Physiological and Biochemical Responses of Mentha spp. to Light Spectrum and Methyl Jasmonate in a Controlled Plant Factory Environment

Peppermint (Mentha spp.) produces bioactive metabolites under stress. Light spectrum and methyl jasmonate (MeJA) are important factors influencing growth, physiology, and antioxidant defense. In this study, peppermint was cultivated under different light spectra and foliar MeJA concentrations in a controlled environment. Plants exposed to a balanced RGB (1:1:1) spectrum showed the greatest morphological development, with plant height (35.99 cm), canopy width (21.24 cm), and chlorophyll content (29.64 SPAD) significantly higher than those in other treatments. Foliar application of MeJA produced concentration-dependent effects: 2.0 mM increased photosynthetic rate to 6.49 µmol m−2 s−1 compared with 4.52 µmol m−2 s−1 in the control, 2.5 mM resulted in the highest fresh and dry biomass (24.82 g/plant and 2.42 g/plant, respectively), and 1.5 mM yielded the highest total phenolics (20.22 mg GAE/g DW) and antioxidant activity (60.97%). These findings demonstrate that peppermint responses to MeJA are strongly dose dependent and that light quality modulates growth by reducing stress compared with monochromatic spectra. Overall, the results suggest that integrating balanced light spectra with optimized MeJA concentrations can improve both biomass and secondary metabolite accumulation, supporting peppermint production under controlled conditions.