The Effect of Light Spectrum on the Morphology and Cannabinoid Content of Cannabis sativa L.
Gianmaria Magagnini, Gianpaolo Grassi, Stiina Kotiranta
Medical Cannabis and Cannabinoids, 2018, 1, 19–27
DOI : 10.1159/000489030
Cannabis sativa L. flowers are the main source of Δ-9- tetrahydrocannabinol (THC) used in medicine. One of the most important growth factors in cannabis cultivation is light; light quality, light intensity, and photoperiod play a big role in a successful growth protocol. The aim of the present study was to examine the effect of 3 different light sources on morphology and cannabinoid production. Cannabis clones were grown under 3 different light spectra, namely high-pressure sodium (HPS), AP673L (LED), and NS1 (LED). Light intensity was set to ∼450 μmol/m2/s measured from the canopy top. The photoperiod was 18L: 6D/21 days during the vegetative phase and 12L: 12D/46 days during the generative phase, respectively. At the end of the experiment, plant dry weight partition, plant height, and cannabinoid content (THC, cannabidiol [CBD], tetrahydrocannabivarin [THCV], cannabigerol [CBG]) were measured under different light treatments. The experiment was repeated twice. The 3 light treatments (HPS, NS1, AP673L) resulted in differences in cannabis plant morphology and in cannabinoid content, but not in total yield of cannabinoids. Plants under HPS treatment were taller and had more flower dry weight than those under treatments AP673L and NS1. Treatment NS1 had the highest CBG content. Treatments NS1 and AP673L had higher CBD and THC concentrations than the HPS treatment. Result were similar between experiments 1 and 2. Our results show that the plant morphology can be manipulated with the light spectrum. Furthermore, it is possible to affect the accumulation of different cannabinoids to increase the potential of medicinal grade cannabis. In conclusion, an optimized light spectrum improves the value and quality of cannabis. Current LED technology showed significant differences in growth habit and cannabinoid profile compared to the traditional HPS light source. Finally, no difference of flowering time was observed under different R:FR (i.e., the ratio between red and far-red light).
Keywords : Cannabis sativa L. · LED · Light spectrum · Cannabinoid content
Cultivating Cannabis sativa L. (Cannabaceae) differs from other horticultural plants by the end product that is harvested. The total yield cannot be rated only by the weight of the flowers; the chemical composition of the end product is also in the interest of the producers and end users. Different cannabis chemotypes contain numerous chemical compounds, such as cannabinoids, which are known to exert various pharmacological effects. Morphology and cannabinoid profile are dependent on genetic and environmental factors. For a medicinal cannabis producer, a continuous and uniform yield and production of a specific cannabinoid compound or a ratio between the different cannabinoids throughout the canopy and between growth cycles is important. Therefore, more and more professional medicinal cannabis producers are moving from greenhouses to indoors, into controlled and closed growth chambers. In growth chambers, it is possible to adjust temperature, humidity, light intensity, light spectrum, and air CO2 concentration. One of the most important growth factors in cannabis cultivation is light. Light quality, light intensity, and photoperiod play a significant role in a successful growth protocol.
Growing indoors also improves the pest management and reduces the susceptibility of the crop to natural conditions, such as bad weather. In addition to the environmental factors, the regulatory authorities also increasingly push licensed producers towards producing, packaging, and labeling their products indoors at the producer’s site. As said, indoor production offers the ability to cultivate year round under stable conditions resulting in up to 6 harvests per year. This makes indoor cropping 15–30 times more productive than outdoor cultivation . Also, the historically illegal nature of cannabis has pushed the cultivation inside into artificial environments due to the fear of being caught committing a crime . In addition to the positive effects of environmental control, indoor production minimizes the risk of cross-pollination with other nearby crops, particularly industrial hemp, to guarantee flowers without fertilization or seed maturation. On the other hand, indoor cannabis cultivation is energy intensive due to the high light demand and cooling of the closed environment. Cannabis is a plant adapted to high irradiance levels and warm temperatures. Chandra et al.  demonstrated that the highest photosynthetic efficiency was achieved under ∼1,500 PPFD (Photosynthetic Photon Flux Density) and 25–30 ° C; however, there is no evidence that a higher photosynthesis rate equals higher flower yields. It is also questionable whether such a high light intensity (1,500 PPFD) is economically feasible in terms of energy costs put into lighting and cooling. Indoor cannabis agriculture has in fact been classified as one of the “most energy intensive industries in the U.S.” . Lighting alone consumes 79–86% of the total electricity use [5, 6] in the cannabis farms. It has been calculated that 1% of the total energy consumption in the USA is for cannabis cultivation, and in top production states, such as California, the equivalent value is 3% . Often, cannabis production sites have separate facilities or rooms for each growth phase due to the different photoperiods and other environmental demands. There are 3 distinct phases in cannabis cultivation: propagation phase, vegetative growth phase, and flowering phase. In the interview study conducted by Sweet , it was noted that 600–1,000 W high-pressure sodium (HPS) lights were the most commonly used lighting source in Washington State during the flowering phase. In contrast, a wide variety of lighting types were reported to be used in the vegetative rooms, such as fluorescent light bulbs (CFL or