Characteristics of guaiacyl-syringyl lignin in reaction wood in the gymnosperm Gnetum gnemon L.

Abstract

Abstract Gnetum gnemon L. is a unique gymnosperm species showing angiosperm-like features in terms of its morphology and chemical composition of the cell wall. Xylan is the main hemicellulose component, and its lignin is primarily composed of syringyl (S) and guaiacyl (G) units and small amounts of p-hydroxyphenyl (H) units. In the present study, in addition to branch, root, bark, and leaf samples, the reaction wood (RW) taken from the leaning stem of G. gnemon, was investigated mainly by alkaline nitrobenzene oxidation, ozonation and NMR spectroscopy. The leaning stem was wider on the lower side of the wood stem (lsW) than on the upper side (usW), similar to the case for compression wood (CW) in gymnosperms. The usW contained lignin with a higher S/G ratio, and β-O-4 structure had a higher erythro/threo ratio, while both ratios decreased around the periphery of the stem towards the lsW. The lignin content was higher towards the lsW. Overall, the lignin composition in the RW of this tree was similar to that in the tension wood of angiosperms. The H-units were minor components in the lignin, but the content was higher towards the lsW, which resembles the distribution of the H-units in a gymnosperm CW.