Structural and chemical changes of cell wall types during stem development: consequences for fibre degradation by rumen microflora

Abstract

Legume and grass stems decrease substantially in digestibility as they mature. This review evaluates how anatomical and chemical factors restrict digestion of cell walls in legume and grass stems. Cells that make up legume stems fall into 2 groups: cells with high (≅ 100%) digestibility (e.g. cortex and pith) and cells that appear indigestible (e.g. xylem). The digestibility of xylem cells is restricted by the highly lignified secondary walls (SW). Although cortex and pith cells 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 accessible to rumen microorganisms. Analysis of tissue and cell architecture in grasses strongly supports the hypothesis that observed poor digestion of 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 sclerenchyma SW degradation in vivo can be explained by limited movement of bacteria into sclerenchyma cells and low surface area on interior walls. For example, the ratio of surface area to total cell wall volume for sclerenchyma cells is 100-fold lower than for mesophyll cells. Apparent relationships of some wall constituents–chemical structures to wall digestibility may be the result of the increasing SW and, therefore, may simply reflect limitations imposed by anatomical structure.