## 4 Conclusions  
We propose a model based on the results as shown in Figure 7. The background WL illumination was still important for photosynthesis to provide sugars for terpene biosynthesis in GTs. We then speculate that the increase in cyclic monoterepenes in the GTs was probably due to the direct activation of processes of terpene biosynthesis. Studies have revealed that light modulated the biosynthesis of terpen in many plant species. For example, a RL receptor, phytochrome, was shown to promote monoterepene production in thyme seedlings [32]. In cannabis plants, the content of a meroterpenoid, cannabinoid, was increased by BL treatment with a short photoperiod (12 h light/12 h dark) [33]. This indicates that photoreceptors or related factors, such as transcription factors, might directly or indirectly facilitate the enzymatic processes of terpene biosynthesis in secretory cells in GTs. Figure 6: Effects of light supplementation on leaf pigment and photosynthesis in two-week light-supplemented Japanese mint leaves. (**A**) Total chlorophyll (\(n=4\)) and anthocyanin contents (\(n=4\)) were quantified using a spectrophotometer. (**B**) Photosynthetic parameters, \(\mathrm{Fv}/\mathrm{Fm}\), and \(\mathrm{PSII}\) quantum yield on leaves were monitored using a PAM chlorophyll fluorometer (\(n=8\)–\(13\)). The data were obtained from independent experiments as biological replications. Error bars indicate standard deviations from the mean. Different alphabets indicate significant differences according to [PERSON]’s HSD test; \(p<0.05\). Recently, GT research is spotlighted because certain secondary metabolites produced by plants have high pharmaceutical value [4; 5]. A lot of chemical compounds for producing medicines still rely on plant-derived starting materials. For example, _Artemisia annua_ is recognized as a precious plant resource of artemisinin, an antimalarial agent, stored in GTs. Since a net synthesis of artemisinin has great difficulty and costly, the cultivation of _Artemisia_ plants is necessary to obtain the compound. In addition to the plant, many other medicinal plants store valuable second metabolites, including terpen in GTs. It is known that the contents of the compounds in GTs are dependent on the growing environment [8]. Therefore, a considerable effort is being paid to cultivating medicinal plants in an artificially modified environment to maximize the contents of desired compounds in GTs. Results obtained from Japanese unit as a model of GTs metabolism showed that light, which is one of the critical environmental factors, promotes monoterpene synthesis, and it will be helpful for GT research. Although we first speculated that the BL affects specific terpene biosynthesis, all four monoterpenes (pulegone, mentofuran, menthoe, and menthol) were significantly increased. It suggests that the BL might stimulate the biosynthesis and accumulation of terpene precursors such as DMAPP or IPP in GTs (Figures 1 and 7). These compounds are the necessary starting materials for all higher plant monoter biosynthesis, including _Artemisia_ plants, as mentioned above. Thus, the information on supplement lighting obtained from the Japanese unit study will be helpful to boost the contents of valuable terpene compounds in medicinal plants for industrial cultivating conditions. As no genomic information on the Japanese unit used in the study was provided, genetic verification was not conducted in this study. Therefore, in future research, further details on physiological regulation by the light perception in the Japanese unit need clarification. Figure 7: Schematic of glandular trichomes on Japanese unit leaves. Background WL provides energy for photosynthesis and provides a sugar source, which is necessary for terpene biosynthesis in glandular trichomes. LED light supplementation may promote the terpene biosynthetic pathway in glandular trichome cells through photoreceptor activation. S: secretory cells; ST: stalk cells; SC: storage cavity. **Author Contributions:** Conceptualization, K.Y. and M.M.