Light Spectra Have Minimal Effects on Rooting and Vegetative Growth Responses of Clonal Cannabis Cuttings

Authors:
Melissa Moher School of Environmental Science, University of Guelph, Guelph, Ontario, Canada, N1G 2W1

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David Llewellyn School of Environmental Science, University of Guelph, Guelph, Ontario, Canada, N1G 2W1

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Scott Golem HEXO Corp., 120 Chem. de la Rive, Gatineau, Quebec, Canada, J8M 1V2

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Elizabeth Foley HEXO Corp., 120 Chem. de la Rive, Gatineau, Quebec, Canada, J8M 1V2

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Steve Dinka HEXO Corp., 120 Chem. de la Rive, Gatineau, Quebec, Canada, J8M 1V2

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Max Jones Department of Plant Agriculture, University of Guelph, Guelph, Ontario, Canada, N1G 2W1

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Youbin Zheng School of Environmental Science, University of Guelph, Guelph, Ontario, Canada, N1G 2W1

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Abstract

Until recently, most clonal cannabis (Cannabis sativa) has been propagated using fluorescent lights. Transitioning to light-emitting diodes (LEDs) may be a viable alternative to fluorescent lighting, enabling cultivators to provide specific spectrum treatments to enhance rooting while also saving energy. Vegetative stem cuttings of ‘Gelato-27’, ‘Grace’, and ‘Meridian’ were rooted for 15 days under various combinations of blue (B), red (R), ultraviolet-A (UVA) LEDs, phosphor-converted white (W) LEDs, and a fluorescent (F) control treatment, each with a canopy-level photosynthetic photon flux density (PPFD) of 200 µmol·m−2·s−1 and 16-hour photoperiod. The photon flux ratios of blue (B; 400–500 nm) and red (R; 600–700 nm) narrowband LED treatment combinations were (1) BR, fixed spectrum of B15:R85; (2) B, B75:R25 on day 0–2 followed by B15:R85 on day 2–14; (3) B+UVA, B75:R25 on day 0–2 followed by B15:R85 on day 2–14 plus 15 µmol·m−2·s−1 of UVA on day 7–14; (4) B50, B15:R85 on day 0–7 followed by B50:R50 on day 7–14. The W and F treatments both had static spectra. After the propagation period (i.e., plug stage), a portion of the cuttings under each treatment × cultivar combination were destructively harvested and the remainder were transplanted and grown vegetatively for an additional 21 days (i.e., transplant stage) under a PPFD of ≈275 µmol·m−2·s−1 from ceramic metal halide fixtures and then destructively harvested. Although there were no spectrum treatment effects on the percentage of cuttings that rooted, root index values were higher in cuttings grown under B+UVA vs. F. Further, relative root dry weights of plugs from the B, B+UVA, B50, and F treatments were higher than the W treatment. At the end of the plug stage, there were no spectrum treatment effects on the chlorophyll content index, cuttings grown under the B treatment had thicker stems compared with BR and W treatments, and cuttings grown under the F treatment exhibited the lowest percentage of new aboveground growth. None of the aforementioned spectrum treatment effects from the propagation stage persisted post-transplant. The use of LEDs is a promising, energy-efficient alternative to fluorescent lighting for cannabis propagation and B-enhanced spectrum treatments appear to enhance the rooting performance of clonal cannabis cuttings.

Drug-type cannabis is an economically valuable crop that is often grown in controlled environments (CE), such as greenhouses with supplemental lighting or indoors with 100% electrical lighting, as these environments can produce higher quality crops than outdoor production. CE production allows cultivators to grow cannabis year-round with less concern about sub-optimal weather, including stabilizing crop light exposure and the avoidance of adverse weather (Zheng 2021). Further, CE production enables greater control of the daylength, which is a key production parameter for both inhibiting and inducing flowering in commercial cannabis production, depending on the growth stage. The use of electric lighting in indoor cannabis cultivation facilitates the production of uniform plants with optimal (for cultivators’ individual production protocols) morphological attributes (Moher et al. 2022; Potter 2014).

Cannabis production begins with a propagation period to establish transplants. Propagation can be accomplished via seed (Farag and Kayser 2017), tissue culture (Wang et al. 2009), or clonal cuttings from vegetative tissues (Campbell et al. 2019; Caplan et al. 2018). Most commercial CE cannabis cultivators opt for clonal stem cuttings since they can be harvested from stock mother plants and can provide uniform transplants with predictable growth and cannabinoid composition (Coffman and Gentner 1979; Potter 2009; Small 2017). The canopy-level PPFD [400–700 nm (µmol·m−2·s−1)] is usually low during this stage (Fluence 2020; Lumigrow 2017) to minimize water stress as cuttings establish a root system. Once sufficiently rooted, plants are transplanted into larger containers to provide space for the root systems to enlarge appropriately to support the plant through the subsequent vegetative and flowering stages. Robust root systems help cuttings withstand transplant shock and endure the higher light intensities that are commonly used in the vegetative stage (Fluence 2020; Lumigrow 2017), especially under increasing transpiration loads associated with rapid vegetative growth.

The CE horticulture industry has adapted electric lighting technologies that were initially developed for human vision, such as high-pressure sodium, metal halide, and florescent for use in various crop production capacities. These technologies are each characterized as having fixed spectral output but with very disparate spectrum distributions (Dutta Gupta and Agarwal 2017). Importantly, the spectral output of these traditional technologies has not been optimized for light utilization by plants. Of these technologies, fluorescent lighting is used most commonly in indoor propagation systems (Bilodeau et al. 2019) because it is economical at a low intensity and is amenable to use in multitiered production systems that maximize space use efficiency. Recently, cultivators have been opting for LED technologies to replace the traditional horticultural lighting technologies because of their longer lifetimes, higher energy efficiency, and ability to provide customized spectra that are designed specifically for plant production (Nelson and Bugbee 2014; Singh et al. 2015; Yeh and Chung 2009). Customized spectra leverage the narrowband emissions of different diode colors by varying the proportions of different types of LEDs within a fixture. Further, some fixtures leverage the inherent dimming capabilities of LEDs to enable dynamic spectral control at a fixture level by manipulating the photon flux of individual LED color channels within a given fixture.

Combinations of light from blue (B, 400–500 nm) and red (R, 600–700 nm) wavebands commonly dominate the spectral output of horticultural LED fixtures. This is because B and R wavelengths have been reported to have the highest photosynthetic quantum yield (McCree 1972) and combinations of B and R do not often elicit abnormal morphological or physiological responses (Bantis et al. 2018; Kong et al. 2018). Further, photons from R wavelengths often comprise 75% to 95% of the photon flux in B+R spectra (Jones-Baumgardt et al. 2019; Kong and Zheng 2018; Randall and Lopez 2014; Ying et al. 2020), because R photons are more energy efficient to produce than B (Kusuma et al. 2020). The magenta-hued environments resulting from B+R spectra may be uncomfortable to work under and can make it difficult to evaluate crop health. Therefore, phosphor-converted light from “white” LEDs are also often added to B+R to provide broader spectrum distributions, despite their inherently lower efficacy (Kusuma et al. 2020). By design, white LEDs already contain a significant amount of B, so narrowband B LEDs may not be used in these spectrum combinations.

