The pivotal role of light’s quality (spectral) composition in plant growth and physiological functions has been paramount since the dawn of photosynthesis research [32]. Recently, this field has exploded with intensity due to revolutionary advancements in computer-controlled lighting systems and the sophisticated documentation of photosynthetic activity. These strides are fortified by groundbreaking insights into the molecular mechanisms of photo regulation, governed by an array of photoreceptors and pigment systems [32].

The profound impact of varying light spectra is rooted in (i) the precise activation of distinct photoreceptors, (ii) the disparate efficiency of spectral components in driving robust photosynthesis, and (iii) the varying depth of spectral penetration into plant foliage (Figure 1) [32]. These dynamic factors orchestrate chloroplast movements, optimize light capture, and elevate photosynthetic capacity and metabolic processes [32]. Blue light, for example, dramatically boosts the biosynthesis of chlorophyll a, and alters the Chl a/b ratio by regulating key gene expressions [33]. The synthesis and catabolism of proline, a critical compound for stress adaptation and redox balance, are also profoundly influenced by light quality [17,33,34]. Carbohydrate concentration, a vital indicator of cell division activity, is powerfully correlated with the intensity of photosynthetically active radiation (PAR) (Figure 1) [35,36]. Blue light dramatically enhances total soluble carbohydrates (TSC) and starch accumulation, while red–blue light treatments significantly boost both fresh and dry biomass in plants. Blue light (400–500 nm) typically results in smaller, more compact leaves and shorter stems. Greenlight (510–585 nm) effectively mitigates lower leaf loss, whereas red light (620–700 nm) can inhibit flowering [37]. In stark contrast, a combination of red and far-red light (700–780 nm) forcefully triggers flowering and elongates leaves and stems [37,38].

A low red-to-far-red light ratio deactivates phytochrome b, spikes gibberellin (GA) levels, and obliterates DELLA proteins. Despite its detrimental impact on quantum yield, supplemental UV-A radiation can dramatically elevate photosynthetic rates and biomass accumulation in C. sativa [37,39]. Blue light profoundly influences chlorophyll biosynthesis, plant height, and stomatal opening while driving the accumulation of phenylpropanoid-based compounds without altering the plant morphology of the C. sativa plant [40]. Red light, however, drastically transforms plant morphology and physiology, vigorously stimulating stem growth and flowering but failing to enhance secondary metabolism [40]. The precise balance of blue to red light is crucial for maximizing growth, pigment production, and antioxidant capacity in vegetable plants, although this balance is species-specific [40]. Plants exhibit species-specific responses to light stress, mediated by a diverse array of photoreceptors. Red light markedly boosts stomatal conductance, net photosynthesis, intercellular CO₂ levels, and transpiration rates, while blue light suppresses these parameters by inducing stomatal closure [40]. A combination of red and blue light can severely stifle growth and photosynthesis in C. sativa, increasing MDA and proline content, and protecting cellular organelle membranes (Figure 1) [40].

The spectrum of light wields profound power over photosynthesis, directly and indirectly altering leaf structure, size, senescence rate, photosynthate transport, stomatal conductance, and chlorophyll composition [41]. Blue light, in particular, exerts a significant influence on plant height, stomatal opening, circadian rhythms, photomorphogenesis, flowering time, and chlorophyll biosynthesis [42]. Despite the widespread practice of indoor C. sativa farming, the precise effects of red and blue LED lighting on CBD (cannabidiol) synthesis remain shrouded in uncertainty [43]. Uncovering the optimal red and blue LED ratios could revolutionize CBD yield in indoor C. sativa cultivation [8]. The impact of LED light wavelengths on C. sativa growth and cannabinoid synthesis is nothing short of transformative. Experiments involving six distinct LED treatments (with varying red-to-blue light ratios and photosynthetic photon flux density) revealed that LED2 (R/B 1.61/1) and LED5 (R/B 16.8/1) effectively sustained plant height, stem diameter, and leaf numbers compared to high-pressure sodium lighting. LED2 dramatically boosted CBD content in both leaves and flowers, while LED5 significantly enhanced CBD levels in leaves. These findings underscore that specific red and blue LED ratios can drastically influence C. sativa growth and cannabinoid synthesis, positioning LED2 and LED5 as prime candidates for maximizing CBD yield [8]. In the realm of pest management, the objective is to obliterate yield reduction and uphold optimal CBD/THC (tetrahydrocannabinol) ratios [44].

