## 3 Phytotoxic Effects of Phenolic Compounds
### Flavonoids--A Diverse Group of Compounds with Phytotoxic Activity  
Flavonoids account for more than 10,000 known compounds, many of them with allebotathic potential. They are synthesized via the phenylpropanol or acetate-malonate pathway [176]. All flavonoids share the basic C6-C3-C6 structural skeleton, which consists of two aromatic C6 rings (A and B) connected by a three-carbon unit usually organized as a heterocyclic ring (C) containing an oxygen atom [146]. They are typically categorized into several subgroups based on the carbon of the C ring on which the B ring is attached and the degree of unsaturation and oxidation of the C ring [177]. Flavonoids in which the B ring is attached at position 3 of the C ring are referred to as isoflavones, while those with the B ring linked at position 4 are classified as neoflavonoids. Flavonoids with the B ring attached at position 2 can be further categorized into several subgroups, depending on the specific structural characteristics of the C ring. Flavonones and flavanols are characterized by a single bond between the second and third carbon atoms of the C ring, whereas flavones, flavonols, isoflavones, and anthocyanins feature a double or aromatic bond at this position. Chalcones are characterized by A and B rings linked by an open C ring (three-carbon bridge). In plants, flavonoids occur as aglycones, glycosides, and methylated derivatives [177; 178]. Glycosides can be either O-linked or C-linked. The variant of flavonoid glycoside is based on the number of positions on the flavonoid for glycosylation, the degree of glycosylation, and the number of sugars involved in glycosylation [146]. Flavonoids have various physiological functions in plants, such as regulating seed germination and seedling growth, attracting pollinators, influencing flower and fruit coloration, and defense against abiotic stress. Flavonoid accumulation has been observed in response to abiotic factors such as heat, cold, UV radiation, and nutrient deficiencies. Additionally, flavonoids play protective roles through detoxification, antimicrobial activity, and phytoalexin production [179]. Flavonoids are known as antioxidants due to their characteristic low redox potential. They can therefore reduce strong free radicals such as superoxides, alkyl radicals, and hydroxyl radicals [180]. The position of the B ring, along with the number and placement of hydroxyl groups on the catechol group of the B ring, influence the antioxidant activity of flavonoids [181]. The functional hydroxyl groups of flavonoids can donate electrons via resonance to stabilize free radicals and provide antioxidant protection [182]. Flavonoids are mostly found in the vacuole in glycosylated form [168]. They are excreted from the surface of leaves and other aerial parts of the plants [183] and roots [176],so they can act directly in the rhizosphere to affect other plants. In general, the main phytotoxic effects of flavonoids are inhibition of seed germination and seedling growth, inhibition of auxin transport and energy metabolism, including blocking mitochondrial and chloroplast functions, hyperpolarization of cell membranes, and influencing enzyme activity [180]. Examples of the phytotoxicity of flavonoids are numerous and include a large number of plant species, of which we will mention only a few. One of the good examples of the effects of flavonoids is plants that exhibit both autotoxic and allelopathic effects, such as alfalfa (_Mediago sativa_ L.) [176]. Other crops such as wheat (_Triticum aestivum_ L.), tomato (_Lycopersicon esculentum_ Mill.) [184], barley (_Hordeum sativum distichum_) [180], radish (_Raphanus sativus_ L.), and garden cress (_Lepidium sativum_ L.) [185] were also evaluated. The flavonoids had different negative effects on the seed germination and radical elongation of these plants. Garden cress was more sensitive to the treatments than radish [185].