Hemp root system architecture and allometric relationships vary between monoecious and dioecious cultivars

Hemp (Cannabis sativa L.) cultivation in the United States is reemerging following legalization in the 2018 Farm Bill. Cultivation of hemp for fiber, grain, and flowers has encountered many challenges, including stand establishment and unknowns regarding ecosystem services the crop may provide. Hemp proponents have touted hemp as being carbon neutral or negative, but there is limited research on its root system, the most direct pathway for carbon into soil. Therefore, we sought to measure root system architecture across a subset of commercially available cultivars and to measure aboveground traits to develop allometric relationships that could be used for carbon market estimates. Two monoecious (Fibror 79 and Futura 75/83) and two dioecious cultivars (New West Genetics 2730 and Yuma) were selected from the University of Kentucky's Hemp Trials in 2022 and 2023. Plants (n = 16 total per cultivar; eight per sex for dioecious cultivars) were randomly selected and excavated at early senescence. Plant height, stem basal diameter, aboveground biomass, and root biomass were quantified for development of allometric relationships. Root systems were imaged in PhotoCapture360 and analyzed for root traits, such as total length and average diameter, with GiARoots. Basal diameter was predictive of both aboveground and belowground biomass (r² = 0.69 and 0.77, respectively). Additionally, female and monoecious root systems had smaller diameters but were twice as large as male root systems in mass and surface area. Overall, this study developed predicative equations that may be useful in quantifying hemp's growth and carbon capture potential by flowering behavior.