Terpene yield of native spearmint ( <i>Mentha spicata</i> ) increases proportionally to photosynthetic photon flux density

Native spearmint ( Mentha spicata L.) is a high-value medicinal crop cultivated for its terpene-rich essential oil, yet limited research exists on its response to photosynthetic photon flux density (PPFD) in controlled environments (CEs). This study evaluated the effects of canopy-level average photosynthetic photon flux density (APPFD) on the biomass, essential oil production, and morphology of M. spicata “Spanish” grown in a nutrient film technique system under sole-source LED lighting. An intensity gradient was employed where plants were exposed to APPFD levels ranging from 176 to 827 µmol·m −2 ·s −1 . Shoot dry mass, leaf dry mass, and total terpene yield increased linearly with the increase of APPFD, while essential oil concentration increased asymptotically. The linear increase in terpene yield was strongly correlated with an increase in leaf biomass rather than terpene concentration. No effects of APPFD on plant height or width were observed, but there was a statistically significant decrease in mainstem and internode length as APPFD increased. Leaves were smaller and thicker under higher APPFD, but total leaf area per plant increased. This study provided novel insight into the growth and terpene production of M. spicata “Spanish” under sole-source LED lighting in CEs and informs the development of lighting strategies aimed at enhancing essential oil production.