H. YAMAMOTO1), J. RUELLE1)2), Y. Arakawa1), M. Yoshida1), B. CLAIR2), J. GRIL2)
1) Dept. Bio-materials Sciences, School of Bio-agricultural Sciences, Nagoya University, Nagoya 464-8601, JAPAN (hiro@agr.nagoya-u.ac.jp)
2) Laboratoire de M'ecanique et G'enie Civil, Universit'e Montpellier 2, CNRS, 34095 Montpellier, FRANCE


Introduction

The mechanism responsible for unusual mechanical properties of tension wood gelatinous fiber (G-fiber) was investigated. We discussed origins of high tensile growth stress, high drying shrinkage, and rapid increase of Young's modulus due to drying, in association with microscopic structure of gelatinous layer (G-layer). Anatomical, crystallographic, and micromechanical approaches were employed.


Materials and Method

Released strain of the surface growth stress (= maturation strain) was measured in the tilted stems of some Japanese hardwood species, using the strain-gauge method. After that, tension and normal wood specimens were sampled, and the moisture dependencies of the axial Young's modulus and axial dimensions were measured. Results were analyzed in relation to the anatomical property and X-ray crystallographic behavior of the specimens. Observed results were analyzed by using a G-fiber mechanical model, and microscopic properties of the G-layer and the lignified layer were quantitatively predicted.


Result and Discussions

It was revealed that the longitudinal growth strain (maturation strain) was higher in the G-layer than in the lignified layer (L-layer); the ratio of increase in the longitudinal Young's modulus with drying and the longitudinal drying shrinkage displayed similar patterns. At the same time, we found that the lattice distance of the [200] plane in the cellulose crystallite increased with drying, moreover, the half-width of the [200] diffraction peak increased with drying, which was remarkable in the tension wood. Those results suggest the following mechanism on the microscopic behaviors of the G-layer.

In the green state, the polysaccharide matrix in the G-layer behaves as a water-swollen gel in the living xylem; however, it is transformed into a condensed and hard-packed structure by strong surface tension during moisture desorption, which is a form of xero-gelation. However, in the L-layer, condensation and subsequent xero-gelation of the polysaccharide matrix was prevented by the hydrophobic lignin that mechanically reinforces the matrix. This causes characteristic hygro-mechanical behaviors of the G-layer. A certain transformation in the gel structure would occur during the cell wall maturation, which cause axial contraction in the maturing G-layer to be a high tensile growth stress in the G-fiber.

Key words: Gelatinous layer, crystallography, growth stress, cellulose microfibril, micromechanics