Effect of Bursaphelenchus xylophilus on the water potential and water-splitting complex of photosystem II of Pinus sylvestris seedlings

The process by which Bursaphelenchus xylophilus causes death of Scots pine ( Pinus sylvestris) seedlings was studied by monitoring daily changes in soil and needle water potential and variable chlorophyll a fluorescence (a measure of the status of the photochemical water-splitting complex of photosystem II) over 168 h following treatment, and root water potential at the end of the experiment. In addition to inoculation with Bursaphelenchus xylophilus, four controls consisting of wound, wound and water, wound and B. xylophilus culture filtrate, or unwounded checks were included. Soil water potential of all treatments, and variable chlorophyll a fluorescence of the control treatments did not change during the study. Inoculation with B. xylophilus, however, resulted in a decline of variable chlorophyll a fluorescence and needle water potential 24–36 h after inoculation; whereas, the needle water potential of the controls increased with time. The wound and B. xylophilus culture filtrate and wound and B. xylophilus treatments resulted in significantly greater root water potential than those of the unwounded controls. Wilt and P. sylvestris seedling death were observed at from about −1·5 to −2·0 MPa; however, significant changes in variable chlorophyll a fluorescence occurred before significant changes in water potential were detected. Thus, the high sensitivity of the variable chlorophyll a fluorescence assay enables early detection of the onset of physiological changes leading to seedling death. The data show that one of the first consequences of B. xylophilus infection is an effect on water-relations which, in turn, affecs related physiological processes.