Cell wall structure of tensile flexure wood fibers in <i>Populus x euramericana</i>

Abstract In response to gravitropic environmental stimuli, woody angiosperms develop tension wood (TW). Typical TW fibers are characterized by an inner gelatinous cell wall (c.w.) layer (G-layer). On the other hand, in response to mechanical stimuli, woody plants develop flexure wood. Flexure wood formed under tension in angiosperms is termed tensile flexure wood (TFW). Similarly to TW, TFW represents an increased source of non-recalcitrant cellulose for biofuel production. Histochemical, SEM, and Raman microspectroscopic analyses of TFW of juvenile Populus x euramericana formed in response to severe long-term static bending were performed. The presence of lignin in the G-layer, higher syringyl/guaiacyl ratio, and differences in lignin structure in TFW compared to normal wood, revealed by histochemical analysis, was confirmed by Raman microspectroscopy. Additionally, Raman microspectroscopy indicated differences in cellulose and hemicellulose structure and pectin methylesterification. The G-layer and compound middle lamella contribute to the response to bending by opposite shifts of the orientation-sensitive cellulose band attributed to the glycosidic C–O–C bond. SEM micrographs revealed the c.w. ultrastructure of TFW fibers. The presented findings encourage further investigation of TFW in terms of both changes in c.w. structure as a mechanical acclimation to stem bending and its potential for biofuel production.