Given the prescribed advantages of horticultural LED technologies, there is a need to evaluate the efficacy of using the various spectral combinations available with LEDs, in comparison with fluorescent lighting, for the propagation phase of clonal cannabis. Just as optimum light intensity levels can vary at different stages of production (Moher et al. 2022; Rodriguez-Morrison et al. 2021), the ideal light spectrum distribution may depend on a crop’s growth stage. For example, the use of high proportions of B at the start of clonal propagation may promote faster root initiation (Gil et al. 2020). The addition of UVA (315–400 nm) may increase root biomass and make the plants more resistant to transplant stress (Verdaguer et al. 2017). Adding higher proportions of B proximal to transplant may aid with promoting compact morphology (i.e., short internodes) of transplants (Ying et al. 2020). With the capability of enhanced LED technologies to modify the spectral output, it is possible that a lighting protocol that uses a temporally-dynamic spectrum (i.e., varying the photon flux ratios over time) may enhance the rooting performance of cannabis cuttings vs. a fixed spectrum, whether the fixed spectrum is provided by fluorescent or LED sources. Since there is limited scientific information related to lighting during the propagation of clonal cannabis, the effects of spectrum during this dynamic phase of plant development is unknown.

The overall objective of this study was to compare a range of LED spectrum treatments, both fixed and temporally dynamic, with fluorescent lighting for the propagation stage of clonal cannabis. The overall hypothesis is that some LED spectral combinations, either fixed or dynamic, will be superior to fluorescent lighting for producing cannabis transplants. Specific objectives were 1) compare fixed-spectrum distributions that are commonly used in horticultural LEDs (e.g., B mixed with R, phosphor-converted W) with fluorescent lighting for the rooting, growth, and transplant performance of clonal cannabis cuttings; and 2) investigate the effects of temporally-dynamic vs. fixed LED and fluorescent spectrum treatments on the rooting, growth, and transplant performance of clonal cannabis cuttings.

Materials and Methods

Plant material.

Uniform clonal vegetative stem cuttings (hereafter referred to as cuttings) of three cannabis cultivars were used as treatment plants: ‘Gelato-27’ (short, bushy, and slow rooting), ‘Grace’ (relatively short and moderate rooting), and ‘Meridian’ (tall, stretched, and fast rooting). Cuttings of ‘Gelato-29’ were used as border plants in all plots. All cuttings were taken from 3-month-old mother plants that had been grown indoors under a PPFD of 400 µmol·m−2·s−1 provided by metal halide fixtures (NXT-LP CMH; PL Light Systems Inc, Beamsville, ON, Canada), 18/6 h on/off photoperiod, 65% relative humidity (RH), and 24/20°C light/dark. Cuttings with three to four nodes and 10 to 16 cm in length were taken using sterilized scissors, and basal ends were cut on a 45-degree angle and scarred using a sterilized scalpel. The basal end of each cutting was coated with rooting hormone (StimRoot #1; Master Plant-Prod Inc., Brampton, ON, Canada) and then inserted into a cylindrical rockwool plug (3.6-cm diameter × 4.0-cm height; Grodan, Milton, ON, Canada) contained in 50-cell propagation trays (0.5 × 0.3 m, 50 Plug Pre-filled; A.M.A Horticulture Inc., Kingsville, ON, Canada). The rockwool plugs were presoaked in a preventive biological fungicide (RootShield WP; Bioworks, Victor, NY, USA) at 0.45 g·L−1 in sterile reverse osmosis (RO) water, resulting in an electrical conductivity (EC) of 0.7 dS·m−1 and a pH of 5.2. The propagation trays were covered with transparent vented plastic lids (7-inch Propagation Dome; Mondi Products, Vancouver, BC, Canada) and placed in flats without drainage holes (NR Open Flat; FloraPlast, Chatham, ON, Canada).

Growing conditions.

The experiment was conducted in a commercial cannabis production facility in Southern Ontario, Canada. Three growth enclosures (5.9 × 4.1 × 2.7 m) were constructed, each consisting of two benches (5.9 × 1.8 m) that were separated by 0.5 m and encompassed with panda film (Vivosun, City of Industry, CA, USA), with the black side facing inward, to block ambient light from entering the enclosures (Fig. 1). Each enclosure was divided into six plots (three plots per bench), each equipped with its own light fixtures. Each plot consisted of two propagation trays with ‘Gelato-29’ cuttings located in every edge-adjacent cell (26 cells) in each tray. The remaining cells comprised the inner three rows (i.e., 3 × 8 cells) of the tray, into which cuttings of each of the three treatment cultivars were placed, with each cultivar randomly allocated to one row on a per-tray basis. With this arrangement, there were 16 subsamples per cultivar in each plot. The cuttings were misted with RO water on the foliage at the time of insertion (day 0) and then once on each of days 2, 4, and 6. On day 8, 0.8 L of nutrient solution was added to the bottom of each tray and topped up with nutrient solution thereafter, as needed. The nutrient solution was composed of Dutch Nutrients Gro A and Gro B (Homegrown Hydroponics, Toronto, ON, Canada), each at 5 mL·L−1 in RO water, resulting in an EC of 1.7 dS·m−1 and a pH of 5.8. On day 14, the cuttings were divided into two groups that were either harvested to evaluate rooting or transplanted and grown for an additional 21 d to evaluate transplant success (described later in this article).

Fig. 1.
Fig. 1.

The experimental set up depicting (A) the inside of one enclosure (i.e., block) that is encapsulated by panda film, (B) a bench that consisted of three plots, and (C) two trays of cuttings under the white LED treatment (W), centered under the LED fixtures. Photos are from different blocks.

Citation: HortScience 58, 2; 10.21273/HORTSCI16752-22

Data loggers (HOBO MX2301A; Onset Computer Corporation, Bourne, MA, USA), positioned at fixture level in the center of each enclosure, were used to record temperature and RH on 60-s intervals throughout the propagation period. The daytime temperature and RH were 26 ± 3 °C and 30% ± 9%, 26 ± 3 °C and 30% ± 8%, and 25 ± 2 °C and 30% ± 8% [mean ± standard deviation (SD), n = 14,400] for the three enclosures, respectively. The nighttime temperature and RH were 23 ± 2 °C and 37% ± 5%, 23 ± 2 °C and 36% ± 5%, and 23 ± 2 °C and 36% ± 5% (mean ± SD, n = 7200) for the three enclosures, respectively. The inside of the propagation lids were wiped down with sterile cloths each time the cuttings were evaluated for rooting (described later in this article) to remove condensed water for pathogen suppression. Periodic point measurements of the aerial environment within the propagation domes were taken with a miniature thermo-hygrometer (E100; Mondi Products, Vancouver, BC, Canada) (e.g., Fig. 1B, dome on the far left).

Spectrum treatments.

