STRUCTURAL AND CHEMICAL-CHANGES OF CELL-WALL TYPES DURING STEM DEVELOPMENT - CONSEQUENCES FOR FIBER DEGRADATION BY RUMEN MICROFLORA

Legume and grass stems decrease substantially in digestibility as they mature. This review evaluates how anatomical and chemical factors res trict digestion of cell walls in legume and grass stems. Cells that ma ke up legume stems fall into 2 groups: cells with high (congruent to 1 00%) digestibility (e.g. cortex and pith) and cells that appear indige stible (e.g. xylem). The digestibility of xylem cells is restricted by the highly lignified secondary walls (SW). Although cortex and pith c ells may develop SW or thickened primary walls, digestibility is high because these cell types do not undergo lignification. In contrast. as grass stems mature, SW thickening and lignification occur in all main cell types. However, lignified SW in grass is readily digested when a ccessible to rumen microorganisms. Analysis of tissue and cell archite cture in grasses strongly supports the hypothesis that observed poor d igestion of lignified SW in, vivo is due to limits imposed by anatomic al structure. Compositional limitation to wall digestion lies in the l ignified, indigestible middle lamella-primary wall. This structure con fines SW digestion to inner (lumen) surfaces of cells with an open end . Low sclerenchyma SW degradation in vivo can be explained by limited movement of bacteria into sclerenchyma cells and low surface area on i nterior walls. For example, the ratio of surface area to total cell wa ll volume for sclerenchyma cells is 100-fold lower than for mesophyll cells. Apparent relationships of some wall constituents-chemical struc tures to wall digestibility may be the result of the increasing SW and , therefore, may simply reflect limitations imposed by anatomical stru cture.