Variations in chemical composition and structure of macromolecular components in different morphological regions and maturity stages of Arundo donax

Abstract

Arundo donax plants were manually separated into fractions of different morphological regions (internodes, nodes and foliage) at different stages of maturity and submitted to chemical composition studies. General chemical composition was determined by established methods. The polysaccharides were fractionated by successive extractions of holocellulose with aqueous KOH solutions. The sugar composition was determined by hydrolysis of polysaccharides followed by GC analysis of neutral sugars as alditol acetates and spectrophotometric determination of uronic acids. In situ lignins, milled wood lignins (MWL) and dioxane lignin were characterised by permanganate oxidation followed by GC and GC-MS analysis of the methylated oxidation products and by quantitative 13C NMR spectroscopy. The results of general chemical analysis evidenced the different relative abundance of holocellulose, lignin, proteins, extractives and ashes in internodes, nodes and foliage, at different stages of maturity. In internodes, nodes and foliage about 70–80% of hemicelluloses (21–30% o.d. material) were easily extracted with 5% KOH aqueous solutions. The analysis of hemicelluloses indicated that they are constituted mainly by arabinoglucuronoxylans with a xylose:arabinose:uronic acid ratio of 91–93:5–7:2 for internodes. The high content of xylose in Arundo donax stem (24–27% o.d. material) and the easy extraction of hemicelluloses opens new perspectives for the use of this reed as a source of pentosans. The results obtained by the permanganate oxidation method indicated that Arundo donax lignins are essentially H-G-type with approximate H:G:S proportions of (32–36):(59–61):(5–8) in internodes. The H units are constituted mainly by esterified p-coumaric acid. These results were confirmed by quantitative 13C NMR spectroscopy of isolated dioxane lignin. The quantity of condensed structures in in situ lignin decreases from the older to the younger parts of the stems and is much higher in nodes than internodes.