The experiment was a randomized complete block design with six spectrum treatments and three blocks. All treatments had a daily photoperiod of 16 h (0600–2200 HR) and a target PPFD of 200 µmol·m−2·s−1 at the surface of the propagation lids (Table 1). The spectrum, intensity, and uniformity were measured on a 0.1 × 0.1-m square grid (i.e., 25 locations), centered under the light fixtures, to cover the space occupied by the two propagation trays in each plot. Spectrum and intensity were measured using a radiometrically calibrated spectrometer (XR-Flame-S; Ocean Optics, Dunedin, FL, USA) coupled to a CC3 cosine-corrector with a 1.9 m × 400 µm ultraviolet-Vis optical fiber. The PAR-Spec subroutine in SpectraSuite (Ocean Optics) was used for spectrum and intensity characterizations that were measured at the beginning and end of the experiment.

Table 1.

The photon flux ratios, photosynthetic photon flux density (PPFD; µmol·m−2·s−1), uniformity, and phytochrome photostationary state (PPS) of the different spectrum treatments during the 15-day rooting phase of the cannabis cuttings.

Table 1.

Photon flux ratios of UVA (315–400 nm), blue (B, 400–500 nm), green (G, 500–600 nm), red (R, 600–700 nm), and far red (FR, 700–800 nm) were used to define the spectrum combinations in each treatment (Table 1). Within each treatment, only wavebands with photon flux integrals that comprise ≥ 0.5% of the total photon flux were used to define each respective treatment. Further, although UVA and FR wavebands are described in the spectrum treatments, wavelengths outside the photosynthetically active radiation (i.e., PAR; 400–700 nm) spectrum were excluded from the intensity (i.e., PPFD) measurements. Some treatments had fixed spectra throughout the entire propagation period, and others comprised dynamic spectral combinations that varied temporally throughout the propagation period. The treatments were as follows: 1) F, fixed fluorescent light spectrum (control); 2) BR, fixed spectrum of B15:R85; 3) B, B75:R25 on day 0–2 followed by B15:R85 on day 2–14; 4) B+UVA, B75:R25 on day 0–2 followed by B15:R85 on day 2–14 plus 15 µmol·m−2·s−1 of UVA on day 7–14; 5) B50, B15:R85 on day 0–7 followed by B50:R50 on day 7–14; and 6) W, fixed white spectrum (5700K). The initiation of the light spectrum treatments on day 0 and all subsequent spectrum changes occurred between 1000 and 1400 HR (i.e., during the middle portion of a given day’s photoperiod).

Several lighting technologies were used, alone or in combination, to provide the spectrum treatments. The control treatment used fluorescent lighting to match one of the most common propagation practices of commercial cultivators. The control plots each had two fluorescent fixtures, with each fixture containing 4 × 1.2-m T5 fluorescent tubes (F28W/T5/835/ECO; GE Lighting, East Cleveland, OH, USA). The spectral combinations in all other treatments were provided by LEDs. The UVA spectrum (custom-made LED bars; Yunustech, Mississauga, ON, Canada) had a peak wavelength and full-width at half-maximum intensity (FWHM) of 385 nm and 11 nm, respectively. The intensity of the UVA LEDs was modified using constant current dimmers. The remaining spectrum combinations were all provided by commercial LED bars (Toplight-Targeted Spectrum; Lumigrow, Emeryville, CA, USA), which contained three spectrum channels: blue, white, and red. The intensity of each channel was controlled with pulse-width modulation using software (smartPAR Horticulture Lighting Software Version 1.21.2; Lumigrow). The peak wavelengths and FWHM were 445 nm and 17 nm for blue and 665 nm and 16 nm for red channels, respectively. The white channel, which was composed of phosphor-converted blue LEDs, had two peaks with peak wavelengths and FWHM of 440 nm and 23 nm and 595 nm and 135 nm, respectively.

All lighting fixtures in each plot were hung 0.3 m above the top of the propagation lids and centered over the plots. In the control plots, fluorescent fixtures (1.2 × 0.4 m) were hung 0.4 m apart (note, all fixture positioning measurements are “on center”). For the red and blue LED treatments (including UVA), pairs of Toplight LED bars (1.09 × 0.11 m) were spaced 0.4 m apart. For the plots with UVA, two UVA LED bars (0.05 × 0.6 m) were positioned 0.24 m apart (i.e., adjacent to the inside surface of each Toplight fixture). Four TopLight LED bars were evenly spaced 0.18 m apart in the white LED treatment plots.

The mean photon flux ratios, PPFD, within-plot PPFD uniformity (i.e., minimum PPFD/average PPFD), and phytochrome photostationary state [PPS; based on Sager et al. (1988)] were determined from raw spectral data for each treatment using a Microsoft Excel tool (Mah et al. 2019) and are presented in Table 1. The relative photon flux distribution of the different spectral combinations in each treatment (outlined in Table 1) are provided graphically in Supplemental Fig. 1.

Rooting and growth measurements.

The cuttings were investigated regularly for the presence of visible roots. Each plug was investigated 3, 7, 9, 10, 13, and 14 d after insertion for root emergence on all exterior surfaces of each plug. The root coverage percentages (i.e., the percentage of growing media surface area that was obscured by root tissue) were estimated in increments of 5% (i.e., 0% to 5%, 5% to 10%, 10% to 15%, etc.) and were recorded as root index (RI) values from 0 to 11 (Supplemental Table 1) based on the rooting stage parameter described in Birlanga et al. (2015). Logistical constraints prevented the (necessarily destructive) assessments (i.e., prising apart the rockwool plug) of root initiation. After the 14 d of spectrum treatments, all cuttings were evaluated for an array of rooting and aboveground attributes. The RI on day 14 was used to indicate the root density of each cutting at the time of transplant, as well as the overall percentage of cuttings from each treatment × cultivar combination that had developed visible roots (i.e., RI > 0). Chlorophyll content on the youngest fully expanded leaf of each cutting was measured using a chlorophyll meter (SPAD 502; Spectrum Technologies Inc., Aurora, IL, USA). Stem thickness (ST) was measured at the first internode of each cutting using a digital caliper. The presence of new aboveground growth was recorded for each cutting that had new leaves or nodes.

For each cultivar × tray combination, two plants that had RI > 0 were randomly selected and transplanted into rockwool blocks (0.15 × 0.15 × 0.15 m; Grodan) that were presoaked to saturation with rainwater and grown vegetatively (16-h photoperiod) under 277 ± 16 µmol·m−2·s−1 (mean ± SD, n = 6) from metal halide lamps (MED NXTLP RST MH; PL Light Systems Inc., Beamsville, ON, Canada) for 21 d. The initial height, measured from growing media surface to the highest point on the plant, and the number of nodes of each plant were recorded. Plants were randomly arranged 0.24 m apart on a bench and fertigated using a solution composed of Dutch Nutrients Gro A and Gro B (Homegrown Hydroponics), each added at 5 mL·L−1 to rainwater (EC < 0.1 dS·m−1), with a resulting EC of ≈1.8 dS·m−1 and pH of ≈5.7. The rainwater consistently had an EC ≤ 0.1 dS·m−1 and was treated with 100 ppm hydrogen peroxide at least 24 h before use. The transplants were not fertigated for the first 3 d to encourage root growth, then were drip-irrigated twice daily at 2 L·h−1 for 540 s, such that each plant received ≈0.6 L·d−1.

