Clarification of the mechanism of xylem embolism in pine wilt disease through non-destructive observations

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To clarify the mechanism of xylem embolism in pine wilt disease, three non-destructive methods were applied to monitor xylem water potential and cavitation development in Japanese red pine seedlings inoculated with pinewood nematode (Bursaphelenchus xylophilus).; the methods were MRI (magnetic resonance imaging), AE (acoustic emission) and strain gauge. The occurrence of cavitation during the development of symptoms of pine wilt disease was non-destructively monitored using a compact magnetic resonance (MR) microscope system and measurements of acoustic emission (AE). The occurrence of cavitation was then compared to changes in leaf water potential and stem thickness. In the latent stage, when no embolisms were observed, cavitation events were detected by AE during the daytime, when plants were water stressed. This suggests that cavitation in water-stressed pine occurs at the level of the individual tracheid. In the early stage, greater peaks of AE events occurred coincidentally with the occurrence of patchy embolisms on a mass tracheid level. The threshold water potential for this mass cavitation was higher than that in latent stage pine. In the advanced stage, explosive AE events were observed coincidentally with drastic enlargement of embolisms and decreases in water potential. The AE events in the latent stage occurred only during the daytime, but those in the early and advanced stages also occurred at night. The explosive occurrence of cavitation in the advanced stage was thought to be a case of "runaway embolism".