Herbicidal potential of Eucalyptus dundasii on Lolium rigidum Gaud. and Hordeum spp.

Field observation has found that there is limited understory vegetation within the dripline of Eucalyptus dundasii. Research was conducted to assess the allelopathic potential of E. dundasii on two important weeds species, annual ryegrass and barley grass. The eucalyptus leaves were collected and subjected to a distillation process. The distillation process yielded three bioactive fractions, essential oil fraction, water fraction A and water fraction B. These three fractions were phytotoxic to the germination and seedling growth of the annual ryegrass and barley grass. Essential oils caused the most inhibitory effects on germination and shoot growth of both weeds, followed by water fraction B and fraction A. Both weeds had similar responses when exposed to the essential oils. However, the germination and seedling growth of barley grass were more sensitive than annual ryegrass to water fraction A and the germination of annual ryegrass was more sensitive than barley grass when exposed to water fraction B. The phytotoxicity was concentration-dependent. The research has found that although essential oils are highly volatile, these compounds can partly dissolve in water, resulting in the phytotoxic activity identified in the water fraction A. These results indicate that any significant rainfall events will wash off the essential oils into the ground to perform natural chemical defense against understory vegetation, including weeds. Further study on the identification of bioactive compounds might provide chemical leads for the development of new herbicides with new modes of action.