Drought-induced xylem dysfunction in petioles, branches, and roots of Populus balsamifera L and Alnus glutinosa (L) Gaertn

Variation in vulnerability to xylem cavitation was measured within individual organs of Populus balsamifera L. and Alnus glutinosa (L.) Gaertn. Cavitation was quantified by three different techniques: (a) measuring acoustic emissions, (b) measuring loss of hydraulic conductance while air-dehydrating a branch, and (c) measuring loss of hydraulic conductance as a function of positive air pressure injected into the xylem. All of these techniques gave similar results. In Populos, petioles were more resistant than branches, and branches were more resistant than roots. This corresponded to the pattern of vessel width: maximum vessel diameter in 1- to 2-year-old roots was 140 mu m, compared to 65 and 45 mu m in rapidly growing 1-year-old shoots and petioles, respectively. Cavitation in Populus petioles started at a threshold water potential of -1.1 MPa. The lowest leaf water potential observed was -0.9 MPa. In Alnus, there was no relationship between vessel diameter and the cavitation response of a plant organ. Although conduits were narrower in petioles than in branches, petioles were more vulnerable to cavitation. Cavitation in petioles was detected when water potential fell below -1.2 MPa. This value equaled midday leaf water potential in late June. As in Populos, roots were the most vulnerable organ. The significance of different cavitation thresholds in individual plant organs is discussed.