Histochemical study on heterogeneity of lignin in Eucalyptus species I. Effects of polyphenols

In this research, we investigated the structural heterogeneity and the lignin content of Eucalyptus woods by histochemical methods. Since Eucalyptus woods generally contain much polyphenols in the cell walls, we examined the effects of alkali extraction on the quantitative and qualitative analyses of lignin by the Mäule color reaction, ultraviolet microscopy, and thioacidolysis. Sections cut from small blocks were extracted under two alkali extraction conditions (0.4% NaOH, 100°C, 1 h. and 1.0% NaOH, 80°C, 1 h.). The visible light absorption spectra after the Mäule color reaction and ultraviolet absorption spectra were measured before and after these extractions. The results showed that the absorbances of the visible light absorption spectra after alkali extractions were less than those of the spectra before alkali extractions, but no shifts of absorption maxima were observed. Ultraviolet (UV) absorption spectra showed that the absorbances after alkali extractions were less than those before the extractions, particularly at 280 nm. Polyphenols extracted in the alkali solutions had strong absorptions at 280 nm when measured by an UV spectrophotometer. The syringyl-guaiacyl ratio (S/G) as determined by thioacidolysis increased after alkali extraction. These results indicate that polyphenols also contain structures which give guaiacyl type degradation products by thioacidolysis. It appears that alkali soluble polyphenols cause the misunderstandings of lignin contents and the S/G ratio of lignin. Moreover, histochemical study revealed that the alkali soluble polyphenols were distributed more in the vessels and the rays than in the fibers.