Cultivar genetics and light intensity collectively determine the hemp biomass and quality parameters. High biomass production in plants requires adequate photosynthetic carbon fixation, for which light is a key factor (Mills, 2012). When the light intensity is either too low or too high, photosynthesis is inhibited, leading to reduced plant growth and flowering (Barber and Andersson, 1992). When plants absorb light, carbon dioxide and water are converted to glucose and oxygen through photosynthesis, and glucose is subsequently utilized for plant growth (Gertlowski and Petersen, 1993). These sugars serve as fundamental building blocks for the synthesis of primary and secondary metabolites in plants (Stracke et al., 2007).

In our study, stem, leaf, and flower dry mass showed linear increases as light intensity increased from 200 to 600 μmol·m⁻²·s⁻¹. These results are consistent with those of previous studies, which have demonstrated that increased light intensity promotes hemp growth through enhanced photosynthetic capacity. Vanhove et al. (2011) reported that the growth parameters of high-THC cannabis increased linearly with light intensity, with yield per square meter approaching saturation at 600 μmol·m⁻²·s⁻¹. Other studies on drug-type cannabis have extended these findings to higher light intensities. Rodriguez-Morrison et al. (2021) demonstrated linear yield increases across 120–1,800 μmol·m⁻²·s⁻¹, achieving 450% flower yield (g·m⁻²) increases with no light saturation observed, while Llewellyn et al. (2022) found 60% flower dry mass (g/plant) increases when light intensity increased from 600 to 1,000 μmol·m⁻²·s⁻¹ in the high-THC cultivar ‘Meridian’. Additionally, Sae-Tang et al. (2024) demonstrated that medicinal cannabis can efficiently utilize even higher intensities up to 1000 μmol·m⁻²·s⁻¹ without saturation. The observed wider internodal spacing under 200 μmol·m⁻²·s⁻¹ treatment represents a typical shade avoidance response, where plants elongate internodes under low light conditions to optimize light capture (Ghorbanzadeh et al., 2021; Xu et al., 2021).

Higher light intensities simultaneously increased biomass accumulation and cannabinoid production. Our results demonstrate that CBDA, CBD, and THCA exhibited linear accumulation across the 200–600 μmol·m⁻²·s⁻¹ range, with the highest accumulation occurring at 600 μmol·m⁻²·s⁻¹. The 36.88% increase in total CBD content at 600 μmol·m⁻²·s⁻¹ compared to 200 μmol·m⁻²·s⁻¹ demonstrates the positive relationship between light intensity and phytochemical production. This aligns with previous studies on various cannabis types, which show that substantial cannabinoid yields increase at higher light intensities (Rodriguez-Morrison et al., 2021; Llewellyn et al., 2022; Sae-Tang et al., 2024). More importantly, CBD yield per plant showed linear increases across the entire 200–600 μmol·m⁻²·s⁻¹ light intensity range (P_lin < 0.001). Given that previous studies have demonstrated continued linear increases at much higher intensities (up to 1,800 μmol·m⁻²·s⁻¹), further investigation beyond 600 μmol·m⁻²·s⁻¹ is warranted to determine the upper limits of light intensity optimization for CBD production in hemp cultivation. This combined effect of enhanced cannabinoid concentration and increased flower biomass production indicates that light intensity optimization effectively maximizes the harvestable CBD production per plant, providing significant implications for commercial hemp cultivation in controlled-environment systems.

