PHENOLIC CONSTITUENTS OF CELL-WALL TYPES OF NORMAL AND BROWN MIDRIB MUTANTS OF PEARL-MILLET (PENNISETUM-GLAUCUM (L) R BR) IN RELATION TO WALL BIODEGRADABILITY

The cell walls of parenchyma, rind and vascular bundle fractions of pearl millet (Pennisetum glaucum (L) R Br) were isolated from two brown midrib mutants (bmr) 5753 and 5778 and from their normal (N) near-isogenic line. The cell wall content of parenchyma was lower than that of vascular bundle which, in turn, was lower than that of rind. The amounts of ferulic and p-coumaric acids released by NaOH treatment of the cell walls were in the ranges 3-7 mg g-1 and 2-26 mg g-1, respectively. Parenchyma cell walls of the N line had the highest content of p-coumaric acid (26 mg g-1). This content of p-coumaric acid in the N line contrasts with that of bmr 5753 parenchyma (2 mg acid g-1 walls) and bmr 5778 (7 mg acid g-1 walls). The concentration of p-coumaric acid was highest in parenchyma cell walls that had been found to be the least digested. Parenchyma, rind and vascular bundle cells walls of the N line had much higher ratios of p-coumaric acid to ferulic acid than the mutants; rind and vascular bundle walls were less digestible than parenchyma. Small amounts of truxillic acid dimers were released by NaOH from the parenchyma walls of bmr 5778. Treatment of parenchyma, rind and vascular bundle cells walls with purified 'driselase' (containing xylanases and cellulases) released p-coumaroyl and feruloyl trisaccharides. Between 25 and 53 % of the ferulic acid that was released by the NaOH treatment could be accounted for as feruloyl trisaccharide, but only 1-19 % of the p-coumaric acid was accounted for as p-coumaroyl trisaccharide.