Researchers are turning to the novel method of acoustic emissions to quantify drought stress and xylem cavitation in plants. Acoustic emissions (AE) sensors are extremely easy to install, highly robust and the results are statistically simple to analyse and interpret.
but what, exactly, are AE sensors?
AE sensors have been around for many decades and have traditionally been used to measure stress waves in materials and structures. The stress waves are caused when mechanical pressures build in materials and structures. When the mechanical pressure is released, elastic waves propagate away from the area under stress. AE sensors then measure the frequency and magnitude of the waves and these data can be related to the severity of the stress or weakness of the material.
A common application for AE sensors is to measure the stress that concrete pylons of bridges are placed under when there is a load on the bridge – for example, trucks and cars. A concrete pylon maybe under no stress when there is a small truck on the bridge but under large stress when there are multiple trucks or high traffic loads. AE sensors attached to the concrete pylon can detect the stress waves through the concrete when increasingly heavy loads are placed on the bridge.
In plants, AE sensors connected to the stem or trunk can detect the stress of the xylem. That is, a well-hydrated plant is under low stress as its xylem conduits are saturated with water content. Under increasing drought or water deficit, there is an increase in stress as manifested by an increase in xylem embolism and cavitation. When xylem cavitates, a small but measurable wave is released that propagates throughout the plant tissue. The AE sensors can detect the wave by measuring its magnitude and frequency.
The AE sensors can be installed on plants within different experimental treatments, or different species or crop varieties undergoing drought stress, to provide invaluable insights of physiological responses to soil water deficit.
plant physiologists demonstrate AE sensors are quick and easy to install
Bloemen et al (2016) used Vallen Systeme AE sensors in their research on the importance of photosynthetic stems under drought conditions. To quantify cavitation events within a stem segment prior and post drought treatment, they outlined the following method:
- Cut a sample branch from tree.
- Cover exposed end of branch with parafilm to prevent dehydration.
- Take branch to a nearby laboratory and clamp an AE sensor to the middle of the branch.
- Remove parafilm and allow branch to dehydrate. Bloeman et al allowed this step to occur over a 60-hour period but this could vary depending on your species and research questions.
- During the 60-hour period, record the AE signal every second with the Vallen Systeme AMSY-6 AE measurement system and then analyse the data.
See Figure 1 of Laschimke et al (2006) for an example of an AE sensor installed on a plant.
what are typical results from AE sensors?
Data recorded from the AE sensors can be analysed with numerous techniques, depending on the research hypotheses. Bloemen et al, from the above example, used a cumulative frequency statistical approach. That is, over the 60-hour measurement period, they measured the cumulative frequency of AE signals (i.e. cavitation events). Here is an example data graph of such an approach showing the hypothetical results of cavitation events in a drought-stressed and irrigated treatment:
Prior to the onset of the drought stress treatment, the number of AE signals (cavitation events) were the same for both treatments. Then, five days post the experimental treatment the number of cavitation events is significantly higher in the drought-stressed plants. A further measurement 20 days after the experimental treatment shows that cavitation events are significantly less in the drought-stressed plants. This is because, 20 days into the drought there are less hydraulically functional xylem vessels in the drought-stress plants and, therefore, a smaller number of xylem that can cavitate.
AE sensors are increasingly being deployed by researchers for plant physiology experiments. The sensors are easy to install yet the measured data can be analysed in a variety of ways to provide insights into drought-induced physiological processes.
Bloemen et al (2016). Trees: Structure and Function, 30: 63. doi: 10.1007/s00468-014-1132-9
Laschimke et al (2006). Journal of Plant Physiology, 163: 996-1007. doi: 10.1016/j.jplph.2006.05.004
Roo et al (2016). Applied Sciences, 6: 71. doi: 10.3390/app6030071