Ultrasound acoustic emissions from freezing xylem

Abstract

Rachides of Juglans regia L. (Juglandaceae) and one-year-old twigs of Evonymus latifolia (L.) Mill. (Celastraceae) were cooled in air to -25°C, with an ultrasound detector attached to the xylem where peripheral tissues had been peeled off. Ultrasound acoustic emissions started between -4.5 and -14-3°C, as measured with a thermocouple inserted into the xylem. The number of emissions was significantly lower from saturated plant parts than from those frozen at field water potentials. Bench-drying of saturated samples produced significantly less signals than the freezing protocols. These findings are in accordance with the hypothesis that freezing of xylem under tension induces cavitation events. They corroborate earlier work which tried to provide a logical explanation for the seemingly paradoxical cryo-scanning electron microscope observations of changing vessel contents during a daycourse in the field.