STUDIES OF LIGNIN BIOSYNTHESIS USING ISOTOPIC CARBON: XI. REACTIONS RELATING TO LIGNIFICATION IN YOUNG WHEAT PLANTS

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Abstract
L-Phenylalanine-G-C14, p-hydroxycinnamic acid-2-C14, ferulic acid-2-C14, and sinapic acid-2-C14were administered to wheat plants aged both 30 and 73 days. Radioactive vanilloyl- and syringoyl-methyl ketones were then recovered after ethanolysis of the cell wall residues. When corrected for differences in endogenous lignin, the C14dilution values calculated for the younger plants were generally greater, indicating, as expected, a slower rate of lignification. The difference between the younger and older plants was less for sinapic and ferulic acids than for p-hydroxycinnamic acid or phenylalanine. This suggested that slower lignification in young plants may be due not to relative inactivity of an enzyme system at any one stage of the biosynthetic pathway but to the cumulative effect of slower reactions at several stages. Sinapic acid is converted in the younger plants to lignin yielding vanilloyl-, as well as syringoyl-, methyl ketone, suggesting a demethoxylation. Glucose esters of the radioactive phenolic cinnamic acids corresponding to the labelled compound administered were recovered from the plant extracts. A small percentage of the activity in the cell wall residue was in the form of ferulic acid joined by ester linkages.