Maturation Stress Generation in Poplar Tension Wood Studied by Synchrotron Radiation Microdiffraction

Abstract

Tension wood is widespread in the organs of woody plants. During its formation, it generates a large tensile mechanical stress, called maturation stress. Maturation stress performs essential biomechanical functions such as optimizing the mechanical resistance of the stem, performing adaptive movements, and ensuring long-term stability of growing plants. Although various hypotheses have recently been proposed, the mechanism generating maturation stress is not yet fully understood. In order to discriminate between these hypotheses, we investigated structural changes in cellulose microfibrils along sequences of xylem cell differentiation in tension and normal wood of poplar (Populus deltoides 3 Populus trichocarpa 'I45-51'). Synchrotron radiation microdiffraction was used to measure the evolution of the angle and lattice spacing of crystalline cellulose associated with the deposition of successive cell wall layers. Profiles of normal and tension wood were very similar in early development stages corresponding to the formation of the S1 and the outer part of the S2 layer. The microfibril angle in the S2 layer was found to be lower in its inner part than in its outer part, especially in tension wood. In tension wood only, this decrease occurred together with an increase in cellulose lattice spacing, and this happened before the G-layer was visible. The relative increase in lattice spacing was found close to the usual value of maturation strains, strongly suggesting that microfibrils of this layer are put into tension and contribute to the generation of maturation stress.

Gravitropisms and reaction woods of forest trees – evolution, functions and mechanisms

“…Tension wood fibers are the cell type responsible for force generation, and the specialized G-layer is thought to be directly involved in force generation (Mellerowicz & Gorshkova, 2012). It is the last cell wall layer formed during fiber development, is highly enriched in cellulose and largely devoid of lignin, and has a cellulose microfibril angle (MFA) approaching zero,which makes the cell resistant to stretching but permissive to swelling (Clair et al, 2011). It has been reported that some angiosperms produce functional tension wood lacking a G-layer, most notably for a number of tropical rainforest species (Fisher & Stevenson, 1981;Clair et al, 2006).…”

“…Tansley review New Phytologist reported to lack a G-layer will be required to determine whether the G-layer is universally required to produce functional tension wood. While the molecular mechanisms responsible for force generation are still uncertain, at least some key elements have been identified (Mellerowicz & Gorshkova, 2012;Fagerstedt et al, 2014), including cellulose MFA (Norberg & Meier, 1996;Bamber, 2001;Clair et al, 2011), xyloglucan endotransgylcosylase (XET) (Mellerowicz et al, 2008;Mellerowicz & Gorshkova, 2012), and fasciclin-like arabinogalactan proteins (MacMillan et al, 2010). Different hypotheses have been proposed for mechanisms of force generation, most of which are not necessarily mutually exclusive.…”

Mechanical contribution of secondary phloem to postural control in trees: the bark side of the force

“…To ensure vertical growth, trees need both a 'skeletal' system, which is achieved through the stiffness and strength of the trunk material (Niklas, 1992) and a 'motor' system (Moulia et al, 2006) to control plant posture by generating forces to offset the effect of gravity during growth (Alm eras & Fournier, 2009). The motor function is driven by turgor pressure in unlignified organs (Moulia & Fournier, 2009) or results from cell wall maturation in wood (Clair et al, 2011). The vertical posture of trees was long thought to be controlled only by internal forces in the wood during the formation of fibres (Scurfield, 1973;Fournier et al, 2014).…”

“…Tensile stress is generated in the gelatinous layer during maturation. Current description of the underlying mechanism is that swelling of the gelatinous polysaccharide matrix (Chang et al, 2015) bends the cellulose network (Clair et al, 2011;Alm eras & Clair, 2016), putting cellulose microfibrils into tension. In numerous species, the gelatinous layer is no longer visible as it is later lignified, after generation of the tensile force (Roussel & Clair, 2015).…”

Investigating the molecular underpinnings underlying morphology and changes in carbon partitioning during tension wood formation in Eucalyptus

“…Within the secondary cell walls (SCWs) of tension wood fibres, there are marked differences in cellulose properties. Typically, a-cellulose is relatively increased in tension wood Okuyama et al, 1994;Yoshizawa et al, 2000), and the cellulose is more crystalline (Okuyama et al, 1994;Mü ller et al, 2006) and has a marked decrease in microfibril angle (MFA; Okuyama et al, 1994;Washusen et al, 2005;Ruelle et al, 2006Ruelle et al, , 2010Clair et al, 2011).…”