Lignin Composition and Structure Differs between Xylem, Phloem and Phellem in Quercus suber L.

Abstract

The composition and structure of lignin in different tissues—phellem (cork), phloem and xylem (wood)—of Quercus suber was studied. Whole cell walls and their respective isolated milled lignins were analyzed by pyrolysis coupled with gas chromatography/mass spectrometry (Py-GC/MS), two-dimensional nuclear magnetic resonance spectroscopy (2D-NMR) and derivatization followed by reductive cleavage (DFRC). Different tissues presented varied p-hydroxyphenyl:guaiacyl:syringyl (H:G:S) lignin compositions. Whereas lignin from cork has a G-rich lignin (H:G:S molar ratio 2:85:13), lignin from phloem presents more S-units (H:G:S molar ratio of 1:58:41) and lignin from xylem is slightly enriched in S-lignin (H:G:S molar ratio 1:45:55). These differences were reflected in the relative abundances of the different interunit linkages. Alkyl-aryl ethers (β–O–4′) were predominant, increasing from 68% in cork, to 71% in phloem and 77% in xylem, as consequence of the enrichment in S-lignin units. Cork lignin was enriched in condensed structures such as phenylcoumarans (β-5′, 20%), dibenzodioxocins (5–5′, 5%), as corresponds to a lignin enriched in G-units. In comparison, lignin from phloem and xylem presented lower levels of condensed linkages. The lignin from cork was highly acetylated at the γ-OH of the side-chain (48% lignin acetylation), predominantly over G-units; while the lignins from phloem and xylem were barely acetylated and this occurred mainly over S-units. These results are a first time overview of the lignin structure in xylem, phloem (generated by cambium), and in cork (generated by phellogen), in agreement with literature that reports that lignin biosynthesis is flexible and cell specific.

Compositional Variability of Lignin in Biomass

“…The lignin monomeric composition of cork was assessed after isolation by the Bjӧrkman method by Marques [170][171][172][173] and Lourenço et al [125]. The S/G ratio of cork was 0.029 determined by Py-GC/FID and 0.1 by NMR, revealing a strong predominance of guaiacyl units that was also confirmed by FTIR analysis [170].…”

“…For Quercus suber, lignin was characterized in wood, phloem and cork, revealing a distribution of H:G:S quite distinct. Wood and phloem were enriched in S-lignin with H:G:S values of 1:45:55 and 1:58:41, respectively, whereas cork was considerably enriched in G units and with minor proportions of H and S units (2:85:13) [125]. Similar differences were found in the proportions of interunit linkages: wood and phloem presented more alkyl-aryl ethers (β-O-4′) representing 77 and 71% in accordance to the predominance of S units in these tissues, whereas lower value was reported in cork (68%).…”

“…Cork lignin in young trees was characterized by 2D NMR as a HGS-lignin constituted mainly by guaiacyl units, with a H:G:S of 1:43:7 and a S/G ratio of 0.16. The alkyl-aryl ethers (β-O-4′) were the predominant structures (68-77%), but it also contained condensed structures of phenylcoumarans (β-5′, 18-20%) and dibenzodioxocins (5-5′, 1-5%) [125,173]. The cork lignin is partially acylated at the Cγ of the side chain (48-50%), mainly over G units [125,173].…”

“…The alkyl-aryl ethers (β-O-4′) were the predominant structures (68-77%), but it also contained condensed structures of phenylcoumarans (β-5′, 18-20%) and dibenzodioxocins (5-5′, 1-5%) [125,173]. The cork lignin is partially acylated at the Cγ of the side chain (48-50%), mainly over G units [125,173]. Ferulic acid was present in cork lignin (6%) and considered to participate in the crosslinking between lignin and suberin [173].…”

“…The cork oak wood has a lignin content of 23.6%, and its MWL is characterized by a H:G:S molar ratio of 1:44:55, a S/G ratio of 1.2 (from NMR data), predominance of alkyl-aryl ethers (β-O-4′) (77%), lower amounts of condensed linkages and is scarcely acetylated, mainly over S units [125].…”