MAXIMUM SUSTAINABLE XYLEM SAP TENSIONS IN RHODODENDRON AND OTHER SPECIES

The acoustic technique was used in conjunction with the pressure chamber to determine the tensions causing cavitation of xylem sap in leaves of 5 woody angiosperms (Acer pseudoplatanus L., Alnus glutinosa L. Gaertn., Eucalyptus globulus Labill., Fraxinus excelsior L. and Rhododendron ponticum L.) and 3 spp. of herbs (Lycopersicon esculentum Mill., Plantago major L. and Ricinus communis L.). Leaves of most species suffer considerably from cavitation at sap tensions of 1.6-3 MPa [megapascal]. Two of the herbs, Lycopersicon and Ricinus, cavitated extensively at sap tensions below 1 MPa. Additional evidence is presented that clicks, detected by acoustic amplification, are caused by cavitation of sap in the xylem conduits. A rapid method is suggested for determination of sap tensions in cavitating leaves and which is suitable for surveys of the critical sap tension in a large number of species.