Relationships between Water Stress and Ultrasound Emission in Apple (Malus×domesticaBorkh.)

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Abstract
The relationships between ultrasound emissions (AEs) and water relations of both potted and field-grown apple trees were investigated. AEs were more frequent in unirrigated than in irngated trees. Evidence was accumulated that AEs were related to cavitation events in the xylem vessels. The threshold for occurrence of AEs was approximately −1·0 MPa in all types and sizes of tree studied, though fell to −2·0 MPa or lower after a period of water stress. Ultrasound emissions were much more frequent during an initial drying to about −3·0 MPa than in a second drying cycle after rehydration. The reduction in AEs during the second drying cycle was more than expected from a comparison of the magnitude of water loss in the two cycles. The cross sectional area of wood that was conducting was markedly reduced by a single drought to −3·0 MPa. Drilling into the stem at different distances from the sensor was used to provide further evidence that AEs represented cavitation events.