ULTRAVIOLET MICROSCOPY OF LIGNINS IN SPECIFIC CELL-WALLS OF BARLEY STRAW FRACTIONS WITH DIFFERENT RUMEN DEGRADABILITY

Chemical composition of barley straw fractions and ultraviolet (UV) spectra of the individual cell types were investigated in relation to rumen degradabilities. The barley stem was much less degradable than the leaf blade and leaf sheath. The stem contained more cellulose and slightly more lignin than other straw fractions, but less hemicellulose than the leaf sheath. Stem and leaf sheath similarly had higher ratios of xylose to arabinose residues and of p-coumaric acid to ferulic acid. The UV spectra of tissues showed that most cell types had relatively higher UV absorbance at approximately 250 nm and in the range 312-324 nm, and the maximum values were in the range 278-283 nm. The UV absorbance in leaf blade and leaf sheath was concentrated in the metaxylem and protoxylem vessels, and was minimal in the mesophyll, while all of the stem tissues showed extensive absorbance. Thus, the barley stem and leaf sheath were extensively formed of secondary cell walls, but a lesser degree of lignification (except for metaxylem and protoxylem vessels) was thought to be present in the leaf sheath. In contrast, there appeared to be little secondary thickening of the cell walls in the leaf blade. These results suggest that the far lower degradability of stem compared with leaf blade and leaf sheath is closely related to the extent of secondary cell wall thickening and to the pattern of lignin distribution amongst the individual cell types.