Luca De Prato

Hemp (Cannabis sativa L.) is an increasingly important multipurpose crop, valued for its industrial, nutritional, and medicinal uses. Flowering behaviour in hemp, largely governed by photoperiod sensitivity, is a key determinant of crop adaptation, productivity, and end-use quality. In this study, we propose a novel photoperiod-based classification system for hemp varieties, informed by synthesis and comparative analysis of phenotypic data, multi-environment trial observations, and published literature. Varieties are grouped into four functional categories: Photoperiod-Sensitive Pioneers (PSPs), Intermediate Photoperiod Adapters (IPAs), Solstice-Triggered Hybrids (STHs), and Biomass Maximisers (BMax). This framework offers a practical tool for aligning genotype selection with specific environmental conditions and production objectives. It supports the development of targeted breeding programs, optimised crop management, and precision phenological strategies, particularly under diverse photothermal regimes. We outline the agronomic implications associated with each varietal group and highlight areas for further research into the genetic and physiological mechanisms underpinning photoperiod responsiveness. This classification promotes a more systematic understanding of hemp phenology and strengthens its potential contribution to sustainable and climate-resilient agriculture.
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Growing a high-value crop such as industrial hemp (Cannabis sativa L.) in post-mining environments is economically and environmentally attractive but faces a range of biotic and abiotic challenges. An opportunity to investigate the cultivation of C. sativa presented itself as part of post-mining activities on Christmas Island (Australia) to profitably utilise disused phosphate (PS) quarries. Challenges to plant growth and cadmium (Cd) uptake were addressed in this study using potted plants under fully controlled conditions in a growth chamber. A complete nutritional spectrum, slow-release fertiliser was applied to all plants as a control treatment, and two levels of rock PS dust, a waste product of PS mining that contains 35% phosphorus (P) and 40 ppm of naturally occurring Cd, were applied at 54 and 162 g L−1. After 12 weeks, control plants (no PS dust) significantly differed in phenological development, with no flower production, lower aboveground biomass and reduced photosynthesis efficiency than those with P applied as rock dust. Compared with the controls, the 54 g L−1 level of P dust increased shoot biomass by 38%, while 162 g L−1 increased shoot biomass by 85%. The concentration of Δ9-tetrahydrocannabinol also increased with the higher P levels. Cd uptake from PS dust by C. sativa was substantial and warrants further investigation. However, there was no increase in Cd content between the 54 and 162 g L−1 application rates in seed and leaf. Results indicate that hemp could become a high-value crop on Christmas Island, with the readily available rock PS dust providing a source of P.

Hemp (Cannabis sativa L.) has regained worldwide interest as a crop across temperate and subtropical regions. However, there is a paucity of information about the responses of hemp to environmental conditions and interactions between genotype and environment. This study compared the growth responses of a temperate hemp variety (Morphet Late) and three tropical/subtropical varieties (ECO-GH15, ECO-MC16 and ECO-YP16) to tropical daylengths (11.5 h and 12.5 h), temperatures and varying nitrogen (0, 50, 100, 150 kg ha-1 of N) rates. Three experiments under controlled environmental conditions were conducted to test the growth, and cannabinoid production responses to N. Analysis of phenological data revealed significantly different responses between varieties in terms of days to emergence, time to flowering, growth patterns and final biomass showing predominant responses to daylength or temperature depending on the variety. Two tropical/subtropical varieties (ECO-GH15, ECO-MC16) responded similarly under all conditions; the other (ECO-YP16) showed a marked response to temperature rather than daylength, although all varieties had a strong biomass response to N increase. For all varieties, except the temperate variety, the concentration of Δ9-tetrahydrocannabinol (THC), cannabidiol (CBD), and some minor cannabinoids were significantly increased by a one-hour daylength increase. This study highlights the importance of temperature, daylength, and nitrogen for growth, time to flowering, and cannabinoid concentrations of different hemp varieties grown under tropical conditions and shows the importance of selecting low THC varieties for production in tropical/subtropical environments.

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.

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.