Observations on the embolism and survival of Pinus thunbergii seedlings inoculated with Bursaphelenchus xylophilus

Abstract

To elucidate the relationship between seedling survival and the distribution of xylem embolisms after infection with pinewood nematode (PWN), Bursaphelenchus xylophilus, we examined embolised xylem areas using acid fuchsin dye solution in five groups of 3-year-old Pinus thunbergii seedlings which included a susceptible group and four groups with varying degrees of resistance. We confirmed that embolised areas caused by infection with PWN could be delineated accurately using a dye uptake method with appropriate procedures. In all groups, the additional embolisms were visible 1 cm above or below the inoculation wound 45 days after inoculation (DAI). In all stained seedlings, the distribution of embolisms was associated with axial resin canals and not with ray tissues. The difference in survival rates among the resistant groups depended on the distribution of embolisms in the current-year xylem. A susceptible group and two groups with varying degrees of resistance that showed higher percentage loss of water-conducting area at 45 DAI exhibited low survival rate at 5 months after inoculation. In many surviving seedlings, water movement was generally observed in newer tracheids, while embolisms extended to near the cambium in some seedlings of the three groups. Retention of the water-conducting pathway of newer tracheids is thought to be necessary for survival after infection with PWN.