Lignin distribution in coppice poplar, linseed and wheat straw

Lignin distribution was determined by interference microscopy, and by confocal laser scanning microscopy (CLSM) for a range of agricultural residues including coppice poplar, linseed, and wheat straw. Interference microscopy was used to determine the lignin concentration in the middle lamella at the cell corner, and for the secondary wall of libriform fibres in the secondary xylem of poplar and linseed. Wheat was examined in the same way for cortical fibres. In addition the secondary wall of vessel elements was examined for poplar. Confocal microscopy was used to confirm the results from interference microscopy by providing semiquantitative information based on lignin autofluorescence, and by staining with acriflavine. Wheat had the lowest level of lignification, with 31 % lignin in the middle lamella of cortical fibres and 9 % lignin in the secondary wall. Poplar had a lignin concentration of 63 % in the middle lamella and 6 % in the secondary wall of libriform fibres, while linseed had corresponding values of 69 % and 13 %. The secondary wall of poplar vessel elements had a lignin concentration of 25 %. In all three species most of the stem tissue was lignified except for phloem and bark, where present. In linseed the pith was unlignified. In wheat, most of the parenchyma cells were lignified except for a few cells lining the stem cavity. Libriform fibres in poplar and linseed sometimes had an unlignified gelatinous layer in samples containing tension wood. In linseed, lignification was greater in xylem fibres compared to bast fibres. Ray parenchyma cells of poplar and linseed appeared to be lignified to the same extent as xylem fibres.