Morphology and overall chemical characterization of willow (Salix sp.) inner bark and wood: toward controlled deconstruction of willow biomass

The morphology and chemical composition of the inner bark of four willow hybrids were analyzed as a step toward complete willow biomass valorization. The inner bark consisted of highly delignified bundles of thick-walled sclerenchyma fibers and nondelignified surrounding tissue of thin-walled parenchyma cells. In comparison with willow wood fibers, the sclerenchyma fibers were longer, they had a very narrow lumen and their walls were made of up to eight separate layers. One fourth of the dry mass of the inner bark was formed of ash and acetone extractable substances. Although the lignin-to-polysaccharide ratio was similar in the inner bark and wood, their polysaccharide compositions were different. While glucose and xylose were the main monomers in wood, the inner bark had also high arabinose and galactose contents. In addition, more rhamnose was present in the inner bark which was indicative of its higher pectin content.