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Background  Melatonin acts as a signaling hormone and entraining agent in many organisms. We studied the spatiotemporal regulation and influence of light (photoperiods, intensities, and spectral qualities) on melatonin concentration in the medicinal herb  Hypericum perforatum  L. Furthermore, melatonin concentrations in the leaves of eight species of the  Hypericum  genus were compared and analyzed using high-performance liquid chromatography.    Results  Melatonin concentration was found to be the highest in its flowers and leaves. The leaves exhibited a rhythmic variation in melatonin concentration of approximately 24 h under both light–dark entrained (Zeitgeber time) and constant light [circadian time (CT)] conditions, with melatonin concentration peaking at approximately CT6 in the middle of the subjective day. Melatonin concentration was influenced significantly by not only photoperiods but also applied light’s wavelength and intensity. It was approximately six times higher under long-day conditions (18-h light:6-h dark) than under short-day photoperiods (10-h light:14-h dark) and was the highest (131 μg/g fresh weight [FW]) under treatment with blue light at an intensity of 45 µmol·m 2 /s of photons. The melatonin concentration of the two examined  Hypericum spp. , namely  H. kouytchense  Lev. and  H. coris  L., were approximately twice that of  H. perforatum  L.    Conclusion  Our findings provide first insights on melatonin-related functions and mechanisms in the circadian system of  H. perforatum  and useful resources for further melatonin-oriented research and possible applications in agriculture and pharmaceutical industries.

Effects of circadian clock and light on melatonin concentration in Hypericum perforatum L. (St. John’s Wort)