CYTOPATHOLOGICAL RESPONSE IN ROOTS OF PICEA-ABIES SEEDLINGS INFECTED WITH PYTHIUM-DIMORPHUM

Roots of Picea abies seedlings were inoculated with pathogenic Pythium dimorphum. The ongoing lignification process, the accumulation of lig nin, and the distribution of flavanols and condensed tannins (CT), wer e related to the concurrent visual disease symptoms, hyphal colonizati on, and cellular changes. The hyphae ramified in both the cortex and t he stele within 24 h. Three days after inoculation the concentration o f lignin had increased to a level twice that in noninfected tissues. W ith histochemical staining, the accumulation of lignin and the ongoing lignification were co-located in the inner cortex layer 4-6 days afte r infection. Comparison of temporal onset of lignification with the ti ming of the intruding hyphae shows that this defense response occurs t oo late to effectively prevent the pathogen from spreading. Flavanols and CT were also detected in both infected and noninfected roots. Incr eased staining of these compounds in infected roots was often detected in the inner cortex. Electron microscopic studies revealed that at le ast some of these phenolic compounds were localized either as spherule s or as a layer appressed to the tonoplast of the central vacuole.