Xylem cavitation in excised leaves of Malus sylvestris Mill. and measurement of leaf water status with the pressure chamber

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Abstract
A change occurs in the relationship between xylem water potential (Ψx) measured with the pressure chamber and leaf water potential ((Ψw)) when a period of post-excision water loss is allowed before measurement of (Ψx) and (Ψx). When this occurs, water stress is over-estimated by the pressure chamber measurement. Over the same period of water loss, cavitation vibrations have been detected acoustically in excised leaves. It is suggested that the measurement of (Ψx) is affected by emptying of some of the xylem vessels due to cavitation. This would require that additional pressure be applied to a leaf in the pressure chamber in order to measure (Ψx).