## 3 Phytotoxic Effects of Phenolic Compounds
### Phytotoxic Effects of Chalcones and Their Potential as Future Bioherbicides
thaliana_ shoots induced by _trans_-chalcone is linked to early plasma membrane depolarization and significant disruptions in chloroplast structure and function. These changes are accompanied by ROS accumulation, pigment degradation, impaired photosynthesis, and programmed cell death [204]. Transcriptomic analysis revealed that many of the affected genes were transcription factors and genes associated with oxidative stress, xenobiotic detoxification, ABA and auxin biosynthesis, and primary metabolic processes [205]. Kyoto encyclopedia of genes and genomes (KEGG) pathway analysis of transcriptome results indicated that pathways associated with stress (e.g., specialized metabolism, glutathione metabolism, and hormone signal transduction) were mostly affected, as well as amino acid metabolism and sulfur and phenylpropanol metabolism pathways. Proteasome function, endoplasmic reticulum protein processing, and peroxisome function genes were also significantly affected [205]. Research shows that many chalcones exhibit potent herbicidal effects with minimal toxicity to crops. Their efficacy depends on the substituents on the A and B rings, the applied concentrations, and the specificity of the target species or tissues. [PERSON] et al. [206] synthesized 45 different chalcone derivates, among which chalcone 14f, with a thiophenyl group on ring A and a phenoxyacetic acid group on ring B, was the most potent compound against seed germination and growth in Chinese _amarantth (Amaranthus tricolor_ L.) and barnyardgrass (_Echinochloa crus-galli_ L.). At the highest concentration (1600 \(\upmu\)M), this chalcone completely inhibited the seed germination of Chinese amaranth and highly reduced seed germination (42%) and shoot (51%) and root length (79%) in barnyardgrass. [PERSON] et al. [207] reported several naturally occurring chalcone derivates, isolated from the extract of _Ambrosia salsola_, with moderate phytotoxic effects. Salsolol A and B showed phytotoxicity against duckweed (_Lemma paucostata_ Hegelm. ), while the other two structurally similar dihydrohalcones had no activity. The chalcone derivatives, \(2^{\prime}\),\(3^{\prime}\),\(4^{\prime}\),\(3A\)-pentahydroxychalcone and \(2^{\prime}3^{\prime}A^{\prime}\)-trihydroxychalcone are potent inhibitors of phosphoenolpyruvate carboxylase (PEPC), a critical enzyme for carbon fixation and biomass production in the C\({}_{4}\) photosynthetic pathway of many of the world's most problematic weeds, for example, pigweed (_Amarantthus_ species), foxtail (_Setaria_ species), johnsongrass (_Sorghum halepense_), dallisgrass (_Paspalum dilatatum_), crabgrass (_Digitaria_ species), barnyardgrass (_Echinochla_ species), and velvetleaf (_Abutition theophrast_) [208]. At the whole-plant level, both chalcone derivatives, \(2^{\prime}\),\(3^{\prime}\),\(4^{\prime}\),\(3\),\(4\)-pentahydroxychalcone and \(2^{\prime}\),\(3^{\prime}\),\(A^{\prime}\)-trihydroxychalcone, significantly inhibited the growth of the C4 weed _A. retroflexus_ without affecting oilseed rape, a C3 plant [208]. Chalcones and their synthetic derivatives show considerable potential for developing bioherbicidal compounds as alternatives to commercial herbicides, particularly due to their minimal toxicity to crops. Their effectiveness in controlling weed species in small-scale households and organic farming could be significant depending on their structure, dosage, and method of application. However, further investigations of their mechanisms of action, persistence in soil, and impact on non-target plants, the soil microbiome, insects, and other animals are vital for optimizing their efficacy and integration into sustainable weed control practices.