Allelopathy in Canola (Brassica napus L)

Research output: ThesisDoctoral Thesis

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Canola (Brassica napus L) is Australia’s third largest winter crop. Weeds are
major cost for canola production, by reducing yield and quality. Chemical herbicides are cost effective and widely used. However, to reduce the escalating problem of herbicide-resistant weeds and to address environmental concerns, non-chemical weed control tactics with new mode of action are needed for use to include in the canola cropping system. Suppression of weeds by a crop is an important tactic for weed management. Such interference can include competition for resources as well as allelopathy, the production of defensive compounds which escape into the environment through live root secretion and decomposition of plant residues. The aim of this study was to examine the potential for canola allelopathy to suppress weed growth.

Canola stubble phytotoxicity was first evaluated by an aqueous extract
bioassay on annual ryegrass (Lolium rigidum). In addition inter-cultivar and intracultivar canola autotoxicity was also assessed. Canola genotype residues differed significantly in their inhibition of root elongation of annual ryegrass (12% to 62%). Extract phytotoxicity significantly reduced germination as well as the root (81% to 93%) and shoot (0% to 43%) growth of receiver canola. Canola stubble toxicity on ryegrass root growth was dependent upon extract concentration (0%, 25%, 50% and 75%) for each genotype.. Cultivar Charlton was exhibited the greatest inhibitory effect followed by Sardi607, Zhonshu-ang-No4 and Ag-spectrum. Canola stubble extract also showed some differences for inter-cultivar and intra-cultivar inhibition.

The Equal Compartment Agar Method was used to evaluate canola seedling
allelopathy on annual ryegrass from a collection of 60 Brassica napus genotypes
originating from five continents. In addition, 10 non-napus Brassica genotypes were tested. Significant differences were found among these canola genotypes as measured by an inhibition index which ranged from 8% to 55% inhibition of annual ryegrass root growth relative to the control. Canola genotypes originating from different countries also differed significantly in their seedling allelopathy. The screening of 70 genotypes showed that substantial differences were present, from strong (eg. cv. Av-opal) to weak (eg. cv. Barossa) genotypes. This suggested a genetic basis for differential allelopathic potential between genotypes.

Canola allelopathy of 312 genotypes was investigated in the field in 2012, where significant differentiation was observed between genotypes in their ability to suppress weed growth of a natural infestation. Several genotypes showed significant weed suppressive ability against shepherd purse (Capsella bursa-pastoris), Indian hedge mustard (Sisymbrium orientale) and barley grass (Hordeum leporinum), despite short crop height. Crop height and early vigour influenced weed suppression. The combined effects of allelopathy and competition determined the weed infestation levels seen across the genotypes. A strong correlation (r = 0.77**) was found between the weed suppression by 36 genotypes in the field and their allelopathy under laboratory conditions using ECAM. A subsequent field experiment in 2013 containing extreme allelopathic and competitive genotypes was undertaken to better assess the allelopathic effects of canola genotypes, under the influence of two sowing times. Some of the variation in the field performance of genotypes was predicted by their performance in the laboratory assay.

Crop and weed biomass varied significantly between the genotypes in the
field; high-weed-biomass was harvested in Pak85388-502, Av-opal, Av-garnet and Barossa compared with low biomass in Atr-409 and Cb-argyle. The weed biomass also significantly varied between genotypes in early and late sowing times and ranged from 21 g m-2 to 228 g m-2 and from 78 g m-2 to 297 g m-2 respectively. Avopal and Pak85388-502 plots contained significantly lower weed biomass, while Atr409, Cb-argyle and Barossa produced higher weed biomass. Weed biomass in the early sowing was lower than in the late sowing by 25%, 30% and 35% in Atr-409, Cb-argyle and Barossa, respectively.

Genotypes Av-garnet and Av-opal produced the highest grain yields; which
were almost 50% lower in the late sowing time. Despite its high weed suppressive ability, Pak85388-502’s yield was low compared with the other strongly competitive or allelopathic genotypes in both sowing times. This finding suggested that its lack of local adaptation for yield was independent of its expression of allelopathy and competition.

This second year field study also assessed the role of canola interference
ability on the aggressive weed Paterson’s curse (Echium plantagineum). Genotypes that display strong interference, such as Av-opal, Pak85388-502 and Av-garnet, significantly reduced the rosette diameter and delayed the reproductive stage of E. plantagineum, at both early and late sowing times. Canola genotypes Atr-409, Cbargyle and Barossa showed much weaker interference ability.

In order to identify the responsible allelochemicals, an extensive chemical analysis was undertaken by an advanced metabolomics approach. The sensitive LCQTOF-MS technique was employed to identify the root allelochemicals produced by the extreme canola genotypes identified in laboratory and field experiments. Chemical analysis of each genotype was conducted for three plant parts: shoot, root, and root exudates of 13 days-old-seedlings. A total of 2806 metabolite mass signals was identified. The total numbers of secondary metabolites were distributed differently in the root and shoot tissue and root exudates among the genotypes. Roots generally contained a higher number of allelochemicals than shoot tissue and root exudates, depending on genotype. Fourteen pure compounds were identified from root extracts of both allelopathic and non-allelopathic genotypes. Of these only 8 compounds were identified in root exudates as potential allelochemicals. Sinapyl alcohol, p-hydroxybenzoic acid, 3,5,6,7,8-pentahydroxy flavone, jasmonic acid and methyl-jasmonate were isolated from the allelopathic genotypes Av-opal and Pak85388-502. The presence of these compounds in the strongly allelopathic genotypes suggests that they may be involved in the allelopathic activity of canola seedlings.

It is concluded that substantial variability exists in canola germplasm with
regard to canola allelopathic activity, and the production and exudation of
allelochemicals. Canola seedlings produce and exude certain compounds into the growth medium which inhibit the growth of annual ryegrass in the laboratory and other weed species in the field. Canola seedling allelopathy has great potential for weed suppression. The development of canola cultivars with enhanced allelopathic activity could be an important supplement to traditional chemical modes of action, especially when combined with other competitive traits such as early vigour. More research is needed to understand the genetic control of this allelopathic trait and also potential new modes of action for the development of new herbicide groups.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Charles Sturt University
  • Pratley, Jim, Co-Supervisor
  • An, Min, Co-Supervisor
  • Lemerle, Deirdre, Co-Supervisor
  • Luckett, David, Co-Supervisor
Award date19 Dec 2014
Place of PublicationAustralia
Publication statusPublished - 2015


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