Effect of thermal modification on the micromorphology of decay of hardwoods and softwoods by the white rot fungus Pycnoporus sanguineus

Abstract

Abstract Thermal modification (TM) of wood at high temperatures can lead to significantly improved decay resistance against white rot (WR) fungi, but little is known about the mechanism of this improvement at the cellular level. As a contribution to this topic, hardwoods (HWs) (ash, beech) and softwoods (SWs) (spruce, fir) were submitted to the Termovuoto® TM process at 220°C for 3–4 h (shortly TMW220°C) and were degraded by the WR fungus Pycnoporus sanguineus. The results were studied using microscopy techniques. Decay occurred preferentially via cell wall delignification prior to the removal of cellulose and hemicelluloses. Concerning this effect and fungal colonization, there were no essential differences between TMW220°C and the unmodified reference of all wood species. TMW220°C, however, showed a number of characteristic changes in decay morphology. Narrow and distinct concentric transition zones were observable in decayed fiber and tracheid cell walls, which demark the lignified and delignified regions. Moreover, cell wall separation by preferential delignification of compound middle lamellae (CML) was not seen in decayed tracheids. It can be concluded that the Termovuoto TMW220°C process has an impact on the delignification process of P. sanguineus at the cell wall level, which slows down the decay.