THE LIGNIN OF POPULUS NIGRA L. CV. ‘ITALICA’

Abstract

Anatomical investigation showed that callus cultures of Populus nigra L. cv. ‘Italica’ contain lignified parenchyma cells and that large amounts of lignin are localized in intercellular spaces. A tissue-culture strain on a medium containing IAA also showed differentiation of tracheids with lignified walls. In the plant, lignin was found in the primary xylem, secondary xylem, phloem, and periderm. The lignin from primary xylem gave a negative Mäule reaction. Lignins from callus cultures and from the plant were compared and quantitatively determined. A method was developed to calculate the lignin content of tissues from the lignothioglycolic acid yield and the absorption at 250 nm in the so-called difference spectrum. Oxidation with nitrobenzene yielded syringaldehyde (S) and vanillin (V) from tissue cultures, young and old secondary xylem, phloem, and periderm; primary xylem gave vanillin but no syringaldehyde. For tissue cultures, young secondary xylem, phloem, and periderm, the S/V ratio is lower than for differentiated wood. For 2,4-D cultures the S/V ratio was lower than for a strain cultured on an IAA medium. The aldehyde yield, calculated as a percentage of the lignin, is appreciably higher for secondary xylem (42%) than for phloem, periderm, and tissue cultures (12–18%). Xylem lignin contains 9–10% esterified p-hydroxybenzoic acid. Less of this acid is found in the lignin of young secondary xylem (5.2%) and the tissue cultures (2.8–6.7%). Phloem lignin has a still lower p-hydroxybenzoic acid content (0.6%). Lignin rich in p-hydroxybenzoic acid is characterized by a very high absorption in the difference spectrum at 296 nm, strong absorption in the infrared spectrum at 1265 cm-1, and an extra absorption band at 766 cm-1. After hydrolyzation, this lignin lacks the 766 band in the IR spectrum and shows much less absorption at 1265 cm–1. Alkali lignin was prepared by a 16-hour extraction with 0.5 N NaOH at 70 °C. Complete extraction of the lignin from the tissue samples was rarely obtained. Lignin preparations deriving from the various tissue regions of the plant and from the callus cultures show mutual differences. The lignin of the callus cultures shows the greatest resemblance to the lignin of young secondary xylem but is much more resistant to oxidation. In this respect it shows a closer relationship to the lignin from the phloem and primary xylem of the tree. The terms “angiosperm lignin” and “hardwood lignin” applied to a lignin with a high syringaldehyde content are very misleading, because not all the lignified tissues of dicotyledonous trees contain this type of lignin.