## 1 Introduction  
Light is one of the indispensable environmental factors affecting plants' growth and development. It not only provides energy for photosynthesis, but also regulates multiple responses, including germination, seedling photomorphogenesis, circadian rhythms, phototropism, and flowering time [1, 2]. In general, the light environment is divided into three main components: light photoperiod, light spectra, and light intensity. Specifically, light spectra play vital roles in many aspects, such as photosynthesis, chloroplast development, and plant morphogenesis, whose regulated processes are much more complicated than those caused by light photoperiod or intensity [3, 4]. In higher plants, a series of photoreceptors can accurately perceive ambient light signals and regulate various developmental processes. Generally, photoreceptors in plants can be divided into five main categories: phytochromes (PHYA-E), which sense red/far-red lights (600-750 nm); phototropins (PHOT1 and PHOT2); F-box-containing flavin-binding proteins (e.g., ZEITLUPE, FKF1/LKP2); cryptochromes (CRY1-3), which sense blue/UV-A lights (320-500 nm); and UVR8, which senses UV-B light (280-320 nm) [5; 6; 7; 8; 9]. These photoreceptors play unique and/or redundant roles in regulating a wide variety of developmental processes and metabolic pathways. As some of the most abundant photosynthetic pigments, chlorophyll a absorbs light in the wavelength between 400 nm and 662 nm, and chlorophyll b absorbs light in the wavelength between about 453 nm and 642 nm. Therefore, the red (600-700 nm) and blue (400-500 nm) light spectra are often considered to be the most effectively utilized wavelengths for driving photosynthesis [10]. It is interesting to note that plants grown solely under red light exhibit a lower photosynthetic rate than plants grown under white light, resulting from the fact that red light reduces stomatal density, stomatal conductance, and rubisco protein content [11]. In particular, red light induces substantial impairments in plant growth, such as causing reductions in chlorophyll content and leaf area, and provoking excessive stem elongation, which may eventually lead to the reduction in plant biomass [12; 13; 14]. Blue light is responsible for regulating chloroplast development, stomatal opening, and chlorophyll formation. Blue-light-grown plants contain higher amounts of Chl a/b, cytochrome (Cyt) F, and rubisco than plants grown under white light [15]. Moreover, blue light greatly affects the architecture of shoots, resulting in dense, compact plants, as well as increasing vegetative growth [16]. In addition to physiological and morphological effects, red and blue light have great effects on plants' metabolism, including primary metabolism and secondary metabolism. For example, it has been reported that red light inhibits the translocation of photosynthate from leaves, causing leaves to accumulate far greater starch and soluble sugar contents [17; 18; 19]. Greater anthocyanin content was accumulated in strawberries under red light [20]. In soybeans, red light can also promote the biosynthesis of lignin to improve cell wall firmness [21]. Blue light promoted glucosinolate accumulation in pak choi by increasing the precursor amino acid profiles [22], as well as promoting chlorogenic acid synthesis in strawberries [23]. Pepper plants produced greater amounts of epidermal flavonoids when exposed to enhanced blue light [24]. Thus, the effects of different light spectra on plants are distinctly different. _Ocimum basilicum_ (Basil) belongs to the _Lamiaceae_ family, possessing important medicinal and culinary values, and has been widely grown in many warm and temperate countries [25]. Previous studies have suggested that basil has many potential pharmacological effects, such as antioxidant, antipyretic, antidiabetic, anticancer, and anti-stress effects, among others [26].