Characterization of digestion residues of alfalfa and orchardgrass leaves by microscopic, spectroscopic and chemical analysis

Publisher Website
Google Scholar
Abstract
Leaflets of alfalfa (Medicago sativa L) and leaf blade sections of orchardgrass (Dactylis glomerata L) were digested for three consecutive incubation periods of 3 days each with rumen bacteria. Residues were then analyzed by the following methods: (i) scanning electron and light microscopy to define the botanical structures; (ii) ultraviolet absorption microspectrophotometry to characterize phenolic constituents within specific regions; (iii) NMR and midinfrared spectroscopy to determine components, linkages, and functional groups; and (iv) gas‐liquid chromatography for phenolic acids and lignin constituents after treatment with 1 M NaOH (25°C, 24 h) and 1 M NaOH followed by 4 M NaOH (170°C, 2h). Alfalfa residue consisted of extensive amounts of cuticular sheets, a network of vascular bundles, and smaller portions of cuticle overlying lignified structures associated with major vascular bundles. Orchardgrass residue consisted of large and small vascular bundles, usually without the mestome sheath surrounding phloem, and small portions of cuticle. Alfalfa lignin was more condensed but had similar proportion of ether‐linked phenolic acids compared with that of orchardgrass, while the grass lignin had substantially higher concentrations of ester‐linked p‐coumaric and ferulic acids associated with a higher proportion of carbohydrates in the residue. Alfalfa had a higher fatty ester content, likely due to the large amount of cuticle in the residue, and protein, which likely was contained within the lignin. The results provide a detailed characterization of aromatics and linkages within the most recalcitrant tissues of plants and indicate variations between non‐degradable residues of a legume and a grass.