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 H 2 O 2 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.