|Wilson, John - CSIRO, AUSTRALIA|
Submitted to: Australian Journal of Agricultural Research
Publication Type: Review Article
Publication Acceptance Date: September 18, 1996
Publication Date: N/A
Technical Abstract: Legume and grass stems decrease substantially in digestibility and intake as they mature. This review contrasts features of their morphological and structural development and analyzes whether limited digestion of cell walls within tissues is due to changes in structural anatomy rather than wall composition. Cells that make up legume stem internodes fall into two groups: cells with 100% digestibility (cortex and pith) and cells that are apparently 100% indigestible (xylem). Xylem cells develop thick secondary walls whose composition, with a high lignin content, limits their digestion. The cortex and pith cells may develop secondary walls or thickened primary walls but remain unlignified. In contrast, as grass stem internodes mature, secondary wall (SW) thickening and lignification occur in all main cell types. The lignified SW is readily digested when accessible to rumen microorganisms. Analysis of tissue and cell architecture strongly supports the hypothesis that observed poor digestion of these lignified SW in vivo is due to limits imposed by anatomical structure. Compositional limitation to wall digestion lies in the lignified, indigestible middle lamella/primary wall. This structure confines SW digestion to inner (lumen) surfaces of cells with an open end. Low SW degradation in vivo can be explained by slow movement of bacteria to interior wall surfaces and inaccessibility of wall surfaces, especially in particles when considering the total surface area/cell wall volume.