## 3 Phytotoxic Effects of Phenolic Compounds
### Phenolic Acids--Alleochemicals with a Promising Future  
Phenolic acids are a diverse class of plant phenolics produced by shikimic acid via the phenylpropanol pathway [149]. They are usually produced as by-products during the monolignol pathway through the degradation of cell wall polymers such as lignin [149]. In some cases, they can also be produced by microbes [149]. Phenolic acids are divided into cinnamic and hydroxybenzoic acid derivatives [150]. Cinnamic acid compounds have a simple chemical structure of carboxylic acids, consisting of a single benzene ring and a carboxyl group [150]. The most common naturally found and synthetic cinnamic acid derivatives are \(p\)-coumaric acid, caffeic acid, ferulic acid, chlorogenic acid, sinapic acid, cinnamic acid, quinic acid, dihydrocaffeic acid, dihydrocinnamic acid, 4-fluorocinamic acid, 4-chlorocinnamic acid, 3,4-dimethoxyamic acid, and 4-(3,4-dihydroxyphenyl)-3-butten-2-one. Most synthetic cinnamic acid derivatives are mainly produced through synthetic processes [151] or via the metabolism of specific compounds that plants are exogenously supplied with, such as those containing halogens such as chlorophenylalanines [152]. The group of benzoic acid derivatives includes compounds such as hydroxybenzoic acid, vanilic acid, gallic acid, protocatechuric acid, syringic acid, gentisic acid, benzoic acid, ellagic acid, and salicylic acid [153]. In their comprehensive study, [PERSON] et al. [154] highlighted that the presence and positioning of substituents on the benzene ring play a crucial role in determining the phytotoxic potential of phenolic acids. Studies showed that 4-fluorocinamic acid was the most effective among all cinnamic acid derivatives in terms of the inhibition of seed germination and seedling growth in _Setaria viridis_. Indeed, both halogenated derivatives, 4-fluorocinamic acid and 4-chlorocinnamic acid, showed significantly higher activity than \(p\)-coumaric acid,indicating that the halogen atom significantly increased activity compared to the hydroxyl group. Furthermore, the authors concluded that the presence of hydroxyl groups on the benzene ring could reduce phytotoxic activity and that the double bond was not essential for the growth inhibition activity in _S. viridis_ seedlings. In addition, the introduction of a methoxy group at meta and para positions to replace the hydroxyl group on the benzene ring of caffe acid significantly increased phytotoxic activity. The diverse range of phenolic acids present in plant extracts [155; 156; 157] complicates the investigation of the allebotathic effects of individual compounds. Thus, investigating plant extracts makes it challenging to pinpoint the specific contributions of individual allelochemicals. To overcome this challenge, scientists often employ standardized pure compounds. However, individual compounds typically achieve similar effects only when applied at concentrations much higher than those found in plant extracts [158; 159]. Nonetheless, the overall pattern of allebotathic effects is often consistent, and potential synergistic interactions among these compounds should also be considered. The inhibition of seed germination and seedling growth are the most investigated effects of phenolic acids [154; 156; 157; 160; 161; 162; 163; 164]. These morphological changes are the most visible and easily measurable and also the most useful in agriculture. However, more in-depth studies have shown that exogenously applied phenolic acids also cause physiological changes in plants that damage the ultrastructure of cells and impair their normal function [160; 165]. Among the ultrastructural changes, for example, shrinkage and contraction of epidermal cells, together with the formation of larger grooves, channels, and cyst-like structures, were observed in young leaves of _Cassia sophera_ L. after 15 days of treatment with water leaf extracts of _Calotropis procera_ ([PERSON]) [PERSON].