Exogenous Kaempferol Application Enhances Salt Tolerance Over Quercetin in Apocynum pictum Seedlings: An Integrated Multi‐Level Analysis

ABSTRACT Salinity stress limits plant growth by disrupting water uptake and inducing ion toxicity. Natural bioactive molecules, including flavonols, have gained attention for their potential to enhance abiotic stress tolerance. This study aimed to compare the effects of exogenous kaempferol (200 μmol L −1 ) and quercetin (25 μmol L −1 ) on growth and stress mitigation in Apocynum pictum seedlings under salt stress. Although both flavonoids alleviate salt stress in various species, their regulatory roles in A. pictum remain unclear. Both compounds significantly alleviated salt‐induced growth inhibition, with kaempferol showing stronger effects on plant height, leaf number, leaf length, and root length. Ultrastructural analysis revealed distinct protective strategies: kaempferol enhanced cellular structural stability by preserving root cell integrity and promoting cell wall thickening, whereas quercetin maintained mitochondrial integrity and abundance. Kaempferol treatment also resulted in higher superoxide dismutase (SOD) activity, soluble protein accumulation, and photosynthetic pigment levels compared to quercetin. Transcriptomic profiling uncovered distinct regulatory mechanisms: quercetin primarily activated the ABC transporter pathway for ion homeostasis, whereas kaempferol markedly stimulated the flavonoid biosynthesis pathway by upregulating key structural genes (CHS, CHI, F3H, F3'H, LAR), suggesting enhanced flavonoid accumulation. Collectively, kaempferol and quercetin employ different yet effective strategies to confer salt tolerance in A. pictum . This study highlights the potential of exogenous kaempferol as a biosafe strategy to enhance plant stress tolerance in agricultural systems.