## 3 Phytotoxic Effects of Phenolic Compounds
### Phenolic Acids--Alleochemicals with a Promising Future
containing nine phenolic acids (caffeic acid, gentisic acid, catechol, gallic acid, syringe acid, ellagic acid, resorcinol, \(p\)-coumaric acid, and \(p\)-hydroxy benzoic acid) [165]. [PERSON] and [PERSON] [160] investigated the effect of benzoic acid on mustard (_Brassica juncea_ L.). They observed cell damage and irregularities in cell shape and organelle arrangement. In some treatments, dissolution of the middle lamella was observed, while in others, an intact middle lamella with increased wall deposits was present [160]. In addition, chromosomal aberrations and changes in the shape of the interphase and prophase nuclei were observed in the root tips of _Allium cepa_ after treatment with leaf extracts of _C. procera_[165], depending on the concentration. The most common aberrations were despellization in prophase with the formation of micronuclelei, sticky anaphase with bridges, sticky telophase, C-metaphase, etc. [165]. Ghost cells, cells with membrane damage, and cells with heterochromatic nuclei were also noticed in treated plants [165]. While phenolic compounds act as natural antioxidants in plants, high exogenous concentrations can stress target plants, triggering oxidative damage. This includes the activation of defense mechanisms and heightened antioxidant enzyme activity and malonaldehyde content due to lipid peroxidation in cell membranes [166]. [PERSON] et al. [167] demonstrated that exposing cucumber roots to ferulic and \(p\)-coumaric acids increased the levels of hydrogen peroxide and POX activity. Phenolic acids could also disrupt the growth of target plants by affecting photosynthetic processes. The results of [PERSON] [168] indicate that caffeic acid, coumaric acid, ferulic acid, cinnamic acid, and vanillic acid could significantly reduce photosynthetic activity by decreasing Chl content, while [PERSON] et al. [169] demonstrated that benzoic and cinnamic acids reduced leaf transpiration, stomatal conductance, and intercellular CO\({}_{2}\) concentrations. Data on the effects of phenolic acids on plant hormones are scarce and often contradictory. For example, benzoic acid derivatives stimulate the degradation of auxins [170], while hydroxybenzoic acid promotes the degradation of both auxins and gibberellins [171]. [PERSON] and [PERSON] [172] reported that caffeine and chlorogenic acids significantly increased the activity of indole-3-acetic acid (IAA) oxidase in chickpea roots 10 days after treatment. On the other hand, treatment of wheat seedlings with ferulic acid resulted in increased concentrations of IAA, gibberellins, cytokins, and abscisic acid (ABA), which led to the inhibition of seedling growth [173]. When applied at high concentrations, 3,4-dihydroxybenzoic acid inhibited the growth of tobacco callus and roots, as well as the regeneration of shoots, while at lower concentrations, it stimulated callus formation and rooting processes, acting similarly to auxins [174]. The allebotathic role of phenolic compounds in ecosystems has been recognized since ancient times, with phenolic acids often being the dominant components in most plant extracts, defining their allebotathic potential [175]. Despite the well-documented phytotoxic effects, bioherbicides containing phenolic acids have not been commercialized, unlike those based on EOs or their components [29]. Further in-depth studies on their effects, particularly their mechanisms of action, are crucial for paving the way toward their commercialization. As noted, the complexity of plant extract compositions undoubtedly poses a significant challenge in this regard.