kiel, Germany Received 6 January 1995; Accepted 1 May 1995 Abstract The vulnerability of xylem vessels to water stress- induced cavitation was studied by measuring hydraulic conductivity and ultrasound acoustic emissions [AEs) in Fagus sylvatica L. f. purpurea (Ait.) Schneid. and Populus balsamifera L.. The occurrence of xylem embolism in summer was investigated in relation to leaf water potential and stomatal conductance. Populus was extremely vulnerable to cavitation, losing functional vessels due to embolism at water potentials lower than —0.7 MPa. Fagus experienced embolism when water potential fell below —1.9 MPa. Midday water potentials often approached these threshold values. When evaporative demand increased rapidly on sunny days, water loss became limited by low stomatal conductance. Thus water potentials fell only slightly below the threshold values inducing cavitation. Despite the differences in vulnerability, both species tolerated a similar embolism rate of about 10% in the summer. There was no embolism reversal during pro- longed periods of rain. AEs were predictive of loss in hydraulic conductivity, indicating that AEs were mainly confined to vessels. Finally, vessel length distribution, vessel diameter (tangential axis), vessel density, and vessel wall thickness had been determined for both species investigated. Populus had longer and wider vessels than Fagus, whereas vessel wall thickness was similar in both species. Key words: Acoustic emissions, Fagus, Populus, stomataJ closure, xylem embolism. Introduction Xylem sap of plants is usually under a high tension in the growing season. Therefore, water columns may be disrupted (cavitation). Cavitation leads to an air-filled (embolized) xylem conduit (Zimmermann, 1983). As a result of embolism, hydraulic conductance and possibly stomatal conductance is reduced. Some grasses (Poaceae) produce enough root pressure to reverse embolism over-night, so a high embolism rate on a given day can be tolerated (Tyree et ai, 1986; Neufeld et ai, 1992). In trees, however, there is usually no embolism repair during the growing season (Sperry et ai, 1988a; Tyree et ai, 1994). Thus, water potential (f) should not fall signific- antly below the threshold-value inducing cavitation (!PJ. Trees differ widely in their vulnerability to drought- induced cavitation (Tyree and Ewers, 1991; Zotz et ai, 1994). While moderate embolism rates in late summer have been reported for Acer saccharum (Sperry et ai, 1988a), Betula occidentals (Sperry and Sullivan, 1992) and Alnus cordata (Tognetti and Borghetti, 1994), several species of Populus have been described as extremely vulnerable, losing a large amount of the conducting tissue throughout the growing season by embolism (Tyree and Ewers, 1991; Tyree et ai, 1992, 1994). It has been suggested that stomata play an important role in limiting cavitation (Tyree and Ewers, 1991). Jones and Sutherland (1991) argued that tolerance of a slightly reduced hydraulic conductance might be beneficial in order to maximize stomatal aperture and hence short-term produc- tivity, but this remains to be tested. We report experiments made on Fagus sylvatica f. purpurea and Populus balsamifera. We evaluated the vul- nerability of both species to water stress-induced cavita- tion and characterize the relationship between xylem vulnerabilities and the operating ranges of f. Measurements indicated that water potentials in Fagus were often much lower (more negative) than in Populus. 1 To whom correspondence should be addressed. Fax: +49431 8801527. O Oxford University Press 1995