Monitoring of ultrasonic and audible emissions from plants with or without vessels

Our studies on the occurrence, origin, and intensity of audio and ultrasonic acoustic emissions (AAE and UAE) have been extended by examining a range of plants with contrasting anatomical characteristics. Stem samples, excised shoots, and whole plants have been monitored for AAE and UAE simultaneously. AAE was attenuated less, i.e. transmitted further, than UAE in plant materials generally; Distances travelled by AAE considerably exceeded 40 mm in all species tested. UAE were transmitted up to 40 mm in Thuja, a vessel-less species, but only 10 mm in a range of other species, with, and without, vessels. The extent of attenuation seems to depend upon the properties of the woody xylem. UAE were studied in Thuja by Dixon, Grace, and Tyree in 1984. We have extended their findings by studying both UAE and AAE for the first time in Thuja and also Tasmannia, another vessel-less species. These plants produced fewer AAE than UAE; also their rate of AAE production was lower. Seemingly, tracheids can produce AAE: 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 previously that Ricinus petioles produce only AAE whereas stems produce both AAE and UAE. This difference apparently hinges on the attenuation properties of the tissues surrounding the stem-mounted probes. Such probes could detect AAE from petioles distributed over the whole plant whereas UAE were only detectable locally. When excised stem tissue was dehydrated, UAE were produced in abundance. Apparently the dual-probe technique is thus useful for locating sites of cavitation in whole plants. By comparing AAE and UAE totals with anatomical estimates of cavitatable units we have estimated our counting efficiency; selecting highest totals to represent optimal numbers detected per unit volume of stem vascular tissue (calculated from diameter and length measurements). In general, AAE detection ranged from 0.13% to 5.1% whereas UAE were considerably greater, ranging from 27% to 76% (cf. 16% for Chamaecyparis found by Sandford and Grace, 1985).