The remaining six cuttings from each cultivar × tray combination were destructively harvested by cutting the stems at growing media level and oven-drying the root balls (i.e., rockwool plug with roots in individual paper bags) to constant weight at 65 °C (Isotemp Oven Model 655G; Fisher Scientific, East Lyme, CT, USA). Dry weights (DW) were measured using an analytical balance (Mettler Toledo AE 100; Mettler Toledo, Columbus, OH, USA). Since the roots could not be extracted from the rockwool plugs, the combined DW of the rockwool and roots (i.e., root ball) were converted into relative values [relative root dry weight of plugs (RRDWP)] using the formula: root ball DW/DW of the heaviest root ball.

Post-transplantt vegetative growth measurements.

The transplants were harvested 21 d after being placed in the vegetative environment. Increases in height (ΔH) and number of nodes (ΔNN) were determined by subtracting initial values from harvest values of these parameters. Internode length (IL) was determined by dividing ΔH by ΔNN. ST was measured at the first internode using a digital caliper. Chlorophyll content was measured thrice, then averaged, on the youngest fully expanded leaf using a chlorophyll meter (SPAD 502). The total aboveground fresh weight (FW) of each plant was recorded (AX622N/E Adventure Precision Balance; OHAUS Corporation, Parsippany, NJ, USA) and then dried at 65 °C (Isotemp Oven Model 655G) to constant weight to obtain aboveground DW. Aboveground water content (WC) was calculated using the following equation: [(FW − DW)/FW] × 100%. The rockwool blocks from the transplants were dried at 65 °C to constant weight and then the combined DW of the rockwool and roots (i.e., root ball) were converted into relative values [relative root dry weight of transplants (RRDWT)] using the following formula: root ball DW/DW of the heaviest root ball.

Statistical analysis.

Data were analyzed using generalized linear mixed models using SAS (Statistical Analysis Software University Edition; SAS Institute Inc., Cary, NC, USA). Data were subjected to an analysis of variance, followed by Tukey-Kramer’s tests to determine differences among spectrum treatments, cultivars, and spectrum × cultivar combinations at a significance level of P ≤ 0.05. When intracultivar spectrum treatment, intercultivar spectrum treatment, cultivar, and overall experiment means are presented, data are presented as means ± standard error (SE), where n = 3, 9, 18, and 54, respectively.

Results

Rooting.

There were no obvious signs of rooting on the outer surfaces of the plugs until 9 d after the start of the experiment. Despite this time lag, there was a high percentage of rooted cuttings (i.e., RI > 0) at day 14 (Table 2), with an overall experiment mean of 96% ± 0.7%. Although there were no intracultivar spectrum treatment effects on RI, when the data from the three cultivars were combined, the RI was 21% higher in the B+UVA vs. F spectrum treatments (Fig. 2A). In the harvested cuttings, the RRDWP in ‘Grace’ were 8% higher in the B+UVA vs. W treatments (Table 2). Further, when the data from the three cultivars were combined, the RRDWP in the B+UVA, B50, F, and B treatments were 4% higher than the W treatment (Fig. 2B).

Fig. 2.
Fig. 2.

Root index (A), root dry weight of plugs (RRDWP) relative to the heaviest root ball (B), stem thickness (C), and percentage of cuttings that exhibited new aboveground growth (D) for cuttings of three cannabis cultivars rooted under different spectrum treatments for 15 d. 1) F, fixed fluorescent light spectrum (control); 2) BR, fixed spectrum of B15:R85; 3) B, B75:R25 on day 0–2 followed by B15:R85 on day 2–14; 4) B+UVA, B75:R25 on day 0–2 followed by B15:R85 on day 2–14 plus 15 µmol·m−2·s−1 of UVA on day 7–14; 5) B50, B15:R85 on day 0–7 followed by B50:R50 on day 7–14; and 6) W, fixed white spectrum (5700K). The photosynthetic photon flux density (PPFD; 400–700 nm) was ≈200 µmol·m−2·s−1 for all treatments. Bars represent means ± standard error of all three cultivars combined (n = 9). Bars that share the same lowercase letter are not significantly different at P ≤ 0.05, according to Tukey-Kramer multiple comparisons test.

Citation: HortScience 58, 2; 10.21273/HORTSCI16752-22

Table 2.

The effect of spectrum and cultivar on percentage of rooted cuttings, root index (RI), relative root dry weight (RRDWP), SPAD value (i.e., chlorophyll content index), stem thickness (ST), and percentage of plants that exhibited new aboveground growth for cuttings of ‘Gelato-27’, ‘Grace’, and ‘Meridian’ cannabis cultivars, 15 d after cuttings were taken.

Table 2.
Table 3.

The effects of spectrum and cultivar on increase in height (ΔH), increase in number of nodes (ΔNN), internode length (IL), stem thickness (ST), SPAD value (i.e., chlorophyll content index), aboveground fresh weight (FW), aboveground dry weight (DW), aboveground water content (WC), and relative root dry weight (RRDWT) for ‘Gelato’, ‘Grace’, and ‘Meridian’ cannabis transplants, after the 15-day spectrum treatment period (≈200 µmol·m−2·s−1), then grown for 21 d in a vegetative environment under a metal halide spectrum (≈275 µmol·m−2·s−1).

Table 3.

Aboveground growth of rooted cuttings.

Cuttings were evaluated at the end of the 15-day propagation stage to characterize their development before the transition into the vegetative stage. There were no spectrum effects on SPAD value (i.e., a leaf area-based index of chlorophyll content) with an overall mean of 51 ± 0.3; however, the SPAD value in ‘Grace’ was 4% and 6% higher than in ‘Gelato-27’ and ‘Meridian’, respectively, whereas ‘Gelato-27’ had 2% higher SPAD value than ‘Meridian’ (Table 2). ‘Gelato-27’ cuttings had 12% higher ST in the B vs. F treatments (Table 2). Further, across all spectrum treatments, ‘Gelato-27’ stems were 10% and 26% thicker than ‘Meridian’ and ‘Grace’, respectively, and ‘Meridian’ had 14% thicker stems than ‘Grace’ (Table 2). Across all cultivars, the cuttings in the B treatment had 8% thicker stems vs. both BR and W treatments (Fig. 2C). There were no spectrum effects on new aboveground growth within each cultivar, but ‘Gelato-27’ had 8% more cuttings exhibiting new aboveground growth vs. ‘Meridian’ (Table 2). Across all cultivars, 16% more cuttings had new aboveground growth under the W, B, BR, and B+UVA treatments vs. the F treatment (Fig. 2D).

Post-transplant growth.

