Allelopathic effects of Eucalyptus dundasii on germination and growth of ryegrass and barley grass

To overcome the rapid development of herbicide resistance in weeds, there is an increasing demand for alternative weed control. There is limited understory vegetation within the dripline of Eucalyptus dundasii Maiden in fields. We studied the allelopathic potential of E. dundasii on two important Australian weeds species: annual ryegrass (Lolium rigidum Gaudin) and barley grass (Hordeum glaucum Steud.). Distillation of eucalyptus leaves yielded three bioactive fractions: essential oil fraction and the aqueous fractions A and B. These 3-fractions inhibited the germination and seedling growth of annual ryegrass and barley grass. Essential oils proved most inhibitory to germination and shoot growth of both test weeds, followed by the aqueous fraction B and fraction A. Both weeds had similar responses, when exposed to essential oils. However, the germination and seedling growth of barley grass were more sensitive than annual ryegrass to aqueous fraction A and the germination of annual ryegrass was more sensitive than barley grass when exposed to fraction B. The phytotoxicity was concentration-dependent. The phytotoxic effects identified in the aqueous fraction A indicated that significant rainfall could be washed off some of compounds of essential oils into the ground to provide natural chemical defense against understory vegetation. Further study on other Eucalyptus species, followed by the identification of bioactive compounds, might provide chemical leads for the development of new herbicides with new modes of action.