Adya P. Singh

A review on natural products as wood protectant

Traditional wood protection methods employ chemicals that are considered toxic and can adversely affect human health and the environment. Fortunately, serious efforts are being made globally to develop alternative protection methods based on natural products with little or no toxicity, but the progress in implementation of the technologies has been slow because of certain limitations, including discrepancies between laboratory and field performance of natural products, variability in their efficacy related to exposure/environmental conditions, and legislation difficulties due to disagreements globally on setting standards defining the quality of their performance and use. The focus of this review is to present information on the natural compounds that have shown promise for wood protection, and the information is presented under defined interactive categories. In closing, some thoughts are presented on potential use of rapidly evolving technologies, such as nano- and gene-technologies that can lead to significant advances, particularly from the consideration of specificity of natural products and their economic value.

Ray tracheids in Pinus radiata are more highly resistant to soft rot as compared to axial tracheids: relationship to lignin concentration

A visual decay assessment of Pinus radiata wood, which was part of a framing timber in a house in the North Island of New Zealand, indicated the presence of surface decay. Microscopic observations, employing confocal laser scanning microscopy (CLSM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), provided evidence of decay by cavity-forming soft rot (SR) fungi. A comparison of ray tracheids (RT) with axial tracheids (AT) indicated that RT were considerably more resistant to SR than AT. In the heavily degraded regions of wood, where axial tracheid walls contained abundant SR cavities, the walls of RT contained only a few or no cavities. An assessment of lignin concentration in the cell walls by a combination of TEM, confocal fluorescence and UV microscopy provided evidence of greater lignin concentration in the secondary wall of RT as compared to AT, which may explain the observed greater resistance of RT to soft rot.

Visualising impregnated chitosan in Pinus radiata early wood cells using light and scanning electron microscopy

Chitosan, a deacetylated product of an abundant naturally occurring biopolymer chitin, has been used in a range of applications, particularly in food and health areas, as an antimicrobial agent. In the work reported here Pinus radiata wood was impregnated with chitosan as an environmentally compatible organic biocide (Eikenes et al., 2005a,b) to protect wood against wood deteriorating microorganisms and to thus prolong the service life of wooden products. We developed sample preparation techniques targeted to visualise impregnated chitosan within wood tissues using light microscope and field-emission scanning electron microscope (FE-SEM). Sections were viewed with the light microscope without staining with a dye as well as after staining with the dye toluidine blue. Light microscopy was also undertaken on sections that had been stained with 1% aqueous osmium tetroxide (OsO(4)). For SEM observations, the sections were treated with OsO(4) and then examined with the FE-SEM, first in the secondary electron imaging mode (SEI) and then in the backscattered electron imaging (BEI) mode, imaging the same areas of a section in both SEI and BEI modes. The preparation techniques employed and the combined use of light and scanning electron microscopy provided valuable complementary information, revealing that chitosan had penetrated into the cavities (cell lumens, intercellular spaces) of all sizes present within wood tissues and had also impregnated early wood cell walls. The information obtained is discussed in relation to its importance in further development of chitosan formulations and refinement of impregnation technologies to optimise chitosan impregnation into and distribution within wood tissues as well as in assessing chitosan efficacy.

Certain Aspects of Bacterial Degradation of Pinus Radiata Wood

Bacterial degradation of tracheid walls of Pinus radiata wood was examined by transmission electron microscopy. The wall degradation appeared to be of two different forms, one where bacteria were present within tracheid walls forming tunnels as they moved - tunnelling type of degradation, and the other where bacteria degraded the wall from the lumen outwards - erosion type of degradation. The residual material arising from bacterial erosion of the tracheid wall spread to various extents into the lumen and contained mixed bacterial populations of varied forms. Microscopic details of these two degradation forms which involved adjoining wall areas of the same tracheid are described.

Microscopic, biochemical and physiological assessment of the effect of methylene bisthiocyanate on the sapstain fungus Ophiostoma floccosum

In vitro effects of methylene bisthiocyanate (MBT) on hyphal morphology and ultra-structure of Ophiostoma floccosum were examined using differential interference contrast, epifluorescence and transmission electron microscopy (TEM). To understand the mode of action of MBT, experiments were undertaken to measure potassium ion (K+) leakage from cells, oxygen consumption, glucose and ATP levels. Differential interference contrast microscopy indicated that MBT caused rapid changes in O. floccosum hyphae resulting in extensive vaculoation and accumulation of granular materials within the cytoplasm. Epifluorescence microscopy provided evidence that MBT treatment causes a loss in the permeability properties of the plasma membrane. TEM showed retraction of the plasma membrane from the cell wall, aggregation of cytoplasmic contents, vesiculation of membranous components, a dramatic increase in vacuolation, and eventually a complete loss in the integrity of organelles. There was a rapid efflux of intracellular K+ ions from cells, a substantial loss in K+ ions occurring within the first 5xa0min of MBT treatment. The rate of K+ leakage was MBT concentration treatment-time dependent. The study also showed that the effect of lower concentrations of MBT (0.01 and 0.1xa0mM) on respiratory activity was negligible. However, at the same concentrations, glucose consumption and ATP production were affected. Taken together, these observations suggest that the target site of MBT in O. floccosum alters membrane properties and uncouples oxidative phosphorylation from the respiratory chain.

