## 3 Phytotoxic Effects of Phenolic Compounds
### Phytotoxic Effects of Chalcones and Their Potential as Future Bioherbicides  
Chalcones are a class of natural compounds belonging to the flavonoid family characterized by an open-chain structure containing two aromatic rings (A and B) linked by a three-carbon \(\alpha\),\(\beta\)-unsaturated carbonyl system (Figure 4). This unique structural framework imparts chalcones with a high degree of chemical reactivity and multifunctionality, making them key intermediates in the biosynthesis of various flavonoids and isoflavonoids in plants [189]. About 250 chalcone aglycones and O-glycosides have been reported as naturally occurring components in 54 plant families, predominantly in dicotyledonous plants but also in some monocotyledonous plants, pteridophytes, and gymnosperms [190]. Chalcone glycosides are usually found as yellow flower pigments in the genera _Dahilia_, _Coregpis_, and _Cosmos_[191; 192], and in species such as _Paeonia trollioales_ Stapf ex [PERSON], _Dianthus carophyllus_ L, _Aeschmauthus partifolius_ [PERSON], and _Asystasia gangetica_ ([PERSON] [193]. Chalcone-containing plants (Liquorice (_Glycyrrhiza glabra_ [PERSON]), Kava-kava (_Piper methyticum_ [PERSON]), _Bosenbergia rotunda_ ([PERSON], and _Lophira alata_ ([PERSON]. [PERSON]) have been used as traditional remedies for a range of medical issues [194; 195; 196; 197]. Figure 4: The chemical structures of chalcones, dihydrohalcones, and phloem. The phytotoxic potential of chalcones attracted the interest of scientists two decades ago, starting with research on _trans_-chalcone (1,3-diphenyl-2-propen-1-one). [PERSON] et al. [198; 199] demonstrated that _trans_-chalcone intensively inhibits the growth of shoots and roots in 20 annual plant species. Soon after, [PERSON] et al. [200] suggested that one of the key enzymes in the biosynthesis of lignin monomers, 4-coumarate: coenzyme A ligase (4 CL), is a potential target for chalcone-induced inhibition of plant growth. The results of [PERSON] et al. [201] indicated that growth suppression in soybean (_Glycine max_ (L.) Merr.) cells by exogenous chalcone is positively associated with the inhibition of lignin biosynthesis. Negative effects of chalcones on the root growth of _A. thaliana_ seedlings were demonstrated by [PERSON] et al. [202], who reported that _trans_-chalcone at a concentration of 12.5 \(\upmu\)M inhibited root growth by 30% compared to the control, while a concentration of 1200 \(\upmu\)M led to a striking inhibition rate of 94%. Furthermore, a marked reduction in root hair density was also observed, becoming apparent at 25 \(\upmu\)M and pronounced at 50 \(\upmu\)M of _trans_-chalcone. This compound also disrupted starch reserves in amyloplasts, impairing the normal gravitropic response in recipient plants. Apical meristematic cells of _A. thaliana_ roots treated with 35 \(\upmu\)M chalcone (IC\({}_{50}\)) exhibited characteristic markers of programmed cell death, including mitochondrial condensation, organelle disintegration, and chromatin fragmentation. Programmed cell death was further confirmed by acridine orange/ethidium bromide staining, which corroborated the loss of mitochondrial transmembrane potential observed following chalcone exposure [202]. _trans_-Chalcone exhibited phytotoxic effects on various plant species, inhibiting seed germination in _Plantago lancelolate_ L. and _Lactuca sativa_ L. and early root growth in _Amaranthus retroflexus_ L., _Echinochloa crus-galli_ L., and _P. lancelate_[203]. In _A. thaliana_, trans-chalcone impaired both development and adult plant physiology, with its effects varying depending on whether it was applied via watering or spraying [203]. Treatment with _trans_-chalcone also disrupted the aerial part of the plants. For example, _A. thaliana_, _A. retroflexus_, and _Tritcium aestivum_ L. plants exhibited pronounced chlorosis of leaves during the treatment with _trans_-chalcone [202; 203]. The progressive chlorosis of _A.