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Root dieback is a disease affecting seedlings during their first or second growing seasons. It is caused by pathogenic fungi in the rhizosphere in combination with environmental factors that stress the plants. In this thesis the process of root dieback was studied at several levels.The pathogenesis in primary roots of Norway spruce, Picea abies (L.)Karst., challenged with the pathogenic fungus, Pythium dimorphum Hendrix Campbell, was chosen as a model system to study disease development in nurseries, infection process, and host defenee mechanisms.The development of visual disease symptoms was assessed in three different nurseries, where Norway spruce seedlings were inoculatcd with uniform inoculum of either P. dimorphum or Rhizoctonia sp. Among factors tested (irrigation, media, fungicide, inoculated fungi) only the fungal inoculum had significant effeet on viability of seedlings. Application of benomyl fungicide supressed Rhizoctonia spp. but clearly promoted Pythiaeeous fungi.Hyphae of P. dimorphum penetrated the juvenile roots of Norway spruce rapidly and colonized the whole root tissue within 24 hours. Three days after infection, lignin, measured quantitatively as ligninthioglycolic acid, in infected tissues was double the arnount in non-infected tissues. Flavanols and condensed tannins were constitutively present in both infected and non-infected tissues.More than 30 different pathogenesis-rclated (PR) proteins accumulated in root tissues within two days after inoculation. Ten different proteins revealed chitinase activity, three had chitosanase activity and two 6-1,3-glucanase activity. Two of the chitinases were constitutively present, in addition to eight others that only accumulated after infection. Infection-induced chitinases were detectable already the first day after inoculation.Activity of three chitosanases was detected solely in infected roots, from the second day after inoculation. Two 6-1,3-g1ucanases, accurnulated from second and fourth days after inoculati on, respectively.The hypothesis, that proteins (including PR proteins) in Norway spruce are normally complexed with polyphenols and become released upon fungal infection, was forrnulated. It was tested by adding in the extraction buffer of non-infected roots either non-ionic detergent (Triton X-IDO), known to break the non-covalent association of polyphenol-protein complex, or caffeinc, known to release proteins bound in complex with polyphenols by competitively binding to polyphenols.In both cases the amount of released proteins was similar to the amount of PR-proteins accumulated after inoculation with the pathogenic P.dimorphum, thus strongly supporting our hypothesis.We report here on this novel mechanism, detected in roots of a gymnosperm, Norway spruce, and suggest that this can be a strategy in polyphenol-rich species, where this immediate mechanisrn rnay be involved in the first-row defence response.