Volatile Allelochemicals Released by Crucifer Green Manures

Abstract

Several members of the crucifer family (Brassicaceae), including white mustard (Brassica hirta Moench), brown mustard [B. juncea (L.) Coss], black mustard [B. nigra (L.) Koch], leafy turnip (B. campestris L.), rapeseed (B. napus L.), and garden cress (Lepidium sativum L.) were examined for their potential as allelopathic green manure crops. Hemp sesbania [Sesbania exaltata (Raf.) Rydb. Ex A. W. Hill] germination and fresh weight was inhibited by chopped leaf tissues of all green manures tested, including wheat (Triticum aestivum L.), when added to a sandy loam soil. Wheat seed germination was inhibited only by B. nigra, B. hirta, and L. sativum, although none of the treatments reduced fresh weight of germinated seedlings. The major volatiles released by chopped plants were determined by solid-phase microextraction sampling and identified by gas chromatography–mass spectrometry (GC-MS). Volatiles included allyl isothiocyanate (allyl-ITC), 3-butenyl isothiocyanate, benzyl isothiocyanate (benzyl-ITC), cis-3-hexen-1-ol, and trans-2-hexenal. These compounds, together with methyl-ITC (methyl-ITC), β-phenylethyl-ITC, benzaldehyde, β-ocimene, and α-farnesene were tested for inhibition of seed germination of several crop and weed species when applied as volatiles. Of these, allyl-ITC and methyl-ITC were the most inhibitory, completely inhibiting the germination of all species at a headspace gas concentration of 1 ppm in airtight glass containers. Selecting mustard green manures that release high levels of allyl-ITC would appear to be optimal for allelopathic activity, and plants that produce high levels of benzyl-ITC also appear promising.