Water relations, xylem embolism and histological features ofPinus thunbergiiinoculated with virulent or avirulent pine wood nematode,Bursaphelenchus xylophilus
The development process of pine wilt disease caused by Bursaphelenchus xylophilus (Steiner and Buhrer) Nickle, pine wood nematode, was studied ecophysiologically and histologically in relation to pathogenicity of B. xylophilus. Judging from the predawn xylem pressure potential of needles, the heat pulse velocity, and the soil water potential, the control Pinus thunbergii Parl., used for the study, was not water-stressed. Virulent B. xylophilus isolate can kill non-waterstressed pines. In virulent B. xylophilus isolateinoculated pines, the predawn xylem pressure potential of needles abruptly decreased when the colour of 1-year-old needles changed to brown and then the water conducting function of the xylem was lost completely. Avirulent B. xylophilus isolate does not affect the needle colour and the xylem pressure potential of pines. Avirulent B. xylophilus isolate-inoculated pines, however, responded to nematode invasion by decreasing hydraulic conductance of stem and root xylems. In addition, oleoresin exudation slightly decreased. The decreased hydraulic conductance results from embolism of tracheids caused by cavitation in the central part of the xylem. From histological observation, all of the parenchyma cells in virulent B. xylophilus isolateinoculated pines died. In contrast, the parenchyma cells, degenerated in avirulent B. xylophilus isolateinoculated pines, were limited in the embolized region of the xylem. The difference between the response of pine to the virulent B. xylophilus isolate invasion and that to avirulent B. xylophilus isolate invasion indicates that nematode-induced death of pine relates to the death of parenchyma cells, as well as the decrease in xylem hydraulic conductance.