Many medical cannabis researchers have focused on the effects of light intensity on the crop, both in terms of yield and metabolite content. However, little attention has been paid to another variable that is closely connected to increased light intensity: air temperature. This raises the question: would increased air temperature have any effect on flower yield and cannabinoid content? Mexx Holweg, PhD at the Horticulture and Product Physiology Group of Prof. Dr. Leo Marcelis at Wageningen University and Research, asked himself that question to see if there were also ways to provide growers with a practical toolkit to improve their cultivation. With the support of a consortium of companies including Hortilux, Cultilene, Perfect Plants, LetsGrow, and Delphy, Mexx started to dive deeper into the nuances of CEA cannabis cultivation.
"Especially relevant for growers, we wanted to know if increasing air temperature would allow better use of the light," says Mexx. The study delved into photosynthesis, metabolite production, and the uniformity of cannabinoids and terpenes in flowers in the upper and lower canopy, all critical aspects for growers targeting the medical and pharmaceutical market, where uniformity and standardization of the quality of medical cannabis flower is essential.
The experiment
The experiment used two cultivars, one THC-rich and the other more CBD-rich. Throughout the cultivation, Mexx measured photosynthesis rates that could explain the physiological response of medical cannabis to different light intensities and air temperatures, and determined the effect on the development of cannabinoid and terpene content by taking multiple flower samples. "We did that in different stages. For instance, we did it not only during the harvest but also 5 days before harvest, 10 days before harvest, and so on." According to him, this approach is quite unique in cannabis research. "This allowed us to see the development of cannabinoids and terpenes during flower development, instead of measuring at one time point, potentially missing our treatment effects, and to also better understand what the best harvest day would be based on maximum cannabinoid and terpene concentrations."
Relation with lighting
The results showed a decline in photosynthesis as flowering progressed, suggesting that light intensity could potentially be reduced towards the end of the flowering phase without impacting yield. "This indicates a possibility to reduce energy costs," Holweg added.
The findings also pave the way for future research into dynamic lighting strategies. "With fluctuating energy prices, we could adjust light intensity based on the plant's needs and electricity costs," Holweg suggested. This concept aligns with broader research initiatives at Wageningen University, which are exploring dynamic cultivation environments to optimize resource use.
One of the key findings was the relationship between light intensity and the production of assimilates, which are used for plant growth. "If you increase light intensity, you produce more assimilates, which could potentially be directed towards cannabinoid and terpene production," Holweg noted. However, while the yield increased—approximately 1% more flower yield per 1% increase in light intensity within the 600-1200 µmol m-2 s-1 range—there was no corresponding increase in cannabinoid concentration in either the top or bottom canopy.
"The concentrations of the compounds are not so much affected by light intensity but rather by the flower stage," Holweg explained. The study also highlighted that while the amount of assimilates would increase with increasing light intensity, the overall cannabinoid and terpene concentration remained constant, although the total cannabinoid yield per square meter did increase due to an increased flower yield.
Looking into air temperature
Air temperature played a significant role in the study, affecting both flower morphology and metabolite concentration. "We saw that increasing air temperature reduced the accumulation of cannabinoids and terpenes due to abnormal flower growth, identified as inflorescence reversion, also known as fox-tailing," Holweg said. These phenomena led to abnormal growth of young calyxes, with low cannabinoid concentrations, on top of mature flowers with high cannabinoid concentrations, which negatively impacted total compound concentrations and uniformity.
Interestingly, the effect of air temperature varied by cultivar. "For the Original Blitz THC cultivar, higher air temperatures didn't significantly affect flower yield," Holweg observed. "But for the Harmony CBD cultivar, increased air temperature reduced both plant growth and flower yield." This highlights the importance of cultivar-specific responses, or what Holweg referred to as "the plasticity of the genotype to air temperature."
For growers, the study underscores the importance of balancing light intensity and air temperature to optimize yield and quality. "Low air temperature during the flowering phase combined with increased light intensity appears to be the best approach," Holweg concluded. However, the research also highlighted the nuanced responses of different cultivars, emphasizing the need for tailored strategies in cannabis cultivation and further research to find a less boring and more generalizable answer than 'it depends on the cultivar.'
The paper 'High air temperature reduces plant specialized metabolite yield in medical cannabis, and has genotype-specific effects on inflorescence dry matter production' can be found here.