Studies of Lignin Biosynthesis Using Isotopic Carbon: Xii. The Biosynthesis and Metabolism of Sinapic Acid

Sinapic acid-2-C14 is converted by 25-day-old wheat plants to sinapyl lignin 6 times as efficiently as to coniferyl lignin. This factor rises to 16 in heading wheat, indicating lessened ability to demethoxylate the sinapyl nucleus with increasing age. Feeding experiments with 3-(3,4-dihydroxy-5-methoxyphenyl)-acrylic acid (5-hydroxyferulic acid) provided evidence for the participation of this compound in the conversion of ferulic to sinapic acid. In plants at an early stage of lignification 5-hydroxyferulic acid was dehydroxylated to ferulic acid and was converted to coniferyl lignin, but evidence about the conversion of 5-hydroxyferulic acid to coniferyl lignin in more mature plants was conflicting. Demethylation of sinapic to 5-hydroxyferulic acid was demonstrated by means of a trapping experiment. Ferulic acid-2-C14 is reduced to coniferyl aldehyde and coniferyl alcohol in heading wheat plants and cambial tissue cultures of Pinus strobus.