Cell Wall Accessibility and Cell Structure Limitations to Microbial Digestion of Forage

Discussion of limits to ruminant digestion of plant cell walls largely focuses on the lignification and chemical nature of these walls. We examined the anatomical limitations to digestion of thick-walled fiber particles in grasses. Estimates were made of wall surface area to cell wall volume ratio (SA/CWV), rate of bacterial digestion, and accessibility to bacteria for different types of cell walls. The analysis reveals the following: (i) Bacterial digestion of fiber cells can progress only from the interior (lumen) surface because their middle lamella-primary wall region is consistently found to he indigestible. (ii) Because of secondary wall thickness (c. 1-5 μm), we calculate that at best only 0.45 to 0.60 μm of wall thickness (as little as 20% of the wall in some cells) would he digested within the average residence time of fiber particles in the rumen assuming digestion of wall at the fast rate of 0.02 μm h −1 . (iii) This potential rate of wall digestion overestimates that of typical fiber particles in the rumen because these particles are comprised of many hundreds of cells and relatively few will be disrupted by chewing to give bacteria immediate access to cell lumens. (iv) Digestion of thick-walled cells by bacteria is surface-based and sclerenchyma cells have a particularly low SA/CWV ratio. Calculated ratios for single cells are sclerenchyma (1:5) < stem parenchyma (1.9: 1) < mesophyll (6.7 to 13.3:1). (v) During digestion of secondary walls, an accumulation of toxic levels of phenolic monomers in cell lumens and at the digesting surface is unlikely, but more slowly diffusing phenolic-carbohydrate complexes could reach concentrations toxic to bacteria. The structural limitations described are discussed in relation to future research directions