Abstract Low-irradiance nocturnal light is increasingly recognized as a biologically relevant environmental signal; however, the photobiological role of natural moonlight in regulating plant metabolism remains insufficiently characterized. This study investigates the influence of lunar photoperiodicity, with particular emphasis on spectral frequency and intensity during the full-moon phase, on secondary metabolite biosynthesis in selected medicinal plants (Ocimum sanctum, Withania somnifera, and Aloe barbadensis). Plants were exposed to natural full-moon illumination, new-moon dark-night conditions, or frequency-matched low-flux synthetic moonlight under controlled field conditions across multiple lunar cycles, allowing spectral effects to be distinguished from irradiance-dependent responses. Moonlight spectral properties were quantified using spectroradiometric analysis, while major phytochemical classes—including phenolics, flavonoids, alkaloids, and terpenoids—were quantified using validated colorimetric assays and high-performance liquid chromatography (HPLC). Exposure to full-moon light resulted in a statistically significant enhancement of phytochemical accumulation compared with dark-night controls (p ≤ 0.05). Total phenolic and flavonoid contents increased by up to 18% and 12%, respectively, while alkaloid and terpenoid marker compounds also showed consistent elevation across all species examined. Plants exposed to frequency-matched synthetic moonlight exhibited intermediate responses, supporting a wavelength-dependent rather than intensity-driven regulatory mechanism. Targeted gene expression analysis further revealed upregulation of key biosynthetic and regulatory genes associated with phenylpropanoid and terpenoid pathways, indicating photoreceptor-mediated metabolic modulation under nocturnal light exposure. Collectively, these findings provide frequency-resolved experimental evidence that natural lunar light functions as an effective nocturnal photobiological signal capable of modulating plant secondary metabolism. The study highlights applications in plant chronobiology, chrono-agronomy, and optimized medicinal plant cultivation strategies.
Nageswara Rao Dorepalli (Sat,) studied this question.