Manipulating light spectra in indoor cannabis cultivation has revealed its formidable potential to impact plant growth and pest interactions. Increased blue light has been shown to amplify bud yield compared to white light, whereas red–blue light treatment results in towering plants with diminished leaf–stem dry mass and bud yield. Herbivory, meanwhile, slashes bud yield and CBD/THC concentration (Figure 1) [44,45]. Light quality also affects pest biology, indicating its untapped potential as a pest management weapon. For example, aphid-induced damage triggers salicylic acid-mediated defenses, escalating the production of secondary metabolites such as terpenes and cannabinoids (Figure 1) [44,45], which possess lethal insecticidal properties [44]. In the absence of aphids, ramped-up blue light supercharges bud mass and CBD content in C. sativa. Conversely, when aphids are present, white light initially curtails their growth and survival, but red–blue light may bolster cola production and sustain yield quality. However, these outcomes are intricately context-dependent and laden with trade-offs. Employing light quality and herbivory as management tactics harbors colossal potential but demands rigorous exploration for integrated pest management in controlled environments (Table 1) [44]. These groundbreaking discoveries offer potent new tools for optimizing C. sativa cultivation and executing devastatingly effective pest control.

The symbiotic relationship between mycorrhiza and plants is a dominant force in the plant kingdom, thriving across ecosystems and captivating scientific inquiry for over two centuries. Light is not merely a growth factor; it is a catalyst for plant metabolism, igniting a cascade of biochemical transformations that shape every facet of plant development—from morphology to pigment synthesis, phenolic compound production, and hormone regulation [66]. Blue light stands out as a powerhouse, crucial for the large-scale, sustainable production of industrial C. sativa [66,67]. Chloroplast pigments, the vibrant bioactive compounds inherent in nature, are essential for their coloration and potent antioxidant effects. Carotenoids, in particular, are indispensable for shielding cells from oxidative onslaughts (Figure 1) [61,67]. Phenolic compounds, with their formidable radical-scavenging capabilities and protein interactions, are key players in antioxidant defense [68]. In groundbreaking research, various biopreparations, including vesicular-arbuscular mycorrhiza (VAM), Azotobacter chroococum, and Trichoderma spp., were inoculated on C. sativa seeds and/or substrates, then exposed to blue and white light. The findings were stark: blue light dramatically undermined bio-preparation treatments (Figure 1), leading to significantly lower morphological parameter values compared to controls (Table 1) [61]. Yet, blue light-induced higher leaf pigment levels. Crucially, the type of light did not alter the antioxidant properties, including DPPH(2,2-diphenyl-1-(2,4,6-trinitrophenol) hydroxyl), FRAP (ferric reducing antioxidant power), flavonoids, total flavanol content, or phenolic acids (Figure 1) [61]. Biopreparation treatments also did not markedly influence leaf pigment content (Chl a, Chl b, and carotenoids) or phenolic and flavanol content (Figure 1). These revelations demand further investigation, particularly over prolonged periods or under field conditions, to unlock the full potential of bio-preparation treatments with beneficial microorganisms on the Finola C. sativa cultivar [61]. The implications are profound, offering a path to revolutionize C. sativa cultivation with enhanced growth, resilience, and biochemical richness [61].

