Lignin in wheat internodes. 2. Alkaline nitrobenzene oxidation by wheat straw lignin and its fractions

Finely ground 80% ethanol extracted residues of maturing wheat internode sections of the first internode were successively extracted with 96% dioxane to give milled lignin (ML1) arid water soluble (MC1) fractions. The residue was treated with a polysaccharide hydrolase mixture to give soluble fractions (EC1) and a residue (ER1). The ML1 fractions were minor components at all stages of maturation. The yield of the MC1 fractions decreased during maturation whereas the yield of ER1 fractions increased. The acetyl bromide lignin content increased from 11.5 to 16.2% between the youngest and oldest internode sections. The ratio of syringyl to guaiacyl nuclei (S/V) in internode sections and fractions showed a trend towards a slight increase with maturity. The p-hydroxyphenyl to guaiacyl ratios (H/V) were low except for the youngest internode sections, and are comparable to those for gymnosperms and dicotyledonous angiosperms. FTIR spectroscopy of Bjorkman lignins from mature wheat internodes were characterised on the basis of the ratio of peak intensities in the ranges 1420-151O cm-1, 1240-1260 cm-1 and 1040-1130 cm-1, and were found to have values intermediate between pine and eucalypt Bjorkman lignins. The MC1 fraction was rich ill ferulic acid and p-coumaric acid as determined by alkaline nitrobenzene oxidation and UV spectroscopy. This accounts for the high acetyl bromide lignin content of the internodes. The yield of cinnamic acids increased as the internodes matured. The phloroglucinol test, indicative of cinnamaldehyde derivatives, was negative in the youngest internodes but became positive with increasing maturity. The Maule colour reaction (red purple colour) was not given by any wheat internode sample (S/V less than 2). In contrast, samples from dicotyledonous angiosperms (S/V greater than 2) all gave positive tests.