## 3 Light Effect on Growth and Development of _C. sativa_
### Effect of Photoperiod Growth and Metabolism of C. sativa  
The Earth's rotation and revolution create profound 24 h cycles of day and night and dramatic annual shifts in temperature and humidity. These cyclical changes, though modest in tropical and subtropical regions where precipitation defines seasons, are monumental in temperate regions where daylight and temperature variations dictate the four seasons. Daylight is an unfaltering beacon, far more predictable than temperature, enabling organisms to anticipate seasonal changes and masterfully orchestrate their biological functions. Photoperiod, the length of the light period over 24 h, is the maestro of plant growth. Trees, for example, halt their growth in autumn as days shorten, foreseeing the harsh winter [10]. Photoperiods and circadian clocks are the bedrock of plant development and adaptation, synchronizing key developmental transitions with optimal conditions, and ensuring ecological fitness and survival [10] (Figure 1). _Cannabis_ plants, often exhibiting short-day photoperiod responses, initiate flowering as daylight wanes. Despite extensive cultivation and breeding creating significant genotypic variations in photoperiod responses, some cultivars, particularly those bred with equatorial genotypes, are day-neutral or \"auto-flowering\", transitioning to flowering based on physiological age rather than photoperiod [60]. A pioneering study examined the photoperiod responses of multiple indoor-grown cannabis cultivars, observing flowering initiation under varying light durations (12 to 15 h). Flowering, marked by the appearance of three pairs of stigmas at the apex of the primary shoot, occurred in all cultivars under photoperiods up to 14 h. This finding demonstrates that certain drug-type cannabis cultivars can achieve strong flowering responses under extended photoperiods, potentially leading to higher yields [60] (Table 1). Future research should investigate the yield and quality effects (e.g., cannabinoid composition) of photoperiods longer than 12 h on indoor-grown cannabis cultivars brought to commercial floral maturity [65]. These astonishing findingsunderscore the pivotal role of photoperiod in cannabis cultivation, offering a blueprint for optimizing growth conditions to achieve unparalleled yields and superior quality. The strategic manipulation of light duration heralds a new era of cultivation mastery, where cannabis plants can reach their fullest potential. A groundbreaking study assessed the impact of nine distinct flowering photoperiod treatments on the biomass yield and cannabinoid concentration of three medicinal cannabis varieties [59] (Table 1). The varieties included \"Cannatonic\", renowned for its high CBD content, and \"Northern Lights\" and \"Hindu Kush\", both celebrated for their high THC levels. Following an initial 18-day cycle of 18 h light/6 h dark, the nine treatments included standard 12L/12D, a shortened 10L/14D, and an extended 14L/10D photoperiod (Table 1) [59]. The results were astonishing: flower biomass yields soared under the 14L/10D photoperiod across all varieties. However, for the THC-dominant varieties, a static 14L/10D photoperiod led to a significant decline in THC concentration [59]. In stark contrast, the \"Cannatonic\" variety thrived under the 14L/10D treatments, with CBD concentrations skyrocketing, leading to a monumental 50-100% increase in total CBD yield. This pivotal discovery dismantles the long-held belief that a 12L/12D photoperiod is optimal for all cannabis lines, revealing that lengthening the light period during flowering can dramatically enhance yields for specific varieties [59]. Cannabinoid composition and abundance are predominantly governed by genetics [78]. The circadian clock gene expression, which follows diurnal oscillation, does not change with day length, causing peak expression times to vary with the seasons [59; 78] (Figure 1).