A method for measuring xylem hydraulic conductance and embolism in entire root and shoot systems

Current methods for determining the influence of xylem cavitation on hydraulic conductance are limited to unbranched stem or root segments with hydraulic conductances above c. 2 mmol s−1 MPa−1. Lower conductances and/or highly branched systems are encountered in seedlings, arid-land shrubs, herbs, and distal portions of shoot and root systems of trees. In order to quantify the hydraulic impact of cavitation in such systems, existing techniques have been modified. Branched shoot or root systems were prepared for measurement by removal of leaves, or root tips, respectively. The shoot or root system was enclosed in a vacuum chamber with the proximal end protruding and supplied with perfusing solution. Flow through the xylem was induced by chamber vacuum. Hydraulic conductance was determined from the slope of the flow rate versus pressure relationship. Xylem embolism was quantified from the increase in hydraulic conductance following high pressure (100 kPa) perfusion of solution through the plant. Examples are provided of the application of the method to cavitation studies in the cold desert shrub Artemisia tridentata.