Several volatile allelochemicals were identified and characterized from fresh leaf tissues of the invasive croftonweed. A simple bioassay was used to demonstrate the release of volatile allelochemicals from leaf tissues. The bioassays revealed that foliar volatile components of croftonweed exhibited significant effects on the seedling growth of upland rice. Peroxidase (POD) activity, superoxide dismutase (SOD) activity, and root oxidizability rose as the concentration of volatiles increased. Activity for both POD and SOD significantly increased with exposure to 15 g and 20 g of croftonweed leaf tissue for 5 d. Root activity was significant at 10 g compared to the control. The volatile components also stimulated the development of the aerenchyma tissue and inhibited lateral root formation. Leaf volatiles of croftonweed were identified by gas chromatography coupled with mass spectrometry (GC–MS). Some of the compounds identified included α-phellandrene, camphene, ρ-cymene, 2-carene, α-pinene, limonene, and (z)-3-hexen-1-ol. Bioassays showed that four of these compounds could account for the observed phytotoxicity imparted by total leaf volatiles. Limonene, 2-carene, α-pinene and camphene had no phytotoxic effect on shoot elongation. Phellandrene did cause inhibition in shoot growth at all concentrations. Both (z)-3-hexene-1-ol and ρ-cymene inhibited both shoot elongation and root elongation, but the effects of the two compounds on root length were more significant than on the shoot length.

Nomenclature: Croftonweed, Ageratina adenophora (Spreng.) King and H. E. Robins; upland rice, Oryza sativa L.