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 0-2 and/or 0 -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 0-5, 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 0 -5 of arabinose were preserved during digestion and accumulated in residues recovered from the rumen. In all plants examined, the extent of 0-5 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 0-5 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 0-5 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.

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Cells of mesophyll, epidermis and residual fibrous material were obtained from leaves of Italian and perennial ryegrass harvested at different stages of maturity by mechanical disruption of leaf tissue. Mesophyll cells were selectively removed by filtration through 0.045 mm nylon mesh and remaining non-mesophyll cells centrifuged in metrizamide solutions (56-58% wt to vol.) of known density (1.308-1.329 gcm' at 5°C) to obtain a pure epidermis cell fraction and a residual fibre fraction. Whole mesophyll cells contributed 63-72%, epidermis 12-15% and the fibre fraction 15-24% to the total leaf dry matter. Fibre values were higher in late-cut samples. Cell walls were prepared from mesophyll and epidermis cells by disruption and washing to remove cell contents. Fibre cells were judged free of cell contents and received no further treatment. Examination of cell wall preparations by light and electron microscopy showed that both mesophyll and epidermis preparations were essentially free from contaminating material. Mesophyll cell walls were uniformly thin ~ (200 nm) while those of epidermis ranged from 200&3000nm at the outer face, thinning to 300nm or less at the inner surface. An electron-light layer (cuticle) of approximately 200 nm thickness was present covering the outer face of the epidermis. The fibre fraction largely consisted of sclerenchyma, but contained, in addition, other vascular cells, detached annular rings and heavily silicified leaf hairs. Analysis of cell walls accounted for 85-90% of dry matter. Cellulose was the major component of all cell walls examined (approximately 40% of dry matter) with xylose residues accounting for a further 11% of mesophyll, 13.5-17.5% of epidermis and 21-25% of fibre cell walls. Arabinose was low in fibre cells but was present in much higher proportions in mesophyll and epidermis walls. The ratio of arabinose to xylose was approximately 1:1.5 for mesophyll, 1:2.5 for epidermis and 1:7.0 for the fibre fraction. The molar ratio acetyl to xylose remained fairly constant at 1:4 regardless of the grass, cell type or maturity of the sample. The uronic acid content of epidermis was higher than that in other cell types and showed an increase with increasing maturity of the grass, reaching over 9 % in late-cut samples. Total phenolic material represented 2-3% of mesophyll and epidermis cell walls and 6 % of fibre walls. Ferulic acid alone was released from the primary cell walls by saponification and p-coumaric and ferulic acids from the secondary-thickened fibre walls. Crude protein values (Nx6.25) were high in mesophyll cell wall preparations and low in epidermis and fibre cell walls. Amino acid patterns were similar for both grasses and cell types but hydroxyproline was found in greater amounts in fibre cell walls than in either epidermis or mesophyll.

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