## 3 Light Effect on Growth and Development of _C. sativa_
### Effect of Light Intensity in Growth and Metabolism of C. sativa  
Photosynthesis in plants is primarily driven by light within the 400-700 nm spectral range, known as photosynthetically active radiation (PAR). The efficiency of this process is largely dependent on photon irradiance in this range, measured as photosynthetic photon flux density (PPFD), expressed in \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\). According to the International Commission on Illumination, light intensity is synonymous with photon flux density, which quantifies the number of photons hitting a surface per unit time and area. However, in the horticultural literature, PPFD (Figure 1) remains the standard term for photon irradiance within the PAR wavelength range [79]. Photosynthetic tissues experience a decline at light intensities (LI) below their light saturation points (LSP), where photosynthesis reaches its apex. However, for indoor-grown cannabis, whole-plant photosynthesis is optimized when the LI at the upper canopy leaves approaches their LSP. This optimization is facilitated by the attenuation of PAR within the canopy due to self-shading, allowing lower canopy foliage to operate within its optimal LI range. Unlike many other indoor crops, cannabis foliage exhibits an exceptional tolerance for exceedingly high LI, even when exposed to considerably higher PPFD levels than they are accustomed to. _C. cannabis_ leaves possess an extraordinary photosynthetic capacity, adeptly converting Photosynthetically active radiation (PAR) into biomass. This remarkable potential for biomass conversion underscores cannabis's exceptional adaptability and resilience in high-light environments, rendering it an exceedingly efficient crop for indoor cultivation [79]. In a rigorous study, _Camabis_ plants were cultivated for 12 weeks under a strict 12 h light/12 h dark photoperiod with canopy-level PPFDs ranging from 120 to an impressive 1800 umol m\({}^{-2}\) s\({}^{-1}\), using state-of-the-art LED technology. The research aimed to elucidate the intricate connections between light intensity (LI), photosynthesis, yield, and the overall quality of cannabis grown indoors. Findings revealed that leaf light response curves fluctuated with localized PPFD and throughout the flowering cycle, making them unreliable predictors of whole-plant yield responses to LI. Remarkably, dry inflorescure yield increased linearly with rising canopy-level PPFD up to 1800 umol m\({}^{-2}\) s\({}^{-1}\), despite leaf-level photosynthesis saturating at much lower levels. Additionally, the density of the apical inflorescence and the harvest index surged with increasing LI, enhancing marketable tissue quality and significantly reducing waste. LI treatments did not affect cannabinoid potency and had only minor effects on terpene potency, indicating that increasing LI can substantially boost yields while maintaining consistent secondary metabolite profiles (Figure 1). Commercial growers can leverage these robust findings to determine the optimal LI for their production environments, balancing input costs against product value. Future research must expand to include multiple cultivars of both _indica_- and _sativa_-dominant biotypes and explore the synergistic effects of CO\({}_{2}\) and LI on yield. A comprehensive cost-benefit analysis is essential to identify the optimal combination of these critical inputs, ensuring maximum efficiency and profitability in _Camabis_ cultivation [79]. A landmark study on industrial _C. sativa_ assessed various agronomic traits, amino acids, saccharides, and their derivatives under four distinct light intensities: 30, 80, 130, and 180 umol m\({}^{-2}\) s\({}^{-1}\)[47]. The findings were unequivocal: as light intensity increased, levels of amino acids and soluble proteins plummeted, whilst soluble sugars surged. The study pinpointed 130 umol m\({}^{-2}\) s\({}^{-1}\) as the optimal light intensity, yielding the least stress for plant height, free amino acids, and soluble proteins [47].