Abstract
One strategy for improving the treatability of refractory wood species is biological incising, and its efficiency depends on how the microorganisms modify the porous structure of the wood. Evaluation of the bioincised wood treatability on a micro-scale can thus help to better understand the treatability enhancing mechanisms. In the present study, the biodegradation pattern and micro-scale treatability of Loblolly pine (Pinus taeda L.) heartwood were determined after bioincising with the white-rot fungus Physisporinus vitreus (Pers.: Fr.) P. Karsten isolate 136 and bacterium Bacillus subtilis UTB22. Oven-dried specimens with dimensions of 50 mm × 25 mm × 15 mm (L × T × R) were incubated with the microorganisms at (23±2) °C and (65±5) % relative humidity for six weeks. The control and exposed wood blocks were then pressure treated by 1 % fluorescent dye (fluorescein)-containing water to study the treatability pattern under a fluorescence microscope. The longitudinal and tangential air permeability and compression strength parallel to the grain of the specimens were also determined at the end of the incubation period. Scanning electron microscopic (SEM) studies showed that degradation by B. subtilis UTB22 was limited to the pit membranes, but the cell walls were also degraded to some extent by P. vitreus. The fungus caused a higher mass loss compared to the bacterium, whereas the permeability enhancing ability of the bacterium was more pronounced. The fluorescent dye tracer also showed that higher treatability with more uniformity was obtained by B. subtilis UTB22. The improvement in treatability by both microorganisms was mainly due to the degradation of the earlywood tracheids.
Highlights
Scanning electron microscopic (SEM) studies showed that degradation by B. subtilis UTB22 was limited to the pit membranes, but the cell walls were degraded to some extent by P. vitreus
The average mass losses were less than 1 % with B. subtilis UTB22 and 8.4 % with P. vitreus (Table 1)
In contrast to Schwarze and Landmesser (2000), Schwarze et al (2006), and Schwarze et al (2008) who reported a preferential degradation of pit membranes of Norway spruce tracheids at the initial stage of wood colonization with P. vitreus, SEM observations in the current study showed some signs of cell wall degradation (Figure 3a, white arrows)
Summary
Most wood species are not sufficiently durable against wood-destroying organisms and have to be treated with chemical preservatives to ensure the desired service life or to improve other relevant properties like resistance to fire and UV stability. The penetration depth of active ingredients alone is not sufficient to judge the wood treatability; the uniformity of the penetration should be evaluated, e.g., by microscopic studies of the treated wood (Tarmian et al, 2020). Wood decay can occur from the non-uniform impregnation of the wood cells (Omidvar and Schneider, 2004). This type of penetration is sometimes called marble penetration
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