Lignin composition and structure in young versus adult Eucalyptus globulus plants

Abstract

Lignin changes during plant growth were investigated in a selected Eucalyptus globulus clone. The lignin composition and structure were studied “in situ” by a new procedure enabling the acquisition of 2D-NMR spectra on wood gels formed in the NMR tube, as well as by analytical pyrolysis (Py-GC/MS). In addition, milled-wood lignins were isolated and analyzed by 2D-NMR, Py-GC/MS, and thioacidolysis. The data indicated that p-hydroxyphenyl (H) and guaiacyl (G) units are deposited at the earlier stages, whereas the woods are enriched in syringyl (S) lignin during late lignification. Wood 2D-NMR showed that β-O-4′ and resinol linkages were predominant in the eucalypt lignin, whereas other substructures were present in much lower amounts. Interestingly, open β-1′ structures could be detected in the isolated lignins. Phenylcoumarans and cinnamyl end-groups were depleted with age, while spirodienone abundance increased, and the main substructures (β-O-4′ and resinols) were scarcely modified. Thioacidolysis revealed a higher predominance of S units in the ether-linked lignin than in the total lignin and, in agreement with NMR, also indicated that resinols are the most important non-ether linkages. Dimer analysis showed that most of the resinol-type structures comprised two S units (syringaresinol), the crossed G-S resinol appearing as a minor substructure and pinoresinol being totally absent. Changes in hemicelluloses were also shown by the 2D-NMR spectra of the wood gels, without polysaccharide isolation. These include decreases of methyl galacturonosyl, arabinosyl and galactosyl (anomeric) signals, assigned to pectin and related neutral polysaccharides, and increases of xylosyl (which are ~50% acetylated) and 4-O-methylglucuronosyl signals.