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 from Sorghum bicolor (L) Moench bmr6 and bmr18 (watery- to milky-grain stage), Pennisetum americanum (L) Leeke KS81-1089 (soft-dough stage) and Zea mays L bm3 (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 between bmr and 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 if bmr mutants 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 from bmr mutants was confirmed. No etherified p-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 in bmr mutants than in the normal plants. The low content of ferulic acid bridges in bmr mutants may contribute to the elevated digestibilities of their stems.