Allelopathy is a phenomenon in which plants inhibit the growth of neighboring plants by releasing allelochemicals. The aim of our study was to investigate the allelopathic effect of invasive knotweeds, Japanese knotweed (Fallopia japonica) and Bohemian knotweed (F. ×bohemica) on morphological, biochemical, molecular, and ultrastructural characteristics of roots of the tested species garden radish (Raphanus sativus). Extracts from both knotweeds differed in the content of resveratrol. Methanol extracts had a stronger allelopathic effect on radish than individual allelochemicals. In addition, aqueous extracts inhibited seed germination and root growth of 3-day-old radish seedlings. In particular, the content of total glutathione, ascorbate, and proline increased in the roots, indicating signs of oxidative stress. The expression of some genes related to ascorbate-glutathione cycle and programmed cell death were also altered. The extracts also increased the activities of caspase- and metacaspase-like proteases. The cell organization in root tissues was altered. Many cells from the outermost layer of the root cap were destroyed. The most prominent changes at the ultrastructural level were found in the root cap cells: plasmalemma was detached from the cell wall, mitochondria were damaged, endoplasmic reticulum changed and spherical electron dense inclusions appeared in the cytoplasm. The meristem cells were also altered. In long-term exposure, the 10 % extract of Bohemian knotweed showed the strongest inhibitory potential, causing the destruction of almost all plants. The primary root growth of the treated radish was strongly inhibited in the first half, after 14 days the root of the surviving plants reached the control level, oxidative stress related parameters did not increase. Knotweed allelopathy showed an inhibitory effect mainly on the growth of radish seedlings, which induce oxidative stress in the roots, leading to programmed cell death.
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