Audible acoustic emissions (AAE) and ultrasonic acoustic emissions (UAE), produced by stem segments during dehydration in air, have been recorded and compared. We hypothesize that cavitation of xylem sap generally results in the production of a broad band acoustic emission (AE) with a lower cut-off frequency determined by the dimensions of the resonating element. The larger a conduit's dimensions, the lower is the frequency of its major resonance. Thus the vessels, the largest conduits, can be expected to produce both AAE and UAE. Fibres and small cavitating elements such as small tracheids are expected on the other hand to produce only UAE. Most work utilized Acacia tissues but work was extended to other plant tissues from a range of species with differing anatomical characteristics. Evidence supporting our hypothesis shows that AAE and UAE did not coincide in different tissue types or depend simply upon the degree of dehydration. AAE were detected from tissue with intact major conduits (vessels) but not in similar tissue in which these major conduits had been severed, whereas UAE were detected from both types of tissue. In general, our hypothesis that larger conduits produced lower frequency signals and smaller units at the ultrasonic frequencies was supported. We are forced to conclude that some UAE are generated by events other than cavitating vessels or fibres. Possible interpretation of our data is discussed in terms of the size of the cavitating conduits but including differential signal absorption within the tissue.
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