Dual direction allelopathy: The case of canola, wheat and annual ryegrass

The exploitation of allelopathy for enhanced weed suppression is in its infancy despite major advancements in our knowledge of weed suppressive cultivars. Crop species, including wheat and canola have been identified as potentially being weed suppressive against annual ryegrass. However, the contribution of
crop allelopathy to interference is complex to elucidate within a field context. Traditionally, experiments involving allelopathy only investigate one species as the donor and the other as the target, irrespective of the putative duality of allelopathic responses between some species. In this paper we investigate dual direction allelopathic responses between canola and annual ryegrass at the seedling stage utilising laboratory growth assays, specifically the equal compartment agar method (ECAM). By manipulating experimental conditions, such as sowing density, time of sowing and plant proximity, we compare the efficiency of each species to exert an allelopathic effect on the other. Both species exert an inhibitive effect on the other dependant on the starting experimental conditions which they are subjected. We found that increasing the time that each species grows in the agar before introduction of the other species significantly inhibits the growth of the later sown species, up to 83.6% inhibition of canola root length by annual ryegrass. Density also played an important role in determining the level of root inhibition found for both species. Canola and wheat varietal differences were observed in respect to the level of annual ryegrass inhibition found. This paper discusses the implications of these results in the larger context of varietal selection for weed suppression.