Monochromatic blue light stands as a formidable tool for enhancing secondary metabolite production, though it can significantly impair growth traits like inflorescence size. Strategic, intermittent supplementation with blue light at the terminal stage of the growth cycle [30] shows great potential for dramatically elevating THC levels without substantially diminishing biomass [58]. Dichromatic LED lighting emerges as a groundbreaking alternative, mitigating the adverse effects of pure blue light while sustaining secondary metabolite levels comparable to conventional HPS lighting. An innovative study scrutinized the impact of six distinct light spectra from dichromatic B LED lights on the growth traits and secondary metabolite profiles of C. sativa (Babbas Erkle Cookies’) [58]. These spectra included blue (430 nm), red (630 nm), rose (430 + 630 nm, 1:10 ratio), purple (430 + 630 nm, 2:1 ratio), and amber (595 nm) LEDs and an HPS control. Blue light treatment yielded the highest THC concentration but the lowest THC per plant, whereas the HPS treatment produced the highest THC per plant. Blue light also markedly increased cannabigerol (CBG) and terpene concentrations but had a limited effect on cannabidiol (CBD) biosynthesis (Table 1) [58]. These groundbreaking findings empower producers to tailor spectral designs to specific production objectives, thereby optimizing cannabis production efficiency and slashing operational costs. Future research should be on light qualities richer in the amber spectrum and fine-tuning blue light proportions. Hybrid grow rooms integrating HPS and LEDs could substantially boost light intensity and profitability. C. sativa seedlings were meticulously evaluated under 11 light treatments within an advanced aeroponics system, assessing morphological traits, photosynthetic pigments, and osmolytes (Figure 1) [58].

Treatments L10 (R4/B2) and L11 (R2/B2/G2) excelled in leaf and node numbers. Treatments L2 (white), L3 (R8), and L5 (R7) achieved superior leaf length and width. Exceptional shoot lengths were noted in L3, L6 (R6), and L9 (R6), while root development flourished in L1 (natural light), L5, and L9. L3 exhibited the highest levels of chlorophyll a, chlorophyll b, and photosynthetic quantum yield (Fv/Fm) (Table 1) [37,69]. Hierarchical clustering analysis revealed that higher leaf and node numbers resulted in bushier plants with shorter shoots and longer roots. There was a stark negative correlation between photosynthetic traits and osmolytes with root length (Figure 1). Treatments L4, L6, L8 (R5/B2), and L11 displayed increased node numbers and osmolyte content—proline, ascorbic acid, total soluble carbohydrates (TSC), and sucrose—indicating greater branching, inflorescences, and cannabinoid accumulation [37,69] (Table 1). This pioneering study provides a crucial foundation for future research, steering the optimization of light compositions to cultivate C. sativa with ideal phenotypes. Notably, incorporating green light into other spectra enhanced osmoprotectant molecules compared to red and blue combinations alone, heralding a new era in precision agriculture for C. sativa cultivation.

The research underscores the significant impact of light quality on the morphophysiological traits of industrial C. sativa [70]. A key study investigated the effects of different light spectra—white (WL), blue (BL), red (RL), and a 50:50 red–blue mix (RBL)—on C. sativa’s morphology, gas exchange, and antioxidant capacity. The results were remarkable: BL increased shoot fresh biomass by 15.1%, shoot dry biomass by 27.0%, leaf number by 13.7%, stem diameter by 10.2%, root length by 6.8%, and chlorophyll content by 7.4% compared to WL. Additionally, BL significantly improved net photosynthesis, stomatal conductance, and transpiration while lowering lipid peroxidation and antioxidant enzyme activities. Conversely, RL and RBL treatments notably decreased plant biomass and gas exchange parameters while elevating antioxidant enzyme activity [40] (Figure 1). These groundbreaking findings solidify blue light’s role in sustainable, large-scale C. sativa production, though further research is crucial to unravel the precise mechanisms through which light governs C. sativa growth and development.

The rooting of stem cuttings is a cornerstone of plant propagation, profoundly influenced by endogenous auxins, particularly indole-3-acetic acid (IAA) [71,72]. IAA, produced in the shoot apical meristem or young leaves, is transported to the base, sparking cell division and adventitious root formation (Figure 1) [71,72]. Light is critical in this process, supplying energy for photosynthesis and signaling photomorphogenesis. However, high radiation can devastate auxin levels [73]. Far-red light (700–800 nm) stands out by boosting endogenous auxin accumulation via phytochromes, igniting shade avoidance responses, and enhancing auxin synthesis and stem elongation [73]. A revolutionary study on medicinal cannabis delved into the effects of different light spectra (blue, red, far-red) on the adventitious rooting of stem cuttings and their relationship with endogenous auxin and carbohydrates [56]. The study’s revelations were profound: far-red light dramatically improved rooting in one of two experiments, particularly when combined with red or red-blue light [46] (Table 1). Notably, far-red light applied during the initial rooting stage was sufficient to enhance rooting without triggering excessive stem elongation. While the positive effects of far-red on auxin and carbohydrate concentrations likely underline the improved rooting, the study did not find a direct correlation between these factors and rooting success (Figure 1) [46]. These powerful findings underscore the potential of optimizing light spectra for both industrial C. sativa production and medicinal cannabis propagation, heralding a new era of unparalleled efficiency and effectiveness in cultivation practices.

