On the detachment of gelatinous layer in tension wood fiber

The detachment of gelatinous layer (G-layer), often observed on microtome cross sections, leads some authors believe that G-layer cannot act as the driving force of longitudinal shrinkage in tension wood. The aim of this study was to observe the detachment of G-layer along fibres. Green wood block was cut transversely into two samples. One sample was kept in water and the other ovendried. One face being common to both samples, detachment of G-layer has been studied on the same fibres. Observations have been performed after blocking deformation by embedding. It reveals that the detachment of G-layer is a cutting effect produced during the first making of the transverse face of the wood block to be embedded. After 100 μm far from this primary surface of the sample no detachment can be observed. Drying shrinkage does not affect or little this detachment. The result seems to explain well why the detachment of G-layer occurs during sectioning using a conventional sliding microtomy. These observations prove the adhesion of Glayer in massive wood and confirm the active role of Glayer in tension wood properties. Key-words Wood cell wall, cutting effect, gelatinous layer (G-layer), growth stress, tension wood, Populus euramericana B. Clair (corresponding. author.). J. Sugiyama Laboratory of Cell Structure and Function, Wood Research Institute, Kyoto University, Uji-Kyoto 611-0011, Japan B. Thibaut Laboratoire de Mécanique et Génie Civil (LMGC), UMR 5508 CNRS – Université Montpellier 2, Place E. Bataillon, CC 048, 34095 Montpellier CDX 5, France Present adress: Laboratoire de Mécanique et Génie Civil (LMGC), UMR 5508 CNRS – Université Montpellier 2, Place E. Bataillon, CC 048, 34095 Montpellier CDX 5, France Tel: +33-467-14-34-83 Fax: +33-467-14-47-92 e-mail : [email protected] Introduction To maintain verticality, most angiosperms are able to produce highly tensile stressed wood on the upper side of the leaning trunk. The stress asymmetry between both upper and lower sides of the trunk permits then to bend it to recover verticality . This xylem with high tension stress is called tension wood. It is characterised in many species by an unusual cell wall structure with a characteristic layer called gelatinous layer (G-layer). Glayer is known to have a high cellulose content with a high degree of crystallinity 5 and cellulose microfibrils are oriented along the axis of the cell. These differences in chemical composition and structure give to macroscopic tension wood some particular properties in comparison to normal wood, notably a high shrinkage. These high macroscopic shrinkages can find explanations in the properties of G-layer itself. In spite of its structure with microfibrils axially oriented, G-layer is subject to high shrinkage both in transverse and longitudinal directions. However, in order to contribute to the macroscopic behaviour, G-layer has to have a relatively higher elastic modulus in its axial direction and to be in tight adherence with the other layers of the cell. G-layer being often observed loosely attached to normal secondary wall layer , its contribution to macroscopic behaviour, especially to axial shrinkage, has been put in question . The aims of this study were to observe, after blocking deformation by embedding, the detachment of the G-layer from S2-layer along the fibre. Observations were made on never-dried wood and on dried wood in order to evaluate the influence of drying on the G-layer detachment. Materiel and methods