Invasive alien species affect native flora and ecosystems by reducing biodiversitiy and causing economic loss. One of the most invasive species in Slovenia is Japanese knotweed (Fallopia japonica), which has an impact on growth and development of native plant species by fast vegetative reproduction and allelopathic interactions. With this study we aimed to determine whether leaf and rhizome extract of the Japanese knotweed have an allelopathic effect on the growth and cell division of roots of onion (Allium cepa) and which of them has greater inhibitory effect. For this purpose, we grew onions for 2 days in 5% aqueous extracts of dried leaves and rhizomes of Japanese knotweed. After the experiment was completed, we counted and measured onion roots and prepared squeezed preparations of root tips, which were examined under a microscope. We found that both extracts caused a significant decrease in the number and length of the roots; they also reduced mitotic index. The differences in effects between extracts were not significant, only on the number of roots had a leaf extract bigger inhibitory effect, while the rhizome extract had a similar effect like water which was used as control.
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