Interactions between Scots pine heartwood extractives and wood decaying fungi

The heartwoods of many wood species have natural resistance to degradation caused by wood decaying fungi. Many factors can contribute to the natural durability of heartwoods, but the most significant one is usually the presence of biologically active extractives in the wood material. This thesis investigated the interactions between heartwood extractives and wood decaying fungi in detail, using Scots pine as the wood material. The primary objectives of the thesis were to study the mechanisms of action of extractives and to explore the ways in which the extractives interact with fungi within the structure of wood. The mechanism of action studies focused on the antioxidant activity of extractives and on their ability to inhibit the enzymatic hydrolysis of wood polysaccharides. The antioxidant activity measurements showed that Scots pine heartwood extractives are active antioxidants, which means that they may be able to interfere with the radical-based degradative mechanisms used by wood decaying fungi, particularly brown rots. The extractives could also inhibit the action of hydrolytic enzymes, suggesting that they may be able to prevent the conversion of wood polysaccharides to digestible sugars. However, substantial hydrolase inhibition was only seen with a white rot enzyme preparation, which contained some enzyme(s) capable of modifying the heartwood extractives. The interaction studies used confocal Raman spectroscopy imaging to visualise the cellular level distributions of extractives and other chemical components in intact and decaying heartwood. The studies revealed that the phenolic pinosylvins were present throughout the heartwood tissues, suggesting that they have good ability to interact with fungi and their degradative agents during decay. However, the studies on decaying heartwood showed that pinosylvins were extensively degraded during incipient decay. The hydrophobic resin acids were only detected in the lumens of some tracheids and ray cells, but they were found to have higher resistance to degradation than the pinosylvins. Interestingly, the resin-rich extractives deposits found in tracheid lumens appeared to create local areas of reduced degradation in the decaying heartwood. The results of this thesis provide new information on the ways in which extractives can interact with fungi and contribute to natural durability. The results also increase our understanding of the origins of natural durability in Scots pine, the most abundant wood species in Finland.