Reduced Geomagnetic Field Modulates Photosynthetic Electron Flow, Menthol Yield, Flavonoid Photoprotection and ROS Homeostasis in Peppermint (Mentha × piperita L.)

The Earth's Geomagnetic Field (GMF) is rarely considered a major environmental regulator. We investigated the effects of a reduced GMF, or hypomagnetic field (hMF), on the aromatic plant peppermint ( Mentha × piperita L.). We used GC-MS for essential oil analysis and qPCR to assess monoterpene and sesquiterpene gene expression. Photosynthetic efficiency was quantified via OJIP, NPQ, and PAM fluorescence kinetics. We also quantified H 2 O 2 and ROS gene expression ( RBOHD, SOD1, CAT, APX2 ), and photoprotective antioxidant flavonoids (by HPLC-ESI-MS/MS). The hMF significantly reduced menthol content despite the paradoxical upregulation of key menthol biosynthesis genes. Photosynthetic efficiency was impaired, evidenced by lower PSII parameters and reduced photochemical quenching under high light, along with modulated NPQ indicating potential adjustments in regulated photoprotection. However, maximum PSII quantum yield remained unaltered, suggesting no basal stress. Peppermint adjusted ROS homeostasis by increasing ROS generation (upregulation of RBOHD ) and high-efficiency detoxification (upregulation of APX2 and downregulation of CAT ), leading to reduced steady-state H 2 O 2 . This adjustment was associated with a significant decrease in total photoprotective phenolic compounds. In peppermint, the absence of a normal GMF modulated electron transfer chain activity, leading to a metabolic trade-off where resources were reallocated toward maintaining redox balance at the expense of productivity. These findings establish GMF as a critical, fine-tuning factor regulating peppermint’s metabolic homeostasis. • The Geomagnetic Field (GMF) acts as an active environmental regulator, and its reduction (hMF) functions as a systemic stressor in Mentha × piperita. • The hMF modulates photosynthetic electron flow (confirmed by OJIP and NPQ kinetics) and significantly reduces commercially valuable menthol yield despite gene upregulation. • The plant's defence response involves a metabolic trade-off, diverting resources away from secondary metabolites like menthol and photoprotective flavonoids. • This resource reallocation reduces the ability to produce key defence compounds, establishing a link between GMF and plant chemical defence mechanisms. • Data establishes the GMF as a critical, fine-tuning factor regulating metabolic homeostasis in aromatic plants, relevant for applied agricultural research.