Accumulation regularity of polysaccharides and lignin and identification of lignin monomer types in stems of Dendrobium officinale Kimura et Migo

The stems of Dendrobium officinale Kimura et Migo in different years were used as study materials. The contents of polysaccharides, monosaccharides, and lignin were determined, and the relationships among them were analyzed by multiple linear regression analysis. The D. officinale stems were stained using saffron, fast green, and Mäule staining, and the lignin monomer types were determined by Fourier transform infrared (FTIR) and proton nuclear magnetic resonance (¹H-NMR) spectroscopy. As a result, lignin accumulation was significantly positively correlated with glucose content, and significantly negatively correlated with polysaccharide and mannose content and the mannose to glucose ratio. These results suggested that the accumulation of polysaccharides was influenced by the deposition of lignin. The xylem and sheath of the vascular bundles in the D. officinale stems contained lignin. With the increase in years, the thickness of the vascular thick-walled cells in the D. officinale stems increased, and the degree of development of the vascular bundle sheaths increased. Mäule staining and FTIR and ¹H-NMR spectrometry showed that the stems of D. officinale mainly contained guaiacyl lignins (G lignins) and syringyl lignins (S lignins), as well as trace hydroxyl-phenyl lignins (H lignins). G lignins were mainly distributed in the xylem, while S lignins were mainly distributed in the thick-walled cells. The S/G ratio in the D. officinale stems showed an upward trend with increasing years.