THE NEWLY EXTENDED MAIZE INTERNODE - A MODEL FOR THE STUDY OF SECONDARY CELL-WALL FORMATION AND CONSEQUENCES FOR DIGESTIBILITY

The upper five internodes were collected from maize (Zea mays L) inbre d cell lines Co125 and W401 harvested at the same developmental stage, 5 days after silking. Each internode was dissected into ten equal len gths labelled A (top) to J (base). The youngest cells were found in se ction J, which contained the intercalary meristem, and the oldest in s ection A. Internodes 1, 3 and 5 provided material for chemical analysi s and internodes 2 and 4 for degradability measurements. Cell wall mat erial accounted for one-third of dry matter in section J, doubling to two-thirds in the upper half of each internode. Only section J exhibit ed a polysaccharide profile typical of primary cell walls. In all othe r sections, 1,4-linked glucose (approximately 46% of cell wall) and xy lan largely free from side chains (approximately 25% of cell wall) pre dominated. Net accretion of cell wall polysaccharide reached a maximum by segment G and thereafter little additional carbohydrate was deposi ted. Lignification appeared to be separated from the biogenesis of str uctural carbohydrate and continued over much of each internode reachin g a maximum in section C. Degradability measurements, made using a mod ified neutral-detergent cellulase digestibility method, showed substan tial differences between sections. In line Co125, cell wall degradabil ity fell from over 95% in the youngest section (J) to approximately 24 % in section B. Internode 4 of line W401 failed to show the same patte rn of degradabilities, probably because of a sequential rather than si multaneous pattern of internode elongation. Saponifiable p-coumaric ac id appeared to provide a more sensitive marker than lignin of the exte nt of secondary wall development. The inverse relationship between ext ent of lignification in each section and its degradability confirmed t he value of the internode model for the study of secondary wall format ion and its biological consequences.