Transformations of (14C‐lignin) cell walls of wheat by rumen microorganisms

Abstract

AbstractThe lower halves of apical internodes of wheat harvested at the flowering stage were labelled with [U‐14C] phenylalanine (phe) or with [O14CH3] sinapic acid (sin). Cell wall residues (CWR) and saponified residues (SR) were incubated in a fermenter simulating the rumen for 7 days with rumen fluid or without microorganisms (controls). PheCWR was labelled in all lignin units (measured as aldehydes from nitrobenzene oxidation), in phenolic acids and slightly in proteins. Labelling of pheSR was more lignin‐specific. SinCWR and sinSR were specifically labelled in syringyl units of lignin. The fermentation of CWR resulted in phenylpropane‐derived unit losses in the following decreasing order: ferulic acid>p‐coumaric acid>syringaldehyde>vanillin>p‐hydroxybenzaldehyde. If allowance is made for slight losses in controls, 61, 52, 61 and 63% of the phenylpropanes of pheCWR, sinCWR, pheSR and sinSR, respectively, were transformed into an acid‐precipitable fraction, an acid‐soluble fraction and 14CO2. The comparison of pheCWR and sinCWR degradation showed that syringyl units were solubilised into acid‐precipitable molecules to a greater extent than the other lignin units; demethylation of the syringyl units of lignins was also evident from the different productions of 14CO2. Alkali‐resistant lignins of SR were mainly transformed into acid‐precipitable molecules and were weakly degraded. Lignin solubilisation and degradation seem to be governed by different mechanisms which depend on both cell wall structure and rumen microflora.