The transformation of sapwood (SW) into heartwood (HW) during ageing of wood tissues is the result of physiological and biochemical changes initiated in the transition zone (TZ). These changes contribute to the evolution of active (living) wood cells in SW into less/non-active (dead) wood cells in HW. Previous studies established that the biosynthesis of extractive contents is the most prominent process that occurs in the TZ. To improve our understanding of the extent and characteristics of the TZ in oak wood, the present study reports the results of color parameters (using CIELab color space) and molecular structure and composition of polysaccharide and lignin compounds (using FTIR-ATR and Py-GC-MS). For that purpose, six wood cores from individual living oak (Quercus spp.) trees were collected from two forests with similar environmental conditions, located in the Basque Country (Northern Spain). The color data indicated significant differences between SW, TZ and HW by showing that SW samples were characterized by higher hue (h°) and lower redness (a*) values than the HW, and intermediate values for the TZ. They also suggested that the variations of wood color from SW to HW occur gradually, along a wide TZ counting 4-10 measurement points in a row, depending on the tree. Furthermore, FTIR and Py-GC-MS data gave evidence of the variation trends of polysaccharide and lignin contents in the radial direction, through various FTIR ratios (1735/1325, 1590/1735, 1590/1230, and 1230/1325 cm-1) and one pyrolysis ratio (acetic acid/total polysaccharide: Ps01/Tot_Ps). The observed variations in this present study suggest that the cross-sectional transition patterns can be related to the continuous lignification process of xylem parenchyma cells, as well as the storage of polysaccharide compounds. These results contribute to our fundamental knowledge on the TZ, which may be valuable in research and industrial applications where a clear delimitation of sapwood and/or heartwood is required.
Keywords: Chemical change; Color change; Lignocellulose; Quercus spp; Transition zone; Wood formation.
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