LIGHT-INDUCED ISOMERIZATION AND DIMERIZATION OF CINNAMIC ACID-DERIVATIVES IN CELL-WALLS

The role of light in the isomerization and cyclodimerization of the cinnamic acid component of plant cell walls has been examined in a number of ways. Cinnamic acid derivatives were extracted from neutral detergent fibre cell walls by alkaline hydrolysis and quantified by GC. Amounts of these acids in leaf and stem tissues of barley straw which had naturally been exposed to very different levels of radiation were compared. Leaf cell walls contained less p-coumaric acid than stem cell walls, but a higher proportion of this was found as the (Z)-isomer and cyclodimer (4,4'-dihydroxy-alpha-truxillic acid). Samples of wheat straw from a sequence of 10 consecutive years, grown under conditions where meteorological conditions were the major variable, showed considerable year-to-year differences; the (Z)-/(E)-ratio and dimer/monomer ratio varied three-fold. Additional studies compared the direct effect of light on the cinnamic acid derivatives in growing barley seedlings (live), freeze-dried seedlings (dead) and freshly harvested straw samples (dead). There were no significant changes in the cinnamic acid derivatives extracted from the cell walls of dead tissue which had been stored in the dark. Storage in light resulted in increased (Z)-isomer and cyclodimer contents. These changes occurred whether the tissue was alive or dead. They can, therefore, be directly attributed to the effect of light, and do not appear to be mediated by any enzymic reactions.