Allelopathy and Identification of Allelochemicals in the Leaves of <i>Hakea decurrens</i> subsp. <i>physocarpa</i> W.R. Barker

<i>Hakea decurrens</i> subsp. <i>physocarpa</i> is an invasive species from Australia, with morphological, physiological and ecological features that help it colonize and settle outside of its natural habitats. One of these characteristics is allelopathy, which is an interaction that grants a clear competitive advantage to invasive species that has not been studied in <i>H. decurrens</i> subsp. <i>physocarpa</i>. With the aim of understanding the ecological relationships that take place in habitats invaded by this species, it is especially important to know the allelopathic potential of <i>H. decurrens</i> subsp. <i>physocarpa</i> and the compounds that would be involved in this interaction. To this end, the present study quantified the allelopathic activity of the aqueous extract of leaves gathered on four different occasions in the year, as well as of the compounds present in these extracts. The obtained results show a negative effect of <i>H. decurrens</i> subsp. <i>physocarpa</i> samples collected in March, June, September, and December on the germination and growth of <i>Lactuca sativa</i>. Although the negative effect was observed with all extracts, the extract of leaves gathered in September showing the greatest effect on germination (I₅₀ = 0.08 g/mL), and that of leaves collected in June presented the greatest effect on root size (I₅₀ = 0.05 g/mL). As for the composition of these extracts, nine compounds were identified and quantified through HPLC: arbutin, mesaconic acid, isotachioside, 1-O-vanilloyl-beta-D-glucose, syringic acid-4-beta-D-glucopyranoside, quercetin 3-robinobioside-7-glucoside, quercetin 3-rhamninoside, rutin, and isorhamnetin-3-O-rutinoside. There is a correlation between the quantified parameters and the quantity of these compounds in the extracts, but it is difficult to attribute the allelopathic activity of <i>H. decurrens</i> subsp. <i>physocarpa</i> to a particular compound, since this activity may depend on the combination of these compounds. In conclusion, this work demonstrates that the leaves of the invasive species <i>H. decurrens</i> subsp. <i>physocarpa</i> have allelopathic potential, and their toxicity could be due to the combined action of these compounds, which should be analyzed in future studies.