## 4 Discussion
Additionally, apigenin mitigates oxidative damage by activating the antioxidant defense system. [PERSON] et al. also showed that apigenin attenuated oxidative damage by enhancing the antioxidant properties of plant leaves and effectively scavenging ROS generated under salt stress [39]. Similarly, neohesperidin has been shown to enhance antioxidant activity in plants, aiding in the scavenging of reactive oxygen species produced by stress [40]. This enhanced antioxidant capacity is particularly important for plants under salt stress, as it significantly slows oxidative damage caused by reactive oxygen species and further improves salt tolerance. [PERSON] et al. [41] similarly showed that salt-tolerant soybeans contained higher levels of lignans, kaempfer, and butyrate acid than salt-sensitive soybeans, suggesting that they accumulated more flavonoids under salt stress, thereby enhancing antioxidant capacity. [PERSON] et al. [42] also showed that isoquercitrin, kaempfer, and apigenin accumulated with increasing salt concentration in mulberry seeds, suggesting that these flavonoids may be key metabolites in the response to salt stress. In particular, phenylalanine metabolism serves as a crucial starting point for the synthesis of secondary metabolites, which is closely associated with plant stress responsesand plays a key role in flavonoid biosynthesis [43]. Previous studies have demonstrated that NaCl stress significantly increased the levels of L-phenylalanine, cinnamic acid, and \(p\)-coumaric acid in red rice [44]. In the current study, we observed significant up-regulation of L-phenylalanine and succinate metabolites in 'LQ' leaves under salt stress, whereas 'DT' germplasm exhibited a down-regulation trend, suggesting that 'LQ' is more likely to respond to salt stress through phenylalanine metabolism. Moreover, key genes involved in phenylalanine metabolism, including _HPD_, _AOC3_, and _TAT_, were significantly up-regulated in 'LQ', whereas all of these genes were down-regulated in 'DT'. This observation was almost consistent with the changes in metabolite content and was further confirmed through qRT-PCR analysis. qRT-PCR analysis further corroborated this result, with Cluster-52597.4 of the _AOC3_ family also exhibiting an up-regulation trend. Furthermore, phenylalanine deaminase (PAL), the rate-limiting enzyme in the phenylalanine metabolic pathway, plays a crucial role in mitigating various environmental stresses, including pathogen attacks, low temperatures, and heavy metals [45]. The present study demonstrated that the up-regulation of the PAL gene in 'LQ' may have driven the synthesis of cinnamic acid and its downstream secondary metabolites (e.g., flavonoids and lignans), while the PAL gene was more frequently down-regulated in 'DT' germplasm. This significant difference in gene expression patterns further suggested that 'LQ' enhanced the synthesis of secondary metabolites by strengthening the expression of genes involved in phenylalanine metabolism, thereby improving its capacity to adapt to salt stress. Furthermore, succinic acid, as a key organic acid, may have further enhanced the antioxidant capacity of 'LQ' by participating in energy metabolism and the organic acid cycle, thereby helping the plant mitigate the oxidative damage induced by reactive oxygen species generated under salt stress [46]. Similarly, LBP 1402 exhibited an earlier performance than LBP N7 in accumulating metabolites such as phenylalanine and L-tyrosine and activating related genes, demonstrating a more efficient antioxidant response capacity [47]. Additionally, [PERSON] et al. also found that salt stress induced a significant enhancement of glucose-6-phosphate dehydrogenase (G-6-PDH) and PAL activities in cauliflower bean [48]. Isoquinoline alkaloid biosynthesis plays a critical role in stress tolerance, disease resistance, and environmental adaptation in plants by regulating the production of secondary metabolites [49; 50].