## 3 Light Effect on Growth and Development of _C. sativa_
### Light Spectrum in Cannabis Growth, Development, and Metabolism
Rooting for 15 days under each lighting treatment, maintaining a PPFD of 200 umol m\({}^{-2}\) s\({}^{-1}\) with a 16 h photoperiod, yielded remarkable results: while spectrum treatments did not affect rooting percentages, root index values soared under B+UVA compared to F (Table 1). Moreover, root dry weights in B, B+UVA, B50, and F treatments surpassed those in the W treatment [55]. At the plug stage's end, no significant effects on chlorophyll content index were noted. However, cuttings under B exhibited thicker stems than those under BR and W, and F showed the lowest percentage of new aboveground growth. Blue (400-500 nm) and red (600-700 nm) wavelengths in horticultural LEDs deliver the highest photosynthetic quantum yield without triggering abnormal responses [55](Figure 1). In _C. sativa_, secondary metabolites accumulate in the trichomes of female flowers and sugar leaves, synthesized through metabolic pathways: the PKS pathway produces Olivetolic acid (OLA) and the MVA/MEV and MEP pathways produce dimethylallyl diphosphate (DMAPP) and geranyl diphosphate (GPP). GPP, combined with OLA, forms cannabinoid acid (CBGA), the precursor to all cannabinoids, linking monoterpene and cannabinoid biosynthesis (Figure 1). These findings highlight the transformative potential of optimizing light spectra and metabolic pathways to enhance cannabis cultivation, heralding a new era of efficiency and potency [75; 76]. Light, temperature, and various stresses play pivotal roles in directing the allocation of isoprenoid precursors between the MEP and MEV/MVA pathways by modulating enzyme expression. Enhanced light intensity significantly represses the MEV/MVA pathway while stimulating the MEP pathway [77]. Terpene synthases (TPSs) are critical in converting these precursors into specific mono- and sesquiterpenenes, with their activity being profoundly influenced by light. Notable TPSs include TPS-a (caryophyllene and humulene), TPS g (linalool and nerolidol), and TPS-b (\(\beta\)-myrrene, limonene, \(\alpha\)-pinene) (Table 1) [75; 76]. Despite significant advances in understanding TPS genetics, predicting terpene profiles remains challenging due to external factors such as light, temperature, nutrition, and stress. The developmental stage of the plant also significantly impacts terpene and cannabinoid distribution, peaking in reproductive parts during flower maturity [75; 77; 57; 58; 59; 60; 61; 62; 63; 64; 65; 66; 67]. High R/FR light spectra activate phytochrome (Pfr), enhancing MEP pathway enzyme expression and increasing monoterpene and CBDA concentrations in cannabis buds. Different light spectra markedly influence CBDA and terpene concentrations, with variations reflecting interactions between plant morphology and chosen light spectra (Figure 1). Techniques like strategic pruning and defoliation can greatly enhance light-use efficiency and optimize flower exposure [57]. A groundbreaking study on a CBD-dominant _C. sativa_ genotype, FINOLA, examined the effects of long (far-red) and short-wavelength (blue, UV-A, UV-B) radiation on morphology, infloresence yield, and secondary metabolites. This study compared LOW R/FR (ratio of 1) and HIGH R/FR (ratio of 11) treatments and various blue, UV-A, and UV-B radiation levels. The LOW R/FR treatment increased plant height but reduced infloresence yield, while the HIGH R/FR treatment significantly boosted CBD, THCVA (tetrahydrocannabivarin acid), CBGA (Cannabigerolic acid), and terpene concentrations (Figure 1) [50]. Short-wavelength radiation treatments did not affect yield or morphology but increased THCVA concentrations, with UV-B specifically raising myrrene levels. These findings underscore the transformative potential of manipulating the R/FR ratio and incorporating short-wavelength radiation to fine-tune cannabis morphology, yield, and secondary metabolite profiles. Mastery of light spectrum manipulation stands as a powerful tool for optimizing cannabis cultivation, paving the way for unprecedented advancements in yield quality and production efficiency [50].