Spruce cell wall degradation

Figure 33. Spruce tracheids with cell wall degradation proceeding from the lumen (arrows), and tracheids with totally degraded secondary cell walls.

Figure 32. Spruce, slightly degraded cell walls split in the Sj (arrows).

The pit chambers are reduced to what ontogenetically is called the initial pit chamber, and the pit membranes are dissolved or detached. Fengel (15) reports pit membranes in fossil spruce to be quite resistant. However, in tissues where all secondary cell walls have been degraded we have not seen any pit membranes in bordered pits. 

On the whole, the total breakdown of secondary cell walls proceeds as a front parallel to the wood surface. The interface between degraded and nondegraded areas is sharp in oak and ash, and more diffuse in elm and poplar. In spruce it resembles a transition zone several millimeters thick, in which more and more cells become affected until the whole tissue is uniformly and completely degraded. 

The types and relative abundances of cellulose, hemicellulose, and lignin vary among wood cell types, between earlywood and latewood, and within individual cell walls. These distributional differences affect the relative resistance of the various components to degradation and are best worked out for spruce wood tracheids and birch wood fibers, which constitute roughly 95 and 80%, respectively, of the volumes of the corresponding woods (2, 5). 

Figure 6.34 Localization of early brown-rot degradation within a radial thin section of spruce wood with FT-IR imaging microspectroscopy (a) Charge-coupled device (CCD) camera image of 10 tracheids of a degraded spruce wood section from a sample degraded by the brown-rot fungus G. trabeum for 4 weeks, (b) FT-IR pseudocolor spectral absorbance image. Zones of high total absorbance (red) show a high contribution of middle lamella (ML) and primary cell walls (P) and outer layers of secondary...

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