Allium longistylum is a relatively understudied species whose phytochemical composition and biological activities remain largely unexplored. In this study, the first true leaf (FTL) and the second true leaf (STL) of A. longistylum were compared with respect to phenolic composition, antioxidant capacity, antimicrobial activity, and quorum-sensing (QS) inhibition. Total phenolic content (TPC) and total flavonoid content (TFC) were determined spectrophotometrically, while antioxidant activity was evaluated using ABTS and DPPH radical scavenging assays. Antimicrobial and anti-QS activities were assessed against Staphylococcus aureus, Acinetobacter baumannii, and Chromobacterium violaceum. STL exhibited significantly higher TPC and TFC than FTL, consistent with its stronger radical scavenging activity. Both extracts showed moderate antimicrobial activity and reduced violacein production in C. violaceum, indicating interference with QS. UPLC-Q-Orbitrap-ESI-MS/MS profiling tentatively identified several phenolic acids and flavonoid derivatives. HPLC analysis confirmed the presence of selected phenolic compounds, although several prominent peaks in the chromatograms remained unidentified. Many of the compounds detected by UPLC-Q-Orbitrap-ESI-MS/MS and HPLC have previously been reported to exhibit antioxidant, antimicrobial, and anti-QS activities; their presence may therefore contribute to the bioactivities observed in both extracts. However, their contribution to the observed effects remains speculative and requires further validation through targeted isolation and bioactivity testing. The results suggest that A. longistylum is a promising source of phenolic compounds with antioxidant and antimicrobial properties.

The transition from cotyledons to true leaves involves substantial metabolic changes. However, the stage-specific bioactive properties of native ornamental plants have not been thoroughly investigated. This study compared total phenolic and flavonoid contents together with preliminary biological activities of the cotyledon and true leaf stages in Lespedeza cyrtobotrya, Amorpha fruticosa, Flueggea suffruticosa, Ulmus parvifolia, Tripora divaricata, and Paulownia coreana seedlings to optimize their potential for horticultural applications. Total phenolic and flavonoid contents (TPC and TFC) of ethanol extracts were evaluated, and radical scavenging capacities were assessed using ABTS and DPPH assays. Preliminary antibacterial activities were also investigated against Staphylococcus aureus, Acinetobacter baumannii, while potential quorum sensing (QS) inhibition was tested against Chromobacterium violaceum. The transition to the true leaf stage was consistently associated with increased TPC and TFC levels, suggesting increased antioxidant potential in L. cyrtobotrya, A. fruticosa, F. suffruticosa, and U. parvifolia. In contrast, cotyledons of T. divaricata and P. coreana exhibited preliminary antibacterial and antioxidant potential that surpassed those of the true leaves. As the true leaves matured, pigment-suppression activity was observed in selected extracts, suggesting that the true leaf stage may possess potential QS inhibitory activity. These stage-dependent bioactivities provide essential baseline data for optimizing harvest timing and highlight the potential of specific seedling stages for applications in urban horticulture.