Abstract:The allelopathic effect of Parthenium hysterophorus L. leaf aqueous extract and its ethyl acetate fraction, n-butanol fraction and residuary water fraction on the germination, seedling growth, physiological and biochemical mechanism of Abutilon theophrasti Medic. and Echinochloa crusgalli (L.) Beauv. were studied by using indoor Petri dish bioassay method. The germination rate reduced by 86.7% and 66.6% for A.theophrasti and E.crusgalli at 62.5g·L-1 of aqueous extract respectively, and the inhibition rate of root growth was more than inhibition rates of shoot growth and fresh weight, the inhibition rates of root growth were 96.4% and 87.5% for A.theophrasti and E.crusgalli respectively. The ethyl acetate fraction and n-butanol fraction exhibited more inhibitory effect on germination of A.theophrasti and E.crusgalli than residuary water fraction in three different polarity components, and residuary water fraction exhibited the most inhibitory effect on seedling growth for A.theophrasti while ethyl acetate fraction for E.crusgalli. The ethyl acetate fraction had the highest bioactivity, and seed germination of A.theophrasti and E.crusgalli was complete failure at 2g·L-1, and the inhibition of root growth, shoot growth and fresh weight were 82.6%, 90.2% and 78.2% for A.theophrasti respectively while 78.8%, 86.8% and 69.1% for E.crusgalli respectively. The results of physiological and biochemical indices of receptor indicated that the ethyl acetate fraction decreased root activity, improved SOD and POD activities and increased the content of MDA of treated plants.