## 3 Light Effect on Growth and Development of _C. sativa_
### Effect of Photoperiod Growth and Metabolism of C. sativa
As cannabis has been cultivated across diverse latitudes, variations in photoperiod response have been naturally selected, influencing flowering timing and overall performance [59; 78] (Table 1). In summary, this trailblazing study elucidated that cannabis varieties exhibit unique responses to photoperiod length changes, rendering the standard 12L/12D photoperiod suboptimal for many strains. Remarkably, cannabinoid yields (grams per plant) can more than double by increasing the photoperiod from 12 to 14 h during flowering, as evidenced by the Cannatonic line, due to significant gains in both flower biomass and cannabinoid concentration [59; 78] (Table 1). The 14L/10D photoperiod not only achieved exceptional yield benefits but also did so without incurring additional electricity costs compared to the 12L/12D standard. This makes the 14L/10D photoperiod the superior, all-encompassing treatment, ideal for mixed cultivation and untested varieties [59; 78] (Table 1). To enhance growth, _C. sativa_ is frequently subjected to extended photoperiods or continuous light, which can induce photooxidative damage and necessitate substantial adjustments in photosynthetic processes [56]. Chloroplasts, the primary light-sensing organelles, dynamically alter their ultrastructure in response to varying photoperiods. Plants are categorized by their photoperiod responses: long-day, short-day, and day-neutral. Beyond flowering, photoperiod profoundly affects daily photosynthesis, growth, and starch metabolism. The quality and intensity of light are paramount for the activation of PSI and PSII, essential components of photosynthesis [56] (Table 1). Continuous light exposure can impair photosynthesis due to starch accumulation in _C. sativa_ leaves, leading to a deficit in electron acceptors and the production of reactive oxygen (Table 1) species (ROS), resulting in significant oxidative damage, known as photooxidative stress [56]. Compounds such as carotenoids and phenyl lipids in the thylakoid membranes serve as the first line of defense against ROS (Figure 1) [56]. A pioneering study employed chlorophyll fluorescence, lipid peroxidation (TBARS), and FT-IR spectroscopy to examine two cannabis cultivars under white (W) and purple (P) light with varying photoperiods (16/8, 20/4, and 24/0). The 16/8 photoperiod, irrespective of light type, was the most beneficial, yielding the highest physiological efficiency and lowest TBARS content, indicative of minimal thylakoid membrane damage (Table 1) [68]. _C. sativa_ adapts using highly efficient strategies based on the light type and photoperiod duration. White light facilitated greater dissipation of excess light, reducing the burden on PSI [68]. The P20/4 treatment exhibited effective energy dissipation and cyclic electron transport around PSI, indicative of robust repair mechanisms. Even under continuous light (24/0), P (photoperiod) 24/0 maintained efficient electron transport, positively influencing photosynthetic reactions despite noticeable thylakoid membrane damage [68] (Table 1). These findings underscore the critical importance of optimizing light conditions and photoperiods to enhance photosynthetic efficiency and mitigate oxidative damage in _C. sativa_ cultivation. This paradigm shift in light management has the potential to propel _C. sativa_ growth and productivity to unprecedented levels, heralding a new era of agricultural excellence [56]. A meta-analytic approach to determine when growers should switch photoperiods to optimize _C. sativa_ floral biomass and cannabinoid content. Floral biomass was maximized when the long day length photoperiod was minimized (i.e., 14 days), while THC and CBD potency were maximized under long day length photoperiod for -42 and 49-50 days, respectively (Table 1). This work reveals a yield trade-off in _C. sativa_ between cannabinoid concentration and floral biomass where more time spent under long-day lighting maximizes cannabinoid content and less time spent under long-day lighting maximizes floral biomass (Figure 1).