Optimizing the production of cannabinoids in medical cannabis using LED grow lights

An increasing number of controlled environment cannabis growers are transitioning to LED grow lights, which provide increased energy efficiency and efficacy, reduced heat emission, a longer lifespan and spectrum customization, compared to conventional lights, including fluorescent and high-pressure sodium. Photo courtesy of Wheatfield Gardens

Researchers at Wageningen University in the Netherlands studied the effects of different light spectra on cannabis inflorescence development and the impact on the production of medical cannabinoids.

As more states approve the production of cannabis (Cannabis sativa L.) for medical applications, controlling plant growth will become more important to maximize inflorescence weight. Plant specialized metabolites (PSM) are primarily localized in cannabis female inflorescences. PSM are used to treat a variety of medical conditions, including chronic neuropathic pain, nausea, vomiting, multiple sclerosis spasticity, cancer- or HIV/AIDS-related anorexia, and Tourette’s syndrome symptoms. Ensuring consistent quantities and quality levels of PSM, particularly cannabinoids and terpenoids, is the goal of controlled environment growers producing for medical applications.

Since unprocessed cannabis inflorescences are administered directly to patients, it is critical that uniform PSM concentrations in inflorescences are achieved in order to provide constant medical results. This is why consistent controlled environment production is essential for the production of medical cannabis crops.

Spectrum impact on cannabis growth, PSM concentrations

Increasingly controlled environment cannabis growers are transitioning to the use of light emitting diode (LED) grow lights. The benefits of LEDs over conventional lights such as fluorescent and high-pressure sodium, include increased energy efficiency and efficacy, reduced heat emission, a longer lifespan and spectrum customization. LEDs maintain high photosynthetic photon flux densities (PPFD) which allows for effective light manipulation influencing plant dry matter production and development.

Light spectrum affects plant dry matter production and metabolic processes through photoreceptors as well as through its effects on net photosynthesis rate. Various studies with medical cannabis have been conducted to determine the influence of spectrum on PSM concentrations with mixed results. Discrepancies have been identified with cannabinoid concentrations based on the use of varying PPFD and the difficulty in maintaining consistent PPFD across different spectral treatments in these studies.

Studying the effects of different spectra

Controlled environment medical cannabis is commercially grown under different spectra and PPFD. Researchers at Wageningen University in the Netherlands conducted studies with cannabis to investigate the effects of different red wavelengths (640 and 660 nm), white fraction, and spectrum broadness on plant dry matter production and specialized metabolite accumulation. The researchers focused on analyzing plant morphology and photosynthetic responses at low (600 μmol m-2 s-1) and high (1,200 μmol m-2 s-1) PPFD, to identify spectrum-PPFD interactions. Studies were conducted with C. sativa (var. King Harmony) cultivated in two sequential growth cycles.

Researchers sought to determine the effect of growing plants in controlled environment rooms using four spectra: two low-white spectra and two high-white spectra. The low-white spectra differed in red wavelength peaks (100 percent 660 nm, versus 50:50 percent of 640:660 nm), the high-white spectra differed in spectrum broadness. All four spectra were applied at 600 and 1,200 μmol m-2 s-1.

An in-depth analysis of plant morphology and photosynthetic responses was conducted by the researchers to explain the underlying mechanisms responsible for observed treatment effects.

Interaction between spectrum and PPFD

For medical cannabis the Wageningen University researchers determined there was an interaction between spectrum and PPFD on plant dry matter production and inflorescence yield. They found white light with a dual red peak at 640 and 660 nm, compared to white light with a single red peak at 660 nm, increased inflorescence yield and light use efficiency, regardless of PPFD. This increase was primarily due to increased total plant dry matter production and a more open plant habit. White fraction and spectrum broadness had no effect on inflorescence yield, regardless of PPFD. There was no treatment effect on total cannabinoid concentrations, which indicates a potential to maintain consistent PSM quality.

Spectrum or PPFD did not affect the concentration of any specific cannabinoid or the total cannabinoid concentration. White light with a dual red peak of 640 and 660 nm compared to white light with a single red peak at 660 nm increased total terpenoid concentrations at high PPFD. Increasing the white fraction or spectrum broadness, regardless of PPFD, had no effect on total terpenoid concentrations. Total terpenoid concentration was highest five days before harvest. Bleached inflorescences were only found at the tip of apical inflorescences in white light with a dual red peak of 640 and 660 nm at 1,200 µmol m-2 s-1. Bleached inflorescences exhibited increased total cannabinoid concentrations compared to green inflorescences, primarily attributed to cannabidiol (CBD) as tetrahydrocannabinol (THC) was not affected. Inflorescence type did not influence total terpenoid concentrations.

The researchers hypothesized low white spectra at high PPFD could overexcite the photosystems, potentially leading to bleached inflorescences. The mechanism causing this condition has not been determined and warrants further study.

At higher PPFD, white light with a dual red peak of 640 and 660 nm compared to white light with a single red peak at 660 nm, increased terpenoid concentrations. At low PPFD, photosynthetic parameters like maximum photosynthetic rate and quantum yield were increased when grown under white light with a dual red peak of 640 and 660 nm compared to white light with a single red peak at 660 nm. Spectrum had no effect at higher PPFD. The addition of 640 nm with 660 nm shows potential to improve light use efficiency and promote plant dry matter production.

Editor’s note: This article is based on the Frontiers in Plant Science research article, The role of red and white light in optimizing growth and accumulation of plant specialized metabolites at two light intensities in medical cannabis (Cannabis sativa L.).