ISOLATES OF CELL-TYPES FROM SORGHUM STEMS - DIGESTION, CELL-WALL AND ANATOMICAL CHARACTERISTICS

Cell types were separated from internode 5 of sorghum stems to study t he interrelationship between digestion characteristics and cell wall c omposition. Isolates of epidermis (EPI), sclerenchyma (SCL), vascular bundle zone (VBZ), inner vascular bundles (IVB) and pith parenchyma ce lls (PITH) were freeze-dried and ground for analysis. The cell fractio ns were digested in rumen fluid for times between 0 and 96 h, and wall composition measured using detergent extraction procedures. In-vitro dry matter digestibility (g kg-1 after 48 h) of cell fractions was in the order of PITH (849-906) > IVB (794-816) > SCL (692-701) > VBZ (641 -679) > EPI (608-628). Total cell wall content (CWC), indigestible CWC , and lignin content followed the inverse order. Lignin concentration on a dry matter or cell wall basis was highly correlated with indigest ible wall residue after 96 h. The proportion of cell wall digested aft er 96 h was higher for SCL and VBZ cells (61.8-68.2 %) than for PITH c ells (48.4-56.1 %), despite the former having lignin content three to five times higher than that of PITH cells. Clearly, there were differe nces between the cell types in wall composition or chemical linkages b etween wall components that lead to the observed differences in wall d igestion.