Control of Wood Structure
The secondary xylem cells of woody plants, such as tracheids and wood fibres, have cell walls with a highly organized structure. The orientation of cellulose microfibrils in the primary wall determines the direction of cell elongation and expansion, there
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3.1
Summary The secondary xylem cells of woody plants, such as tracheids and wood fibres, have cell walls with a highly organized structure. The orientation of cellulose microfibrils in the primary wall determines the direction of cell elongation and expansion, thereby controlling the shape and size of xylem cells. In contrast, the orientation of microfibrils in the secondary wall, in particular in the thick middle layer (S 2 layer), is closely related to several of the mechanical properties of wood. Thus, the ability to control the orientation of cellulose microfibrils in the secondary wall might allow us to change the quality of wood and its products. There is considerable evidence that the dynamics of cortical microtubules are closely related to the orientation and localization of newly deposited cellulose microfibrils in the differentiating tracheids or wood fibres. Thus, it seems very likely that manipulation of cortical microtubules would allow control of the orientation of microfibrils, with a consequent improvement of wood quality.
3.2
Significance of microfibril orientation for wood structure Wood, which is a renewable resource, has been used for millennia as a raw material.. It is used currently for lumber, furniture, pulp and paper, chemicals and fuels. Wood is produced by the vascular cambium of living organisms, namely trees. Moreover, wood quality within a single species can vary markedly, depending on environmental factors such as climate, soil conditions and the spacing of growing trees, all of which affect the cambial growth of trees. In addition, wood quality varies among species and within individual trees according to cambial age, stem position and distance from the crown. Tree-to-tree variability within members of a species under identical growth conditions shows that genetic factors also influence wood quality. These observations indicate the strong possibility that wood quality might be improved and unified not only by silvicultural treatments, such as pruning, thinning and fertilization, but also by breeding to select genetically desirable trees (Panshin and de Zeeuw 1980; Zobel and van Buijtenen 1989; Zobel and Jett 1995). Recently developed molecular biological approaches also have the potential to improve wood quality, in particular the chemical composition of wood components such as lignin (Higuchi 1997). Differences in wood quality, in particular of mechanical properties, are largely due to differences in wood P. Nick (ed.), Plant Microtubules © Springer-Verlag Berlin Heidelberg 2000
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Control of wood structure
structure. Thus, wood structure is one of most important targets in attempts to control wood quality. The orientation of microfibrils, referred to as the microfibril angle (the angle between cellulose microfibrils and the main cell axis) in the secondary wall is one of the most important ultrastructural characteristics that determine the properties of wood (Cave and Walker 1994). In particular, the angles of the middle layer (S 2 layer), which is the thickest layer in the
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