Cell Wall Lignification and Degradability

During the past century, analytical techniques applied to the study of plant cell wall chemistry have become more refined and increasingly sophisticat­ ed. Concepts attempting to describe the relationship of lignin to other cell wall constituents have evolved in concert with analytical ability. Early no­ tions of cell wall lignification described lignin as an amorphous polymer acting as a glue or cement in plant tissue (Bailey, 1936). Contemporary dogma recog­ nizes a more intimate, direct relationship of lignin with cell wall polysaccha­ rides via specific chemical bonds (Watanabe et aI., 1989). An understanding of the impact of these specific chemical relationships on ruminal cell wall degradation of agronomic crops will be necessary to advance knowledge in ruminant nutrition and plant physiology, and to capitalize on opportunities for precise application of molecular genetic techniques to rumen microbial or plant systems. Nearly 60 yr ago, Buston (1935) studied the relationship between pec­ tin, hemicellulose, and lignin in plants and concluded that "it is not consid­ ered that pectin is a precursor of lignin." Other attempts to elucidate cell wall lignification by Griffioen (1938) indicated "young lignins" from young­ er portions of sunflower (Helianthus annus L.) stalks were less polymerized than "old lignins" present in old stalk portions. Research in the mid-1920s (Waksman, 1926a) recognized that lignins in straw were resistant to fungal and bacterial degradation in the process of soil humus formation. Thus, lig­ nins and tannins from degrading soil organic matter accumulate under anaero­ bic conditions (Waksman, 1926b). Extension of these early studies concluded that any depressing effect of lignin on anaerobic cellulose decomposition "must be due to the manner of its binding with cellulose" (Waksman & Cor­ don, 1938). Partial destruction of "the bond between lignin and cellulose," as well as dissolution of lignin was achieved by pretreating straw with 150 g kg-I ammonium hydroxide for periods of up to 15 d (Nikolaeva, 1938). This pretreatment improved coefficients of digestion for nutrients. However, this improvement was not directly related to the amount of lignin removed