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Use of centrifugal force in the study of xylem cavitation

Two methods were evaluated for using centrifugal force to measure the occurrence of cavitation as a function of negative pressures in xylem. The general protocol was to measure the hydraulic conductivity of xylem segments (stem or root pieces) before and after centring them on a centrifuge rotor and spinning them about their long axis to generate negative xylem pressure. The percentage decrease in conductivity from the initial to final measurement was used to quantify the embolism resulting from cavitation during spinning. In one approach, segments were spun with their ends exposed to air. This method could only be used when xylem conduits were much shorter than the segment. Results from an angiosperm (Betula occidentalis) and a gymnosperm (Abies lasiocarpa) corresponded to previous observations of embolism caused by air dehydration where negative pressure was measured with the pressure chamber. Results also agreed with embolism caused by injection of air into the xylem, in support of the air-seeding hypothesis for cavitation. In a second approach, segments were spun in a rotor designed to keep the segment ends immersed in water during spinning. This gave the same results as for non-immersed segments. Immersing the segment ends allowed measurements on any material, regardless of conduit length, as demonstrated for roots of B. occidentalis. The chief advantage of the centrifugal force method is the rapidity and precision with which any desired xylem pressure can be imposed.