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, ando-,m-, andp-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)-5×10(-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 naturetrans-cinnamic acid,o-, m-, p-coumaric acids, coumarin, and chlorogenic acid inhibit plant growth regardless of their concentration. However, caffeic and ferulic acids can either promote or inhibit plant growth according to their concentration.
Two alfalfa varieties, namely Rasen and Yuba, and rice by‐products of Koshihikari (Oryza sativa L. cv. Japonica), including hull and bran were used for weed control in rice. Rasen and Yuba were sequentially evaluated to have the strongest allelopathic potential among eight common alfalfa varieties in Japan. Rasen, Yuba and rice by‐products in a laboratory experiment exhibited suppression of emergence of lettuce (Lactuca sativa L.), barnyardgrass (Echinochloa oryzicola Vasing.), and monochoria (Monochoria vaginalis Presl var. plantaginea Solms‐Laub.). In a paddy field experiment, at 15 days after application, a dose of 1 ton ha−1 of alfalfa, rice by‐products and the incorporated alfalfa‐rice by‐products significantly inhibited weed growth and reduced weed species, except for rice bran. However, at 40 days after application, the greatest weed control was sequentially recorded with Yuba (95.2%), Rasen (90.5%) and the incorporated Rasen‐hull (88.3%). Rice hull exhibited stronger (51.7%) weed control than the negligible weed control of rice bran (25.1%). Inhibition of both Rasen and Yuba on weed emergence after application became more vigorous, whereas those of rice by‐products were gradually devitalized. Rasen had a maximum increase of rice yield (80.6%) compared to the control and it was slightly higher (9.6%) than the herbicide treatment. Findings showed that Rasen was the most promising material for weed control among those studied. Results denoted that weed control capability of allelopathic materials had an impetuous effect on rice growth and yield.
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