Modulation of growth, essential oil yield, and cytotoxic properties of Cymbopogon martinii through biostimulant application in the sub-humid western Himalayas

Palmarosa (Cymbopogon martinii), an economically important aromatic grass cultivated for its high-value essential oil (EO), is widely used in the perfumery, cosmetic, flavor, and allied industries. It requires sustainable nutrient management strategies to enhance EO yield and quality while minimizing environmental risks. In this context, seaweed-based biostimulants offer a promising alternative to synthetic fertilizers. This study evaluated the effects of foliar application of Gracilaria edulis seaweed extract (G-sap) at five concentrations (0, 3, 6, 9, and 12% v/v) on palmarosa growth and EO production. In addition to agronomic and EO traits, quality-related functional attributes, including Total Phenolic Content (TPC), Total Flavonoid Content (TFC), and cytotoxic activity against A549 (Lung Adenocarcinoma cells) using the MTT assay, were also assessed. The findings showed that G-sap treatments significantly enhanced palmarosa growth and EO production. The 6.0% (v/v) extract (T3) produced the highest biomass and EO content, increasing by 29.08% and 41.11%, respectively, over the control. This treatment also yielded the maximum geraniol content (78.36–85.84%), the principal constituent of palmarosa EO. Biochemical characterization revealed that P3 (6% G-sap; T3) possessed the maximum TPC (360.34 ± 2.64 mg GAE g− 1) and TFC (58.06 ± 0.77 mg QE g− 1), while P1 (control, 0% G-sap; T1) exhibited the lowest levels. Cytotoxicity assays demonstrated that the P3 oil sample exerted strong, dose-dependent inhibitory effects on A549 cells, reaching nearly 100% at 100–200 µg mL− 1 at 24 h; however, this effect was transient at 48 h, whereas P1 displayed comparatively mild effects (~ 10–20%). The study indicates that foliar application of 6.0% G-sap effectively enhances palmarosa growth, biomass, EO yield, and geraniol content. P3 oil sample exhibited superior phenolic and flavonoid levels and, with P2, strong cytotoxicity, indicating functional and therapeutic value. Seaweed-based foliar bio-stimulants thus improve both productivity and bioactivity while supporting ecological and economic sustainability in aromatic crop cultivation.