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Ultra-structural observations on the degradation of wood surfaces during weathering
1987 - IRG/WP 2280
Radiata pine (Pinus radiata D. Don) sapwood was converted into blocks with a transverse face about 5 mm square and measuring 8 mm longitudinally. Transverse (T.S.), Radial (R.L.S.) and Tangential (T.L.S.) surfaces were prepared and specimens exposed to the weather inclined at 45° facing equatorially for periods of between 20-60 days. After 30 days exposure erosion of the middle lamella was observed followed after 40 days exposure by extensive separation of individual fibres at the interface of the middle lamella and secondary wall. Degradation of the S2 layer of the cell wall revealed corrugations orientated parallel to the fibre axis suggesting preferential removal of cell wall components. Further degradation proceeded by progressive delamination and checking of the S2 and erosion of the S3 cell wall layer. In addition to the above changes preferential degradation of the rays was observed in radial (R.L.S.) and tangential (T.L.S.) longitudinal surfaces.
P D Evans, S Thein

A review of the configuration of bordered pits to stimulate the fluid flow
2005 - IRG/WP 05-40315
As the bordered pits have generally been thought to have an influence on the refractory nature of softwoods, structural behaviour of this conducting pathways is discussed according to the published literature. Various theories on the role of bordered pits to axial flow are expounded in respect to preservative treatment. Pit aspiration is also reviewed.
I Usta

Water sprinkled pine wood: A microscope study on boards showing streaking
1993 - IRG/WP 93-10033
Boards sawn from the outer sapwood of pine lumber previously water sprinkled for periods of 10-18 weeks and kiln dried, showed streaking after staining. Two types of enhanced stain uptake were noted: 1) a more concentrated and localized form after 10 weeks and 2) a more diffuse type which developed in later stages of water sprinkling. Areas showing enhanced stain uptake were examined using light and scanning electron microscopy. For all samples, staining was most concentrated in rays (both fusiform and uniseriate) and axial resin canals. Studies suggest that increased permeability and enhanced stain uptake results primarily from microbial (fungal and bacterial) destruction of rays and axial resin canals (i.e. ray parenchyma, epithelia cells, window pit membranes), either by direct attack (i.e. close proximity of bacteria/fungal hyphae) or by diffusion of enzymes (probably cellulases) from microbes distant from sites of hydrolysis. Sapwood offcuts from the outer regions of water sprinkled wood lacking bark showed severe colonization and decay of ray tissues by both bacteria and sapstain fungi. Diffuse stain uptake in boards at later stages of water sprinkling appeared to be secondary and result from bacterial and/or fungal decay of bordered pit membranes.
G F Daniel, T Elowson, T Nilsson, A P Singh, K Liukko

Bordered Pit Imaging
2012 - IRG/WP 12-10773
New findings about bordered pits will be presented using the latest microscopy techniques. Three-dimensional imagery at the nanolevel is used, and short 3-D movies will shown as part of the discussion on this topic to reveal new features that have not previously been reported in pits. The implications for both microorganism penetration through lignified cells as well as preservative penetration in wood will be overviewed in light of the findings.
D Mascheck, B Goodell, H Militz, M Lessard, J Jellison

Studies on the permeability of Norway spruce (Picea abies)
1987 - IRG/WP 2295
It is well known that the bordered pits play a dominating role for liquid transport in softwoods. The mechanism behind pit aspiration during drying or heartwood formation seems to be well understood. The reason why the fixation of the membrane (torus) to the pit opening remains almost irreversible is, however, still unclear. A technique has been developed combining so-called "solvent exchange drying" with scanning electron microscopy. In this way the pit system can be "frozen" and the number of pits in aspiration can be determined. The technique has been used to widen the understanding of pit aspiration. It has also been used to follow pit aspiration during air drying and in connection with efforts to pit aspirated pits by different physical and chemical methods.
I Johansson, K Nordman-Edberg

