Near-null magnetic fields remodel flavonoid profiles and essential oil content in Ocimum basilicum and are associated with altered photochemical performance and ROS homeostasis.

Exposure to hypomagnetic fields (hMF, < 40 nT) represents one of the least-characterised environmental stressors for plants. Using Ocimum basilicum L. as a compact, metabolite-rich model for bioregenerative life-support systems, we compared plants grown under hMF versus geomagnetic field (GMF, ∼44 μT) conditions. hMF significantly enhanced essential oil accumulation, with eugenol and methyl eugenol increasing 1.3- and 1.8-fold, respectively. Despite the increased accumulation of these metabolites, transcripts of key phenylpropanoid biosynthetic genes, including the two methyltransferases (ObCOMT and ObEOMT), were significantly downregulated. Total phenolic content decreased while flavonoid content rose, accompanied by a clear shift toward more polar glucuronide- and rhamnoside-conjugated flavonols. Steady-state H2O2 production declined, together with differential regulation of genes involved in ROS detoxification (downregulation of ObCSD and ObCAT; upregulation of ObFSD1). Photosynthetic performance was affected under hMF: chlorophyll b increased, the chlorophyll a/b ratio decreased, and OJIP fluorescence parameters were consistent with partial PSII reaction-centre inactivation, reduced electron transport efficiency, and markedly lower non-photochemical quenching capacity. In parallel, several stress-responsive transcription factors (ObICE1, ObCBF4, ObDREB5) and protective genes were upregulated. These results indicate that hMF exposure induces coordinated changes in secondary metabolism, photosynthetic performance, and oxidative homeostasis in basil, providing a framework for understanding plant responses to near-null magnetic environments relevant to space agriculture.