Cytochemical and immunocytochemical characterization of wood decayed by the white rot fungus Pycnoporus sanguineus II. Degradation of lignin and non-cellulosic polysaccharides in European ash wood

• Degradation of ash wood (hardwood) by Pycnoporus sanguineus (white rot) was investigated using TEM immunocytochemistry.
• Lignin was preferentially removed in ash xylem cells prior to hemicelluloses (i.e. selective decay) like in spruce (Part I).
• Considerable hemicellulose(s) remained in fibers after complete delignification of compound middle lamella regions.
• Vessel and parenchyma cells showed selective decay with higher resistance than fibers.
• Micromorphologically, ash fibers produce less typical features of selective decay than spruce tracheids (Part I).

Degradation of lignin and non-cellulosic polysaccharides (pectins and hemicelluloses) by the white rot fungus Pycnoporus sanguineus were investigated in European ash wood (hardwood) using transmission electron microscopy coupled with immunocytochemistry. Lignin was preferentially removed in secondary cell walls of ash xylem cells including fibers, vessel and (ray/axial) parenchyma cells prior to degradation of hemicelluloses (i.e. selective decay). Concomitantly, degradation of cell walls occurred by progressive thinning from the lumen surface outwards similar to simultaneous decay. Preferential degradation of middle lamellae before complete degradation of secondary cell walls (i.e. a typical feature of selective decay) was only observed in bi-/triseriate rays by initial attack from pit regions. Vessel and parenchyma cells showed greater resistance to decay than fibers with narrower decay zone produced. A comparison of decay between ash and spruce wood (Part I, softwood) demonstrated that P. sanguineus produces selective decay in both hard- and softwoods but more typical micromorphological features of selective decay are produced in spruce tracheids than ash fibers. This study highlights that degradation of lignin proceeds hemicelluloses in wood cell walls attacked by the selective white rot fungus P. sanguineus.