FORMATION AND STRUCTURE OF LIGNIN IN MONOCOTYLEDONS .4. DEPOSITION PROCESS AND STRUCTURAL DIVERSITY OF THE LIGNIN IN THE CELL-WALL OF SUGARCANE AND RICE PLANT STUDIED BY ULTRAVIOLET MICROSCOPIC SPECTROSCOPY

The ultraviolet (UV) absorption spectra of lignins in various morphological regions of growing culm of sugarcane and rice plant were recorded to examine the heterogeneous distribution of lignin structural moieties. The spectra were compared with those of the mixture of pine milled wood lignin (MWL), p-coumaric and ferulic acid esters in different proportions. The lignification of various tissues takes place at different growing stages depending on the role that the tissue plays in the plant, and the lignins in different morphological regions differ in respect to the concentration and structure. Hydroxycinnamic acids as the characteristic structural moieties of gramineous lignin continuously deposit in the cell wall before and after the initiation of lignin deposition. The ferulic acid residue deposits more rapidly at the early stage than at the late stage of lignification, while p-coumaric acid residue deposits continously throughout the process of lignification. The lignin in the secondary wall of protoxylem vessel consists of mostly guaiacylpropane (G) unit and less than 6% of hydroxycinnamic acid moieties, and the deposition of syringylpropane (S) unit becomes apparent at the late stage of lignification. The lignin in the cell corner of fiber and the secondary wall of metaxylem vessel is composed of G and S units, and the proportion of S to G unit tends to increase with the progress of lignification. The lignin in the secondary wall of fiber contains S and G units in a approximately constant ratio at every stage of lignification.