Weed suppression by release of isothiocyanates from turnip-rape mulch

The allelopathic potential of isothiocyanates (ITC) released by turnip-rape mulch [Brassica rapa (Rapifera Group)-Brassica napus L.] was evaluated. Six different ITCs were identified from chopped turnip-rape by HPLC-DAD/HPLC-MS. All plant parts contained 2-phenylethyl-ITC. In the shoot n-butyl and 3-butenyl-ITC dominated. Younger leaves, flowers, and buds also contained small amounts of benzyl and allyl-ITC. Furthermore, marginal amounts of 4-pentenyl-ITC were detected. In the soil, where turnip-rape mulch was incorporated, only low amounts of ITCs were detected. It was shown that the DT50 of ITCs in soil are very short. Germination tests with weed seeds in aqueous ITC solutions showed, that aryl-ITCs were the most suppressive compounds. Within the alkyl-ITCs, the activity decreased with increasing molecular mass. The susceptibility of different weed species to ITCs mainly depended on seed size. Smaller seeds tended to be more sensitive. Further studies demonstrated a high biological activity of ITCs in the vapor phase, n-Butyl-ITC was more suppressive in the vapor phase than in aqueous solution, while 2-phenylethyl-ITC showed the opposite effect. Results demonstrated that weed suppression observed in the field was probably due to the high amounts of ITCs identified in turnip-rape mulch. Isothiocyanates were strong suppressants of germination on tested species-spiny sowthistle [Sonchus asper (L.) Will], scentless mayweed (Matricaria inodora L.), smooth pigweed (wAmaranthus hybridus L.), barnyardgrass [Echinochloa crusgalli (L.) Beauv.], blackgrass (Alopecerus myosuroides Huds.), and wheat (Triticum aestivum L.)-and probably interact with weed seeds in the soil solution and as vapor in soil pores.