Middle lamella appeared unchanged in the early stages of decay.
The ultrastructural observations presented using [KMn0.sub.4] staining provided evidence that in the surface regions of peracetic-acid-treated boards lignin was completely removed from the cell corner middle lamella regions, though cells were still joined in other regions of middle lamella.
Earlywood is reported to have a higher lignin concentration than latewood for Douglas fir and red pine,(43) which may be related to differences between earlywood and latewood in the proportion of middle lamella to secondary wall.
It begins in the
middle lamella and progresses through the primary cell wall before penetrating into the secondary cell wall (Wilson, 1993).
The tracheids in the surface layers of pretreated boards tended to separate in the region of middle lamella, (1), (2) being more severe in preweathered boards, (1) and this region of cells would be one distinct point of weakness if the coating did not penetrate sufficiently and also uniformly enough to stabilize this region.
The complete removal of lignin from the middle lamella in the preweathered boards, resulting in cell separation, may therefore result in a region that is effective in blocking photodegradation.
The investigators used size exclusion chromatography analysis, carbohydrate analysis and uronic acid assay profiling of the collected fractions to determine the differences between the middle lamella and cell wall pectin.
Based on these observations, the researchers believe that during early ripening, the middle lamella pectin is degraded by pectinases.