Japanese and Bohemian knotweed are invasive alien plants that are known to inhibit the growth of their neighbors with quick growth, vegetative reproduction, and allelopathy. The aim of our study was to evaluate the effect of their rhizome extracts on morphological and biochemical characteristics of radish seedlings. Biochemical analysis of extracts with the thin layer chromatography revealed similar composition for both knotweeds. The germination rate of radish seeds did not decrease after the exposure to aqueous extracts with concentration from 0,5% to 10%. After 7 days of germination, radish roots were the most inhibited, whereas shoots were not affected. Roots were short and thick due to change of shape and size of cortex cells. In treated plants, the structure of the root cap and the apical meristem were changed. Biochemical characteristics of oxidative stress (enzymatic and non-enzymatic antioxidants, lipid peroxidation) were changed but mainly nonsignificantly. Staining with diaminobenzidine showed the increased synthesis of hydrogen peroxide in roots. Root tip ultrastructure was analyzed with transmission electron microscope. Knotweed extracts had a strong effect on root cap cells: cell membrane detached from the cell wall, irregularly shaped nuclei with condensed heterochromatin, ring-shaped mitochondria and round granular endoplasmic reticulum with dense contents were observed. Some cells were dead, in others signs of programmed cell death appeared. Meristem cells were less damaged than root cap, more vacuoles were presented.
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