Changes in lignin structure during earlywood and latewood formation in Scots pine stems

Lignification of earlywood and latewood during annual ring formation in Scots pine stem in the season occurs with opposite dynamics. The subject of this study was to examine the structure of lignin deposited at successive stages of secondary cell wall maturation of these two wood types. Lignin specimens derived from developing cells by the thioglycolic acid were analyzed by the methods of alkaline oxidation, alkaline and acid hydrolysis and IR Fourier spectroscopy. The composition of lignin structural subunits was found to change at each stage of lignification in dependence of the forming wood type. The molar ratio of p-hydroxyphenyl, guaiacyl and syringyl subunits in polymer was modified in earlywood and latewood oppositely. In the course of earlywood cell maturation, syringyl subunits in lignin increased in parallel with p-hydroxyphenyl units. During latewood lignification, syringyl and p-hydroxyphenyl subunits in lignin structure decreased, whereas guaiacyl subunits increased as secondary cell wall maturation. In lignin isolated from both wood types, the ether-bound carbohydrates were more than by an order of magnitude of ester-bound carbohydrates. In early xylem, the content of ether-bound carbohydrates practically did not change during lignin deposition, whereas carbohydrates with ester linkages gradually decreased toward mature xylem. Lignification in latewood was accompanied by the decline of both ester-linked and ether-linked carbohydrates with a sharp increase in the latter in mature xylem. Hemicelluloses, included in such bonds, contained mainly the residues of xylose and arabinose. The composition and the content of hydroxycinnamic acids, taking part in these linkages, also changed in dependence of lignification steps and wood type. The data received are in agreement with the alterations in absorption IR spectra of lignins isolated from the cells at the beginning of lignification and mature xylem. The reasons for the differences in the lignin structure during earlywood and latewood development are discussed.