High air temperature reduces plant specialized metabolite yield in medical cannabis, and has genotype-specific effects on inflorescence dry matter production

Improving and standardizing the production of medical cannabis is essential for developing consistent and uniform medical products. Key challenges are achieving high concentrations of plant specialized metabolites (PSMs), uniformity of PSMs at different positions in the canopy, and high inflorescence dry matter production, while minimizing energy inputs for heating, ventilation, cooling, and electrical lighting. This study evaluated the effects of air temperature and photosynthetic photon flux density (PPFD) on PSM and dry matter production, and photosynthetic efficiency in medical cannabis (Cannabis sativa), 'Original Blitz' and 'Harmony CBD'. Plants were grown in climate-controlled chambers at three PPFD (600, 900, 1200 μmol m⁻² s⁻¹) during the short-day (generative) phase. The experiment spanned four cultivation cycles, with two at a lower temperature (day/night 25/21 °C) and two at a higher temperature (31/27 °C) during the short-day phase. Higher air temperature reduced total cannabinoid concentrations, but had no effects on terpenoids, while enhancing PSM uniformity between upper and lower inflorescences. Further, higher air temperature either decreased inflorescence dry matter production (in 'Harmony CBD') or had no effect (in 'Original Blitz'), thus influencing total cannabinoid yield. Increasing PPFD resulted in a linear rise in inflorescence dry matter production without affecting PSM composition, increasing overall cannabinoid yield. Toward the end of the short-day phase, leaf photosynthesis declined, likely due to leaf senescence. High temperatures caused abnormal inflorescence clusters to develop on top of older inflorescences, disrupting typical maturation and leading to lower cannabinoid levels.