Until recently, clonal cannabis propagation primarily relied on fluorescent lighting [74]. The shift to LEDs marks a revolutionary advancement, allowing cultivators to implement precise spectral treatments that enhance rooting and drastically cut energy consumption [74]. A groundbreaking study explored the effects of various LED combinations—blue (B), red (R), ultraviolet-A (UVA), and phosphor-converted white (W)—against a fluorescent (F) control on vegetative stem cuttings of ‘Gelato-27’, ‘Grace’, and ‘Meridian’. Rooting for 15 days under each lighting treatment, maintaining a PPFD of 200 µmol m⁻² s⁻¹ with a 16 h photoperiod, yielded remarkable results: while spectrum treatments did not affect rooting percentages, root index values soared under B+UVA compared to F (Table 1). Moreover, root dry weights in B, B+UVA, B50, and F treatments surpassed those in the W treatment [55]. At the plug stage’s end, no significant effects on chlorophyll content index were noted. However, cuttings under B exhibited thicker stems than those under BR and W, and F showed the lowest percentage of new aboveground growth.

Blue (400–500 nm) and red (600–700 nm) wavelengths in horticultural LEDs deliver the highest photosynthetic quantum yield without triggering abnormal responses [55] (Figure 1). In C. sativa, secondary metabolites accumulate in the trichomes of female flowers and sugar leaves, synthesized through metabolic pathways: the PKS pathway produces Olivetolic acid (OLA) and the MVA/MEV and MEP pathways produce dimethylallyl diphosphate (DMAPP) and geranyl diphosphate (GPP). GPP, combined with OLA, forms cannabigerol acid (CBGA), the precursor to all cannabinoids, linking monoterpene and cannabinoid biosynthesis (Figure 1). These findings highlight the transformative potential of optimizing light spectra and metabolic pathways to enhance cannabis cultivation, heralding a new era of efficiency and potency [75,76].

Light, temperature, and various stresses play pivotal roles in directing the allocation of isoprenoid precursors between the MEP and MEV/MVA pathways by modulating enzyme expression. Enhanced light intensity significantly represses the MEV/MVA pathway while stimulating the MEP pathway [77]. Terpene synthases (TPSs) are critical in converting these precursors into specific mono- and sesquiterpenes, with their activity being profoundly influenced by light. Notable TPSs include TPS-a (caryophyllene and humulene), TPS g (linalool and nerolidol), and TPS-b (β-myrcene, limonene, α-pinene) (Table 1) [75,76]. Despite significant advances in understanding TPS genetics, predicting terpene profiles remains challenging due to external factors such as light, temperature, nutrition, and stress. The developmental stage of the plant also significantly impacts terpene and cannabinoid distribution, peaking in reproductive parts during flower maturity [57,75,76,77]. High R/FR light spectra activate phytochrome (Pfr), enhancing MEP pathway enzyme expression and increasing monoterpene and CBDA concentrations in cannabis buds. Different light spectra markedly influence CBDA and terpene concentrations, with variations reflecting interactions between plant morphology and chosen light spectra (Figure 1).

