BIODEGRADATION OF CELL-TYPES IN NORMAL AND BROWN MIDRIB MUTANT PEARL-MILLET (PENNISETUM-GLAUCUM (L) R BR) - MICROSPECTROPHOTOMETRIC AND ELECTRON-MICROSCOPIC STUDIES OF WALLS AND WALL LAYERS

Aromatic constituents of cell walls limit the feeding value of forages , but information is lacking on the sites and types of these constitue nts that retard biodegradation. A series of cell types in stems and le af blades of a normal (N) and a brown midrib (bmr) mutant line of pear l millet (Pennisetum glaucum (L) R Br) were analyzed by UV absorption microspectrophotometry from freeze-dried bulk walls and from 4 mum thi ck sections. Cell types were evaluated by scanning and transmission el ectron microscopy for rumen microbial degradation. Generally, N cell w alls had a higher absorbance, hypsochromatically shifted lambda(max), and a higher absorbance ratio (280:320 nm) compared with bmr walls. Sp ectra suggested that ester-linked phenolic acids were present in ligni fied and non-lignified walls of both plant lines. Results are consiste nt with previous information that N cell types have substantially high er concentrations of ester-linked p-coumaric acid and more condensed a romatics than bmr walls. Pronounced variations occurred within regions of some cell types, including the middle lamella region vs. secondary layers of sclerenchyma cells and the outer vs. inner regions of epide rmal cells. Biodegradable cell walls, such as stem parenchyma and epid ermis and parenchyma bundle sheaths of leaf blades, showed less absorb ance than heavily lignified walls. For some walls, UV spectra related well with variations in digestibility, while for other cell walls furt her work is needed to relate spectra to biodegradation.