# Acoustic Emissions to Measure Drought-Induced Cavitation in Plants
## 3 Acoustic Emission (AE) Application to Measure Drought-Induced Cavitation: From Past to Present
### In Vivo Measurements  
Dealing with drought stress is a dynamic process and the resistance of a plant against drought depends on both physiological and anatomical characteristics [21]. To obtain a good understanding of a plant's behavior during drought stress, _in vivo_ measurements on plants are necessary. Conventionalmethods to determine a plants' vulnerability to cavitation are destructive, and their results are recently under intense debate because several artefacts are reported to play a role [40; 41; 105]. In addition, these methods are labor-intensive, which hampers their applicability in the field. The call for non-destructive methods is therefore now more urgent than ever [39]. [PERSON] _et al._[41] recently recommended \(\upmu\)CT as the standard technique to measure hydraulic vulnerability, but this method is not suitable for field applications. Today, the AE technique is used in a destructive way and, thus, subject to similar artefacts as the conventional methods, but in contrast to these established methods, the AE technique has the potential to measure non-destructively, enabling automated and continuous measurements of cavitation in the field. Because of the difficult signal interpretation of AEs, only a few studies have focused on the possibility of _in vivo_ measurements of cavitation on actively growing trees with acoustic sensors. Field measurements on _P. sylvestris_ with a 150 kHz resonant sensor during the growing season showed good relationships between AE activity and stem diameter variation [73] or sap flow [106; 107]. In addition, such continuous measurements will provide valuable information on plant behavior during drought and, more specifically, contribute to the debated process of cavitation recovery [108; 109; 110; 36; 111].  
When using the AE technique on living plants in lab or field conditions, [PERSON] _et al._[80] use a broadband point-contact sensor (20-1000 kHz) with flat frequency response in order to differentiate between the different AE sources. Based on preliminary analysis and clustering, a minimum set of signal frequency features may be selected (weighted peak frequency, frequency centroid and partial powers) that enable discrimination between different AE sources. However, when it is sufficiently demonstrated that the resonant sensor shows good results, its use might be preferred in further practical applications because of the more straightforward data processing and interpretation. A second point of attention for _in vivo_ measurements is the particular nature of cavitation detection. Contrary to the hydraulic method and visualization methods, the AE technique measures changes in the degree of embolism rather than the degree of embolism itself. This principle should be kept in mind when preparing and interpreting AE measurements of cavitation. If the initial state of xylem embolism is known, cumulative AE measurements might result in the degree of embolism if cavitation has not been repaired in the measurement period. This will require a reference number of AE signals at 100% embolism, which may be obtained in a VC or at the end of a drought experiment. Combination of broadband AE measurements with other non-destructive measurements of sap flow, water potential, water content and xylem/phloem shrinkage [20] may yield the largest insight into plant water dynamics during drought. A better knowledge of these dynamics is crucial to feed mechanistic plant and climate models and will help with guiding mitigation of climate change impacts on plants in natural and agricultural communities [112].