HOST AND CELL-TYPE AFFECT THE MODE OF DEGRADATION BY MERIPILUS-GIGANTEUS

Wood degradation by the white-rot basidiomycete Meripilus giganteus (P ers.:Pers.) Karst. was studied in naturally infected and artificially inoculated wood of beech (Fagus sylvatica L.) and large-leaved lime (T ilia platyphyllos Scop.). Semi-thin sections revealed that the seconda ry walls of most fibres contained internal cavities. Three distinct ty pes of cavity formation, which differed not only between hosts, but al so between cell type and location in the annual ring, were identified. Within discoloured wood of naturally infected beech, the structure of the cavities and their formation by the associated hyphae were remini scent of a soft-rot. By contrast, cavity formation in artificially ino culated beech and large-leaved lime wood differed from a soft-rot mode of attack as extensive delignification always preceded cavity formati on, and neither T-branching, L-bending, nor hyphal growth were found w ithin cell walls. The formation of half-moon shaped cavities in beech wood was present only in tension-wood fibres. From large diameter hyph ae, growing within the fibre lumen, numerous fine perforation hyphae e xtended transversely via helical cracks into the cell wall. Subsequent degradation of cellulose within concentric layers of the tension-wood fibres commenced from the apices of perforation hypha. Sections stain ed with ruthenium red and hydroxlamine-ferric chloride, revealed that M. giganteus preferentially degrades pectin-rich regions of the middle lamellae in xylary ray cells. In large-leaved lime, such regions were uniformly located in the middle lamellae of axial and ray parenchyma. In beech wood, degradation of pectin-rich middle lamellae regions com menced after the delignification of secondary walls and resulted in a conspicuous hollowing of multiseriate xylem rays. Plasticity in wood d egradation modes by M. giganteus in large-leaved lime and beech wood r eflects variations in cell wall structure and/or prevailing wood condi tions.