Cannabis Yield, Potency, and Leaf Photosynthesis Respond Differently to Increasing Light Levels in an Indoor Environment

Abstract

Since the recent legalization of medical and recreational use of cannabis (Cannabis sativa L.) in many regions worldwide, there has been high demand for research to improve yield and quality. With the paucity of scientific literature on the topic, this study investigated the relationships between light intensity (LI) and photosynthesis, inflorescence yield, and inflorescence quality of cannabis grown in an indoor environment. After growing vegetatively for 2 weeks under a canopy-level photosynthetic photon flux density (PPFD) of ≈ 425 μmol·m-2·s-1 and an 18-h light/6-h dark photoperiod, plants were grown for 12 weeks in a 12-h light/12-h dark ‘flowering’ photoperiod under canopy-level PPFDs ranging from 120 to 1800 μmol·m-2·s-1 provided by light emitting diodes. Leaf light response curves varied both with localized (i.e., leaf-level) PPFD and temporally, throughout the flowering cycle. Therefore, it was concluded that the leaf light response is not a reliable predictor of whole- plant responses to LI, particularly crop yield. This may be especially evident given that dry inflorescence yield increased linearly with increasing canopy-level PPFD up to 1800 μmol·m-2·s-1, while leaf-level photosynthesis saturated well below 1800 μmol·m-2·s-1. The density of the apical inflorescence and harvest index also increased linearly with increasing LI, resulting in higher-quality marketable tissues and less superfluous tissue to dispose of. There were no LI treatment effects on cannabinoid potency, while there were minor LI treatment effects on terpene potency. Commercial cannabis growers can use these light response models to determine the optimum LI for their production environment to achieve the best economic return; balancing input costs with the commercial value of their cannabis products.

High Light Intensities Can Be Used to Grow Healthy and Robust Cannabis Plants During the Vegetative Stage of Indoor Production

“…However, the present study observed a 24% increase in area-based chlorophyll content, which may indicate that cannabis favours upregulating photosynthetic capacity (i.e., maximizing resource utilization) over the common foliar morphology-based adaptive responses to high light stress. Clonal cannabis' very high photosynthetic capacity (Chandra et al, 2008;Rodriguez-Morrison et al, 2021) appears to be present even at the relatively young vegetative stage (Chandra et al, 2015). In the context of indoor production, the reduction in individual leaf area with increasing LI may also confer an increase in whole-plant net photosynthesis, since a greater proportion of the incident PAR should penetrate deeper into the canopy through inherent reductions in self-shading.…”

“…Following initial setup, the PPFD at the top of each plant was measured and recorded twice weekly using a quantum sensor (LI-180; LI-COR Biosciences, Lincoln, NE, USA), and the fixture hang-heights were adjusted accordingly, to maintain consistent canopy-level PPFDs throughout the trial. Although the layout of the experiment was a RCBD, the trial was conducted as a gradient design (Jones-Baumgardt et al, 2020;Rodriguez-Morrison et al, 2021) with each plant treated as an experimental unit and assigned a LI level consistent with their respective accumulated light histories. To this end, the average PPFD (APPFD) each individual plant received over the trial was obtained by computing the light integrals between each bi-weekly PPFD measurement period, summing these integrals over the entire trial to determine a total light integral (TLI, mol•m -2 ), and then back-calculating to determine APPFD by dividing TLI by the total number of seconds of lighting during the trial (i.e., 3600 s•hr -1 × 16 hr Ruter, 1992).…”

“…In the propagation stage, the photoperiod is generally 18 -24 h (Chandra et al, 2020) and canopy-level photosynthetic photon flux density (PPFD, µmol•m -2 •s -1 ) is usually low (Fluence, 2020;Lumigrow, 2017) to minimize transpiration loss as the clonal plants establish new root systems. After approximately two weeks in propagation, rooted cuttings (i.e., transplants) transition into the vegetative stage (Caplan et al 2018) where they are exposed to similar photoperiods but higher PPFD than propagation to encourage strong vegetative growth to prepare the plants for the flowering stage (Rodriguez-Morrison et al, 2021). After approximately two to four weeks in the vegetative stage, plants are transitioned to a 12-h photoperiod and even higher PPFD to enhance growth and yield.…”

“…The LI during the vegetative stage can influence plant growth attributes such as height, stem thickness, branching, leaf size, leaf thickness, and biomass partitioning (Poorter et al, 2019). Since these attributes affect a crop's robustness as it enters the flowering stage, the vegetative stage LI must be selected to promote the development of the foundational structure (e.g., thicker stems and more nodes) needed to support prolific inflorescence development, which can account for more than half of the total aboveground biomass at peak maturity (Rodriguez-Morrison et al, 2021). The current lack of scientific information related to LI during the vegetative stage has resulted in a broad range of canopy-level PPFDs (e.g., 250 to 650 µmol•m -2 •s -1 ) being recommended to cultivators (Fluence, 2020;Lumigrow, 2017).…”

“…The current lack of scientific information related to LI during the vegetative stage has resulted in a broad range of canopy-level PPFDs (e.g., 250 to 650 µmol•m -2 •s -1 ) being recommended to cultivators (Fluence, 2020;Lumigrow, 2017). Since cannabis can tolerate (Chandra et al, 2008) and even flourish (Rodriguez-Morrison et al, 2021) under very high LI, there is opportunity to elevate PPFD during the vegetative stage to enhance plant structure and shorten the length of the vegetative stage. Therefore, the objective of this study was to determine the effects of a broad range of LI on vegetative stage cannabis morphology and growth attributes, to guide cultivators towards optimizing the LI for their specific production strategies.…”