# Acoustic Emissions to Measure Drought-Induced Cavitation in Plants
## 3 Acoustic Emission (AE) Application to Measure Drought-Induced Cavitation: From Past to Present  
As stated in the introduction, [PERSON] and [PERSON] [42] recorded sounds in plants when they were subjected to dehydration. They detected audible vibrations (<20 kHz) in petioles of dehydrating leaves of diverse plant species by fixing the petiole on a phonograph pick-up needle (Figure 2). Because measurements were often disturbed by environmental noise, the step was made towards AE detection in the ultrasonic frequency range (>20 kHz) by [PERSON] and [PERSON] [43]. This has facilitated various experiments because the problematic ambient noise in the audible range could be electronically filtered. Based on similar sound production by the rupture of plant sap under tension in glass tubes [18; 44], it was hypothesized that the rupture of sap in plant conduits produced the observed sounds during drying. Although extensive circumstantial evidence was provided to support this hypothesis [45; 46; 47; 48], it was realized that sounds may be produced by other mechanisms too. Sounds were observed during drying of plant tissues that do not contain conduits [45; 49], during re-watering [48], and also during freezing [50; 51; 52] and thawing [50; 51]. The AE technique is also applied to monitor internal cracking of wood during drying [53; 54] and soaking [53]. Moreover, [PERSON] _et al._[55] speculated that plants may actively produce sounds for short-distance communication. AE detection is thus an interesting tool, applicable in a wide range of domains. Further scope of this review is, however, on its application in the detection of drought-induced cavitation in plants. Figure 1: Typical vulnerability curve with P12, P50 and P88 representing the xylem water potential at which, respectively, 12%, 50% and 88% of xylem hydraulic conductivity is lost, adapted from [PERSON] and Gartner [30] and [PERSON] _et al._[36]. During dehydration of fresh plant material, AE signal detection was found to be a valid method to measure cavitation in woody branches [35], leaves [56], herbaceous stems [57] and sap wood sections [49], while others found a poor correspondence between hydraulic and acoustic measurements [58; 59]. Especially in angiosperm species, the continued AE activity after loss of most of the hydraulic conductivity was a great cause of concern [31; 60]. Other processes than cavitation in sap-conducting elements that could cause extra AE signals during drought stress have been suggested by various authors: cavitation of fibers, wood tracheids or ray cells [58; 59; 61; 62], mechanical strains [25; 48], and microscopic failure [43; 63; 64]. Moreover, the actual mechanism that causes cavitation-related AE signals is not exactly known. Different processes have also been suggested: vibration of the conduit wall after sudden pressure release due to cavitation [42; 43; 60], oscillation of hydrogen bounds in water after pressure release [43], conversion from liquid water to vapour during cavitation of the water [25], pit membrane rupture [65], torus aspiration (in gymnosperms [43]), the entry of a gas bubble through a pore in the pit membrane [25], and subsequent bubble oscillation [23]. Moreover, as the number of detected cavitation related AE signals may be larger than the number of cavitated conduits [56; 66; 67], different AE-inducing processes might be involved during cavitation. Attempts have been made to use signal features in order to get more insights into the underlying processes. [PERSON] and [PERSON] [61] suggested that vessel length has an influence on the cut-off frequency of the detected signals. They suggested that cavitation in long vessels produced broadband signals with frequencies down to 500 Hz, whereas cavitation in fibers and wood tracheids only produced higher frequencies (>100 kHz). [PERSON] and [PERSON] [60] observed that, when detecting AE signals in the frequency range 50-1000 kHz, the frequency spectra changed towards higher frequencies when the degree of cavitation increased in _Thuja_, _Pinus_, and _Acer_ stems.