Plant Flavonoids: Biosynthesis, Regulation, and Roles in Biotic and Abiotic Stresses

, ROS, and JA/SA/ABA signalling, while late tailoring (hydroxylation, glycosylation, O-methylation, and acylation) modulates solubility, stability, localisation, and bioactivity. Under UV, drought, high temperature, salt stress, freezing, nutrient imbalance, and metal toxicity, distinct chemotypes contribute to photoprotection and to biotic defence as phytoalexins and anti-herbivore deterrents. We propose that flavonoids act not only as redox-active, membrane-protective metabolites but also as signals that reset transcriptional and hormonal programmes; pathogens and insects can blunt this interface via detoxification, efflux, and enzymatic breakdown. Key quantitative gaps include in vivo antioxidant weight relative to enzyme cycles, branch-specific flux partitioning, and links between tissue patterning and protection. Priorities are outlined for deploying stress-responsive flavonoid repertoires to boost crop resilience under combined stresses without yield penalties.