Author ORCID Identifier

https://orcid.org/0000-0001-6027-7990

Date Available

8-4-2025

Year of Publication

2025

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy (PhD)

College

Agriculture

Department/School/Program

Plant and Soil Sciences

Faculty

Dr. Robert Pearce

Faculty

Dr. Arthur Hunt

Abstract

Industrial hemp (IH) establishment has many challenges including: poor emergence, low stand establishment, and variable plant development in direct seeded hemp. In floral hemp production, quality transplant production is a challenge with seedlings often becoming root bound and etiolated. In both direct seeded and transplant hemp, the relationship between root vigor and vegetative growth has not been studied and may prove insightful for managing emergence and establishment challenges. The goals of this study were three-fold: 1) to measure and describe root system architecture (RSA) and root phenotypes observed in selected IH cultivars especially in regard to identifying agronomically important RSA traits; 2) To explore emergence, stand establishment, plant growth, and correlating RSA features to better stand establishment in IH cultivars grown for both dual purpose (DP) and fiber (F) production; and 3) to explore how plant growth regulators (PGRs) may help manage early root and shoot growth in transplanted floral hemp (TFH) and the impacts these chemicals may have on mature plants and floral yield. Both two and three dimensional (2D & 3D) root phenotyping was conducted on a selected population of three hemp morphotypes (Chinese [CH], Grain/Dual [GD], and Fiber/Feral [FF]). Differences in 2D imaging revealed greater median number of roots and root tips in the CH morphotypes compared to the GD. The 3D data revealed differences among morphotypes at discrete timepoints from differences in rooting depth, width, and rooting lengths of both total and thicker roots (0.5 – 1.0 mm). Early stand establishment observations demonstrated: that a high proportion of live seed planted apparently never emerged, uneven emergence and seedling development contributed to further plant losses between establishment and harvest as late emerging smaller plants struggled for resources, and when coupled with RSA data collected on the same group of cultivars some poor stand establishment may be attributed to a lack of robust root growth and development. For transplanted hemp, a gibberellic acid (GA) blocker plant growth regulator, prohexadione calcium (PCa) was used as a substrate drench at rates ranging from 0 to 300 mg L-1 to manage growth of hemp transplants. This PGR, is involved in reducing cell elongation after initial cellular division, and thus acts as a plant growth retardant to try and manage stem etiolation and root binding. Rates of PCa ranging from 75 – 150 mg · L-1 were found to slow root and shoot growth in transplant production. These rates did not negatively impact final floral yields, though higher concentrations (300 mg · L-1), greater abiotic stress conditions, and cultivar selection may impact these results. To overcome early stand establishment challenges in IH, seed emergence, even seedling development, and selecting for cultivars with vigorous rooting depths, greater total lengths, and wider rooting perimeters are needed in direct seeded hemp. In transplanted hemp poor transplant quality due to etiolation and root binding can be managed through utilizing PCa (75 – 150 mg · L-1) without negatively impacting final floral yields. When taken in its entirety, this work provides valuable information for improving hemp management, and also suggests specific areas of research to improve emergence and establishment in direct and transplanted hemp.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2025.313

Funding Information

This project was funded in part by USDA, Agricultural Research Service, Forage-Animal Production Research Unit as part of National Program 215, Grass, Forage, and Rangeland Agroecosystems, project number 5042-21500-001-000D.

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