Ultrastructural characteristics of cell wall disintegration of Pinus spp. in the windows of an old Buddhist temple exposed to natural weathering

An ultrastructural study was conducted to examine the disintegration pattern of cell wall layers during natural weathering. Pine wood samples from the wooden windows in an old Buddhist temple in the Republic of Korea were investigated by transmission electron microscopy (TEM) and ultraviolet (UV) microscopy. The first disintegration during the weathering process occurred in the cell corner middle lamella and extended to the compound middle lamella. Dissolution of warts and loosening of the S3 layer occurred almost simultaneously with the degradation of the middle lamella. After degradation and separation of the S3 layer from the S2 layer, defibrillation appeared in the S1 layer. The S2 layer was the most resistant against weathering. However, the S2 layer was also defibrillated at the later stage of weathering. Defibrillation in the S2 layer at the last stage of weathering suggested that lignin was degraded first, followed by hemicellulose and cellulose. UV microscopic work showed that lignin in tracheids was linearly decreased from the surface to the inner parts of the cells. TEM and UV microscopy exhibited that lignin in the middle lamella was much more susceptible to weathering than that in the secondary wall, thus suggesting the chemistry of lignin in the middle lamella to be different from that in the secondary walls.