After the 21 d post-propagation vegetative period, there were no intra- or intercultivar spectrum effects on the evaluated post-transplant growth attributes, with overall averages (± SE, n = 54) of 42 ± 0.5 cm, 10 ± 0.1, 4.3 ± 0.08 cm, 8.7 ± 0.07 mm, 56 ± 0.3, 96 ± 1.2 g, 21 ± 0.3 g, 78 ± 0.1%, and 0.97 ± 0.002 for ΔH, ΔNN, IL, ST, SPAD value, aboveground FW, aboveground DW, aboveground WC, and relative RRDWT, respectively (Table 3).

Discussion

This trial was designed to evaluate the effects of light spectrum on the rooting, growth, and transplant success of clonal cuttings from different cannabis cultivars that reportedly had varying rooting rates. Traditional fluorescent lighting along with five LED spectra, composed of two fixed-spectrum (W and BR) and three dynamic-spectrum (B, B+UVA, and B50) treatments, were investigated. Some of the spectrum treatment selections were based on previous findings in the literature that high proportions of B can promote root initiation (Gil et al. 2020) and make transplants more compact (Ying et al. 2020), whereas UVA can promote root growth and plants resistant to transplant stress (Verdaguer et al. 2017). Fixed-spectrum LEDs have the advantage of infrastructural and logistical simplicity, whereas dynamic-spectrum treatments have the potential for a higher level of control in using spectrum as a production tool to manipulate crop growth and morphology.

Although there were no intercultivar effects on percentage of rooted cuttings, RI, and RRDWP, there were minimal intracultivar differences in the RRDWP of ‘Grace’ clones. However, when all rooting parameters (rooting percentage, RI, RRDWP, and RRDWT) were examined, there were no intercultivar differences even though the three cultivars were reported to have different rooting behaviors, according to our industry partners. Post-transplant, there were no intracultivar differences in the measured parameters, and the cultivars behaved according to normal growth habits. For example, ‘Gelato-27’ had the lowest increase in height, but also the highest increase in number of nodes (consequently, the shortest internodes) at the end of the vegetative stage. The lack of intracultivar spectrum effects might be due to the short time when the plants are actively growing during the propagation period. As demonstrated in this study, the total propagation period was only 2 weeks and the active growing period must be shorter than that. Studies on floriculture crops also have shown only minimal spectrum treatment effects on both cuttings and seedlings during the propagation stage, which did not remain evident at the finishing stage of production (Craver et al. 2019; Currey and Lopez 2013). Therefore, the minimal spectrum treatment effects observed during the rooting may be transient in nature.

Across all three cultivars, B+UVA appeared to promote rooting (Fig. 2A and B) and, in most cases, the LED treatments promoted the growth of aboveground tissues, compared with the F treatment. There was concern that relatively higher thermal radiation output of fluorescent lighting had the potential to favor aboveground growth attributes in the F treatment. However, periodic measurements of temperature and humidity inside the propagation domes (data not shown) indicated adequate intertreatment uniformity in aerial conditions. Further, aboveground growth attributes were generally lower in the F vs. LED treatments. Generally, the various LED treatments comprised primarily of photons from B and R wavebands (commonly called magenta or “blurple”) produced higher quality rooted cuttings than the broader spectra W LED and fluorescent treatments. Given these results and the potential for >3-times higher efficacy (i.e., conversion of electrical energy into PAR photons; μmol·J−1) of modern horticultural LED vs. fluorescent lighting technologies (DesignLights Consortium 2021; Nelson and Bugbee 2014; Shelford and Both 2021), there are compelling arguments for using LED technologies for the propagation of clonal cannabis.

From the results of the present study, the potential benefits of magenta vs. white fixed spectra, or temporally dynamic spectra (with or without UVA) vs. fixed spectra are less clear. More research is needed to investigate whether dynamic-spectra protocols can have practical benefits in the clonal propagation of cannabis transplants. Fixtures with high proportions of photon flux from B and R LEDs should have higher efficacy than fixtures with relatively high proportions of photon flux from phosphor-converted W LEDs (DesignLights Consortium 2021; Kusuma et al. 2020). Further, in comparing B with R photons, Westmoreland et al. (2021) noted that flowering cannabis yield per area was positively correlated, but yield per energy cost was negatively correlated with increasing B fraction in the spectrum, due to higher energy per photon and lower efficacy of B vs. R LEDs. To compromise between energy efficiency and color rendering, some modern horticultural LED technologies contain a limited proportion of W LEDs to promote improved color rendering while still maintaining the relatively high efficacy emblematic of fixtures composed predominantly of B and R LEDs. Accordingly, the reported efficacies are normally higher than fixtures composed predominantly of W LEDs and lower than fixtures composed entirely of B and R LEDs (DesignLights Consortium 2021). For a given production system, the optimum LED spectrum distribution for cannabis production will depend on the growth stage and cultivators’ specific production goals. In the relatively short propagation stage, the energy efficiency benefits of spectra with high proportions of narrowband R and B may be largely offset by low color rendering (Park and Runkle 2018) and reduced occupant comfort under magenta-hued lighting spectra.

Conclusion

The use of different light spectra derived from LEDs were generally as good or better than fluorescent lighting for the rooting and post-transplant stages of clonal cannabis production. This confirms the viability of transitioning away from older fluorescent lighting technology to more energy-efficient LEDs for the propagation stage of cannabis. All the tested LED treatments can be used in cannabis cutting rooting propagation, although the B and B+UVA appeared to be slightly better in certain aspects.

References Cited

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    • Search Google Scholar
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    • Search Google Scholar
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    • Search Google Scholar
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    • Search Google Scholar
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    • Search Google Scholar
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    • Search Google Scholar
    • Export Citation
  • Kong, Y., Stasiak, M., Dixon, M.A. & Zheng, Y. 2018 Blue light associated with low phytochrome activity can promote elongation growth as shade-avoidance response: A comparison with red light in four bedding plant species Environ. Exp. Bot. 155 345 359 https://doi.org/10.1016/j.envexpbot.2018.07.021

    • Search Google Scholar
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  • Kong, Y. & Zheng, Y. 2018 Variation of phenotypic responses to lighting using a combination of red and blue light-emitting diodes versus darkness in seedlings of 18 vegetable genotypes Can. J. Plant Sci. 99 2 159 172

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  • Kusuma, P., Pattison, P.M. & Bugbee, B. 2020 From physics to fixtures to food: Current and potential LED efficacy Hortic. Res. 7 1 1 9 https://doi.org/10.1038/s41438-020-0283-7

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    • Search Google Scholar
    • Export Citation
  • Moher, M., Llewellyn, D., Jones, M. & Zheng, Y. 2022 Light intensity can be used to modify the growth and morphological characteristics of cannabis during the vegetative stage of indoor production Ind. Crop Prod. 183 114909 https://doi.org/10.1016/j.indcrop.2022.114909

    • Search Google Scholar
    • Export Citation
  • Nelson, J.A. & Bugbee, B. 2014 Economic analysis of greenhouse lighting: Light emitting diodes vs. high intensity discharge fixtures PLoS One 9 6 e99010 https://doi.org/10.1371/journal.pone.0099010

