Soft Rot Decay of Cengal (Neobalanocarpus heimii) Heartwood in Ground Contact in Relation to Extractive Microdistribution

IRG/WP 03-10501

A P Singh, A H H Wong, Yoon Soo Kim, Seung-Gon Wi, Kwang Ho Lee

The heartwood of cengal (Neobalanocarpus heimii, fam. Dipterocarpaceae) is naturally durable. A square-sawn utility pole specimen of cengal heartwood, after 30 years in ground contact, showed 10-15 mm surface decay all around the ground line position, accompanied with isolated surface termite attack at the decayed region. Light and transmission electron microscopy (TEM) of the decayed regions provided evidence of wood cell wall degradation by cavity-forming soft rot fungi. In the outermost layers, where such decay was most severe and severely discolored, all tissue types were degraded. However, in regions with moderate decay, differences in tissue types were observable in the extent of cell wall degradation. The presence of relatively intact vessels and parenchyma cells among heavily degraded fibres suggested that fibres were more susceptible to decay than vessels and parenchyma. Middle lamella was the only cell wall region which remained intact in all cell types which were severely degraded. Analysis of cengal heartwood revealed high contents of extractives soluble in hot water (14.1% m/m) or methanol (20.3% m/m), lignin (31.4% m/m) and phenolic materials (23.5% m/m), while microscopic observations provided information on the microdistribution of extractive materials. Vessels, fibres and parenchyma cells (both ray and axial parenchyma) all contained extractives in their lumen, but in variable amounts. The bulk of extractives were present in the lumina of fibres and parenchyma cells, filling them partially or completely. In vessels, only a thin layer of the extractives was present coating the warty layer. Brown coloration of cell walls in the light micrographs and their electron dense appearance in the TEM micrographs suggested that cell walls were also impregnated with extractives. This feature was typical of all cell types, but the amount and distribution of extractives was variable, judging by the variations in the coloration and density of cell walls. Overall, cell wall density of vessel and parenchyma cell walls appeared to be greater than that of fibre walls. TEM observations also provided evidence that pit membranes connecting parenchyma cells were well coated and impregnated with extractives, which was also found in the narrow canals of fibre pit pairs. Fungal hyphae were present in the extractive masses localized in cell lumina, and indications were that the extractives did not completely inhibit fungal growth. The microscopic observations suggested a close correlation between extractive microdistribution and the pattern and extent of cell wall degradation. In addition to fungitoxicity, the physical constraint of the extractive material present in cengal heartwood cells is likely to have a profound effect on restricting growth and pathways for cell wall colonization by soft rot fungi, thus conferring natural wood protection. Collectively, these observations suggest that both extractive content and their microdistribution are crucial in controlling wood decay, and should be closely examined to elucidate the role of heartwood extractives in wood durability and as natural preservatives.

Keywords: Tropical hardwood, Neobalanocarpus heimii, cengal heartwood, natural durability, soft rot, extractive microdistribution, microscopy

Conference: 03-05-18/23 Brisbane, Australia

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