Correlation between acoustic emission, water status and xylem embolism in pine wilt disease.

The occurrence of cavitation events and embolism during the latent, early stage and the late developmental stages of pine wilt disease was monitored nondestructively by acoustic emission (AE) and high-resolution magnetic resonance microscopy, respectively, and the results were compared with changes in leaf water potential and stem thickness. In the latent stage of the disease, when no embolisms were observed, cavitation events were detected by AE during the daytime in water-stressed Japanese black pine (Pinus thunbergii Parl.) seedlings, indicating that cavitation occurred at the individual tracheid level. In the early stage of the disease, an increase in the frequency of AE events occurred coincidentally with the occurrence of patchy embolisms at the mass tracheid level. The threshold water potential for such mass cavitation was higher than that causing cavitation of individual tracheids during the latent stage of the disease. In the advanced stage of the disease, explosive AE events were observed coincidentally with drastic enlargement of embolized areas and decreases in water potential. The AE events in the latent stage occurred only during the daytime, whereas, in the early and advanced stages of the disease, they also occurred at night. The explosive occurrence of cavitation in the advanced stage was thought to be a case of "runaway embolism."