Allelopathic Activity of <i>Annona reticulata</i> L. Leaf Extracts and Identification of Three Allelopathic Compounds for the Development of Natural Herbicides

Using plant-based allelopathic compounds might be a potent substitute to help mitigate the effects of synthetic herbicides. <i>Annona reticulata</i> L. is often planted for its fruit in residential gardens. This plant is well-documented for its diverse ethnomedicinal uses. However, there is no information in the literature on the allelopathic potential of <i>A. reticulata</i> leaves. Therefore, the allelopathic potential and relevant allelopathic compounds of <i>A. reticulata</i> leaves were investigated in this study. The bioassays were carried out using a completely randomized experimental layout (CRD), and the resulting data were analyzed using one-way ANOVA at <i>p</i> ≤ 0.05. Aqueous methanol extracts of <i>A. reticulata</i> leaves significantly inhibited the growth of three dicots and three monocots (<i>Lepidium sativum</i> L., <i>Medicago sativa</i> L., <i>Lactuca sativa</i> L., <i>Echinochloa crus-galli</i> (L.) P. Beauv., <i>Lolium multiflorum</i> Lam., and <i>Phleum pratense</i> L., respectively). The level of growth inhibition was proportional to the <i>A. reticulata</i> extract concentration. Three compounds were purified through different chromatographic steps, and their structures were determined using spectroscopy and identified as loliolide, 5-hydroxy-3,4-dimethyl-5-pentylfuran-2(5H)-one, and 3,4-dihydroxyphenylethanol. The 5-hydroxy-3,4-dimethyl-5-pentylfuran-2(5H)-one had the greatest effect on suppressing cress root growth, while loliolide had the greatest effect on suppressing timothy shoot growth. The values for 50% seedling growth suppression showed that the compound with the maximum inhibitory activity was loliolide, followed by 5-hydroxy-3,4-dimethyl-5-pentylfuran-2(5H)-one and 3,4-dihydroxyphenylethanol. Therefore, this result suggests that the three compounds might be responsible for the allelopathic effects of <i>A. reticulata</i> leaf extracts, and these compounds have the potential to be used to develop effective bioherbicides.