The mechanical properties and molecular dynamics of plant cell wall polysaccharides studied by Fourier-transform infrared spectroscopy

Polarized one- and two-dimensional infrared spectra were obtained from the epidermis of onion (Allium cepa) under hydrated and mechanically stressed c onditions. By Fourier-transform infrared microspectroscopy, the orientation of macromolecules in single cell walls was determined. Cellulose and pecti n exhibited little orientation in native epidermal cell walls, but when a m echanical stress was placed on the tissue these molecules showed distinct r eorientation as the cells were elongated. When the stress was removed the t issue recovered slightly, but a relatively large plastic deformation remain ed. The plastic deformation was confirmed in microscopic images by retentio n of some elongation of cells within the tissue and by residual molecular o rientation in the infrared spectra of the cell wall. Two-dimensional infrar ed spectroscopy was used to determine the nature of the interaction between the polysaccharide networks during deformation. The results provide eviden ce that cellulose and xyloglucan associate while pectin creates an independ ent network that exhibits different reorientation rates in the wet onion ce ll walls. The pectin chains respond faster to oscillation than the more rig id cellulose.