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Published online by Cambridge University Press: 04 August 2010
SUMMARY
Concurrent measurements of cavitation by the ultrasound acoustic emission technique and sap velocity by the thermoelectric heat pulse method were carried out in the field on three woody species (Quercus pubescens, Quercus ilex and Alnus cordatd) characterized by different wood structure. The plant water status was assessed by measuring xylem water potential and stomatal conductance. A good correspondence was found between the patterns of sap velocity and cavitation rate. A threshold-type relationship was observed, in Alnus cordata, between water flow and cavitation rate. In some cases temporal lags between ultrasound emission and sap velocity were observed: several factors may account for these lags, including the possibility that cavitation of xylem conduits may be a rather patchy phenomenon and that different xylematic volumes might have been sensed by the ultrasound and heat pulse transducers.
INTRODUCTION
The formation and spreading of gaseous emboli through the xylem are recognized as common events in water stressed plants (Milburn, 1979; Tyree & Sperry, 1989a). In particular, it is widely held that the increase of xylematic tension, which is caused by the drop of water potential between the soil and the atmosphere, frequently induces cavitation, i.e. the breakage of water columns and the formation of gas bubbles in the lumina of xylem conduits.
The mechanism of cavitation is still being debated, although the hypothesis that cavitation is caused by the aspiration of air bubbles through the intervessel pit membranes (the so-called air seeding hypothesis) is widely supported by experimental evidence (Sperry & Tyree, 1988; Sperry, Tyree & Donnelly, 1988).
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