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  • Park, Y. & Runkle, E.S. 2018 Spectral effects of light-emitting diodes on plant growth, visual color quality, and photosynthetic photon efficacy: White versus blue plus red radiation PLoS One 13 8 e0202386 https://doi.org/10.1371/journal.pone.0099010

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  • Potter, D.J. 2009 The propagation, characterisation and optimisation of Cannabis sativa L. as a phytopharmaceutical (PhD Diss) King’s College London London, UK

    • Search Google Scholar
    • Export Citation
  • Potter, D.J. 2014 A review of the cultivation and processing of cannabis (Cannabis sativa L.) for production of prescription medicines in the UK Drug Test. Anal. 6 1–2 31 38

    • Search Google Scholar
    • Export Citation
  • Randall, W.C. & Lopez, R.G. 2014 Comparison of supplemental lighting from high-pressure sodium lamps and light-emitting diodes during bedding plant seedling production HortScience 49 5 589 595 https://doi.org/10.21273/HORTSCI.49.5.589

    • Search Google Scholar
    • Export Citation
  • Rodriguez-Morrison, V., Llewellyn, D. & Zheng, Y. 2021 Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment Front. Plant Sci. 12 456 https://doi.org/10.3389/fpls.2021.646020

    • Search Google Scholar
    • Export Citation
  • Sager, J.C., Smith, W.O., Edwards, J.L. & Cyr, K.L. 1988 Photosynthetic efficiency and phytochrome photoequilibria determination using spectral data Transactions of the Amer. Soc. Agr. Biol. Eng. 31 6 1882 1889 http://10.13031/2013.30952

    • Search Google Scholar
    • Export Citation
  • Shelford, T.J. & Both, A.J. 2021 On the technical performance characteristics of horticultural lamps AgriEngineering. 3 4 716 727 https://doi.org/10.3390/agriengineering3040046

    • Search Google Scholar
    • Export Citation
  • Singh, D., Basu, C., Meinhardt-Wollweber, M. & Roth, B. 2015 LEDs for energy efficient greenhouse lighting Renew. Sustain. Energy Rev. 49 139 147 https://doi.org/10.1016/j.rser.2015.04.117

    • Search Google Scholar
    • Export Citation
  • Small, E. 2017 Medical marijuana: Theory and practice 275 350 Cannabis: A complete guide. CRC Press Boca Raton, FL, USA

  • Verdaguer, D., Jansen, M.A.K., Llorens, L., Morales, L.O. & Neugart, S. 2017 UV-A radiation effects on higher plants: Exploring the known unknown Plant Sci. 255 72 81 https://doi.org/10.1016/j.plantsci.2016.11.014

    • Search Google Scholar
    • Export Citation
  • Wang, R., He, L., Xia, B., Tong, J., Li, N. & Peng, F. 2009 A micropropagation system for cloning of hemp (Cannabis sativa L.) by shoot tip culture Pak. J. Bot. 41 2 603 608

    • Search Google Scholar
    • Export Citation
  • Westmoreland, F.M., Kusuma, P. & Bugbee, B. 2021 Cannabis lighting: Decreasing blue photon fraction increases yield but efficacy is more important for cost effective production of cannabinoids PLoS One 16 3 e0248988 https://doi.org/10.1371/journal.pone.0248988

    • Search Google Scholar
    • Export Citation
  • Yeh, N. & Chung, J.P. 2009 High-brightness LEDs–energy efficient lighting sources and their potential in indoor plant cultivation Renew. Sustain. Energy Rev. 13 2175 2180 https://doi.org/10.1016/j.rser.2009.01.027

    • Search Google Scholar
    • Export Citation
  • Ying, Q., Kong, Y., Jones-Baumgardt, C. & Zheng, Y. 2020 Responses of yield and appearance quality of four Brassicaceae microgreens to varied blue light proportion in red and blue light-emitting diodes lighting Sci. Hortic. 259 108857 https://doi.org/10.1016/j.scienta.2019.108857

    • Search Google Scholar
    • Export Citation
  • Zheng, Y. 2021 Soilless production of drug-type Cannabis sativa Acta Hortic 1305 376 382 https://doi.org/10.17660/ActaHortic.2021.1305.49

Supplemental Fig. 1.
Supplemental Fig. 1.

Relative spectral photon flux distribution for cannabis cuttings rooted under the different spectrum treatments for 15 days: 1) F, fixed fluorescent light spectrum (control); 2) BR, fixed spectrum of B15:R85; 3) B, B75:R25 on day 0–2 followed by B15:R85 on day 2–14; 4) B+UVA, B75:R25 on day 0–2 followed by B15:R85 on day 2–14 plus 15 μmol·m−2·s−1 of UVA on day 7–14; 5) B50, B15:R85 on day 0–7 followed by B50:R50 on day 7–14; and 6) W, fixed white spectrum (5700K).

Citation: HortScience 58, 2; 10.21273/HORTSCI16752-22

Supplemental Table 1.

The conversion from estimated root coverage percentages observed on all surfaces of the rockwool plug to root index (RI).

Supplemental Table 1.
  • Fig. 1.

    The experimental set up depicting (A) the inside of one enclosure (i.e., block) that is encapsulated by panda film, (B) a bench that consisted of three plots, and (C) two trays of cuttings under the white LED treatment (W), centered under the LED fixtures. Photos are from different blocks.

  • Fig. 2.

    Root index (A), root dry weight of plugs (RRDWP) relative to the heaviest root ball (B), stem thickness (C), and percentage of cuttings that exhibited new aboveground growth (D) for cuttings of three cannabis cultivars rooted under different spectrum treatments for 15 d. 1) F, fixed fluorescent light spectrum (control); 2) BR, fixed spectrum of B15:R85; 3) B, B75:R25 on day 0–2 followed by B15:R85 on day 2–14; 4) B+UVA, B75:R25 on day 0–2 followed by B15:R85 on day 2–14 plus 15 µmol·m−2·s−1 of UVA on day 7–14; 5) B50, B15:R85 on day 0–7 followed by B50:R50 on day 7–14; and 6) W, fixed white spectrum (5700K). The photosynthetic photon flux density (PPFD; 400–700 nm) was ≈200 µmol·m−2·s−1 for all treatments. Bars represent means ± standard error of all three cultivars combined (n = 9). Bars that share the same lowercase letter are not significantly different at P ≤ 0.05, according to Tukey-Kramer multiple comparisons test.

  • Supplemental Fig. 1.

    Relative spectral photon flux distribution for cannabis cuttings rooted under the different spectrum treatments for 15 days: 1) F, fixed fluorescent light spectrum (control); 2) BR, fixed spectrum of B15:R85; 3) B, B75:R25 on day 0–2 followed by B15:R85 on day 2–14; 4) B+UVA, B75:R25 on day 0–2 followed by B15:R85 on day 2–14 plus 15 μmol·m−2·s−1 of UVA on day 7–14; 5) B50, B15:R85 on day 0–7 followed by B50:R50 on day 7–14; and 6) W, fixed white spectrum (5700K).

