The Acoustic Detection of Cavitation in Fern Sporangia

Cavitation of water in the annular cells of fern sporangia wasmonitored simultaneously by acoustic detection and visual analysisusing a light microscope. The number of cavitable annular cells was found to be 21.3 ±1.7 (n=40). The number and timing of ultrasonic acoustic emissions(UAE) received from a fully hydrated sporangium allowed to dehydrateon the acoustic probe closely approximated the average numberof cavitable annular cells. The number and timing of audibleacoustic emissions (AAE) was found to be lower than the numberof UAE. AAE were associated with cavitation-induced forwardmovements of the sporangium rather than individual cavitationevents within each annular cell. The results are discussed in view of an hypothesis proposedby Ritman and Milburn (1988) which describes the relationshipbetween the frequency of acoustic emission and the size of theresonating element. Key words: Cavitation, acoustic detection, sporangia     

Acoustic Emissions from Plants: Ultrasonic and Audible Compared

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 cavitationof xylem sap generally results in the production of a broadband acoustic emission (AE) with a lower cut-off frequency determinedby the dimensions of the resonating element. The larger a conduit'sdimensions, the lower is the frequency of its major resonance.Thus the vessels, the largest conduits, can be expected to produceboth AAE and UAE. Fibres and small cavitating elements suchas small tracheids are expected on the other hand to produceonly UAE. Most work utilized Acacia tissues but work was extended to otherplant tissues from a range of species with differing anatomicalcharacteristics. Evidence supporting our hypothesis shows thatAAE and UAE did not coincide in different tissue types or dependsimply upon the degree of dehydration. AAE were detected fromtissue with intact major conduits (vessels) but not in similartissue in which these major conduits had been severed, whereasUAE were detected from both types of tissue. In general, ourhypothesis that larger conduits produced lower frequency signalsand smaller units at the ultrasonic frequencies was supported.We are forced to conclude that some UAE are generated by eventsother than cavitating vessels or fibres. Possible interpretationof our data is discussed in terms of the size of the cavitatingconduits but including differential signal absorption withinthe tissue. Key words: Cavitation, acoustic emission, ultrasound, plants, water stress     

Monitoring of Ultrasonic and Audible Emissions from Plants With or Without Vessels

Our studies on the occurrence, origin, and intensity of audioand ultrasonic acoustic emissions (AAE and UAE) have been extendedby examining a range of plants with contrasting anatomical characteristics.Stem samples, excised shoots, and whole plants have been monitoredfor AAE and UAE simultaneously. AAE was attenuated less, i.e. transmitted further, than UAEin plant materials generally; Distances travelled by AAE considerablyexceeded 40 mm in all species tested. UAE were transmitted upto 40 mm in Thuja, a vessel-less species, but only 10 mm ina range of other species, with, and without, vessels. The extentof attenuation seems to depend upon the properties of the woodyxylem. UAE were studied in Thuja by Dixon, Grace, and Tyree in 1984.We have extended their findings by studying both UAE and AAEfor the first time in Thuja and also Tasmannia, another vessel-lessspecies. These plants produced fewer AAE than UAE; also theirrate of AAE production was lower. Seemingly, tracheids can produceAAE: vessels are not essential for audio emissions. AAE and UAE were monitored in Ricinus, a woody plant with vessels.AAE production tended to predominate. We have shown previouslythat Ricinus petioles produce only AAE whereas stems produceboth AAE and UAE. This difference apparently hinges on the attenuationproperties of the tissues surrounding the stem-mounted probes.Such probes could detect AAE from petioles distributed overthe whole plant whereas UAE were only detectable locally. Whenexcised stem tissue was dehydrated, UAE were produced in abundance.Apparently the dual-probe technique is thus useful for locatingsites of cavitation in whole plants. By comparing AAE and UAE totals with anatomical estimates ofcavitatable units we have estimated our counting efficiency;selecting highest totals to represent optimal numbers detectedper unit volume of stem vascular tissue (calculated from diameterand length measurements). In general, AAE detection ranged from0.13% to 5.1% whereas UAE were considerably greater, rangingfrom 27% to 76% (cf. 16% for Chamaecyparis found by Sandfordand Grace, 1985). Key words: UAE, AAE, xylem vessels, cavitation