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

The role of light in the isomerization and cyclodimerization of the ci nnamic acid component of plant cell walls has been examined in a numbe r of ways. Cinnamic acid derivatives were extracted from neutral deter gent fibre cell walls by alkaline hydrolysis and quantified by GC. Amo unts of these acids in leaf and stem tissues of barley straw which had naturally been exposed to very different levels of radiation were com pared. Leaf cell walls contained less p-coumaric acid than stem cell w alls, but a higher proportion of this was found as the (Z)-isomer and cyclodimer (4,4'-dihydroxy-alpha-truxillic acid). Samples of wheat str aw from a sequence of 10 consecutive years, grown under conditions whe re meteorological conditions were the major variable, showed considera ble year-to-year differences; the (Z)-/(E)-ratio and dimer/monomer rat io varied three-fold. Additional studies compared the direct effect of light on the cinnamic acid derivatives in growing barley seedlings (l ive), freeze-dried seedlings (dead) and freshly harvested straw sample s (dead). There were no significant changes in the cinnamic acid deriv atives extracted from the cell walls of dead tissue which had been sto red in the dark. Storage in light resulted in increased (Z)-isomer and cyclodimer contents. These changes occurred whether the tissue was al ive or dead. They can, therefore, be directly attributed to the effect of light, and do not appear to be mediated by any enzymic reactions.