Microspectrophotometry and Digestibility of Alkali-Treated Walls in Bermudagrass Cell Types

Phenolic compounds within plant cell walls reduce forage quality, hut information on the site and type of these compounds is needed to determine precise mechanisms influencing forage digestibility. The present study was undertaken to evaluate the loss of phenolic compounds from walls of various cell types after treatment with NaOH using microspectrophotometry and to relate this loss to digestibility by rumen microorganisms. Cell walls of ‘Coastal’ bermudagrass [Cynodon dactylon (L.) Pers.] leaf-blade sections were untreated or treated successively with 0.1 M and 1.0 M NaOH to release phenolic acids. Sections were then digested in vitro with rumen fluid. Walls of parenchyma bundle sheath, sclerenchyma, and mestome sheath from undegraded sections were examined by ultraviolet (UV) absorption microspectrophotometry and evaluated with histochemical stains for phenolics. Walls of untreated parenchyma bundle sheaths gave UV absorption spectra similar to p-coumaroyl or feruloyl arabinoxylotrisaccharides, and the absorption was low after 0.1 M NaOH treatment and essentially absent after 1.0 M NaOH treatment. A crude estimation of ferulic acid equivalents was calculated from Beer's Law assuming that all absorbance in parenchyma bundle sheath was due to p-coumaroyl or feruloyl groups. Sclerenchyma and mestome sheath walls contained esterified and/or etherified p-coumaric and/or ferulic acids and a relatively high proportion of lignin (i.e., condensed phenolic polymers). Cell walls appeared to be substantially distorted and partially degraded after 0.1 M NaOH treatment; digestion of walls treated with 1.0 M NaOH resulted in mostly unrecognizable residues. Absorption maxima were still present in undegraded mestome walls after 1.0 M NaOH treatment. Microspectrophotometry of alkali-treated walls indicated that p-coumaroyl and feruloyl groups appeared to be mostly responsible for limiting digestion in certain cell types (e.g., parenchyma bundle sheath), while the more refractory walls (e.g., mestome sheath) contained esterified and/or etherified p-coumaric and ferulic acid and more recalcitrant aromatic compounds.