The origin of maturation stress in tension wood: using a wide range of observations to assess hypothetic models

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1 Action FP0802 The origin of maturation stress in tension wood: using a wide range of observations to assess hypothetic models Tancrède Alméras, Bruno Clair, Joseph Gril Laboratory of Mechanics and Civil Engineering CNRS - Univ. Montpellier (France)

2 #1 Wood maturation and growth stresses in trees Wood maturation stress is set at the end of wood formation Induced tension (balanced by compression in inner wood) Accumulation of growth stress in the stem Biomechanical function: optimisation of bending strength

3 #2 Wood maturation and growth stresses in trees Highly pre-stressed wood = reaction wood Asymmetric distribution of maturation stress around the stem Generates a bending moment Biomechanical function: stem actuation

4 #3 Wood maturation and growth stresses in trees Technological consequences Field of growth stress cracks, warps Reaction woods associated to heterogeneity

5 #4 Consequences of maturation stress at the microscopic level Stress appears in cell-walls during their formation Detailed mechanism: not clear yet, several hypothetic models Induces a microscopic heterogeneous stress field in the wall Between cell-wall layers Between constituents of a layer In the tree: stress in a layer Cut wood sample: stress contrast Determines the initial state of wood as a material Pre-stresses may have consequences on wood behaviour!

6 #5 Maturation stress generation in G-layer tension wood Tension wood mechanical design Many species: presence of a gelatinous layer (G-layer) Microfibrils parallel to the cell axis Unusual matrix composition (no lignin, xyloglucans, AGP ) G What does generate large axial tensile stress during its formation? S2 S1 CML+P

7 #6 Implication of the G-layer Amount of G-layer is correlated with maturation stress Fang et al IAWA Journal Maturation strain (%) Clair et al Holzforschung Similar results: Yamamoto et al J. Wood Sci.

8 #7 Implication of the G-layer Apparent paradox between structure and function Cellulose is synthesised free of stress Crystalline cellulose is not modified after synthesis How is tension induced inside crystalline cellulose??? G S2 S1 CML+P CSC G-layer Cytoplasm

9 #8 Hypothetic mechanisms Contraction of amorphous cellulose regions Okuyama et al Annals Forest Sci. Bamber 2001 IAWA Journal Yamamoto et al Wood Sci. Tech.

10 #9 Hypothetic mechanisms Lateral swelling of G-layer and interaction between layers Goswami et al The Plant Journal

11 #10 Microscopic stress field in tension wood cells AFM topography in water Selective enzymes Goswami et al The Plant Journal Clair et al IAWA Journal Clair et al IAWA Journal

12 #11 Longitudinal Tangential Effect of moisture changes Macroscopic drying shrinkage Drying shrinkage of layers Normal wood Tension wood Wet Normal >>> Tension wood Dry Fang et al Wood Sci. Tech. Clair et al Holzforschung

13 #12 Hypothetic mechanisms Contraction of the matrix due to moisture changes Presence of AGP Bowling and Vaughn 2008 American Journal of Botany Lower water content

14 #13 Hypothetic mechanisms Formation of microfibril aggregates Interaction with xyloglucans Mellerowicz et al Annals of botany

15 #14 Hypothetic mechanisms Other mechanisms involving interaction between fibrils Matrix shrinkage + stiff zones

16 #15 Hypothetic mechanisms Other mechanisms involving interaction between fibrils Active bridges?

17 #16 Hypothetic mechanisms Other mechanisms involving interaction between fibrils Matrix swelling + stiff bridges

18 #17 Micro-mechanical model of maturation stress generation Dimensional change in the constituents Embedded network model Multilayer cell model Alméras et al Holzforschung Macroscopic stress/strains

19 #18 Model parameters and needed data Physico-chemical changes during maturation Chemical composition Microfibril network structure Thickness and MFA of each layer Kinetics of layer formation In vivo boundary conditions Dimensional change in the constituents Embedded network model Multilayer cell model Macroscopic stress/strains Stiffness of the constituents Stiffness of each layer Stress induced in each layer Microscopic stress/strains

20 Physico-chemical changes during maturation Chemical composition Microfibril network structure Thickness and MFA of each layer Kinetics of layer formation In vivo boundary conditions Dimensional change in the constituents Embedded network model Multilayer cell model Macroscopic stress/strains Stiffness of the constituents Stiffness of each layer Stress induced in each layer Microscopic stress/strains Thank you for your attention