Crj20086 1801..1809

Grass degradability declines as cell wall and lignin concentrations increase during maturation. The role of tissue development and lignification in decline of stem degradability was examined in maize (Zea mays L.) internodes sampled at 10 stages of growth from early elongation through plant physiological maturity. The fourth elongated internode above ground level was collected from three maize hybrids grown in a 2-yr, replicated field trial at St. Paul, MN. Internode cross-sections and ground samples were incubated in vitro for 24and 96-h with rumen microbes. Tissue degradation was examined by light microscopy and degradability of cell wall polysaccharide components from ground internodes was determined. All tissues in elongating internodes were completely degradable except for protoxylem vessels, which was the only lignified tissue. After elongation, degradability of all tissues declined markedly except for phloem, which never lignified. Tissues with thick, lignified secondary walls (sclerenchyma and rindregion parenchyma) required longer incubation times for observable degradation. Cell wall polysaccharide components were highly degradable in immature internodes, but degradability declined after elongation and reached a minimum by Sampling Date 8, with glucose and xylose residues having the greatest reductions in degradability. Cell wall polysaccharide degradation was related to lignin concentration and ferulate cross-linking in a complex manner. MILK AND MEAT PRODUCTION by ruminant livestock is largely dependent on forages (Galyean and Goetsch, 1993). Because forages generally have high concentrations of cell wall material, degradability of cell walls often determines how much digestible energy ruminants will have available for productive functions. The decline in cell wall degradability of forages associated with maturation is well documented. This reduction in cell wall degradability is thought to be caused primarily by progressively greater lignification of plant cell walls during development (Jung and Deetz, 1993). However, the degree to which the different plant parts and tissues lignify varies dramatically (Wilson, 1993). These tissue-specific differences in lignification may account for the limited negative correlation of lignin concentration with cell wall degradability observed with forage samples of similar maturity. As grass stems develop, sclerenchyma and the rindregion parenchyma tissues develop thick, lignified cell walls (Wilson, 1993). These lignified grass tissues are degraded slowly, but after extended periods of degradation only a very thin residual wall layer remains undegraded (Engels, 1989). This pattern of extensive degradation of thick-walled, lignified tissues is markedly different than what has been observed in legumes. Xylem tissue of alfalfa (Medicago sativa L.) stems has thick, heavily lignified walls and is virtually nondegradable (Jung and Engels, 2002). The reason for this difference among these forages in degradability of lignified tissues is unclear. Legumes generally have higher concentrations of lignin than do grasses of similar maturity, although some of this reported difference results from the poor recovery of grass lignin in the acid detergent lignin method (Hatfield et al., 1994). Both types of forages undergo a shift in composition from primarily guaiacyl-type lignin to mixed syringyl-guaiacyl lignin during maturation (Jung and Deetz, 1993). While lignified legume and grass cell walls are both rich in cellulose and xylans, grasses contain very little pectin (Moore and Hatfield, 1994) and legumes do not contain appreciable amounts of ferulates (Jung and Deetz, 1993). Early studies with a synthetic lignin model (polyeugenol) complex with cellulose indicated that crosslinking was necessary for lignin to inhibit cellulose degradation by both isolated cellulase and rumen microbes (Gressel et al., 1983; Jung and Ralph, 1990). Both ferulate monomers and dimers have been identified as being cross-linking agents in grasses (Iiyama et al., 1990; Ralph et al., 1992, 1994). Ralph et al. (1995) demonstrated that ferulate esters in perennial ryegrass (Lolium perenne L.) act as the initiation site where lignin polymerization begins. Therefore, ferulate-mediated cross-links, formed during cell wall development, have the potential to influence cell wall degradability. In a series of studies using a maize (Zea mays L.) cell culture model system, Grabber and coworkers demonstrated that cross-linking of arabinoxylan to lignin by ferulate monomers and dimers reduced polysaccharide degradability by isolated enzymes (Grabber et al., 1995, 1998a, 1998b). In maize basal-stem internodes collected at silking, both Klason lignin and ferulate ether (the only form of cross-linking that can currently be quantified) concentrations were only weakly negatively correlated with cell wall degradability (Jung and Buxton, 1994). However, in elongating maize internodes, ferulate ether concentration was strongly negatively correlated with cell wall degradability, whereas degradation of postelongation internodes was more strongly correlated with Klason lignin concentration (Jung et al., 1998). Neutral detergent fiber degradability of smooth bromegrass (Bromus inermis Leyss) plants, selected for divergent ferulate ether and Klason lignin concentrations, was negatively impacted by both cell wall components. However, these components were at least partially independent of one another in their effects, and ferulate ethers H.G. Jung, USDA-ARS Plant Science Res. Unit and U.S. Dairy Forage Res. Center Cluster, Univ. of Minnesota, Dep. of Agronomy and Plant Genetics, 411 Borlaug Hall, 1991 Upper Buford Circle, St. Paul, MN 55108; M.D. Casler, USDA-ARS U.S. Dairy Forage Res., 1925 Linden Drive West, Madison, WI 53706. Received 2 Mar. 2006. *Corresponding author ([email protected]). Published in Crop Sci. 46:1801–1809 (2006). Forage & Grazinglands doi:10.2135/cropsci2006.02-0086 a Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: LSD, least significant difference. R e p ro d u c e d fr o m C ro p S c ie n c e . P u b lis h e d b y C ro p S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 1801 Published online June 20, 2006