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 constituents that retard biodegradation. A series of cell types in stems and leaf blades of a normal (N) and a brown midrib (bmr) mutant line of pearl millet (Pennisetum glaucum (L) R Br) were analyzed by UV absorption microspectrophotometry from freeze-dried bulk walls and from 4 jlm thick sections. Cell types were evaluated by scanning and transmission electron microscopy for rumen microbial degradation . Generally, N cell walls had a higher absorbance, hypsochromatically shifted Amax' and a higher absorbance ratio (280 :320 nm) compared with bmr walls . Spectra suggested that ester-linked phenolic acids were present in lignified and non-lignified walls of both plant lines. Results are consistent with previous information that N cell types have substantially higher concentrations of esterlinked p-coumaric acid and more condensed aromatics 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 epidermal cells . Biodegradable cell walls, such as stem parenchyma and epidermis and parenchyma bundle sheaths of leaf blades, showed less absorbance than heavily lignified walls. For some walls, UV spectra related well with variations in digestibility, while for other cell walls further work is needed to relate spectra to biodegradation .