  • Bantis, F., Smirnakou, S., Ouzounis, T., Koukounaras, A., Ntagkas, N. & Radoglou, K. 2018 Current status and recent achievements in the field of horticulture with the use of light-emitting diodes (LEDs) Sci. Hortic. 235 437 451 https://doi.org/10.1016/j.scienta.2018.02.058

    • Search Google Scholar
    • Export Citation
  • Bilodeau, S.E., Wu, B., Rufyikiri, A., MacPherson, S. & Lefsrud, M. 2019 An update on plant photobiology and implications for cannabis production Front. Plant Sci. 10 296 https://doi.org/10.3389/fpls.2019.00296

    • Search Google Scholar
    • Export Citation
  • Birlanga, V., Villanova, J., Cano, A., Cano, E.A., Acosta, M. & Pérez-Pérez, J.M. 2015 Quantitative analysis of adventitious root growth phenotypes in carnation stem cuttings PLoS One 10 7 e0133123 https://doi.org/10.1371/journal.pone.0133123

    • Search Google Scholar
    • Export Citation
  • Campbell, L.G., Naraine, S.G.U. & Dusfresne, J. 2019 Phenotypic plasticity influences the success of clonal propagation in industrial pharmaceutical Cannabis sativa PLoS One 14 3 e0213434 https://doi.org/10.1371/journal.pone.0213434

    • Search Google Scholar
    • Export Citation
  • Caplan, D., Stemeroff, J., Dixon, M. & Zheng, Y. 2018 Vegetative propagation of cannabis by stem cuttings: Effects of leaf number, cutting position, rooting hormone and removal of leaf tips Can. J. Plant Sci. 98 5 1126 1132 https://doi.org/10.1139/cjps-2018-0038

    • Search Google Scholar
    • Export Citation
  • Coffman, C.B. & Gentner, W.A. 1979 Greenhouse propagation of Cannabis sativa L. by vegetative cuttings Econ. Bot. 33 2 124 127 https://doi.org/10.1007/BF02858280

    • Search Google Scholar
    • Export Citation
  • Craver, J.K., Boldt, J.K. & Lopez, R.G. 2019 Comparison of supplemental lighting provided by high-pressure sodium lamps or light-emitting diodes for the propagation and finishing of bedding plants in a commercial greenhouse HortScience 54 1 52 59 https://doi.org/10.21273/HORTSCI13471-18

    • Search Google Scholar
    • Export Citation
  • Currey, C.J. & Lopez, R.G. 2013 Cuttings of Impatiens, Pelargonium, and Petunia propagated under light-emitting diodes and high-pressure sodium lamps have comparable growth, morphology, gas exchange, and post-transplant performance HortScience 48 4 428 434 https://doi.org/10.21273/HORTSCI.48.4.428

    • Search Google Scholar
    • Export Citation
  • DesignLights 2021 DesignLights Consortium Medford, MA, USA

  • Dutta Gupta, S. & Agarwal, A. 2017 Artificial lighting system for plant growth and development: Chronological advancement, working principles, and comparative assessment 1 25 Dutta Gupta, S. Light emitting diodes for agriculture. Springer Nature Singapore http://10.1007/978-981-10-5807-3

    • Search Google Scholar
    • Export Citation
  • Farag, S. & Kayser, O. 2017 The Cannabis plant: Botanical aspects 3 12 Preedy, V.R. Handbook of cannabis and related pathologies. Academic Press Cambridge, MA, USA https://doi.org/10.1016/C2013-0-18721-1

    • Search Google Scholar
    • Export Citation
  • Fluence 2020 Cannabis cultivation guide Fluence Austin, TX, USA

  • Gil, C.S., Jung, H.Y., Lee, C. & Eom, S.H. 2020 Blue light and NAA treatment significantly improve rooting on single leaf-bud cutting of Chrysanthemum via upregulated rooting-related genes Sci. Hortic. 274 109650 https://doi.org/10.1016/j.scienta.2020.109650

    • Search Google Scholar
    • Export Citation
  • Jones-Baumgardt, C., Llewellyn, D., Ying, Q. & Zheng, Y. 2019 Intensity of sole-source light-emitting diodes affects growth, yield, and quality of Brassicaceae microgreens HortScience 54 7 1168 1174 https://doi.org/10.21273/HORTSCI13788-18

    • Search Google Scholar
    • Export Citation
  • Kong, Y., Stasiak, M., Dixon, M.A. & Zheng, Y. 2018 Blue light associated with low phytochrome activity can promote elongation growth as shade-avoidance response: A comparison with red light in four bedding plant species Environ. Exp. Bot. 155 345 359 https://doi.org/10.1016/j.envexpbot.2018.07.021

    • Search Google Scholar
    • Export Citation
  • Kong, Y. & Zheng, Y. 2018 Variation of phenotypic responses to lighting using a combination of red and blue light-emitting diodes versus darkness in seedlings of 18 vegetable genotypes Can. J. Plant Sci. 99 2 159 172

    • Search Google Scholar
    • Export Citation
  • Kusuma, P., Pattison, P.M. & Bugbee, B. 2020 From physics to fixtures to food: Current and potential LED efficacy Hortic. Res. 7 1 1 9 https://doi.org/10.1038/s41438-020-0283-7

    • Search Google Scholar
    • Export Citation
  • Lumigrow 2017 LED grower’s guide for cannabis Lumigrow Emeryville, CA, USA

  • Mah, J.J., Llewellyn, D. & Zheng, Y. 2019 Protocol for converting spectrometer radiometric data to photon flux units http://www.ces.uoguelph.ca/TechNotes.shtml

    • Search Google Scholar
    • Export Citation
  • McCree, K.J. 1972 The action spectrum, absorptance and quantum yield of photosynthesis in crop plants Agric. Meteorol. 9 191 216 https://doi.org/10.1016/0002-1571(71)90022-7

    • Search Google Scholar
    • Export Citation
  • Moher, M., Llewellyn, D., Jones, M. & Zheng, Y. 2022 Light intensity can be used to modify the growth and morphological characteristics of cannabis during the vegetative stage of indoor production Ind. Crop Prod. 183 114909 https://doi.org/10.1016/j.indcrop.2022.114909

    • Search Google Scholar
    • Export Citation
  • Nelson, J.A. & Bugbee, B. 2014 Economic analysis of greenhouse lighting: Light emitting diodes vs. high intensity discharge fixtures PLoS One 9 6 e99010 https://doi.org/10.1371/journal.pone.0099010

    • Search Google Scholar
    • Export Citation
  • Park, Y. & Runkle, E.S. 2018 Spectral effects of light-emitting diodes on plant growth, visual color quality, and photosynthetic photon efficacy: White versus blue plus red radiation PLoS One 13 8 e0202386 https://doi.org/10.1371/journal.pone.0099010

    • Search Google Scholar
    • Export Citation
  • Potter, D.J. 2009 The propagation, characterisation and optimisation of Cannabis sativa L. as a phytopharmaceutical (PhD Diss) King’s College London London, UK

