Methylation analysis of mesophyll,epidermis, and fibre cell-walls isolated from the leaves of perennial and italian ryegrass

Intact, finely milled mesophyll, epidermis, and fibre cell-walls prepared from the leaves of perennial and Italian ryegrass have been subjected to methylation analysis. Methylation of the cell-walls led to a consistently higher recovery of glucose residues than that obtained by analysis of monosaccharide residues as their alditol acetates. Values for other sugars were in close agreement. The partially methylated sugars formed were consistent with the presence, in order of decreasing concentration, of cellulose, (glucurono)arabinoxylan, xyloglucan, rhamnogalacturonan, (1→3),(1→4)-linked glucan, (1→4)-linked galactan, and (1→3),(1→6)-linked galactan. The relative proportions of these polysaccharides differed between the various types of cell. Arabinoxylan comprised 21.6%, 26.7%, and 36.5% of the total sugars recovered from mesophyll, epidermis, and fibre cell-walls, respectively. Mixed-linked glucan and rhamnogalacturonan were found in epidermis walls in amounts 2- to 3-fold higher than in other cell-walls. The xylan backbone of arabinoxylan was more heavily substituted in primary than in secondary-thickened (fibre) cell-walls. Arabinose, found largely as terminal residues in the cell-walls, carried various amounts of alkali-labile substituents, particularly at position 5. The extent of 5-substitution reflected the phenolic content and was substantially higher in fibre cell-walls. The methylation data, coupled with the analytical data for uronic acids and non-carbohydrate components, accounted for ～98% of the cell-wall dry matter.