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Japanese knotweed (Fallopia japonica) (Houtt.) Ronse Decr. and Bohemian knotweed (F. ×bohemica) (Chrtek and Chrtková) J. P. Bailey are invasive plants in Slovenia. We studied the effects of aqueous extracts [0.5 %, 1 %, 2 %, 5 % and 10 % (w/v)] from rhizomes of F. japonica and F. ×bohemica on the seeds of radish (Raphanus sativus) and examined the morphological and biochemical changes during germination and early growth of seedlings. Germination and early growth of radish were monitored 3, 5 and 7 days after treatment. Extracts of these two knotweed taxa delayed seed germination and strongly reduced the length of the primary root, but had less effect on shoot growth. These extracts triggered stress-induced morphogenic responses in the treated radish seedlings, stimulating the formation of lateral roots at low concentrations and causing inhibition at high concentrations. The extract concentration and not the knotweed taxon influenced the biochemical markers of oxidative stress in the radish. Total antioxidative capacity was increased in treated radish seedlings. The extract of F. japonica had a greater impact on the radish morphology than that of F. ×bohemica, with similar influences on the biochemical parameters. High pressure liquid chromatography identified emodin, resveratrol, catechin and epicatechin in the rhizomes of both knotweed species. Their contents were species dependent.

Allelopathy represents a valuable biochemical strategy in plant-plant interactions among different plants, e.g. among crops and weeds. It is an important strategy in the colonization of many invasive alien plants. Allelopathic plants affect the growth of other plants in the vicinity through the release of secondary metabolites (allelochemicals) into the soil. In particular, many allelochemicals suppress the root growth of target plants, but little is known about the mechanism involved in root growth inhibition. In this review, we will highlight the mechanism of root suppression involving: (i) Alterations in auxin homeostasis affecting polar auxin transport and root gravitropism, (ii) Biochemical and physiological processes in inhibited roots associated with oxidative stress due to direct production and accumulation of reactive oxygen species or suppression of antioxidative response and (iii) The ultrastructural modifications in root tip exposed to allelochemicals that drastically suppress the cell division and eventually lead to shorter roots of target plants.

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Allelopathic plants release secondary compounds into the soil that then suppress the growth of nearby plants. Allelopathy has been shown for the invasive Japanese knotweed (Fallopia japonica) and Bohemian knotweed (F. × bohemica). The aggressive and dominant invaders represent a serious threat to the local plant communities outside their native range. Here, we analysed the phenols in the knotweed rhizomes using nuclear magnetic resonance. We also evaluated the allelopathic potential of methanol extracts of F. japonica and F. × bohemica rhizomes and compared these with the effects of the individual knotweed phenols resveratrol, epicatechin and emodin, and their mixture. Rhizomes of both knotweeds contained similar amounts of epicatechin and emodin, with 24% higher resveratrol in F. × bohemica. Only the F. × bohemica methanol extract inhibited radish (Raphanus sativus) seed germination. After 3 days of treatments with 10% (w/v) extracts of both knotweeds, radish seedlings showed up to 70% shorter roots. In contrast, root growth of seedlings treated with the individual phenols resveratrol, epicatechin and emodin, and their mixture, was inhibited by up to 30%, similar to the 1% knotweed extracts. Biochemical parameters of oxidative stress also increased in the roots of treated seedlings, with high levels of malondialdehyde in particular indicating lipid peroxidation. Total antioxidative capacity was also increased in seedlings exposed to 0.6 mg/mL resveratrol and emodin. This study shows higher allelopathic potential of the knotweed methanol extracts compared to the individual phenols and their mixture.

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