INTERACTIONS BETWEEN NORWAY SPRUCE (PICEA-ABIES) AND HETEROBASIDION-ANNOSUM .1. INFECTION OF NONSUBERIZED AND YOUNG SUBERIZED ROOTS

Nonsuberized and young suberized roots of Norway spruce (Picea abies) were inoculated with an S-strain of Heterobasidion annosum for 2-20 da ys. The roots were sectioned for light microscopy and transmission ele ctron microscopy. They were susceptible to infection at both stages of development. The host reaction to delimit the infection was the forma tion of a necrotic ring barrier in the outer cortex. In cases where th e inner cortex also became infected, fungal hyphae accumulated just be fore the endodermis, which acted as a new barrier. Only in nonsuberize d roots did the hyphae succeed in penetrating the stele, and within 3 days after inoculation the stele was almost completely digested. Other resistance reactions included accumulation of phenolic deposits, seco ndary thickening of cell walls, and formation of papillae. The hyphae were able to grow within and across cell walls. The presence of transl ucent areas around penetrating hyphae possibly reflected the digestive action of fungal enzymes. The hyphae showed signs of degeneration whe n entrapped in encompassing cellular deposits.