    • Search Google Scholar
    • Export Citation
  • Potter, D.J. 2014 A review of the cultivation and processing of cannabis (Cannabis sativa L.) for production of prescription medicines in the UK Drug Test. Anal. 6 1–2 31 38

    • Search Google Scholar
    • Export Citation
  • Randall, W.C. & Lopez, R.G. 2014 Comparison of supplemental lighting from high-pressure sodium lamps and light-emitting diodes during bedding plant seedling production HortScience 49 5 589 595 https://doi.org/10.21273/HORTSCI.49.5.589

    • Search Google Scholar
    • Export Citation
  • Rodriguez-Morrison, V., Llewellyn, D. & Zheng, Y. 2021 Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment Front. Plant Sci. 12 456 https://doi.org/10.3389/fpls.2021.646020

    • Search Google Scholar
    • Export Citation
  • Sager, J.C., Smith, W.O., Edwards, J.L. & Cyr, K.L. 1988 Photosynthetic efficiency and phytochrome photoequilibria determination using spectral data Transactions of the Amer. Soc. Agr. Biol. Eng. 31 6 1882 1889 http://10.13031/2013.30952

    • Search Google Scholar
    • Export Citation
  • Shelford, T.J. & Both, A.J. 2021 On the technical performance characteristics of horticultural lamps AgriEngineering. 3 4 716 727 https://doi.org/10.3390/agriengineering3040046

    • Search Google Scholar
    • Export Citation
  • Singh, D., Basu, C., Meinhardt-Wollweber, M. & Roth, B. 2015 LEDs for energy efficient greenhouse lighting Renew. Sustain. Energy Rev. 49 139 147 https://doi.org/10.1016/j.rser.2015.04.117

    • Search Google Scholar
    • Export Citation
  • Small, E. 2017 Medical marijuana: Theory and practice 275 350 Cannabis: A complete guide. CRC Press Boca Raton, FL, USA

  • Verdaguer, D., Jansen, M.A.K., Llorens, L., Morales, L.O. & Neugart, S. 2017 UV-A radiation effects on higher plants: Exploring the known unknown Plant Sci. 255 72 81 https://doi.org/10.1016/j.plantsci.2016.11.014

    • Search Google Scholar
    • Export Citation
  • Wang, R., He, L., Xia, B., Tong, J., Li, N. & Peng, F. 2009 A micropropagation system for cloning of hemp (Cannabis sativa L.) by shoot tip culture Pak. J. Bot. 41 2 603 608

    • Search Google Scholar
    • Export Citation
  • Westmoreland, F.M., Kusuma, P. & Bugbee, B. 2021 Cannabis lighting: Decreasing blue photon fraction increases yield but efficacy is more important for cost effective production of cannabinoids PLoS One 16 3 e0248988 https://doi.org/10.1371/journal.pone.0248988

    • Search Google Scholar
    • Export Citation
  • Yeh, N. & Chung, J.P. 2009 High-brightness LEDs–energy efficient lighting sources and their potential in indoor plant cultivation Renew. Sustain. Energy Rev. 13 2175 2180 https://doi.org/10.1016/j.rser.2009.01.027

    • Search Google Scholar
    • Export Citation
  • Ying, Q., Kong, Y., Jones-Baumgardt, C. & Zheng, Y. 2020 Responses of yield and appearance quality of four Brassicaceae microgreens to varied blue light proportion in red and blue light-emitting diodes lighting Sci. Hortic. 259 108857 https://doi.org/10.1016/j.scienta.2019.108857

    • Search Google Scholar
    • Export Citation
  • Zheng, Y. 2021 Soilless production of drug-type Cannabis sativa Acta Hortic 1305 376 382 https://doi.org/10.17660/ActaHortic.2021.1305.49

Melissa Moher School of Environmental Science, University of Guelph, Guelph, Ontario, Canada, N1G 2W1

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David Llewellyn School of Environmental Science, University of Guelph, Guelph, Ontario, Canada, N1G 2W1

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Scott Golem HEXO Corp., 120 Chem. de la Rive, Gatineau, Quebec, Canada, J8M 1V2

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Elizabeth Foley HEXO Corp., 120 Chem. de la Rive, Gatineau, Quebec, Canada, J8M 1V2

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Steve Dinka HEXO Corp., 120 Chem. de la Rive, Gatineau, Quebec, Canada, J8M 1V2

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Max Jones Department of Plant Agriculture, University of Guelph, Guelph, Ontario, Canada, N1G 2W1

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Youbin Zheng School of Environmental Science, University of Guelph, Guelph, Ontario, Canada, N1G 2W1

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Contributor Notes

We thank Ontario Centres of Excellence and HEXO Corp. for financial support, and HEXO Corp. for providing the plant material and production space for this experiment. We also thank Bluelab Corporation for their measurement tools, and Madelyn Hinde and Allison Slater for their outstanding technical support.

Y.Z. is the corresponding author. E-mail: yzheng@uoguelph.ca.

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  • Fig. 1.

    The experimental set up depicting (A) the inside of one enclosure (i.e., block) that is encapsulated by panda film, (B) a bench that consisted of three plots, and (C) two trays of cuttings under the white LED treatment (W), centered under the LED fixtures. Photos are from different blocks.

  • Fig. 2.

    Root index (A), root dry weight of plugs (RRDWP) relative to the heaviest root ball (B), stem thickness (C), and percentage of cuttings that exhibited new aboveground growth (D) for cuttings of three cannabis cultivars rooted under different spectrum treatments for 15 d. 1) F, fixed fluorescent light spectrum (control); 2) BR, fixed spectrum of B15:R85; 3) B, B75:R25 on day 0–2 followed by B15:R85 on day 2–14; 4) B+UVA, B75:R25 on day 0–2 followed by B15:R85 on day 2–14 plus 15 µmol·m−2·s−1 of UVA on day 7–14; 5) B50, B15:R85 on day 0–7 followed by B50:R50 on day 7–14; and 6) W, fixed white spectrum (5700K). The photosynthetic photon flux density (PPFD; 400–700 nm) was ≈200 µmol·m−2·s−1 for all treatments. Bars represent means ± standard error of all three cultivars combined (n = 9). Bars that share the same lowercase letter are not significantly different at P ≤ 0.05, according to Tukey-Kramer multiple comparisons test.

  • Supplemental Fig. 1.

    Relative spectral photon flux distribution for cannabis cuttings rooted under the different spectrum treatments for 15 days: 1) F, fixed fluorescent light spectrum (control); 2) BR, fixed spectrum of B15:R85; 3) B, B75:R25 on day 0–2 followed by B15:R85 on day 2–14; 4) B+UVA, B75:R25 on day 0–2 followed by B15:R85 on day 2–14 plus 15 μmol·m−2·s−1 of UVA on day 7–14; 5) B50, B15:R85 on day 0–7 followed by B50:R50 on day 7–14; and 6) W, fixed white spectrum (5700K).

 

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