Luca De Prato

Christmas Island, Australia, is a remote tropical island that relies predominantly on phosphate mining income. Given that mining operations will decline during the next decade, there is an urgent need to explore alternative industries to maintain local employment. This need could be met through activities such as a transition from mining to the production of high-value crops established in post-mined quarries. However, agriculture post-mining presents a series of challenges to growing crops, such as the lack of appropriate nutrients and organic matter, and the potential for soil compaction and the presence of heavy metals. Industrial hemp (Cannabis sativa L.) grows well in contaminated substrates, making it an ideal candidate for evaluation in the transition from mining to agriculture on Christmas Island. Hemp is a crop gaining increasing attention due to its wide range of commercial products and the relaxing of legal restrictions associated with its secondary metabolites. Preliminary experiments with hemp on Christmas Island identified a range of challenges to plant growth, such as the early onset of flowering, stunted growth, nutrient deficiency symptoms, low yield, and fluctuations in cannabinoid concentrations. Although some research has been carried out on hemp globally, there is limited information on optimal conditions for flowering, growth and nutrition, and cannabinoid production under tropical/subtropical environmental conditions. The current study examined a range of hemp varieties in terms of time to flowering, growth, biomass and seed yields, as well as cannabinoid production, in response to a series of abiotic stressors present under the tropical conditions of Christmas Island in postphosphate mined soil. Before undertaking these experiments, an improved method for extraction and detection of cannabinoids was developed. Following this, to investigate the challenges associated with growing hemp on Christmas Island, selected tropical/subtropical varieties (ECO-GH15, ECO-MC16 and ECO-YP16), bred by Ecofibre Limited, and the temperate hemp variety (Morphet Late) were evaluated under different daylengths, temperatures, nutrition and heavy metal substrate concentrations, in controlled growth rooms, to determine physiological, cannabinoids and growth responses. The combination of daylength and temperature affected time to flowering, and consequently, final biomass. Under short daylength (11.5 h and 12.5 h), temperature was shown to be the main driver for time to flowering, depending on genotype. Cannabinoid concentrations were affected by one extra hour of daylength, and varieties responded differently. These experiments revealed the responses of diverse ecotypes and the importance of environment by genotype interactions. Soil nutrient content impacted time to flowering, photosynthesis, leaf gas exchange and plant growth, resulting in significant differences in biomass, seed and cannabinoid production. The addition of nitrogen was associated with higher above ground biomass. Plants not receiving nitrogen had greatly diminished development and growth. Phosphate deficiency delayed time to flowering and flower development. Increasing phosphate concentrations and slow-release fertiliser application increased net photosynthesis under high photosynthetically active radiation (1800 μmol m-2s-1). Potassium deficiency resulted in increased stomatal conductance and gas exchange, possibly due to impaired stomatal function. The investigation of potential cadmium uptake confirmed that hemp could grow in soils containing cadmium and that the plants limit its translocation to leaves and seeds with increasing cadmium concentration in the soil. Overall, this work demonstrates the importance of genotype by environment studies to identify appropriate ecotypes and optimal plant nutrition to achieve consistent production from hemp in tropical/subtropical conditions. The results of this study contribute to our understanding of hemp production in terms of time to flowering, cannabinoid production, and physiology while providing a potential economic opportunity for Christmas Island. These findings could also inform other remote communities at similar latitudes with post-mining soils regarding the requirements for growing hemp as a high-value crop. Remote communities could take advantage of multipurpose applications of hemp as a viable option for high-value products. Knowledge of hemp cannabinoid production as affected by the environment will be essential, globally, to tailor cannabinoids for particular needs.