Histochemistry of Lignin Deposition during Sclerenchyma Differentiation in Alfalfa Stems

The spatial deposition of lignin within the cell walls layer of sclerenchyma is distinct from the lignification pattern of secondary xylem cell walls. Histochemical staining and autofluorescence of the cell walls during alfalfa internode ageing clearly showed that lignin deposition first occurs in the secondary cell walls of the differentiating sclerenchyma. This is in contrast to the widely reported lignification pattern in secondary xylem where lignins are incorporated in the primary cell walls prior to the secondary walls. Spatial variations in the onset of lignin deposition are accompanied by differences in the frequency of the di-methoxylated lignin monolignol (i.e. syringyl unit) depending on the nature of the tissue. Modifications of the lignin composition are also related to the physiological stage of alfalfa internodes.