Effects of drought stress and severe pruning on the reaction zone induced by single inoculations with a bark beetle associated fungus (Ophiostoma ips) in the phloem of young Scots pines

The objective of this study was to test the effect of water stress and prun ing on the resistance of young Scots pines (Pinus sylvestris) to a bark bee tle: associated fungus. Six-year-old potted trees were either pruned (70% o f needles removed) or subjected to several successive episodes of severe wa ter stress, prior to inoculation of inner bark with. the fungus Ophiostoma ips (Rumb.) Nannf., which is usually associated with the bark beetle Ips se xdentatus Boern. Well-watered, nonpruned trees served as, controls. Predawn needle water potential reached -2.5 MPa and net CO2 assimilation rates wer e reduced to almost zero during each water stress episode. The length of th e reaction zones around inoculation points reached 3-4 cm after 3 weeks. It was higher during Spring than during Autumn. Impact of water stress on the length of the reaction zone was Very limited and independent of the number of drought episodes that had been imposed prior to inoculation. The only v isible change was a slight decrease when the inoculation was done during th e period of maximum water stress intensity. Growth of the pathogen in the p hloem was not affected by water stress. Correspondingly, pruning had no eff ect on either of these two parameters. Induced reaction zones accumulated m onophenolic compounds that were undetectable in unwounded phloem. These inc luded pinocembrin and pinolsylvin and its monomethylether known to contribu te to the defence against the fungus. Neither draught nor severe pruning in duced any change in the nature or concentrations of these compounds in the unwounded phloem or in the reaction zone.