Morpho-physiology and cannabinoid concentrations of hemp (Cannabis sativa L.) are affected by potassium fertilisers and microbes under tropical conditions

Industrial hemp (Cannabis sativa L.) is a crop with the potential for multiple products. However, there is a lack of information regarding the effects of some key nutrients and soil microbiology. Potassium (K) and microbes can affect plant physiology, growth, and secondary metabolite production. A glasshouse experiment in a fully controlled growth room was undertaken to assess K requirements and effects on physiology on a tropical/subtropical variety of industrial hemp, ECO-GH15, bred for Australian conditions by Ecofibre (Brisbane, QLD, Australia). Increasing rates of K of conventional fertiliser liquid applications were applied to hemp plants and compared to a slow-release form containing soil microbes (SRK) under short daylength (12.5 h), simulating a tropical environment. Application of 11, 43 and 129 ppm of conventional fast-release potassium K as potassium sulphate (K₂SO₄), or a slow (controlled) release form (131 ppm of K), were applied to plants and their growth parameters compared. A series of physiological and growth response data was collected, including photosynthesis response to increasing active radiation (PAR or PPFD) and carbon dioxide (CO2), and cannabinoid production. Plant growth responses, such as stalk diameter, height, and shoot dry weight, increased with SRK application, while no differences were observed between the three increasing conventional K rates. The lower conventional K rate (11 ppm) resulted in increased photosynthetic activity up to 600–700 PPFD. Physiological data showed that the increasing K rate produced less efficient plants in terms of PPFD and CO₂ utilisation. Cannabinoid analysis showed an increase in cannabidiol (CBD) and cannabidivarin (CBDV) at the higher K rate, while the SRK increased the production of delta-9-tetrahydrocannabinol (THC) and similar cannabinoids. This study revealed that sustained nutrient application improved plant photosynthesis and gas exchange regulation resulting in increased plant growth and cannabinoid production under tropical conditions, and helped to better understand the role of K in plant stress and physiological efficiency.