Glycosidic linkages of legume, grass and cereal straw cell walls before and after extensive degradation by rumen microorganisms

Abstract

AbstractThe technique of methylation analysis for the location of glycosidic linkages in cell wall polysaccharides was applied directly (after dry milling in liquid nitrogen) to whole wheat and barley straws, cell wall preparations from early‐ and late‐cut perennial ryegrass and white clover, and their extensively degraded residues recovered from the rumen. The addition of an internal standard, methyl‐β‐D‐allo‐pyranose, to milled samples enabled the recovery of partially‐methylated sugars to be quantified, and the recovery of parent sugars from methylated material to be compared with values obtained by direct estimation of monosaccharides by the alditol acetate method. Recovery of sugars from methylated samples was generally higher than that predicted by the alditol acetate method, particularly from digested residues.All four samples of Gramineae showed similar proportions of glycosidic linkages in which (1 →4)‐linked xylose and glucose units accounted for more than 80% of total identified partially‐methylated sugars. The proportions of glycosidic linkages found in digested residues of straws were essentially the same as those found in the parent material. In residues of the more digestible ryegrass there was a two‐ to three‐fold increase in (1→4)‐linked xylose units without branch points at the O‐2/O‐3 positions, the proportions of these branch points being substantially reduced. A corresponding reduction in the proportion of (1→2)‐, (1→3)‐and terminally‐linked arabinose units was also found. The proportion of both (1→4)‐ and (1→3)‐linked glucose units were also reduced in digested residues. These changes were greater in the early‐cut than in the late‐cut sample. White clover differed from the Gramineae in a number of respects. Galactose units predominantly in the form of (1→4)‐, (1→6)‐ and terminally‐linked units, were the major non‐cellulosic sugars present. The proportion of all forms of galactose was reduced in digested residues, but (1→6)‐linked units appeared more resistant to degradation than (1→4)‐linked units. Like ryegrass, the proportion of xylose units increased in digested residues from clover, while the proportion of glucose decreased. The proportion of (1→4)‐linked mannose units, initially present at levels ten times as great as in the Gramineae, was also substantially reduced.