Techniques like strategic pruning and defoliation can greatly enhance light-use efficiency and optimize flower exposure [57]. A groundbreaking study on a CBD-dominant C. sativa genotype, FINOLA, examined the effects of long (far-red) and short-wavelength (blue, UV-A, UV-B) radiation on morphology, inflorescence yield, and secondary metabolites. This study compared LOW R/FR (ratio of 1) and HIGH R/FR (ratio of 11) treatments and various blue, UV-A, and UV-B radiation levels. The LOW R/FR treatment increased plant height but reduced inflorescence yield, while the HIGH R/FR treatment significantly boosted CBD, THCVA (tetrahydrocannabivarin acid), CBGA (Cannabigerolic acid), and terpene concentrations (Figure 1) [50]. Short-wavelength radiation treatments did not affect yield or morphology but increased THCVA concentrations, with UV-B specifically raising myrcene levels. These findings underscore the transformative potential of manipulating the R/FR ratio and incorporating short-wavelength radiation to fine-tune cannabis morphology, yield, and secondary metabolite profiles. Mastery of light spectrum manipulation stands as a powerful tool for optimizing cannabis cultivation, paving the way for unprecedented advancements in yield quality and production efficiency [50].

The adaptability of C. sativa through significant physiological and biochemical changes was rigorously studied under 10 different LED light spectra (a combination of red, blue, green, far-red, UV, and natural light) using an advanced aeroponic system [32]. The R7/B2 light treatment propelled cannabidiol (CBD) levels to unprecedented heights, coupled with an increased photosynthetic rate and substantially lower levels of reactive oxygen species, total phenols, flavonoids, DPPH radical scavenging capacity, and antioxidant enzyme activities (Figure 1) [32]. Under these conditions, THC and CBD showed minimal stress responses, while THCA emerged as the primary stress marker, closely followed by CBDA. Notably, CBD concentrations surged under treatments L4 (R7), L5 (R7), and L8 (R5/B2/G1), with THC levels consistently surpassing those under natural light across all spectra. CBDA (cannabidiolic acid) synthesis peaked in L3 (R8), L4 (R7/B2), L6 (R6/B2/G1), and L8. The L4 treatment delivered staggering concentrations of CBD, CBDA, THC, and THCA while minimizing stress markers. In stark contrast, L6 resulted in lower CBD and THC but higher THCA and CBDA, alongside intensified stress response activity [62]. Green light emerged as a pivotal factor in CBD and CBDA synthesis, with far-red and UV-A light displaying complex, nuanced effects. Photosynthetically active radiation spans 400 to 700 nm, with red (600–700 nm) and blue (420–450 nm) wavelengths wielding the most significant influence due to their optimal absorption by chlorophylls a and b, complemented by carotenoid absorption (Figure 1) (Table 1) [62].

Recent revelations suggest that a full spectrum (white light) dramatically outperforms monochromatic red and blue lights in promoting plant growth and functionality [62]. LED light spectra profoundly impacted every measured morphological, physiological, and chemical parameter [62] (Table 1).

The zenith of inflorescence yields was achieved under red and blue light at a 1:1 ratio, with similar success observed at a 1:4 blue/red ratio in two of the three varieties tested [15]. Conversely, white light with a 1:1 blue ratio yielded the lowest output. Light spectrum intricately shaped the chemical profile, with CBGA showing the most robust response, particularly under blue-rich light compared to far-red-rich HPS light [15]. Major cannabinoids like CBDA, THCA, and CBCA (cannabichromene acid) were also significantly affected, although responses were cultivar-specific and less pronounced than for CBGA. Blue light unequivocally promoted compact plant morphology, surpassing the influence of red/blue ratios (Table 1) [15]. The study uncovered cultivar-dependent responses, highlighting genetic variance and unveiling new avenues for breeding cannabis varieties optimized for light-induced responses [15]. Water relations and oxidative stress parameters remained largely unaffected by light quality across all varieties, underscoring the minimal impact of light spectra on Cannabis water relations [15]. The full spectrum light significantly enhanced inflorescence yield compared to blue/red light, reinforcing the hypothesis that the light spectrum is a powerful tool for manipulating plant development and cannabinoid profiles, offering unparalleled opportunities to fine-tune cannabis and cannabinoid production (Table 1) [15]. These findings illuminate the transformative potential of strategically optimized light spectra in revolutionizing Cannabis cultivation, maximizing both yield and cannabinoid content while effectively managing plant stress responses. This paradigm shift in horticultural practices paves the way for unprecedented advancements in the efficiency and potency of Cannabis production.