Micromechanical detection of growth stress in wood cell wall by wide-angle X-ray diffraction (WAX)

The present study is aimed at the detection of mechanical stresses generated in the cellulose microfibril (CMF) crystals situated in the secondary wall (S-2) of living cells. Green wood specimens were boiled in water to release the internal stress in the CMF by the hygrothermal softening of the lignin-hemicellulose matrix (MT). Thereafter, the changes in d(200) and d(004) lattice spacings of crystalline cellulose were observed in boiled and nonboiled samples by wide-angle X-ray diffraction. The d(200) lattice spacing increased, whereas d(004) lattice spacing decreased. The results show that a mechanical stress still remained in the CMF and the MT region in the green cell wall, even after releasing the macroscopic surface growth stresses by removal of the wood block from the living stem. The interpretation is that CMF generates tensile stress in the longitudinal direction, and surrounding MT substances generate compressive stress in the living cell wall, which compresses the CMF in the lateral direction. The results confirm the "unified hypothesis" for explaining the mechanism of growth stress generation.