Growth stresses are highly controlled by the amount of G-layer in poplar tension wood

To determine how gelatinous fibres and gelatinous layers contribute to the magnitude of longitudinal growth stress in tension wood, anatomical measurements of gelatinous fibres were carried out on poplar tension wood (Populus I4551). It was found that (a) no gelatinous fibres were observed under a growth strain level of 0.06 to 20 0.08%; (b) almost 100% of the non-conductive tissues contained gelatinous fibres above a growth strain level of 0.15 to 0.19%; and (c) the area of fibres, the area of fibres with gelatinous layers per unit of tissue area, and the thickness of the gelatinous layers predominantly influence the magnitude of growth stress. 25 2 IAWA Journal INTRODUCTION Trees produce asymmetric growth stresses to maintain the vertical orientation of the main stem or the angle of a branch, in order to receive sufficient light or in 30 response to a strong dominant wind. This is usually achieved by the production of reaction wood, often combined with eccentric growth. While gymnosperms produce compression wood on the lower side of leaning stems, angiosperms produce tension wood generating high tensile stresses on their upper side (Wardrop 1964; Fisher & Stevenson 1981). Both strategies allow strongly 35 heterogeneous growth stress distribution at the periphery of stems, generating the bending moments required to control their shape. Normal wood fibres are composed of a thin primary wall and a thick secondary wall divided into 3 sub-layers; the S 1 , S 2 and S 3 layers. In many hardwood species such as beech, poplar, oak and chestnut, tension wood contains fibres with a 40 special morphology and chemical composition due to the development of the so-called gelatinous layer (G-layer) (Onaka 1949) that replaces the S 3 layer and a part or the whole of the S 2 layer (Saiki 1971). The G-layer is known to have a high cellulose content with a high degree of crystallinity (Norberg & Meier 1966; Côté et al. 1969) and to contain microfibrils oriented along the axis of the cell 45 (Fujita et al. 1974). There is some disagreement about the origin of growth stresses in wood While it is known that some species do not need to produce G-layer to induce high growth stresses (Okuyama et al. 1994; Yoshida 50 et al. 2000; Clair et al. 2006b), tension wood with a G-layer is a good model for trying to understand growth stress generation. In this paper we will concentrate …