Concentrations of the stilbene glucosides astringin and isorhapontin, which ranged from 5 to 50 mg/g fresh weight in Picea abies bark, decreased in response to in vivo infection with the root-rot fungus Heterobasidion annosum. The initial concentration of the stilbene astringin was negatively correlated with the depth of hyphal enetration. Resin acid contents increased following infection, but were not correlated with t I ! e depth of hyphal penetration. The spatial distribution of stilbene glucosides in spruce stem, root collar and root bark, and the seasonal variations in concentration, were estimated.
Although the suberized rhytidome and phellem of Norway spruee bark appeared to be the strongest barriers against penetration by Heterohasidion annosum, artificial inoculations on standing trees showed that the living phefloderm and phloem bark were also able to contain the fungus. Thiekened cell walls and excretions in the lumen of neerotic cells, as well as morphological changes in the infeeting hyphae, indicated that this protection is of both mechanical and chemical character. Rate of wound periderm formation was correlated to bark thickness and tree age.
Norway spruce (Picea abies) seedlings, 2-4 years old, were subjected to different degrees of stress. Drought stress affected the predis osition to infection by Heterobasidion annosum in the same way in root wood, stem wood ancfstem bark. At high water potentials (-3 to -5 bar) resistance was high, at intermediate levels (-5 to -15 bar) it was low but increased a ain at water potentials below -8 to -15 bar. Neither light nor oxygen deficiencies appeared to kave adverse effects on resistance. Total length of unsuberized roots was a sensitive indication of strain on the seedlings.
Seedlings of Norway Spruce and Scots Pine were inoculated with S and P strains of the root rot tungus Heterobasidion annosum. Infection frequencies through the living bark layers were not related to subgroup affiliation in neither spruces nor pines. Growth rates in vitro were not related to growth rates in vivo. Oxygen concentration had a stronger effect on the growth rates of P than on S strains.
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