Weed Suppression by Release of Isothiocyanates from Turnip-Rape Mulch

wick et al., 1983). Living plants do not actively release high amounts of ITCs (Börner, 1961), because glucosiThe allelopathic potential of isothiocyanates (ITC) released by nolates are located in the vacuole and myrosinase is turnip–rape mulch [Brassica rapa (Rapifera Group)–Brassica napus L.] was evaluated. Six different ITCs were identified from chopped bound to the cell wall (Björkman, 1976). As long as this turnip–rape by HPLC-DAD/HPLC-MS. All plant parts contained separation exists, there is only a low ITC content in the 2-phenylethyl-ITC. In the shoot n-butyl and 3-butenyl-ITC domicells (Tang, 1971). Larger amounts of ITCs can only nated. Younger leaves, flowers, and buds also contained small amounts be released by the breakdown of the cells, e.g., during of benzyl and allyl-ITC. Furthermore, marginal amounts of 4-pendecomposition of dead plant material (Bell and Muller, tenyl-ITC were detected. In the soil, where turnip–rape mulch was 1973), or even faster by incorporating green plant mateincorporated, only low amounts of ITCs were detected. It was shown rial into the soil. If Brassica spp. plant tissues are incorthat the DT50 of ITCs in soil are very short. Germination tests with porated into the soil, it is possible to control weeds in weed seeds in aqueous ITC solutions showed, that aryl-ITCs were the following crop by ITCs released from the mulch the most suppressive compounds. Within the alkyl-ITCs, the activity decreased with increasing molecular mass. The susceptibility of differ(Brown and Morra, 1995; Boydston and Hang, 1995; ent weed species to ITCs mainly depended on seed size. Smaller seeds Al-Katib et al., 1997). This might be a chance to reduce tended to be more sensitive. Further studies demonstrated a high the use of herbicides and could be an additional tool to biological activity of ITCs in the vapor phase. n-Butyl-ITC was more control herbicide-resistant weeds. suppressive in the vapor phase than in aqueous solution, while 2The objectives of this study were to evaluate the allelphenylethyl-ITC showed the opposite effect. Results demonstrated opathic potential of ITCs released by turnip–rape that weed suppression observed in the field was probably due to the mulch. Therefore, the content of ITCs in different parts high amounts of ITCs identified in turnip–rape mulch. Isothiocyanates of turnip–rape, and in the soil after incorporation was were strong suppressants of germination on tested species—spiny determined, and the phytotoxicity of different ITCs on sowthistle [Sonchus asper (L.) Hill], scentless mayweed (Matricaria inodora L.), smooth pigweed (Amaranthus hybridus L.), barnseveral weed species—spiny sowthistle [Sonchus asper yardgrass [Echinochloa crusgalli (L.) Beauv.], blackgrass (Alopec(L.) Hill], scentless mayweed (Matricaria inodora L.), urus myosuroides Huds.), and wheat (Triticum aestivum L.)—and smooth pigweed (Amaranthus hybridus L.), barnprobably interact with weed seeds in the soil solution and as vapor yardgrass [Echinochloa crusgalli (L.) Beauv.], blackin soil pores. grass (Alopecurus myosuroides Huds.), and wheat (Triticum aestivum L.)—was investigated. S of the Brassica family are often used as cover MATERIALS AND METHODS or green manure crops (de Almeida, 1985; Brown Plant and Soil Material and Morra, 1995; Boydston and Hang, 1995; Al-Khatib Winter turnip–rape [Brassica rapa (L.) var. rapa ssp. oleifera et al., 1997; Krishnan et al., 1998). Superficial incorpora(DC.) Metzg. cv. Perko] was planted (10 kg ha2) 20 Aug. tion (upper soil surface, 0–10 cm) of green mulch of 1997 at Hohenheim University, Stuttgart, Germany. The soil these species caused a temporary weed suppression, type was a loam (pH 6.7) with an organic matter content of which is probably due to secondary plant metabolites 1.8%. The average plant density in April 1998 was 66 plants (Al-Khatib et al., 1997). Brassica spp. contain high m2. On 24 Apr. 1998 the shoots of the plants were cut off amounts of glucosinolates (Fenwick et al., 1983). Al(2–4 cm above soil level) and 1 wk later, the turnip–rape mulch though the biological activity of these secondary plant was incorporated 10 cm deep into the soil with a rotary tiller. metabolites is very low (Fenwick et al., 1983), they play Plant and soil material for chemical analysis and germination a key role for weed suppression as they can be converted tests were taken from this field (fresh plant samples 23 Apr. to the corresponding isothiocyanates (ITC) by the plant 1998 and soil samples 7, 15, and 28 d after mulch incorporation) and stored at 2208C until analysis. enzyme myrosinase. Besides ITCs, also other breakdown products of glucosinolates (nitriles, thiocyanates, and oxazolidinethiones) can occur, depending on variGermination Tests ous factors [e.g., pH, Fe(II) concentration, side chainGermination tests were performed in glass petri dishes substitution, and plant species] (Bones and Rossiter, (9-cm diam.) containing two layers of filter paper (9-cm diam., 1996). The main breakdown products at pH 7 are ITCs Macherey-Nagel), moistened with 8 mL of distilled water and (Bones and Rossiter, 1996), which are phytotoxic (Fen50 seeds of the test species were placed on the filter paper. For different concentrations the hydrophobic ITCs were initially dissolved in methanol and then known amounts of the methaJ. Petersen, current address: Inst of Sugar Beet Research, Holtenser Landstr. 77, 37079 Goettingen, Germany; and R. Belz, F. Walker, and K. Hurle, Inst. of Phytomedicine, Weed Science Dep., Hohenheim Abbreviations: DT, disappearance time; ED, effective dosage; HPLC, Univ., 70593 Stuttgart, Germany. Received 29 Nov. 1999. *Correhigh-performance liquid chromatography; ITC, isothiocyanate; MS, sponding author ([email protected]). mass spectrometry; PAD, photodiode array detector; TSM, thousand seed mass. Published in Agron. J. 93:37–43 (2001).