Spectral characteristics of lignin and soluble phenolics in the near infrared- a comparative study

Prediction of canopy chemical compounds including lignin based on reflectance or absorbance in the near infrared has been demonstrated using hyperspectral data. Little attention has been paid to the fact that many types of plant phenolics have chemical structures similar to lignin and may have similar spectral responses in the near infrared. To address this problem we have analysed near infrared spectral signatures of dry powder standards of lignin and tannin, dissolved lignin, tannin and other soluble phenolics, lignin and soluble phenolics extracted from Norway spruce needles, as well as wooden blocks containing varying amounts of tannin. Our results provide solid evidence that the spectral signatures of lignin and tannin are very similar and that spectral features and wavelengths used previously for lignin determination are in fact likely to be due to tannin and lignin reflectance features. This similarity may greatly affect near infrared assessments of canopy lignin concentrations and impact analyses for which these predictions are used. Additionally, certain near infrared reflectance properties associated with increasing levels of foliar damage may be due to increased levels of tannins in affected mesophyll cells. We have also identified the existence of a threshold lignin concentration below which spectrometry cannot precisely detect lignin.