A comparison between high and conventional incision densities for improving preservative treatment of Douglas-fir heartwood lumber
1993 - IRG/WP 93-40009
Incising is required for the preservative treatment of most wood species from the western United States; however, there is considerable debate about the density of incisions required to achieve adequate treatment. Previous studies have shown that the incision effect on treatment is relatively narrow, suggesting that incision densities should be significantly increased over current practices and stimulating the development of at least 2 competing high density incisors. In this report, we compare ammoniacal copper arsenate and chromated copper arsenate treatment of end-matched Douglas-fir lumber incised using one of these high density incisors and a conventional lumber incisor. As expected, higher density incisions improved the depth and uniformity of treatment; however, even this process could not improve CCA treatment to the point where it would meet the American Wood Preserver's Association Standard for treatment of dimension lumber.
S T Lebow, J J Morrell

Wood structure in relation to excessive absorption - a literature survey
1971 - IRG/WP 300
This literature study, composed in connection with 'the structure of wood in relation to the excessive absorption', started from the idea that the subject only could be dealt with successfully if the structure was taken into account in relation to a normal absorption pattern in so far as this knowledge can enlighten the understanding of the abnormal situation. The normal penetration pattern is dealt with in comparison of increasing absorption caused by using several different methods with the drying technique and especially caused by micro-organisms. It transpired that bacteria especially play an important role in excessive absorption. They degrade the wood tissue to such an extent that openings develop which can easily be passed unhindered
S M Jutte

Hydrolysis of bordered pits during colonization of conifers by brown-rot fungi
1995 - IRG/WP 95-10103
Brown-rot decay results in rapid reduction in degree of polymerization (DP) of holocellulose with concomitant strength loss (MOR) without removing lignin. Development of new methods of wood protection will require focusing on early events in the sequence of depolymerization. Bordered pit membranes (sapwood) represent a readily available source of non-lignified carbohydrate, ie. pectin and cellulose. Commercial pectinases (Pectinol) and Trichoderma sp. have been shown to degrade pit membranes and increase penetration of preservatives. Brown-rot fungi have previouely been shown to produce oxalic acid (OA) during the decay process. Plant pathogens have been shown to degrade pectin by the synergistic action of OA and polygalacturonase (PG). The OA solubilizes the pectin by chelating the Ca++ and the PG hydrolyses the a-1,4 linkages. We have demonstrated the ability of Postia placenta, Gloeophyllum trabeum and Serpula incrassata to use pectin as a sole carbon sourse and to produce OA and PG on both liquid media and wood. Aspergillus niger and Trichoderma sp. also produce PG on wood but no OA or weight loss. The optimal pH of brown-rot polygalacturonase activity is circa 4.0. As the pH of the wood drope below pH 4, due to acid production during decay, there is a progressive decrease in PG activity and the possibility of acid catalyzed hydrolysis of pit membranes is suggested by increased permeability. We hypothesize that pectin utilization is an essential step during incipient brown-rot decay which helps to initiate fungal metabolism and promote the spread of fungal hyphae between tracheids.
F Green III, J L Tschernitz, T A Kuster, T L Highley

Use of Variable Pressure Scanning Electron Microscopy for in situ Observation of Degradation of Wood Surfaces during Artificial Weathering
2012 - IRG/WP 12-20489
Variable pressure scanning electron microscopy (VPSEM) is capable of observing electrically non-conductive materials like wood without the need to render the surface conductive by applying a coating of gold, platinum or carbon. Hence, there is the possibility that VPSEM could be used to non-destructively follow the structural changes occurring at wood surfaces during weathering, because specimens can be observed initially (before weathering), and then re-examined after exposure to the weather. This study uses VPSEM to examine the time-dependent micro-structural changes occurring at untreated and treated wood surfaces exposed to artificial weathering. Cryptomeria japonica (sugi) sapwood was treated by grafting a UV absorber (UVA) to the wood. We compared the micro-checking of bordered and half-bordered pits at treated (grafted) and untreated wood surfaces exposed to weathering. We hypothesised that the grafting treatment would restrict micro-checking of pits. VPSEM revealed significant micro-checking of pits at untreated surfaces exposed to artificial weathering for only 30 h. The dimensions of pit micro-checks increased when untreated samples were exposed to weathering for an additional 30 h. Pit micro-checking occurred more slowly in sugi sapwood treated with a UV absorber. We conclude that pit micro-checking at wood surfaces exposed to artificial weathering is restricted by grafting a UVA to the wood. We further conclude that VPSEM is a valuable tool to follow the time-dependent micro-structural changes occurring at wood surfaces exposed to weathering.
F Hatae, Y Kataoka, M Kiguchi, H Matsunaga, J Matsumura

