*Hokkaido Research Center, Forestry and Forest Products Research Institute, Hitsujigaoka-7, Toyohira-ku, Sapporo 062-8516, Japan; **Wood Research Institute, Kyoto University, Uji, Kyoto 612-0011; ***Department of Forestry and Wildlife Management, University of Massachusetts, Amherst MA 01003-4210, USA.

During transpiration, xylem sap in water conduits is under tension. Water columns break under extreme tension and form bubbles (emboli). In healthy plants, water columns recover by rehydration of the conduits when the tension is reduced. However, in trees infected with wilting diseases, sap ascent stops because the xylem does not rehydrate. We observed embolism in light-microscope sections of pine (Pinus densiflora ) inoculated with pine wood nematodes (Bursaphelenchus xylophilus). Embolism was recorded on videotape while acoustic emissions (AEs) were monitored with an AE transducer attached to the sections. We converted the ultrasonic AEs to audible signals and recorded them on the audio track of the videotape. Following the slow emptying of tracheids injured during sectioning, a bubble emerged within each intact tracheid, abruptly swelled, and filled the whole tracheid. The AEs coincided with almost all of the rapid bubble development(See Movie&Sound). This result supports the idea that AEs detected in the trunks of living trees are produced by embolism in tracheids. In the second experiment, the time necessary for rehydration was measured after the addition of water to dehydrated xylem sections. Suggesting that certain substances that inhibit bubble dissolution may be released into the xylem after infection with pine wood nematode, the rehydration time is longer in tracheids of trees infected for one month than in healthy trees or trees infected for 10 days to two weeks.

MOVIE & SOUND: Embolism in trackeids
Acoustic emission at bubble formation (embolism) was converted into sound.

Pinus densiflora xylem (Radial section)
        <------ Longitudinal direction of tracheids ------>