Résumé
ABSTRACTThe aim of the present study is to improve the understanding of the evolution of fibreproperties and ultrastructure caused by the retting process at the plant cell wall scale. Twocomplementary structural investigating technic are presented here, namely using X-RayDiffraction (XRD) and solid state Nuclear Magnetic Resonance (NMR). An estimation of thecellulose crystallinity index by XRD measurements, confirmed by NMR, shows an increaseof 8% in crystallinity with retting mainly due to the disappearance of amorphous polymer. Inaddition, NMR investigations show a compaction of inaccessible cell wall polymers,combined with an increase in the relaxation times of the C4 carbon. This densificationprovides a structural explanation for the observed improvement in mechanical performance ofthe secondary wall of flax fibres during the field retting process.INTRODUCTIONDuring retting, the various microorganisms present in the soil colonize the flax stems. Theenzymes they secrete, and particularly polygalacturonases, gradually degrade the pecticcompounds located mainly in the middle lamellae of the fibre bundles. This damage to theplant tissues structure makes it easier to extract the fibre bundles during the scutching stage.An over retting results in the loss of the cellulose that makes up the fibres (Placet et al. 2017),arguably due to secretion of cellulase. To obtain information on the supramolecular structureof cellulose fibrils in the plant cell walls, the model of Larsson and Wickholm (Larsson et al.,1997) could be used for deconvoluate the C4 region between 77 and 92 ppm. In addition, themechanical properties of flax cell walls during could be investigated by AFM Peak ForceQuantitative Nano-Mechanical property mapping (AFM PF-QNM) (Goudenhooft et al. 2018).Here we explore the evolution of the flax cell wall behaviour during a controlled dew rettingphase. Structural and mechanical investigations were conducted at the cell wall scale using XRayDiffraction (XRD) and solid-state Nuclear Magnetic Resonance (NMR), whilenanomechanical measurements were performed by nanoindentation and AFM PF-QNM).RESULTS AND CONCLUSIONSThe macroscopic illustration of the flax lots is displayed in Fig 1. NMR results show asignificant evolution in the cell wall ultrastructure of flax during the retting stages. Theimprovement in the mechanical performance of plant cell walls can be explained by thecompaction of inaccessible zones (Fig 2).