Resistance of Alstonia scholaris vestures to degradation by tunnelling bacteria
1992 - IRG/WP 92-1547
Electron microscopic examination of vessels and fibre-tracheids in the wood of Alstonia scholaris exposed to tunnelling bacteria (TB) in a liquid culture showed degradation of all areas of the secondary wall. The highly lignified middle lamella was also degraded in advanced stages of TB attack. However, vestured pit membranes and vestures appeared to be resistant to degradation by TB even when other wall areas in Alstonia scholaris wood cells were severely degraded. The size comparison indicated vestures to be considerably smaller than TB, and we suspect that this may primarily be the reason why vestures in Alstonia scholaris wood were found to be resistant to degradation by TB.
A P Singh, T Nilsson, G F Daniel

Observations on preservative penetration in poplar
1991 - IRG/WP 3662
In the course of studies on the CCA treatment and durability of poplar timber a zone of refractory wood at the sapwood/heartwood boundary was observed. Further studies were conducted to investigate the causes of this refractory behaviour. The initial results have suggested that the presence of lipid storage products in the ray tissue may lead to impermeability, either by directly blocking pits or by preventing pit damage on drying. More work should be carried out before firm conclusions are drawn on the significance and causes of this effect.
R J Murphy, S U Din, M J Stone

Modelling hyphal growth of the bio-incising fungus Physisporinus vitreus
2010 - IRG/WP 10-10710
The white-rot fungus Physisporinus vitreus preferentially degrades the pit membranes of bordered pits in tracheids and subsequently enhances wood permeability. Thus, P. vitreus can be used to improve the uptake of wood preservatives and environmentally-benign wood modification substances. This process can be used to enhance the use and sustainability of native conifer wood species by the wood industry. Mathematical modelling in combination with laboratory experiments is a powerful and efficient investigation method that enables a deeper insight into complex interactions between biological systems and their environment. The objective of mathematical modelling is not to develop an extremely complex system of equations in an attempt to mimic reality. Rather, it is applied to reduce a complex (biological) system into a simpler (mathematical) system that can be analyzed in more detail and from which key properties can be identified, isolated and investigated. In addition, a verified mathematical model enables to optimize hyphal growth and impact of the P. vitreus. Enhanced uptake of wood preservatives and environmentally-benign wood modification substances of Norway spruce wood and optimized quality control processes would be of importance for the usability, durability and sustainability of wooden structures in civil engineering. In this paper we present a mathematical model of hyphal growth and expansion of P. vitreus in heartwood of Norway spruce. The model enables to investigate the global penetration front of the fungus in wood as function of the control parameters as well as its shape and penetration depth. This model will serve to assist the choice of pellet concentration and reaction times that are required to induce a defined degree of wood permeability by the fungus.
M Fuhr, C Stührk, F W M R Schwarze, M Schubert, H J Herrmann

Copper Nanoparticles in Southern Pine Wood Treated with a Micronised Preservative: Nanodistribution of Copper in the Pit Membrane and Border of an Earlywood Bordered Pit
2011 - IRG/WP 11-30566
Copper nanoparticles can penetrate the cell walls of unlignified parenchyma cells in southern pine wood treated with a micronised wood preservative, but they are excluded from lignified tracheid walls. This paper extends these observations to include the cell wall layers of the bordered pit. Focused ion beam and ion milling were used to make an ultra-thin section of the cell wall layers of an earlywood bordered pit excised from southern pine wood that had been treated with a micronised wood preservative. High resolution transmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy in combination with energy dispersive analysis of X-rays were used to detect and examine the penetration of the torus and pit border by copper. Copper was more abundant in the torus than in the pit border, but the depth of penetration of copper in both cell wall layers was approximately the same, ~100nm. High resolution transmission electron microscopy was unable to detect crystalline material in either the torus or pit border. Therefore we conclude that copper nanoparticles are unable to penetrate the torus and border of the bordered pit in accord with our previous observation that nanoparticles are excluded from the cell walls of lignified tracheids.
H Matsunaga, Y Kataoka, M Kiguchi, P Evans