T5), metal halide bulbs (MH), HPS lamps, induction bulbs, light-emitting diodes (LED), or a combination of different lighting types. During the propagation phase, the most commonly used lighting source is fluorescent light . When using older technology, such as HPS or fluorescent light, the spectrum is seldom adjusted according to the plants’ needs: the technology has been originally developed for totally different applications, such as street or office lighting. In the horticulture and crop science industry, it has been long known that one can manipulate plant morphology and metabolism with the light spectrum. For example, blue light has been shown to decrease internode length and enhance compactness of various species [9, 11], whereas far-red and green wavelengths have been shown to induce shade avoidance syndrome symptoms, including stem and leaf elongation and premature flowering . A recently published paper from the Czech Republic also concluded that cannabis plants grown under a red and blue light spectrum had shorter internodes and a smaller leaf area compared to a white light source . However, the paper does not give more specific information about the spectra used. In addition to morphological changes, light spectrum and irradiance
level also have an impact on plant metabolism. The plant receives signals from the light environment
through photoreceptors. Phytochromes, cryptochromes, phototropins, and UVR8 are the most well-studied photoreceptor groups found in higher plants. Phytochromes are the red- and far-red-sensing photoreceptors which regulate, for example, flowering, shade avoidance syndrome behavior, and germination in many species. Cryptochromes and phototropins are regulated mainly by blue and green wavelengths [14, 15]. UVR8 is responsible for UV-B-induced responses. Short wavelength irradiation
has been shown to enhance the plant defense mechanism by inducing metabolic activity, such as phenolic compound synthesis. Phenolic compounds, including anthocyanins, found especially in red-colored leaves, have been shown to accumulate in lettuce leaves under short-wavelength blue and UV light. Many phenolic compounds are part of the plants’ defense mechanism, which are synthesized under environmental stress. Short-wavelength irradiation and high photon flux irradiance are examples
of light-related environmental stress. Several cannabinoids have also been suggested to be involved in the plant defense mechanism and to have antioxidant properties, including Δ-9-tetrahydrocannabinol (THC) and cannabidiol (CBD)  as well as cannabigerol (CBG) . Bouquet  hypothesized that cannabis resin has a protective sunscreen function. However, the glands and the secreted resin are accumulated on the lower leaf surface instead of the upper surface and in the perigonial bracts in the inflorescence which should be more susceptible to sun light . While light quality may have an effect on the cannabinoid synthesis, cannabis yields are thought to strongly correlate with increasing light intensity [3, 20]. However, light intensity did not seem to affect the cannabinoid concentration when plants were grown under different light intensities under HPS light [21, 22]. In the studies by Vanhove et al.  and Potter and Duncombe , it was concluded that THC concentrations of flower material could be primarily linked to cannabis variety instead of cultivation method. In both studies, an increasing irradiance level correlated positively with flower dry weight, which resulted in higher total cannabinoid yield in the high irradiance treatments. However, the effects of different light qualities, or spectral composition, on cannabinoid synthesis and concentration in floral parts remain elusive. There are no recent light-related studies conducted with cannabis and based on cannabinoid profiles. However, already in very early studies in 1983, Mahlberg and Hemphill  concluded that in different light environments it was possible to manipulate the cannabinoid content of C. sativa L. measured in young leaves. The authors used colored filters to alter the light spectrum and concluded that the THC content of leaves from plants grown under shaded daylight and filtered red and blue light did not differ significantly from the THC content in daylight controls, while leaves from plants grown under filtered green light and darkness contained significantly lower levels of THC than those from plants grown in sunlight. The research and equipment at that time was not specific enough to thoroughly explain the effect of wavelength areas on cannabinoid content and the effect of lighting conditions on cannabis potency is still not clear. The first study related to light quality and cannabinoid content was conducted by Fairbairn and Liebann , who concluded that no increase of cannabinoids was found in a Nepalese variety grown in a greenhouse with or without supplemental lighting (HPS or UV lamps). Cannabis growers have been interested in UV light for a long time; however, the relationship between cannabinoids and UV-B is not as direct as first proposed. Increased concentrations of THC, but not of other cannabinoids,
were found with UV-B treatment in both leaf and floral tissues of drug-type plants [20, 25]. In contrast, none of the cannabinoids in fiber-type plants were affected by UV-B radiation. In a more recent study, hemp leaves were exposed to UV-C radiation and analyzed for changes in secondary metabolite biosynthesis . While no remarkable change in the cannabinoid content was observed, significant increases in dehydrostilbenes and cinnamic acid amide derivatives were found. The limited data available on the appropriate light source for cannabis production underscore the importance of studying technological developments in horticultural lighting. The objective of this study was to examine the effects of lightspectral quality on cannabis morphology and cannabinoid content in the female flowers under artificial growing conditions. Two lighting technologies (HPS and LED) and 3 different light spectra were used in this study.