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Plant Bioacoustics
  Figure 3. a: Playback of caterpillar feeding vibrations increased the induced response of A. thaliana to herbivore damage compared with no- vibration controls: *P < 0.05; error bars 95% confidence; N = 44/bar (43 for rosette center); N.S., not significant. b: Gray scale map showing the increase in chemical response in the playback leaves (pbl) and same-age systemic leaves (sl), expressed as the percent change from the levels in control plants. c: Relationship between the amplitude of chewing vibration playbacks and the chemical defensive response (N = 22). GS, glucosinolates. Reproduced from Appel and Cocroft, 2014.
 the leaves. In other words, sound had a “priming” effect on the chemical defenses of the plant. The caterpillar sounds were initially recorded with a laser Doppler vibrometer and then reproduced using piezoelectric actuators supported under a leaf, mimicking the duty cycle and source level of the original signals with as much fidelity as possible. Two plants received playback while an additional two had an actuator attached but no playback. Playbacks of wind and leafhopper insect calls (which occupy a similar spectral range as the caterpillars) were also conducted. Each test plant had two leaves removed and were tested for chemical defense expression; one leaf had the piezoelectric actuator attached, and the other (systemic) leaf was selected from the part of the rosette most distant from the activated leaf. Figure 3 illustrates that the authors found that chemical defense concentrations rose 32% in the directly activated leaves and 24% in the more distant leaves and that increas- ing the amplitude of the playbacks corresponded with increasing concentrations of chemical response. Playbacks of the other two sounds yielded responses statistically indis- tinguishable from the control.
Monica Gagliano is another researcher who has consistently published work on potential plant responses to sound, with a particular focus on root growth in response to sound play- backs. Her work seems representative of modern playback experiments. After some initial work showing that bean sprout roots grow toward 200-Hz playback tones, Gagliano and her colleagues (2017) described how garden pea roots
responded differentially to accessible water, flowing (but inaccessible) water, and recorded sounds of flowing water. To demonstrate this, the authors gave growing plant roots an opportunity to grow into one of two plastic trays, one of which served as a control, and the other was exposed either to a PVC tube containing inaccessible moving water (while keeping the soil temperature constant) or to a speaker broadcasting back the circulating water sound (Figure 4). They found strong evidence that the presence of water circu- lating in a PVC pipe attracted root growth even though the water was not accessible and the temperatures remained the same (scenarios TS1 and TS2). Oddly enough, they found the presence of the embedded speaker and MP3 playback device seemed to repeal root growth regardless of whether and what type of sound was played (scenarios TS3-TS6).
They then modified the experiment so that both plastic trays were outfitted with playback systems but with only one broadcasting flowing water sound (scenarios TS7-TS9). Only then did they note a preference in root growth toward the water playback compared with a speaker generating no signal (TS9), but the roots didn’t seem to be able to distin- guish between flowing water noise and white noise (TS8). The authors speculate that magnets in the attached speaker might have influenced the response of the roots as well.
Both of these experiments illustrate the difficulties involved in measuring potential plant responses to sound. Isolating the effects of sound and vibration from other potential envi- ronmental cues (temperature, chemical release, physical
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