INFECTION OF NORWAY SPRUCE (PICEA-ABIES (L) KARST) SEEDLINGS WITH PYTHIUM-IRREGULARE BUISM AND PYTHIUM-ULTIMUM TROW - HISTOLOGICAL AND BIOCHEMICAL RESPONSES

We have studied the reaction of Picea abies seedlings to infection wit h Pythium. The highly virulent species Pythium ultimum and the less vi rulent species Pythium irregulare germinated on the root and hypocotyl surface, formed appressoria and penetrated through the stomata as wel l as through the epidermis. No major differences in the growth of both fungal species were observed during the early events of colonization. The less virulent species formed about 25% more appressoria suggestin g that the fungus experienced difficulties with penetration. Differenc es were observed in the response of the host plant to infection. Autof luorescence, possibly related to deposition of lignin or lignin-like m aterials increased more in cortical and endodermal tissue colonized wi th the highly virulent P. ultimum than with the less virulent I! irreg ulare. Chitinase activity was highest in the tissues mast extensively colonized by the fungus. In addition, a systemic increase of chitinase activity was also detected. Interestingly, chitinase activity increas ed systemically in cotyledons which were never in contact with the pat hogen, indicating the translocation of a systemic signal. Salicylic ac id was also detected in spruce seedlings; its level increased in roots during infection with the less virulent P. irregulare.