Substituent groups linked by alkali‐labile bonds to arabinose and xylose residues of legume, grass and cereal straw cell walls and their fate during digestion by rumen microorganisms

Abstract

The distribution of alkali labile substituents of the neutral sugar components of hemicellulose in unfractionated cell walls of early‐ and late‐cut perennial ryegrass, wheat and barley straw, and white clover, before and after extended incubation in the rumen, was determined by a modified methylation technique. Xylose residues were found to be extensively substituted, with 50% of residues in the Gramineae and 75% in white clover carrying substituent groups on O‐2 and/or O‐3. Acetyl groups could account for 50–70% of substitutions, the remainder being to unknown cell wall components. The distribution of alkali‐labile substituents of xylose was the same in the original plant material as in residues recovered from the rumen, and was not affected by delignification under conditions which preserved the acetyl content of cell walls. Arabinose residues were heavily substituted at O‐5, with 40–60 % of residues from the Gramineae, but only 10% of residues from the white clover, carrying substituent groups at this site. Linkages to O‐5 of arabinose were preserved during digestion and accumulated in residues recovered from the rumen. In all plants examined, the extent of O‐5 substitution strongly correlated with the proportion of total phenolics (r=0.844) and total phenolics minus phenolic acids (r=0.779) present. The correlation was improved when restricted to the Gramineae (r=0.879 and 0.881 respectively). Within the Gramineae, the increase in the extent of substitution to O‐5 of arabinose correlated well with the increase in the proportion of total material in digested residues (r=0.899). As correlations were retained or improved when values for phenolic acids were subtracted from total phenolics, and as the proportion of phenolic acids decreased during digestion, it is suggested that linkages to O‐5 of arabinose represent one form of lignin‐carbohydrate bonding. The importance of such linkages to the control of polysaccharide degradation and the effect of their cleavage by alkali‐treatment is discussed.