INTERACTIONS OF TRANS-CINNAMIC ACID, ITS RELATED PHENOLIC ALLELOCHEMICALS, AND ABSCISIC-ACID IN SEEDLING GROWTH AND SEED-GERMINATION OF LETTUCE

Phenolic compounds have been identified as the most common allelochemicals produced by higher plants. Inhibitions of cinnamic acid, its related phenolic derivatives, and abscisic acid (ABA) on seedling growth and seed germination of lettuce were studied. trans-Cinnamic acid, and o-, m-, and p-coumaric acids inhibited the growth of etiolated seedlings of lettuce at concentrations higher than 10(-4) M and seed germination above 10(-3) M. Coumarin inhibited seedling growth and seed germination at 10(-5) M or above. Chlorogenic acid inhibited seedling growth above 10(-4) M, but did not inhibit seed germination at 10(-5)-5 X 10(-3) M. LoW Concentrations (below 10(-3) M) of caffeic and ferulic acids promoted the elongation of hypocotyls, but higher concentrations (over 10(-3) M) inhibited seedling growth and seed germination. These phenolic compounds and abscisic acid had additive inhibitory effects both on seedling growth and seed germination. The inhibition on lettuce was reversed by caffeic and ferulic acids at concentrations lower than 10(-3) M except for the inhibition of germination by coumarin. These results suggest that in nature trans-cinnamic acid, o-, m-, p-coumaric acids, coumarin, and chlorogenic acid inhibit plant growth regardless of their concentration. However, ever, caffeic and ferulic acids can either promote or inhibit plant growth according to their concentration.