DIGESTION KINETICS OF PARENCHYMA AND SCLERENCHYMA CELL-WALLS ISOLATED FROM ORCHARDGRASS AND SWITCHGRASS

Both cell-wall chemistry and anatomical structure determine the digestion characteristics of cell types in forages. The objectives of this study were to determine the digestion kinetics of cell types apart from anatomical factors, and to relate these digestion kinetics to previously determined cell-wall constituents. Parenchyma and sclerenchyma cell walls, isolated from plant parts of orchardgrass (Dactylis glomerata L.) and switchgrass (Panicum virgatum L.) at four growth stages, were milled (digestion > 150-mu-m) and incubated in buffered rumen fluid for 0 to % h. Lag time (L) of cell types ranged from 4.0 to 8.0 h, rate constant (k) from 0.035 to 0.100 h-1, and indigestible residue (IR) from 89 to 595 g kg-1. Differences between L of cell types were small. The k of parenchyma was almost-equal-to 50% greater (P < 0.01) than that of sclerenchyma. The concentration of IR increased (P < 0.05) during plant maturation, particularly for stem cell types. The IR of orchardgrass parenchyma was less (P < 0.01) than that of sclerenchyma at immature growth stages, but not at late anthesis. In contrast, IR of switchgrass parenchyma was greater (P < 0.01) than that of sclerenchyma. Among cell types, IR concentrations were lowest (P < 0.05) for leaf sheath parenchyma and leaf blade sclerenchyma. The L and k of both cell types and IR of sclerenchyma were poorly correlated to neutral sugar, esterified phenolic acid, and lignin concentrations. The IR concentration of parenchyma was correlated (P < 0.01) with most cell-wall constituents, suggesting that fiber digestibility could be improved by altering the cell-wall composition of this cell type. The rate and extent of digestion of sclerenchyma was much greater than reported from histological studies, suggesting that the anatomical configuration of this cell type, rather than its cell-wall composition, is the major factor limiting its digestion.