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Extractive micro-distribution in Douglas fir sapwood and heartwood: Effects on cell wall porosity
Douglas fir has distinctly coloured heartwood as a result of extractive deposition. Heartwood formation is known to affect durability and treatability with preservatives as well as other types of wood modification involving infiltration with chemicals. Using fluorescence microscopy, we have studied the distribution of extractives in sapwood and heartwood of Douglas fir showing that extractives infiltrate the cell wall of heartwood tracheids thereby reducing porosity. Several different types of extractive are localised to heartwood cell walls, resin canals and wood rays using fluorescence microspectroscopy and spectral unmixing. Förster resonant energy transfer measurements show that accessibility of lignin lining cell wall micropores is reduced by about 50% as a result of extractive infiltration of cell walls in heartwood relative to sapwood. Conventional histochemistry confirms the presence of flavonoids and tannins in both sapwood and heartwood.