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 H₂O₂ 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, indicated 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 H₂O₂. 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.