Abstract
Water in plant xylem is often superheated, and therefore in a meta-stable state. Under certain conditions, it may suddenly turn from the liquid to the vapor state. This cavitation process produces acoustic emissions. We report the measurement of ultrasonic acoustic emissions (UAE) produced by natural and induced cavitation in corn stems. We induced cavitation and UAE in vivo, in well controlled and reproducible experiments, by irradiating the bare stem of the plants with a continuous-wave laser beam. By tracing the source of UAE, we were able to detect absorption and frequency filtering of the UAE propagating through the stem. This technique allows the unique possibility of studying localized embolism of plant conduits, and thus to test hypotheses on the hydraulic architecture of plants. Based on our results, we postulate that the source of UAE is a transient "cavity oscillation"' triggered by the disruptive effect of cavitation inception.Received: 14 March 2011, Accepted: 23 April 2012; Edited by: V. Lakshminarayanan; Reviewed by: C. Negreira, Laboratorio de Acústica Ultrasonora, Universidad de la República, Uruguay; DOI: http://dx.doi.org/10.4279/PIP.040003Cite as: E. Fernández, R. J. Fernández, G. M. Bilmes, Papers in Physics 4, 040003 (2012)
Highlights
The cohesion-tension theory suggests that water in the xylem of transpiring plants is under tension with a hydrostatic pressure below atmospheric and, most of the time at “negative” values [1]
We report the measurement of ultrasonic acoustic emissions (UAE) produced by natural and induced cavitation in corn stems
Under sufficiently high tension, xylem may fail to maintain this state, causing liquid water to turn into vapor in a violent way
Summary
The cohesion-tension theory suggests that water in the xylem of transpiring plants is under tension with a hydrostatic pressure below atmospheric and, most of the time at “negative” values [1]. According to its phase diagram, water under these conditions is overheated (i.e., in a metastable state). It should not be in the liquid but in the vapor phase [3]. Under sufficiently high tension (i.e., low pressures caused by water deficit), xylem may fail to maintain this state, causing liquid water to turn into vapor in a violent way. This phenomenon, usually known as cavitation, causes the embolism of the conduits, reducing tissue hydraulic conductivity and exacerbating plant physiological stress [4,5]. Some herbaceous species are known to sustain cavitation almost every day, repairing embolism during the night, while most woody species preclude cavitation occurrence by a combination of stomatal behavior and anatomical and morphological adjustment [6]
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