Cornell University Research on Hemp Responses to Light Treatments, Part 1

This column shares the latest findings from academic researchers exploring the impacts of emerging technology on hemp grown for cannabidiol (CBD).

Limited experiments have analyzed the responses of hemp (yield and CBD content) to daily light integral (DLI) and light quality. Three autoflowering hemp cultivars were grown in a greenhouse under four DLI treatments to determine response (flower yield and CBD content) to light. A follow-up experiment explored photoperiodic hemp response to greenhouse supplemental light spectrum (90:10 red:blue; 60:40 red:blue; white Light Emitting Diode [LED]; and High-Pressure Sodium [HPS]) on flower yield and content of CBD and tetrahydrocannabinol (THC).

Cornell researchers in Dr. Neil Mattson’s laboratory have performed several experiments with cannabinoid hemp that have uncovered insights useful to cannabis growers. Their work has been funded by the New York State Energy Research and Development Authority (NYSERDA) (1) and managed by the Greenhouse Lighting and Systems Engineering (GLASE) consortium (2). This article describes the results of two 2021 experiments.

Impact of DLI on Hemp Yield and CBD Content

Cannabis sativa for either THC or CBD is considered a high light requiring crop despite little information available in the scientific literature.

Traditionally, it is recommended to provide 30-40 mol·m^-2·d^-1 daily light integral (DLI) making this a higher light requiring crop than tomatoes or other fruiting crops. Some information is available in the literature for photoperiodic (such as how short days induce flowering) cannabis. For example, Rodriguez-Morrison, et al. (2021) reported linear increases in flower dry yield as the DLI increased from 8.6 to 77.8 mol·m^-2·d^-1. Across this broad DLI range a 0.5% increase in yield was reported for every 1% increase in DLI. Amazingly, cannabis responded positively to more light than is provided by the sun on a sunny day in the middle of summer (maximum possible DLI of 65 mol·m^-2·d^-1). As opposed to typically grown photoperiodic cannabis, autoflowering cannabis cultivars are seed-propagated and do not require short days to flower (photoperiod insensitive). No information is available in the scientific literature on the response of autoflowering cannabis to DLI.

The objective of this experiment was to determine the flower yield and cannabinoid (CBD and THC) response of three autoflowering hemp cultivars to DLI in greenhouse production.

Method

The experiment used Cornell’s Light and Shade System Implementation (LASSI) lighting control algorithm (3) in four adjacent glass greenhouses at Cornell University in Ithaca, New York to grow three autoflowering hemp cultivars under DLI treatments of 15, 20, 25, and 30 mol·m^-2·d^-1 (see Figure 1). Seeds of autoflowering cultivars AutoCBD (supplier, Phylos), Maverick (supplier, Kayagene) and Pipeline (supplier, Kayagene) were germinated in moist paper towels on July 18, 2021. Two days later on July 20, seeds of each cultivar were planted in two-gallon containers with a peat-perlite substrate (Lambert LM-111) and placed into one of four greenhouse sections. A quantum sensor placed in the center of each greenhouse measured light intensity and was used by LASSI to make supplemental lighting and shade decisions to reach the greenhouse DLI targets. The experiment was arranged as a randomized complete block design with plants arranged in six blocks (each with one plant of each cultivar), in each of the four greenhouses. During the experimental period, plants were drip irrigated as needed with a fertilizer prepared from Jack’s 5-12-26 (JR Peters) and Calcium Nitrate 15.5-0-0 (Yara). Nutrient solution pH was maintained at 5.5–5.8 and electrical conductivity (EC) was maintained at 2.0 dS/m. Plants were harvested 84 days after seeding, air dried for 10 days, and flowers were separated from leaves and stems. Cannabinoid content of the apical meristem was analyzed using high-performance liquid chromatography (HPLC).

Results:

• When cultivars were grouped together, optimum flower dry weight was reached at 20 mol·m^-2·d^-1 and did not further increase at higher DLIs. Individual cultivar responses were not statistically significant but exhibited patterns of optimal DLI for yield at 20 mol·m^-2·d^-1 for AutoCBD and Pipeline and 30 mol·m^-2·d^-1 for Maverick.
• DLI did not significantly impact the percentage of CBD or THC. However, for both cannabinoids there was a pattern for Maverick that increasing DLI led to an increase in CBD/THC, for example, CBD increased from 7.5 to 10% as DLI increased from 15 to 30 mol·m^-2·d^-1.

The Cornell researchers found that DLI responses of autoflowering CBD hemp are cultivar specific. Some cultivars (such as AutoCBD and Pipeline) appear to have lower DLI requirements than other cultivars (such as Maverick). In general, these autoflowering (non-photoperiodic) cultivars seem to have lower DLI requirements than photoperiodic hemp, though further research is required to study additional cultivars (especially directly comparing autoflowering to photoperiodic), as well as across a wider range of DLIs.

