Pre-transplant ozonated water soaking boosts growth and artemisinin yield in hydroponic Artemisia annua via hormesis

Artemisinin (ART), a key antimalarial compound from Artemisia annua L., remains in limited supply, motivating improved production strategies. This study evaluated a single pre-transplant dip in ozonated water (OW) as an abiotic elicitation strategy for hydroponically grown A. annua and identified the concentration that maximized ART yield without reducing biomass. Seedlings were immersed for 40 s in OW at 0, 0.5, 1.0, 1.5, or 2.0 mg L −1 before transplantation into a deep-flow hydroponic system, and responses were evaluated at 2 and 4 weeks after transplanting (WAT). Growth exhibited a hormetic response: at 4 WAT, shoot dry weight increased from 6.25 g plant −1 in the control to 11.13 and 12.74 g plant −1 at 0.5 and 1.0 mg L −1 , respectively, whereas 2.0 mg L −1 reduced net photosynthesis, stomatal conductance, and intercellular CO 2 concentration. Antioxidant responses were concentration- and time-dependent: superoxide dismutase peaked at 1.0 mg L −1 at 2 WAT, catalase at 0.5 mg L −1 at 4 WAT, and 2,2-diphenyl-1-picrylhydrazyl radical-scavenging activity at 0.5 mg L −1 at both times. At 4 WAT, 0.5 mg L −1 increased ART concentration from 5.35 to 9.31 mg g −1 dry weight (DW) and per-plant yield from 33.44 to 103.60 mg plant −1 , corresponding to increases of 74.1% and 209.8%, respectively. The ART-forming-to-competing precursor ratio rose from 1.42 in the control to 6.08 at 0.5 mg L −1 , consistent with a shift toward ART biosynthesis. Thus, a single 40-s dip in 0.5 mg L −1 OW may provide a simple elicitation strategy for ART production in closed hydroponic and vertical-farming systems.