Abstract
Chrysanthemoides monilifera subsp. monilifera (boneseed), a weed of national significance in Australia, threatens indigenous species and crop production through allelopathy. We aimed to identify phenolic compounds produced by boneseed and to assess their phytotoxicity on native species. Phenolic compounds in water and methanol extracts, and in decomposed litter-mediated soil leachate were identified using HPLC, and phytotoxicity of identified phenolics was assessed (repeatedly) through a standard germination bioassay on native Isotoma axillaris. The impact of boneseed litter on native Xerochrysum bracteatum was evaluated using field soil in a greenhouse. Collectively, we found the highest quantity of phenolic compounds in boneseed litter followed by leaf, root and stem. Quantity varied with extraction media. The rank of phenolics concentration in boneseed was in the order of ferulic acid > phloridzin > catechin > p-coumaric acid and they inhibited germination of I. axillaris with the rank of ferulic acid > catechin > phloridzin > p-coumaric acid. Synergistic effects were more severe compared to individual phenolics. The litter-mediated soil leachate (collected after15 days) exhibited strong phytotoxicity to I. axillaris despite the level of phenolic compounds in the decomposed leachate being decreased significantly compared with their initial level. This suggests the presence of other unidentified allelochemicals that individually or synergistically contributed to the phytotoxicity. Further, the dose response phytotoxic impacts exhibited by the boneseed litter-mediated soil to native X. bracteatum in a more naturalistic greenhouse experiment might ensure the potential allelopathy of other chemical compounds in the boneseed invasion. The reduction of leaf relative water content and chlorophyll level in X. bracteatum suggest possible mechanisms underpinning plant growth inhibition caused by boneseed litter allelopathy. The presence of a substantial quantity of free proline in the target species also suggests that the plant was in a stressed condition due to litter allelopathy. These findings are important for better understanding the invasive potential of boneseed and in devising control strategies.
Highlights
The invasion of exotic species is one of the most important challenges experienced by native ecosystems
In methanol extracts we found the highest concentrations of catechin [1.01μg/ mg (38 μg/mL)] and p-coumaric acid [0.45 μg/mg (17 μg/mL)] in leaf, the highest concentrations of phloridzin (3.36 μg/mg (126 μg/mL)] in root, and the highest concentrations of ferulic acid [5.83 μg/mg (219 μg/mL)] in litter (Fig 1)
The relative phenolic concentration in boneseed was ranked as ferulic acid > phloridzin > catechin > p-coumaric acid
Summary
The invasion of exotic species is one of the most important challenges experienced by native ecosystems. It is generally accepted that successful invasion by exotic species is determined by the characteristics of the invaded habitat [1] and biological attributes of the invader, including allelopathy [2, 3]. Phenolic compounds are a class of the most important. They are ubiquitous in all plant organs that have been the subject of chemical, biological, agricultural, and medical studies in last few decades [9,10,11,12]. The identification and quantification of specific allelochemicals is imperative in characterising the basis of allelopathy in plants and may serve as a source of novel, natural herbicides, pesticides and medicines
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