Lignin and Hydroxycinnamic Acids in Walls of Brown Midrib Mutants of Sorghum, Pearl Millet and Maize Stems

Abstract

The characteristics of walls from stems of brown-midrib (bmr) mutants fromSorghum bicolor(L) Moenchbmr6andbmr18(watery- to milky-grain stage),Pennisetum americanum(L) Leeke KS81-1089 (soft-dough stage) andZea maysLbm3(early-dent stage) with respect to the types of linkages of hydroxycinnamic acids to wall polymers and to structural features of their lignins were investigated. The lignin content of all mutants, determined using the acid detergent lignin procedure, was significantly lower than that of their normal counterparts. There was, however, no significant differences in total lignin contents betweenbmrand normal lines as determined by the acetyl bromide procedure or the sum of the acid-insoluble (Klason) lignin and acid-soluble lignin. It is suggested that this behaviour could be explained ifbmrmutants are characterised by higher amounts of lignin with a lower degree of polymerisation than normal lines. The lowered S/V ratio and lowered total yield of alkaline nitrobenzene oxidation products in lignin frombmrmutants was confirmed. No etherifiedp-coumaric acid was found in any sample tested, except the normal line of pearl millet. The concentration of etherified ferulic acid, which is probably involved in ester-ether bridges between lignin and polysaccharides, was lower inbmrmutants than in the normal plants. The low content of ferulic acid bridges inbmrmutants may contribute to the elevated digestibilities of their stems.