Ultrastructural patterns of beech leaf degradation by Sporotrichum pulverulentum

Degradation of beech leaves by the white rot fungus Sporotrichum pulverulen tum was investigated for 4 weeks under laboratory conditions. Observations by transmission electron microscopy revealed that the degradation patterns depended on the nature of the foliar tissues and on the stage of decay. Fib res, parenchyma cells and specific zones of the sclerenchyma tissue, corres ponding to the cells located between vessels, were strongly degraded, while vessels and epidermal, cells were more resistant to degradation. During th e early stages of degradation, a selective removal of components from the l ignin and hemicellulose-rich layers was observed in the cell walls of scler enchyma fibres and parenchyma cells. At a more advanced stage of decay, a s imultaneous disappearance of all cell wall layers (primary wall, secondary wall, middle lamella and cell corners) occurred, irrespective of cell type. In parenchyma cells, removal of the intracellular brown pigments occurred prior to degradation of the cell walls, while in epidermis, the cell walls were altered first. In sclerenchyma cells surrounding the vessels, hyphae w ere found to be closely associated with decayed areas, while in all the oth er tissues, there was no contact of the fungus with lignocellulosic compoun ds and polyphenols, suggesting a diffusion of the fungal degradative enzyme s. A fibrillar mucilagenous matrix, often detected between hyphae and cell walls, might make this diffusion easier. Cellulase activity was detected in the decayed tissues by the release of reducing sugars localized in the vic inity of the leaf cell walls and often somewhat distant from the hyphae. Th e role of the fungal enzymes involved in degradation of cell wall component s and brown pigments in beech leaves is discussed. (C) 2001 Editions scient ifiques et medicales Elsevier SAS.