Babitha Jampala, Mullens, Ashley
Hemp is a valuable new agricultural crop grown for seed/oil, fiber, and metabolites. Hemp belongs to the genus Cannabis in which plants produce a unique class of compounds called cannabinoids. These cannabinoids help in chemical defense in the human body and also have pharmaceutical and psychoactive properties. There are around 120 cannabinoids, but the two most important cannabinoids are delta-9 tetrahydrocannabinol (THC) and cannabidiol (CBD). Hemp according to the Agricultural act of 2014 was defined as a plant Cannabis sativa L. with a THC concentration of not more than 0.3% on a dry weight basis. The plants that have a higher percentage of THC are regarded as marijuana types. With only two known compounds, there is great potential to uncover its full chemical structure and evaluate improvement of traits to take advantage of its full suite of agronomic and medicinal value. With strong interest in Louisiana in developing hemp as an agricultural crop, research for the development of better traits in warranted.
Traditional breeding in addition to the use of molecular markers will help in speeding up the process of development of hemp varieties suitable for the needs of Louisiana stakeholders. Development and application of molecular breeding techniques can help understand and determine important traits. There are many traits in hemp that gives it an important crop status. It can be grown in most climates and requires minimal care. Hemp varieties exhibit considerable differences in height, maturity, oil content, seed size, and cannabinoid profile. The traits of interest are fiber length and quality, adaptability, pest and disease resistance, oil content, and cannabinoid profile.
Hemp plants can grow up to 1-5 m depending on growing conditions and their genetic makeup. Is stems are erect, usually branched with woody interiors and hollow internodes. For fiber, relatively unbranched stems with long internodes are preferred. Hemp stalks contain two important fractions: the bast fiber and hurd. The separation of bast fibers from the inner hurd called retting, which is an important post-harvest process for hemp. Development of varieties containing high cellulose content in the bast fiber and low pectin and lignin cross-linkages may decrease the time required for retting and improve fiber strength and quality. Understanding the genetic regulation of fiber traits in hemp will help in the development of varieties for the fiber industry. A recently published study by Guerriero, G. et al. (2017) gave some possible genetic insight into fiber development.
Hemp is an annual, dioecious, and is a highly cross-pollinated crop. Male and female plants are utilized differently for different end products. For CBD oil extraction, female plants are preferred, where the economic part is the flower. Whereas, for fiber production mostly male plants are utilized. For seed production, a large population of female plants along with some desirable male plants are required, which, help in pollination. There is a high demand for determination of sex in hemp prior to planting to ensure the quality of the end product. Mandolino, G. et al, (1999) developed genetic marker for determination of male sex in dioecious hemp plants. By using these genetic markers and developing new SNPs based on the previously developed markers will help in the identification of sex in hemp plants prior to planting. The identification process can substantially reduce costs because more male plants grown to full maturity only means less economic product and loss of growing space and production time.
It is imperative to understand the biosynthesis of cannabinoids in hemp to keep the THC levels below 0.3% to be in the legal limit. In the biosynthesis pathway, cannabigerolic acid is the substrate on which two different enzymes act, which determines the end product. If the enzyme THCA synthase acts on cannabigerolic acid, the product formed is THCA, which, accompanied with heat forms THC. If the enzyme CBDA synthase acts on cannabigerolic acid, the product formed is CBDA which later undergoes decarboxylation into CBD. Genetic understanding of the genes underlying the enzymes is necessary to keep the levels of THC low. In addition, other factors controlling the different aspects of cannabinoid biosynthesis in the plant need to be investigated in detail.
For grain production, hemp plants that are short, with low to no branching and early maturing are preferred. The traits that need to be considered for hemp grown for grain/oil are short-stature, high yield, uniform seed size, and improved shattering resistance. Plant height can be marginally controlled by plant spacing in the field. High-density planting of hemp will reduce branching and can be harvested mechanically using farm equipment reducing labor. However, seed size is highly variable among hemp cultivars. Selecting varieties that can produce uniform seeds that are larger in size can improve the yield of hemp. Recently, the use of hemp seed as human and animal food has improved. Improving the taste of hemp seed using different terpene profile can expand the market options for the hemp farmers.
Understanding and improving all the above-mentioned traits along with resistance to pests and diseases, improving the adaptability, and providing better post-harvest storage will prove hemp to be a potential agricultural crop for the state of Louisiana that can contribute to the states agricultural economy.
Guerriero, G., Behr, M., Legay, S., Mangeot-Peter, L., Zorzan, S., Ghoniem M and Hausman, JF. (2017) Transcriptomic profiling of hemp bast fibers at different developmental stages. Nature Scientific reports 7: 4961.
Mandolina, G. et al. (1999) Identification of DNA markers linked to the male sex in dioecious hemp (Cannabis sativa L.) Theor. Appl. Genet. 98:86-92.