Submitted to: International Symposium on Wood and Pulping Chemistry
Publication Type: Proceedings
Publication Acceptance Date: June 12, 2003
Publication Date: N/A
Technical Abstract: Apoplastic pH and monolignol polymerization rate are thought to influence the formation of lignin and its interactions with structural polysaccharides. Primary walls of maize were suspended in pH 4 or 5.5 buffers and synthetically lignified by gradual ¿end-wise¿ or rapid ¿bulk¿ polymerization of coniferyl alcohol. Coniferyl alcohol was more efficiently incorporated into cell walls as end-wise polymers at pH 5.5 (93%) than as bulk polymers formed at either pH (59-68%). End-wise lignification was less efficient at pH 4 (48%) due to inactivation of cell wall peroxidase. Thioacidolysis of cell walls revealed that end-wise polymers had 1.8 to 2.6 fold more ether inter-unit linkages and 70% fewer end-groups than bulk polymers. Low pH enhanced the formation of ether linkages, particularly for end-wise polymers. End-wise and bulk polymers depressed the enzymatic degradability of cell walls to the same degree, indicating that both types of lignin have similar interactions with structural polysaccharides. In contrast, lowering apoplastic pH from 5.5 to 4.0 during lignification reduced the degradability of cell walls by about 25%. Further studies indicated that this pH-dependent depression in degradability was related to benzyl ester and ether cross-links formed via lignin quinone methide intermediates. Consequently, lignin-matrix interactions are influenced by apoplastic pH during lignification but not by the rate of monolignol polymerization.