## 3 Phytotoxic Effects of Phenolic Compounds
### Flavonoids--A Diverse Group of Compounds with Phytotoxic Activity
Detailed studies on the influence of flavonoids on seed germination and growth included the model plant _Arabidopsis thaliana_ ([PERSON] [PERSON], as well as a large number of weed species such as spotted knappeved (_Centaurea maculosa_ Lam. ), diffuse knappeved (_Centaurea diffusa_ Lam. ), Dalmatian toadflax (_Linaria adalmatic_ L.), and kochia (_Kochia scoparia_ L.) [184]. In addition, catechins secreted by the invasive plant _C. maculosa_ inhibit the seed germination and growth of _C. diffusa_ and _A. thaliana_[180]. Not all flavonoids are necessarily inhibitory for the treated plants. Similarly to phenolic acids, it is also known that the application of flavonoids in small amounts can have a stimulatory effect, while higher concentrations have an inhibitory effect, a phenomenon known as hormesis [185]. In the case of garden cress, flavone and 3-OCH\({}_{3}\)-flavone in particular inhibited radical elongation, while quercetin, \(\beta\)-naphthoflavone, catechin, catechol, and the flavanones naringin and hesperetin promoted radical elongation in the same species [185]. Most flavonoids had an inhibitory effect on root growth, including flavones (7-OCH\({}_{3}\) flavones, chrysin dimethyl ether, and 3\({}^{\prime}\),\(A^{\prime}\)-di-OCH\({}_{3}\) flavones), morin, and 7-OCH\({}_{3}\) flavanones [185]. These tests are useful for comparing phytotoxic effects with the flavonoid structure. Thus, flavones were found to have the strongest inhibitory effect on seed germination and seedling growth because they contain the 4-oxo functional group in their structure, as well as a 2,3-double bond in the C-ring and one or more hydroxyl groups [185]. Flavonols, which have a similar structure to the flavones and contain a hydroxyl group in a C-3 position, also show a strong inhibitory effect. Furthermore, the presence of a 4-carbonyl functional group combined with a C-2-C-3 double bond is crucial for sustaining their effects. It is assumed that the most active compounds are the flavonoids, of which the methylated ones are the most potent. Conversely, biological activity is not affected by glycosylation or by the absence of a 3-OH group in ring C and a 5-OH group in ring A. [PERSON] et al. [185] hypothesize that the catechol orientation in the B-ring could be responsible for the biological effects. One of the most important mechanisms of action is the influence of flavonoids on the regulation of auxin transport and degradation [176]. Depending on their structure, flavonoids can influence auxin degradation through IAA oxidases and peroxididases [176]. In addition, they affect the polar transport of auxin and thus impair the root growth of the treated species. Certain flavonoids have been reported to stimulate the destruction of IAA by IAA oxidase due to specific structural requirements. These compounds can also inhibit or interfere with the mode of action of plant hormones such as auxin [186]. Processes related to auxin transport and energy metabolism in plants are interconnected and a number of physiological changes occur under the action of flavonoids, including disruption of cellular respiration and chloroplast function, hyperpolarization of cell membranes, alteration of the pH of intercellular spaces, impairment of enzyme activity, etc. [176; 187]. Some flavonoids are potent inhibitors of energy metabolism and block the functions of mitochondria and chloroplasts. For example, flavones have been shown to impair ATP formation in plant mitochondria. Flavonoids have an affinity for many enzymes and other proteins in plants, including those required for mitochondrial respiration, where they lead to the inhibition of nicotinamide adenine dinucleotide hydrogen (NADH) oxidase and disruption of the balance of reactive oxygen species [177]. In addition, flavonoids take on a negative charge in the intracellular medium at a neutral pH. At low concentrations, these compounds can promote cell growth, perhaps due to more effective utilization of cellular enzymes, proteins, and electron carriers.