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 the 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 layer and the outer part of the S2 layer. Subsequent layers were found with a lower microfibril angle (MFA), corresponding to the inner part of the S2 layer of normal wood (MFA approximately 10°) and the G layer of tension wood (MFA approximately 0°). In tension wood only, this steep decrease in MFA occurred together with an increase in cellulose lattice spacing. The relative increase in lattice spacing was found close to the usual value of maturation strains. Analysis showed that this increase in lattice spacing is at least partly due to mechanical stress induced in cellulose microfibrils soon after their deposition, suggesting that the G layer directly generates and supports the tensile maturation stress in poplar tension wood.

Solvent Polarity and Internal Stresses Control the Swelling Behaviour of Green Wood During Dehydration in Organic Solution

“…2422 wood slightly swelled during serial organic solvent substitution. For each solvent, a stable value of the strain (-0.02% to +0.1%) was rapidly attained, which was the same order as what is released with hygrothermal treatments (Clair 2011). Tension wood shrunk greatly (up to 0.5 %) and seemed to continue to shrink at the end of the substitution without reaching an asymptote.…”

“…Along the tangential direction (Fig. 2a), the value of tangential strain (ε T ) was positive both in normal wood and tension wood, which means that the specimens expanded in tangential direction in all the organic solutions in the same order as what was measured after hygrothermal treatment (Clair 2011). As the solvent concentration increased from 0 to 70%, the specimens swelled and reached a maximum swelling.…”

“…The shrinkage pattern also depends on the large mechanical stresses, called maturation stresses, which originate in living trees during the cell maturation process (Clair 2011). In the case of normal wood, newly formed fibers tend to contract longitudinally and to expand transversely, but this strain is restrained by their adherence to the older and stiffer wood cells.…”

Tree growth stress and related problems

“…The unified hypothesis was a consistent theory that explained the growth stress generation not only in normal wood but also in compression wood or tension wood. However, conclusive evidence demonstrating tensile-stress generation in the polysaccharide framework is still confined to few examples [55,56,91]. The same can be said for compressive stress in matrix region [91,92].…”

“…The scale are 10 lm creation of free surface results in stress redistribution in its vicinity, which accounts for both the apparent deformation and easy detachment of the G-layer from the lignified wall after the action of sectioning initiated the crack propagation. Based on their findings, we can consider that the characteristic behavior of G-fiber can be, indeed, attributed to the intrinsic property of G-layer [42,44,46,48,[51][52][53][54][55][56].…”