Phytotoxicity of canola residues: Release of water-soluble phytotoxins

Phytotoxicity of root, stem and leaf residues of canola (Brassica napus L.) was studied in a laboratory incubation for 8 wk. Bioassays were conducted with corn (Zea mays L.), barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.) seeds to determine the relative toxicity of residue extracts. The release of volatile fatty acids (VFAs) and phenolic compounds (PCs) during incubation was studied to assess the role of these compounds in phytotoxicity. Canola residue extracts significantly inhibited (P less than or equal to 0.01) seedling growth (coleoptile and radicle lengths) of all crop species tested, with leaf and root residues causing the greatest and least toxicity, respectively. Incubation temperature had no impact on the toxicity of extracts. All residue extracts had VFAs prior to and during the incubation, with leaf residues producing relatively high levels of VFAs. The release of VFAs declined drastically during the incubation period. The release of PCs, measured as total phenolic compounds in extracts, was highest in leaf residues and remained higher than those of root or stem residues. Toxicity of residue extracts was not related to the amounts of VFAs and PCs found. However, toxicity appeared to be most related to the presence of total phenolic compounds in residue extracts.