Investigation of the ester‐ and ether‐linked phenolic constituents of cell wall types of normal and brown midrib pearl millet using chemical isolation, microspectrophotometry and 13C NMR spectroscopy

Abstract

AbstractThe cell walls of rind, parenchyma and vascular bundle fractions of pearl millet (Pennisetum glaucum (L) R Br) were isolated from two brown midrib mutants and their normal (N) near‐isogeneic line. The walls were sequentially treated with 1 M NaOH at 25°C for 20 h to determine ester‐linked phenolic acids and then with 4 M NaOH at 170°C for 2 h to determine ether‐linked phenolic constituents. The untreated walls and their residues resulting from each treatment were analyzed by microspectrophotometry and 13C NMR spectroscopy. The amount of ester‐linked p‐coumaric acid was determined by chemical analysis and found to be two to six times higher in the N line with no difference among lines in ferulic acid content. Ether‐linked ferulic acid was about 30% higher in the rind in the N line and ether‐linked p‐coumaric acid was only slightly higher with the greatest difference found in the rind tissue. Microspectrophotometry of the untreated tissues showed absorption maxima at 232–238 nm. 288–292 nm and 312–324 nm. Treatment with 1 M NaOH generally reduced or eliminated the 312–324 nm absorption, with 4 M NaOH removing the remainder of the 288–292 nm absorption. 13C NMR confirmed these reductions of aromatic functionalities by alkali treatments. The combination of techniques provides excellent correlation of two types of spectral data with chemical identification and quantitation and establishes that bmr mutants have less ester‐linked p‐coumaric acid and less ether‐linked ferulic acid, thus providing a better understanding of the factors contributing to biodegradability.