Lignin Composition and Structure Differs between Xylem, Phloem and Phellem inQuercus suberL.
The composition and structure of lignin in different tissues-phellem (cork), phloem and xylem (wood)-ofQuercus suberwas 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 variedp-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.
Centro de Estudos Florestais, Instituto Superior de Agronomia, Universidade de Lisboa Lisboa, Portugal.
Ana
A
Lourenço
Consejo Superior de Investigaciones Científicas, Instituto de Recursos Naturales y Agrobiología de Sevilla Seville, Spain.
Jorge
J
Rencoret
Centro de Estudos Florestais, Instituto Superior de Agronomia, Universidade de Lisboa Lisboa, Portugal.
Catarina
C
Chemetova
Centro de Estudos Florestais, Instituto Superior de Agronomia, Universidade de Lisboa Lisboa, Portugal.
Jorge
J
Gominho
Consejo Superior de Investigaciones Científicas, Instituto de Recursos Naturales y Agrobiología de Sevilla Seville, Spain.
Ana
A
Gutiérrez
Consejo Superior de Investigaciones Científicas, Instituto de Recursos Naturales y Agrobiología de Sevilla Seville, Spain.
José C
JC
Del Río
Centro de Estudos Florestais, Instituto Superior de Agronomia, Universidade de Lisboa Lisboa, Portugal.
Helena
H
Pereira
DFRC; NMR; Py-GC/MS; Quercus suber; cork; milled lignin; phloem; xylem