Chemical characterization of cork, phloem and wood from different Quercus suber provenances and trees

Sustainability of cork oak (Quercus suber) forests is threatened by biotic and abiotic factors and characterization of potentially differing genetic resources has therefore gained importance. This work addresses the chemical variation of the three tissues of cork oak stems – cork, phloem and wood – in relation to tree and provenance, looking for genetic chemical diversity and for physiological derived differences. The three tissues differ with cork clearly differentiating regarding summative composition, component ratios and monomeric composition. Cork is the only tissue where suberin is present (42.3% o.d. mass) as the main cell wall component, and it has a high content of extractives (11.7%) with significant proportion of lipophilic compounds. Phloem is more lignified than wood (38.0% vs. 23.4%) and has less polysaccharides (49.1% vs. 64.6%) with glucose-to-other sugars relation of 1:1.3 in phloem and 1:0.7 in wood. Analytical pyrolysis showed that lignification is a heterogeneous process and the lignin monomeric composition depends on tissue and cell type: cork lignin has a H:G:S ratio of 1:2.5:0.3 and S/G ratio of 0.12, while phloem and wood lignins have mainly G and S units with a S/G ratio of respectively 1.1 and 2.3. No significant differences were found between the three provenances, but some chemical variation occurred between the trees within a provenance. NIR spectroscopy and principal component analysis differentiated cork, phloem and wood, while the dispersion within each group highlighted the significant tree variability, while provenances were a non-significant factor of chemical variation.