NEW EVIDENCE FOR LARGE NEGATIVE XYLEM PRESSURES AND THEIR MEASUREMENTBY THE PRESSURE CHAMBER METHOD

Pressure probe measurements have been interpreted as showing that xyle m pressures below c. -0.4 MPa do not exist and that pressure chamber m easurements of lower negative pressures are invalid. We present new ev idence supporting the pressure chamber technique and the existence of xylem pressures well below -0.4 MPa. We deduced xylem pressures in wat er-stressed stem xylem from the following experiment: (1) loss of hydr aulic conductivity in hydrated stem xylem (xylem pressure = atmospheri c pressure) was induced by forcing compressed air into intact xylem co nduits; (2) loss of hydraulic conductivity from cavitation and embolis m in dehydrating stems was measured, and (3) the xylem pressure in deh ydrated stems was deduced as being equal and opposite to the air press ure causing the same loss of hydraulic conductivity in hydrated stems. Pressures determined in this way are only valid if cavitation was cau sed by air entering the xylem conduits (air-seeding). Deduced xylem pr essure showed a one-to-one correspondence with pressure chamber measur ements for 12 species (woody angiosperms and gymnosperms); data extend ed to c. -10 MPa. The same correspondence was obtained under field con ditions in Betula occidentalis Hook., where pressure differences betwe en air- and water-filled conduits were induced by a combination of in situ xylem water pressure and applied positive air pressure. It is dif ficult to explain these results if xylem pressures were above -0.4 MPa , if the pressure chamber was inaccurate, and if cavitation occurred b y some mechanism other than air-seeding. A probable reason why the pre ssure probe does not register large negative pressures is that, just a s cavitation within the probe limits its calibration to pressures abov e c. -0.5 MPa, cavitation limits its measurement range in situ.