Correlative light and scanning electron microscopy of the same sections gives new insights into the effects of pectin lyase on bordered pit membranes in Pinus radiata wood.

Bordered pits are structures in the cell walls of softwood tracheids which permit the movement of water between adjacent cells. These structures contain a central pit membrane composed of an outer porous ring (margo) and an inner dense and pectin-rich disc (torus). The membrane is overarched on each side by pit borders. Pits may be aspirated, a condition where the torus seals against the pit border, effectively blocking the pathway between cells. In living trees this maintains overall continuity of water conduction in xylem by sealing off tracheids containing air. Drying of timber results in further pit aspiration, which reduces wood permeability to liquid treatment agents such as antifungal chemicals. One possible way to increase permeability is by treating wood with pectin lyase to modify or remove the torus. The effectiveness of this treatment was initially evaluated using light microscopy (LM) of toluidine blue stained wood. Pectic material is coloured pink-magenta with this stain, and loss of this colour after treatment has been interpreted as indicating destruction of the torus. However, correlative light (LM) and scanning electron (SEM) microscopic observations of identical areas of toluidine blue stained sections revealed that many unstained pits had intact but modified tori when viewed with SEM. These observations indicate that LM alone is not sufficient to evaluate the effects of pectin lyase on pit membranes in wood. Combining LM and SEM gives more complete information.

COMMENT ON THE STRUCTURE OF OSMIUM TETROXIDE-CHITOSAN COMPLEX

Reaction of an aqueous solution of osmium tetroxide with the chitosan polymer model, the oligomer tetradeca(β(1→4)2-amino-2-deoxyglucopyranose)-β(2→5)mannitol, afforded a brown solid, the structure of which was proposed to contain osmium imido groups, −N˭Os. The product showed a new, distinct absorption band at 850 cm−1 in the FTIR spectrum, while the 13C CP-MAS NMR spectrum showed a lower signal relative intensity ratio for C2:C3/5 compared with that in the NMR spectrum of the starting oligosaccharide due to a downfield shift of C2 where allylic to the -N˭Os bond. Results from quantum chemical calculations were used to compare computed properties of the previously described “osmium tetroxide-chitosan complex” in which osmium tetroxide was coordinated to chitosan amino ligands with single dative N→Os bonds, and the product prepared in this study, proposed to have covalently-bonded imido bonds between osmium and nitrogen. The consequences of multiple bonding of two chitosan N to the Os atom in the 5d 3 6s hybrid tetrahedral configuration and with the C-N˭Os bond angle greater than 150° required one of the carbohydrate pyranose rings attached to Os to adopt a boat conformation.

Probing the Wood Coating Interface at High Resolution

Wood is a versatile biomaterial used in a wide range of applications. Although wood is strong and durable the products made from it when placed in service, particularly in outdoor environments, can deteriorate within a relatively short time because of exposure to weathering factors, such as solar radiation, rain and decay microorganisms. Application of coatings to the exposed surfaces of wood products, that can prevent solar radiation and water from reaching wood tissues, can provide protection from wood deteriorating factors. Coating adhesion, which is among the factors that play an important role in determining the performance of an applied coating, is related to chemical and physical interactions with wood, the latter involving coating attachment to wood via penetration into surface tissues, where cell lumens and cell wall delaminations have an important role.

Resistance of the S1 layer in kempas heartwood fibers to soft rot decay

Naturally durable heartwoods, where available, continue to be used as support structures in environments considered hazardous, particularly in ground contact. However, durability of heartwoods against wood decay microorganisms varies. Therefore, it is important to evaluate heartwood products for their in-service performance in order to maximise benefits derived from this valuable natural resource of limited supply. In the work presented, wood pieces from a kempas (Koompassia malaccensis) utility pole that had been placed in service in an acidic soil in Malaysia, and in time had softened at the ground-line position, were examined by light and transmission electron microscopy to evaluate the cause of deterioration. Light microscopy (LM) provided evidence of extensive attack on fibre cell walls by cavity-producing soft rot fungi. Transmission electron microscopy (TEM) revealed in greater detail the distribution and micromorphologies of cavities as well as their relationships to the fine structure of fibre cell walls, which consisted of a highly electron dense middle lamella, a moderately dense S1 layer and a multilamellar S2 layer with variable densities, reflecting differences in lignin concentration. The resistance of the moderately dense S1 layer to soft rot was a feature of particular interest and is the main focus of the work presented. The resistance appeared to be correlated with high lignification of the outermost region of the S2 wall, interfacing with the S1 layer, an unusual cell wall feature not previously described for normal wood.