Insights into the lignin and extractives changes and the discoloration mechanism in plantation teak wood under xenon exposure

Teak heartwood will turn into an attractive golden color under exposure to light, which is a necessary process before its processing and utilization. Herein the discoloration mechanism of xenon lamp aged teak was investigated based on the changes in lignin and extractives. After xenon exposure, the lignin within the S2 layer, middle lamella, and cell corners all underwent changes. Lignin and extractives were degraded and oxidized, with an increase in conjugated and non conjugated carbonyl groups. The β-O-4 bonds within lignin decreased, and the doubling of the syringyl/guaiacyl (S/G) ratio (0.25→0.55) promoted chromophore formation. Flavonoid metabolites contribute ∼38 % of the total color difference ( ΔE ). The increase of the extractives including vanillin, syringaldehyde, acetoguaiacone, cyanidin 3-rutinoside-5-glucoside, myricetin, and maclurin, and the decrease of 2-methyl-9,10-anthracenedione, squalene, vernoflexuoside, panaxatriol, 4′-methoxychalcone and tephrosin were related to discoloration. Extractives delay lignin photolysis through antioxidant activity. The lightness increased, the color became more yellow, the saturation increased, and the surface of the teak heartwood appeared gold. Based on the results, the relationships between color and chemical composition were established by nonlinear fitting of chromaticity parameters with FT-IR characteristic peaks and the S/G ratio. The results confirmed that conjugated carbonyl content is exponentially positively correlated with ΔE ( R 2 = 0.92), while lignin content is negatively correlated with L * ( R 2 = 0.85). Totally, this study reveals the synergistic mechanism between the increased S/G ratio of lignin and the accumulation of flavonoid metabolites during the photochromism of plantation teak through multi-scale characterization for the first time. A quantitative model between chemical components and chromaticity parameters was established. The findings of the work affords crucial understandings of the comprehensive effects of lignin and extractives on the discoloration of teak heartwood under light. These insights are of value for the further utilization of its unique color characteristics. • The lignin and extractives of teak heartwood degraded under xenon exposure. • Average UV absorbance of cell walls increased from 0.37 to 0.40 under xenon exposure. • Syringyl/guaiacyl ratio more than doubled, and the content of β-O-4 decreased by 18 %. • Nine color-related extractives accumulated, and six photosensitive substances degraded, synergistically driving the golden discoloration of teak heartwood.