## 3 Light Effect on Growth and Development of _C. sativa_
### Effect of Light Intensity in Growth and Metabolism of C. sativa
White light with dual red peaks at 640 and 660 nm significantly outperforms white light with a single red peak at 660 nm, dramatically enhancing inflorescence yield and light use efficiency across all PPFD levels. This enhancement is attributed to superior total plant dry matter production and optimized plant architecture, maximizing photon capture [65]. Critically, the white light fraction and spectrum breadth exert no influence on inflorescence yield, with cannabinoid concentrations remaining consistent, ensuring premium PSM quality. At elevated PPFD, dual red peak light markedly increases terpenoid concentrations; at lower PPFD, photosynthetic efficiency improves, showing no effect at higher PPF (Table 1) [65]. Incorporating 640 nm alongside 660 nm markedly boosts light use efficiency and dry matter production [65]. As PPFD escalates, light energy seldom limits dry matter production, but pigment overexcitation can induce ROS formation, causing photo-oxidative damage toward photoinhibition (Figure 1). A higher white fraction, with a well-balanced red-to-blue ratio and enhanced green fraction, significantly mitigates photoinhibition risk, promoting superior quantum yields at high PPFD by ensuring balanced light absorption across pigments. This approach is pivotal for preventing photoinhibition and maximizing plant productivity [65]. The prevailing belief that increasing light intensity (LI) and ultraviolet radiation (UV) can significantly boost cannabis secondary metabolite levels lacks substantial scientific backing, leaving cultivators uncertain about lighting optimization [80]. A seminal study has now illuminated this area, revealing that cannabis plants subjected to an intense combination of UV and PPFD (via LED) demonstrated remarkable growth, with aboveground biomass metrics increasing by 1.3-1.5 times in the highest PPFD treatments compared to the lowest. The 'Meridian' cultivar's inflorescence dry weight surged by 1.6 times. Moreover, plants in the highest PPFD treatment directed significantly more biomass to infloresces tissues, resulting in a 7% increase in the harvest index (Table 1) [80]. Notably, the study detected no UV treatment effects on aboveground biomass metrics or inflorescence cannabinoid concentrations. However, sugar leaves exposed to UVA + UVB treatment exhibited a 30% increase in THC concentrations, although total THC in these foliar tissues remained unaffected by UV [80]. This study highlights the profound impact of UV and high PPFD on cannabis growth and secondary metabolite production, underscoring the critical need for the meticulous optimization of these protocols. Designing an optimal UV exposure protocol for cannabis cultivation requires the meticulous consideration of several factors: spectrum, intensity, daily duration, and total exposure time relative to the harvest period. The scheduling of UV treatments within the PAR photoperiod is also crucial. Researchers must assess not only inflorescence yield and secondary metabolite composition but also the significant morphological impacts of UV on developing infloresces and related tissues, such as the density and composition of glandular trichomes (Figure 1). _Cannabis_ plants thrive under high canopy light intensities in indoor settings [80]. This study highlighted the economic benefits of maximizing canopy-level PPFD. However, it found no commercially significant advantages from UV radiation exposure [80]. Given the myriad potential UV exposure strategies, further research is essential to determine if UV can become a viable commercial tool in indoor cannabis production and to establish the most effective treatment protocols for commercial application. Daylength-extension (DE) lighting is a vital technique in the cannabis industry, employed to enhance plant size and produce cuttings by extending the vegetative stage and controlling flowering [54]. Despite its importance, growers have encountered persistent issues with incomplete or transitional infloresces in several cannabis cultivars, even under extended photoperiods.