The cannabinoid content of cannabis is primarily determined by genetic factors dependent on variety, tissue type, position of tissue, and the growth stage. However, environmental conditions are the other major factors that contribute to the production and accumulation of cannabinoids. Environmental stresses, such as light, temperature, water deficit, nutrition, heavy metals, phytohormones, soil bacteria, and biotic stresses can all greatly affect the production of THC and CBD, for example. With a series of stress experiments, a recent study conducted by Colorado State University set out to investigate the effects of short-term environmental stresses on the onset of cannabinoid production in young immature flowers.
The stress experiments
For the analysis, the local Colorado hemp variety that is known to produce around 5–10% CBD, named Green-Thunder, was grown in a greenhouse maintained at 20–25 oC under a 16 h light/8 h dark cycle at a humidity level of 50–70%. Several types of stress experiments were set up. For the mechanical wounding experiment, on day 1, three fully expanded fan leaves, at least three nodes from the top were punctured using a 1/4″ round hole punch for 12 holes per leaf. For herbivore damage, a total of twenty 3rd instar caterpillar larvae of tobacco hornworm Manduca sexta were placed on the leaves of six hemp plants that were covered with a mesh net bag for 5 days. For the heat treatment, excess heat was generated by a heater that maintained the grow tent at 45–50 oC for 7 days. For the drought experiment, a water deficient condition was simulated by irrigating twice a week with 200 ml which is approximately 20% relative water content in the soil, while control plants were grown under well-watered condition.
Drought stress: 80% CBD and THC decrease
So what were the results of the different tests? The study found that the 5 days of mechanical wounding did not affect the production of any of the cannabinoids during the initial stage of flowering. However, after 5 days of herbivore treatment, there was a significant difference in concentration between day 1 and day 6 of CBGA (control: 308 μg/g; treatment – 24 μg/g), CBG (control: 69 μg/g; treatment: 52 μg/g), and CBD (control: 755 μg/g; treatment: 194 μg/g) between the control and treatment plants. The 7 days of heat treatment at 45–50 oC significantly reduced the production of CBGA during this observed window (control: 206 μg/g; treatment: 182 μg/g) and CBG (control: 21 μg/g; treatment: − 112 μg/g). Notably, the largest change was observed after 7 days of drought stress, when plants showed a 40% greater accumulation of CBG (control: 336 μg/g; treatment: 622 μg/g), and a significant decrease (70–80%) in CBD (control: 1182 μg/g; treatment: 297 μg/g) and THC amounts (control: 3927 μg/g; treatment: 580 μg/g).
The researchers note that further studies are necessary to validate if the observed changes apply to mature buds and to understand the hypothesized underlying gene regulation causing these increases or decreases in cannabinoid concentrations.