The seedling root response of annual ryegrass (Lolium rigidum) to neighbouring seedlings of a highly-allelopathic canola (Brassica napus)

• Canola (Brassica napus L.) shows allelopathic action to annul ryegrass seedlings.
• The allelopathic effect of canola is greatest at high canola density.
• Ryegrass showed a distinct pattern of root growth plasticity in presence of canola.
• Ryegrass roots are able to recognise hetero-specific canola roots.

ABSTRACTLaboratory experiments were conducted under controlled conditions to examine the avoidance responses of seedling roots of annual ryegrass (Lolium rigidum) to the presence of seedling roots of a highly-allelopathic canola (rapeseed) genotype (Brassica napus cv. Av-opal). The postulated concentration gradient of the canola (donor) allelochemicals was altered by changing the seedling density and/or the proximity in the nutrient-free agar medium. In addition, activated carbon was tested for its ability to nullify the effect of the allelochemicals. The ryegrass (receiver) roots initially grew towards the canola roots and then grew away from them, showing bent or curved growth, with generally shorter and thicker roots. This effect was greatest at high canola density and at the closer proximity, but was negated by the addition of activated carbon. These findings add considerable weight to the conclusion that certain genotypes of canola exuded allelochemicals which are detected by the annual ryegrass roots causing an active response that modifies and inhibits the growth of this weed.