Salt Stress Re-Routes Terpenoid and Flavonoid Metabolism in Peppermint (Mentha × Piperita L.)

Salt stress is a major abiotic factor limiting crop productivity. This study investigated the physiological and biochemical responses of peppermint (Mentha × piperita L.) to increasing NaCl concentrations (25, 50, and 100 mM). Terpenoids and flavonoids were analysed by GC-MS and HPLC-ESI-MS/MS, respectively, alongside Real-Time PCR for key biosynthetic genes (menthol biogenesis/Chalcone isomerase) and chlorophyll fluorescence measurements (OJIP, quantum yield, NPQ and PAM). Total terpenoid production decreased significantly (41% at 25 mM, 75% at 100 mM NaCl). Key compounds like menthone (365-71 g/kg), menthol (35-19 g/kg), and 1,8-cineole (58-19 g/kg) were reduced, although pulegone and menthofuran initially increased at moderate salt levels (25 mM) before declining at 100 mM. Mechanistically, this suppression was due to a strategic metabolic bottleneck: upstream terpenoid biosynthetic genes (DXS, GDPS, LS, and L3OH) were upregulated, while terminal-step genes (like menthone/menthol reductase) were progressively downregulated. This metabolic shift correlated strongly with severe photosynthetic impairment, evidenced by reduced quantum yield (0.82-0.61) and altered chlorophyll quenching (0.45-0.28). This impairment likely reduced the precursor supply for terpenoid synthesis. The plant also exhibited selective flavonoid adjustment. While total phenolic content (12.5-7.8 mg GAE g^-1 FW) and regulatory gene expression were reduced, the biosynthesis of specific flavonoid conjugates increased (+1.5-fold at high NaCl). These findings reveal that salt stress in peppermint triggers sophisticated and selective metabolic reallocation, severely impacting commercially valuable terpenoids and overall photosynthetic efficiency.