Plant phytochemicals are closely linked to the primary photosynthetic pathways. The content of cannabinoids, a carbon-based phytochemical, is highly dependent on robust plant growth and photosynthetic efficiency. Glucose produced through photosynthesis enters the glycolytic pathway, generating precursors for phytochemical synthesis (Aharoni and Galili, 2011). This process leads to the formation of acetyl-CoA, which plays a key role in OA biosynthesis via the hexanoate pathway (Flores-Sanchez and Verpoorte, 2008). Recent molecular studies have provided quantitative evidence of these biochemical connections. The critical role of hexanoyl-CoA availability has been demonstrated through quantification studies showing levels of 15.5 pmol·g⁻¹ fresh weight in hemp flowers, with accumulation patterns directly paralleling CBDA production (Stout et al., 2012). Metabolic engineering has identified specific bottlenecks in OA biosynthesis, achieving an 83-fold increase in production by targeting acetyl-CoA carboxylase, pyruvate dehydrogenase bypass systems, and NADPH-generating malic enzymes (Luo et al., 2022). Although metabolomic research has shown enhanced secondary metabolite production at moderate light intensities for certain compounds, our study demonstrated that cannabinoid-specific optimization requires higher intensities.

Our gene expression analysis revealed an increased expression of cannabinoid biosynthesis genes under high light intensity, providing molecular evidence for enhanced cannabinoid production. The expression patterns revealed two distinct clusters: the MEP pathway (GPPS) and the hexanoate–cannabinoid biosynthesis pathway, which included OLS, OAC, PT, THCAS, CBDAS, and CBCAS. While GPPS showed the highest expression at 400 μmol·m⁻²·s⁻¹, reflecting optimal primary energy production from light, genes involved in cannabinoid biosynthesis were maximally expressed at 600 μmol·m⁻²·s⁻¹. In particular, CBDAS, the primary gene responsible for CBDA synthesis, showed significant upregulation under 600 μmol·m⁻²·s⁻¹, directly correlating with the observed increase in CBDA and total CBD content. This coordinated upregulation of the hexanoate–OA–CBGA–cannabinoid biosynthesis pathway under high light intensity explains the enhanced accumulation of cannabinoids and demonstrates that light intensity is a key factor in hemp phytochemical optimization. These findings align with recent reports in molecular studies that UV radiation can upregulate both CBDAS and THCAS expression by 4-fold, while enhancing OAC and OLS expression in cannabis cell cultures (Mansouri et al., 2024). Furthermore, comprehensive RNA-seq analysis has revealed that early enzymatic steps, particularly CBGA production, appear to be more rate-limiting than terminal synthase activities, such as THCAS, providing new insights into cannabinoid biosynthesis bottlenecks (Apicella et al., 2022). However, the preferential CBDA accumulation despite concurrent CBDAS and THCAS upregulation under high light intensity can be attributed to cultivar-specific genotypic and enzymatic characteristics. Hemp cultivar ‘Queen Dream’ can possess functional CBDAS alleles while harboring hypoactive THCAS variants (Singh et al., 2021), which determines the CBD-dominant chemotype regardless of transcript expression levels (Grassa et al., 2021; Ren et al., 2021). The enhanced CBGA precursor availability under 600 μmol·m⁻²·s⁻¹ is preferentially directed toward CBDA synthesis through superior CBDAS enzymatic efficiency in such cultivars, demonstrating that cannabinoid profile determination involves both transcriptional and post-transcriptional regulatory mechanisms.

The molecular mechanisms underlying light-regulated cannabinoid production involve complex spectral-dependent responses. Recent spectral studies have demonstrated that LED treatments rich in blue and UV-A wavelengths can produce 26–38% higher THC content than traditional HPS lighting at equivalent intensities (Islam et al., 2021). These spectrum-dependent responses suggest that optimizing both light intensity and spectral quality could further enhance cannabinoid production beyond the intensity optimization demonstrated in our study. Future research should also consider spectral optimization, as dual red peak spectra (640 + 660 nm) have shown superior performance compared with single-peak configurations (Holweg et al., 2024).

This study focused on the ‘Queen Dream’ cultivar. Future research should validate these light intensity management findings across diverse hemp genotypes and chemotypes. In addition, investigating the interactions between light intensity and spectral quality could refine cultivation protocols to maximize cannabinoid production in controlled environmental systems.