Hydrophobic and Hydrophilic Extractives in Norway Spruce and Kurile Larch and Their Role in Brown-Rot Degradation.

Sophie Füchtner,Lisbeth Garbrecht Thygesen,Theis Brock-Nannestad,Annica Pilgård,Annika Smeds,Maria Fredriksson

doi:10.3389/fpls.2020.00855

Abstract

Extractives found in the heartwood of a moderately durable conifer (Larix gmelinii var. japonica) were compared with those found in a non-durable one (Picea abies). We identified and quantified heartwood extractives by extraction with solvents of different polarities and gas chromatography with mass spectral detection (GC-MS). Among the extracted compounds, there was a much higher amount of hydrophilic phenolics in larch (flavonoids) than in spruce (lignans). Both species had similar resin acid and fatty acid contents. The hydrophobic resin components are considered fungitoxic and the more hydrophilic components are known for their antioxidant activity. To ascertain the importance of the different classes of extractives, samples were partially extracted prior to subjection to the brown-rot fungus Rhodonia placenta for 2–8 weeks. Results indicated that the most important (but rather inefficient) defense in spruce came from the fungitoxic resin, while large amounts of flavonoids played a key role in larch defense. Possible moisture exclusion effects of larch extractives were quantified via the equilibrium moisture content of partially extracted samples, but were found to be too small to play any significant role in the defense against incipient brow-rot attack.

Highlights

Wood used as building material in an outdoor environment (i.e., European Standard EN 335-1:2006 use class 3–4) is frequently exposed to high relative humidity or wetting, highly increasing the risk of degradation by wood-degrading micro-organism
In order to investigate the influence of hydrophobic and hydrophilic extractives on water sorption and brown-rot degradation, we prepared milled samples and sticks of Norway spruce and Kurile larch extracted with only hydrophobic or only hydrophilic solvents, as well as extracted with all four solvents
The extraction efficiency was lowest for the ethanol fraction, indicating that ethanol-soluble analytes are harder to remove from the wood, which might be connected to their localization

Summary

Introduction

Wood used as building material in an outdoor environment (i.e., European Standard EN 335-1:2006 use class 3–4) is frequently exposed to high relative humidity or wetting, highly increasing the risk of degradation by wood-degrading micro-organism. The resistance of wood to degradation in such a setting is mainly determined by its inherent durability (natural or artificial) and its moisture sorption properties, apart from environmental factors and design (Brischke et al, 2006; Meyer-Veltrup et al, 2017; Brischke and Alfredsen, 2020). For the heartwood of many species and all of the sapwood this is not the case, and artificial wood protection is needed (Kutnik et al, 2017). In Europe, many of the old wood-preservatives were banned due to their high toxicity, and Extractives’ Functions in Wood Degradation research and development focused their efforts on more environmentally benign ways to protect wood (Schultz and Nicholas, 2002; Singh and Singh, 2012). One approach is to achieve a better understanding of the mechanisms underlying natural durability in trees, and the current study aims to contribute to this

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