DISTRIBUTION OF GUAIACYL AND SYRINGYL LIGNINS IN NORMAL AND COMPRESSION WOOD OF BUXUS-MICROPHYLLA VAR INSULARIS NAKAI

The distribution of guaiacyl and syringyl lignins in the secondary xylem tissues of normal and compression wood of Buxus microphylla var. insularis Nakai was examined by visible light (VL) microspectrophotometry coupled with the Maule and Wiesner colour reactions and by UV-microspectrophotometry, and compared with normal wood of Betula ermani Cham. Buxus formed compression wood on the lower side of the leaning stems, and the secondary walls of the vessels and fibre-tracheids showed excessive lignification, resembling the S2 (L) layer of compression wood tracheids in gymnosperms. In normal wood of both species, the Maule colour reaction indicated that in Betula the secondary walls of fibres contain larger amounts of syringyl units in the lignins than other tissues, and that in Buxus the secondary walls of fibre-tracheids contain both syringyl and guaiacyl units. The vessel walls of both species contained higher amounts of guaiacyl units. Heterogeneity of the syringyl-lignin distribution was found in the secondary walls of Buxus fibre-tracheids. In compression wood of Buxus, on the other hand, the spectra of the secondary walls of the vessels and fibre-tracheids after the Maule reaction showed low absorbances compared with the normal wood, whereas, after the Wiesner reaction, their secondary walls gave high absorbances. In addition, the UV-absorption maximum of the secondary fibre walls shifted from 274 nm to 279 nm, and the UV-absorbances of the vessel and fibre-tracheid walls greatly increased in compression wood. The results obtained in the present study demonstrated that in normal Buxus wood the secondary walls of the vessels and fibre-tracheids contain both guaiacyl and syringyl units, though the syringyl unit is a minor constituent in the vessel walls, and that both cell types increase their contents of guaiacyl units, especially in the outer parts of the secondary walls during their changes from normal wood to compression wood. The present study also suggested that the Wiesner reaction may be used for examining the content of lignin and the proportion of guaiacyl to syringyl units in lignins.