Photobiology of medicinal cannabis : pharmaceutical compounds and crop morphology

Medicinal cannabis (Cannabis sativa) is a high-value crop whose economic potential relies on optimising the yield of specialised metabolites, particularly cannabinoids and terpenoids, concentrated in female inflorescences. In controlled-environment agriculture, fine-tuning environmental factors, especially light intensity, spectral composition, and air temperature, is key to maximising both inflorescence and metabolite yields. This thesis investigates the physiological mechanisms underpinning inflorescence development and metabolite production, with a focus on how light and temperature modulate plant morphology, photosynthesis, and metabolite accumulation across developmental stages. Mother plant management was studied in relation to cutting propagation. While mother plant age did not impact rooting success, light intensity during mother plant cultivation influenced carbohydrate reserves in cuttings, with mixed effects on rooting across genotypes. Higher light intensity during propagation consistently improved root dry mass. During the flowering phase, air temperature and light intensity exerted significant but genotype-dependent effects. Elevated temperatures reduced cannabinoid concentrations due to abnormal inflorescence formation but improved metabolite uniformity across plant canopy levels. High temperatures also variably affected inflorescence yield across genotypes. In contrast, increasing light intensity linearly enhanced inflorescence yield and total metabolite output per area, without diminishing metabolite concentration. Late-stage photosynthetic decline suggests potential for adaptive lighting strategies to conserve energy toward the end of flowering. Light spectrum further modulated productivity. Combining red light peaks at 640 nm and 660 nm improved photosynthesis and inflorescence yield compared to a single red peak, while broader spectra at high light intensity achieved similar outcomes. Cannabinoid concentrations remained stable across spectral treatments, though terpenoid levels increased with dual red-peak lighting under high intensity highlighting spectrum-by-intensity interactions. Finally, supplemental ultraviolet (UV-A + UV-B) radiation exerted minimal or inconsistent effects, depending on background light intensity and plant acclimation. At lower light intensities, UV slightly enhanced metabolite content near harvest but reduced inflorescence yield. Under higher light intensity, UV modestly improved inflorescence yield without altering metabolite levels, suggesting diminishing returns as plants acclimate to high background light. In conclusion, this research demonstrates that medicinal cannabis tolerates high light intensities while maintaining light-use efficiency and supports the use of dynamic lighting and integrated environmental strategies to optimise yield and resource use. While light spectrum influences architecture and terpenoid accumulation, cannabinoid levels are more strongly governed by genotype and inflorescence developmental stage. The findings emphasise the value of tailoring cultivation practices to balance productivity, sustainability, and quality, advocating for a broader definition of quality that encompasses the full spectrum of bioactive metabolites beyond DELTA9-THC.