BIOINCISION OF Gmelina arborea Roxb. HEARTWOOD FOR PERMEABILITY IMPROVEMENT TO CHEMICAL TREATMENT

Abstract

Gmelina arborea is a versatile but moderately refractory timber species whose heartwood is impermeable to chemical treatment due to tyloses deposition which occlude its vessels. This makes its utilisation unsustainable for a broad range of technical applications. Bioincision is an emerging procedure capable of improving the permeability of wood to fluid treatment, but information on its suitability in improving the permeability of Gmelina arborea heartwood is limited. Therefore, the effect of bioincision on permeability, anatomical and mechanical properties of Gmelina arborea heartwood was investigated. Five 34-year-old Gmelina arborea trees were harvested at the University of Ibadan plantation. A bolt of 300 cm was obtained from the base of each tree. Heartwood of each bolt was extracted and converted into standard dimensions. Wood samples (n=100) were bioincised with two white-rot fungi: Inonotus dryophilus (ID, 999) and Ganoderma adspersum (GA, CBS109416) for 9 weeks, while unincised samples (n=50) served as control. Ten samples (20 mm x 20 mm x 60 mm) each from bioincised and unincised treatments were pressure-impregnated for 90 minutes with Tanalith (5.5% concentration) for permeability tests. Thereafter, samples from bioincised and unincised (n=30) were pressure-treated with liquid dye and depth of penetration was evaluated. Ten samples (10 mm x 10 mm x 10 mm) each were obtained from bioincised treatments to assess effect of fungi on wood anatomy. Tanalith absorption (Kg/m3) and retention (Kg/m3) were calculated. Axial Penetration Depth (APD, mm); Axial Penetration Area (APA, %); Tangential Penetration Depth (TPD, mm); Radial Penetration Depth (RPD, mm); Lateral Penetration Area (LPA, %) were measured. Modulus of Elasticity (MOE, MPa); Modulus of Rupture (MOR, MPa); Maximum Compressive Strength (MCS//, N/mm2); Janka Hardness (JH, N) of unincised and bioincised samples were determined using standard methods. Data obtained were analysed using descriptive statistics and ANOVA at α0.05. Unincised samples had least absorption (112.0±8.5) and retention (6.2±0.5), while GA incised samples had highest (135.0±16.6 and 7.4±0.9), respectively. The APD significantly varied from 7.0±1.7 (unincised samples) to 31.2±4.1 (GA incised samples), implying higher degradation of vessel tyloses by GA. The APA increased from 0.07±0.03 (unincised samples) to 3.1±1.9 (ID incised samples). The TPD ranged from 0.4±0.2 (unincised samples) to 3.0±2.0 (GA incised samples), indicating increased lateral penetration. The RPD were 0.1±0.1 and 4.8±2.2 in unincised and GA incised samples, respectively. The LPA varied significantly from 0.001±0.001 (unincised samples) to 17.0±9.4 (GA incised samples). Both fungi induced delamellation of axial parenchyma cells and degraded tyloses within the wood vessels. Fungal hyphae of GA and ID were predominantly found in vessels and ray parenchyma cells. The MOE ranged from 7305.0±298.1 (GA incised) to 7771.1±256.0 (unincised samples). The MOR varied significantly from 70.8±6.1 to 77.9±3.9 in GA incised samples and unincised samples, respectively. The MCS// were 42.2±1.8 and 43.1±1.4 in GA incised samples and unincised samples, respectively. The JH increased from 2241.0±115.5 (unincised samples) to 2593.1±208.7 (GA incised samples). Bioincising Gmelina arborea heartwood with Ganoderma adspersum effectively dissolved occlusions within the vessels. Wood bioincision improved permeability, with negligible effects on the mechanical properties.