Mechanisms of weed suppression in wheat (Triticum aestivum L.) and Canola (Brassica napus L.) in Southeastern Australia

This project was designed to assess the use of competitive canola and wheat cultivars for weed suppression under southern Australian field conditions in an effort to apply IWM strategies for weed management and provide fundamental knowledge on the mechanisms underlying crop interference with broadacre weeds. Replicated canola and wheat cultivar field trials were conducted in Condobolin and Wagga Wagga in 2014-2016. Crop and weed growth were monitored at selected phenological growth stages by assessment of above-ground canopy traits including early vigour, canopy closure, crop height, crop and weed biomass, leaf area index (LAI), Normalised difference vegetation index (NDVI), and light interception. In addition, for wheat, shoot and root tissues and rhizosphere soil samples were collected for metabolic profiling. Extracts of plant tissue and soil were analysed for secondary metabolites associated with weed suppression using liquid chromatography-mass spectrometry, with a particular focus on benzoxazinoids (BXs) and related microbially-produced metabolites.
Wheat cultivar and location influenced the production of wheat biomass, early vigour, leaf area index, light interception, crop height and yield, and weed number and biomass. Early crop vigour and biomass accumulation were strongly and inversely correlated with accumulation of weed biomass in both year and location, suggesting these traits were associated with weed interference. Accumulation of weed biomass was inversely related to early crop vigour, crop biomass, NDVI, height, LAI and the light interception at both locations.
Cultivar differences were also observed in canola canopy architecture and yield; early season biomass, light interception, LAI and crop vigour exhibited both year and location interactions. Cultivars with the greatest biomass, light interception, LAI, and visual vigour were most weed suppressive. Although crop and weed biomass accumulation differed significantly among cultivars for both location and year, weed biomass was strongly and inversely related to crop cultivar biomass production. Hybrid cultivars exhibited up to 50% less weed biomass in contrast to open-pollinated cultivars.
Metabolic profiling provided key information related to biosynthesis and release of metabolites associated with weed suppression in commercial field-grown wheat under standard production practices, in comparison to cereal rye and the heritage wheat cultivar Federation, both recognised for their ability to effectively suppress weeds. Up to 15 individual BXs including BX glycosides, lactones, and hydroxamic acids were detected in roots and soil. Both qualitative and quantitative differences in BXs were observed and were dependent on cultivar, crop growth stage, season and location.
Phytotoxic microbial metabolites (the phenoxazinones 2-amino-3H-phenoxazin3-one (APO), 2-actylamino-3H-phenoxazin-3-one (AAPO), 2-amino-7-methoxy-3H-phenoxazin-3-one (AMPO), 2-acetylamino-7-methoxy-3H-phenoxazin-3-on (AAMPO)) were detected and were transformed in the wheat rhizosphere from benzoxazolinones produced by roots and exudates through the action of soil microbiota in both locations. Abundance was dependent on cultivar, phenology, season and location. These findings first demonstrate that production of phenoxazinones can occur at ecologically important concentrations in Australian soils, such that weed suppression by certain wheat cultivars may be facilitated under field conditions. Further research is required to determine if 1) cultivars expressing production of BX metabolites including hydroxamic acids may be targeted for enhanced weed control through biosynthetic modification or 2) if soil microbial transformants may be regulated to encourage production of the potently active phenoxazinones.