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 wails. We e xamined 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 accessib ility to bacteria for different types of cell walls. The analysis reve als the following: (i) Bacterial digestion of fiber cells can progress only from the interior (lumen) surface because their middle lamella-p rimary wall region is consistently found to be indigestible. (ii) Beca use of secondary wall thickness (c. 1-5 mu m), we calculate that at be st only 0.45 to 0.60 mu m of wall thickness (as little as 20% of the w all in some cells) would be digested within the average residence time of fiber particles in the rumen assuming digestion of wall at the fas t rate of 0.02 mu m h(-1). (iii) This potential rate of wall digestion overestimates that of typical fiber particles in the rumen because th ese particles are comprised of many hundreds of cells and relatively f ew will be disrupted by chewing to give bacteria immediate access to c ell lumens. (iv) Digestion of thick-walled cells by bacteria is surfac e-based and sclerenchyma cells have a particularly low SA/CWV ratio. C alculated ratios for single cells are sclerenchyma (1:5) < stem parenc hyma (1.9:1) < mesophyll (6.7 to 13.3:1). (v) During digestion of seco ndary walls, an accumulation of toxic levels of phenolic monomers in c ell lumens and at the digesting surface is unlikely, but more slowly d iffusing phenolic-carbohydrate complexes could reach concentrations to xic to bacteria. The structural limitations described are discussed in relation to future research directions.