Impact of Greenhouse Supplemental Light Quality on Hemp Yield and CBD Content

Very little information is available in the scientific literature on the impact of greenhouse supplemental light quality on hemp yield or cannabinoid content. Anecdotal evidence suggests that higher blue light can be used to keep plants shorter (desirable in greenhouse environments), and that higher blue light may impact cannabinoid concentration (CBD or THC), but one recent paper found that increasing blue fraction of photons from 4 to 20%, did not impact plant height, decreased flower yield by 12%, and did not impact THC/CBD concentration (4). With CBD hemp, it is desirable to promote higher CBD accumulation; however, if the same treatment also causes THC concentration to be higher that may be undesirable if it passes the 0.3% threshold.

The objective of this experiment was to determine the impact of greenhouse supplemental light quality (including 90:10 red:blue LED, 60:40 red:blue LED, white LED, and HPS) on the plant morphology, flower yield, and content of CBD and THC for photoperiodic hemp cultivars. In addition, due to lights made available from a supplier we were able to include additional treatments: R:B LED with an added far-red peak and R:B LED with an added UV-A peak.

Method

Stem tip cuttings of 10-16cm length of cultivars ‘TJ’s CBD’ and ‘T2’ were taken from mother plants for vegetative propagation. The cuttings were dipped in Clonex rooting hormone and placed into 3.8 x 3.8 cm rock wool cubes, and put into 1020 trays. Trays were then placed into a misting system that provided a 10-second misting of water every 15 min. The cuttings received an 18-hr photoperiod that consisted of natural daylight as well as supplemental HPS lighting to avoid early flowering. Cuttings of both cultivars were maintained under these conditions in the propagation house for three weeks. Upon successfully rooting the plants, they were first transplanted into 10 cm pots and subsequently into 11-liter (three-gallon pots) with LM-111 all-purpose potting mix (LM-111, Lambert Peat Moss, Rivière-Ouelle, Canada). After transplanting, the plants were moved to grow benches and provided with HPS lights at 250 µmol·m^-2·s^-1 for vegetative growth for three weeks. During this stage, plants received an 18-hr photoperiod. Following the vegetative growth period, plants were moved under their respective lighting treatments and a 12-hr photoperiod was commenced to initiate flower induction (see Figure 2). Watering took place on an as-needed basis with a 200 mg/L nitrogen fertilizer made with 15-5-15 Cal Mag Jack’s Professional LX Water-Soluble Fertilizer (JR Peters Inc., Allentown, Pennsylvania). Greenhouse temperatures were maintained with day/night temperatures of 25.5/16.5 °C. Supplemental lighting treatments were provided for 10 weeks at 200 µmol·m^-2·s^-1 for 12-hr daily resulting in a supplemental DLI of 8.6 mol·m^-2·d^-1 in addition to sunlight.

Results:

• The lighting treatments had an impact on mean dry flower weight for TJ’s CBD but not T2. The HPS and White LED treatments had greater mean dry flower weights than the 60:40, 90:10, Far-red and Ultraviolet A (UVA) treatments.
• Lighting treatments showed an effect on TJ’s CBD cultivar but not the T2 cultivar. Within the TJ’s CBD cultivar, the white LED treatment had higher THC concentrations than 60:40, 90:10, Far-red and UVA treatments while the Far-red treatment also had lower THC concentrations than HPS.
• White LED light treatment results are very promising for replacing less energy efficient HPS fixtures with white LEDs.

Please note that commercial producers should always evaluate new production practices at a small scale before widespread adoption as each operation’s cultivars and growing environment may differ substantially from an experimental setting.

Continue following the GLASE hemp research in the “Cultivation Classroom” column throughout this year. A future article will describe findings from research conducted in 2022 and 2023 that explored hemp responses to lighting and CO₂ enrichment controls.

Acknowledgments

GLASE would like to acknowledge the following Cornell University individuals who conducted the research: Nicholas Kaczmar and Paul Reum.

References

1. https://www.nyserda.ny.gov/.
2. https://glase.org/.
3. https://glase.org/wp-content/uploads/2020/03/Technical-Reportissue5.pdf.
4. https://doi.org/10.1371/journal.pone.0248988.

About the Columnist

Gretchen Schimelpfenig, PE, is a Senior Energy Engineer at Energy Resources Integration (ERI), a clean energy consulting firm developing a sustainable future for our planet through cost-effective energy management. She leads emerging technologies studies exploring innovative solutions to save electricity and natural gas in commercial, industrial, and agricultural buildings. Gretchen is also the Executive Director of Cornell University’s Greenhouse Lighting and Systems Engineering (GLASE), an academic consortium pioneering climate-smart agricultural technology demonstrations and workforce development. She has worked with over 250 growers and researchers to optimize greenhouses and indoor farms through her emerging technology research, efficiency program implementation, and consultation with growers across North America. Gretchen is the former Technical Director of Resource Innovation Institute (RII). She is a licensed Civil Professional Engineer in California and Vermont.

How to Cite This Article

Schimelpfenig, G., Cornell University Research on Hemp Responses to Light Treatments, Part 1, Cannabis Science and Technology, 2024, 7(1), 14-17.