Responses of Water-Stressed Pinus thunbergii to Inoculation with Avirulent Pine Wood Nematode (Bursaphelenchus xylophilus): Water Relations and Xylem Histology

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Abstract
The effect of water-stress conditioning on water relations and histological features ofPinus thunbergii Parl. inoculated with avirulent isolate ofBursaphelenchus xylophilus (Steiner and Buhrer) Nickle, pine wood nematode, were investigated. Pines were kept under 8 days cycle of severe water stress. One-half of the water-stressed pines died as a result of infection by avirulent pine wood nematode and water stress tended to induce increased susceptibility and/or decreased resistance of pines to avirulent pine wood nematode. In dead pines, the water conducting function of xylem was lost, and all of the parenchyma cells died. In surviving pines, the xylem hydraulic conductivity and the xylem water content were significantly reduced (12 to 23% and 77 to 83%, respectively) compared to controls. Safranin dye perfusion of excised axis stem segments indicated that the water conductance was limited to the very narrow peripheral area of xylem. Embolism caused by cavitation in the tracheids occurred in the central part of xylem and in that dysfunctional region of the xylem the axial parenchyma cells surrounding the epithelial cells, and ray parenchyma cells partly degenerated but the epithelial cells survived. The disruption of tracheid shape observed in surviving pines indicates that avirulent pine wood nematode temporarily disturbed cell division of the cambium. Considering the differences in responses between dead pines and surviving pines after inoculation with avirulent pine wood nematode, the death of water-stressed pines apparently resulted from death of cells, in particular the vascular cambium and the loss of xylem hydraulic function by cavitation.