As the hemp industry grows, producers face the challenge of cultivating a crop that has received comparatively little scientific study, and that can become unusable – and illegal – if it develops too much of the psychoactive chemical THC.

In a new study, Cornell researchers have determined that a hemp plant’s propensity to “go hot” – become too high in THC – is determined by genetics, not as a stress response to growing conditions, contrary to popular belief.

Credit: Justin James Muir/Cornell University

“Often that issue of going hot has been blamed on environment,” said Larry Smart, senior author of the study and professor in the horticulture section of the School of Integrative Plant Science.

“[People thought] there was something about how the farmer grew the plant, something about the soil, the weather got too hot, his field was droughted, something went wrong with the growing conditions,” Smart said. “But our evidence from this paper is that fields go hot because of genetics, not because of environmental conditions.” 

The study, “Development and Validation of Genetic Markers for Sex and Cannabinoid Chemotype in Cannabis Sativa L” was published Jan. 10 in Global Change Biology-Bioenergy.

Smart and his team conducted field trials at two sites, in Ithaca and Geneva, New York, studying the genetics and chemistry of 217 hemp plants. They found that differences in growing conditions between the sites had no significant influence on which chemicals the plants produced. But when they compared the CBD (cannabidiol) and THC levels of each of the plants against their genomes, they found very high correlation between their genetics and the chemicals they produced.

Hemp and cannabis are both of the family Cannabis sativa, but hemp plants produce low levels of THC (0.3% or less), whereas cannabis plants typically contain 5% to 20% THC. Hemp has high levels of the medically useful chemical CBD, while high-THC cannabis contains minimal CBD.

Jacob Toth, first author of the paper and a doctoral student in Smart’s lab, developed a molecular diagnostic to demonstrate that the hemp plants in the study fell into one of three genetic categories: plants with two THC-producing genes; plants with two CBD-producing genes; or plants with one gene each for CBD and THC.

To minimize the risk of plants going hot, hemp growers ideally want plants with two CBD-producing genes.

“The molecular assays developed in this paper provide useful tools in breeding hemp,” Toth said. “To keep THC levels low, ensuring a lack of THC-producing genes will be important for the development of future compliant cultivars. Molecular testing is also much quicker and less expensive than current methods, and it can be done on seedlings instead of mature plants.”

While conducting the research, the team also discovered that as many as two-thirds of the seeds they obtained of one hemp variety – which were all supposed to be low-THC hemp – produced THC above legal limits.

The researchers hope their work will help address this problem by providing breeders with easy-to-use genetic markers that can be utilized much earlier on seedlings and both sexes of plants. CBD and THC are produced by only females, but breeders may be using a male plant for cross pollination without knowing if it has genes for THC production, until it appears in their female offspring, Toth said.

The team also developed genetic markers to determine the sex of hemp plants prior to flowering, since the sexes of young plants are indistinguishable. “This technology is, at this point, too expensive for farmers to use on an entire field, but it will be very useful for breeders who want to separate males and females early on to better control cross-pollination,” Smart said.

Smart said future research in his lab will focus on breeding hemp cultivars – for CBD, grain and fiber – that are high-yield, legally compliant and adapted to New York’s growing conditions.

Also contributing were postdoctoral researcher Craig Carlson and doctoral student George Stack, from Smart’s lab; Rebecca Wilk, field coordinator for Cornell AgriTech; Don Viands, associate dean and director of academic programs and professor of plant breeding and genetics; Jamie Crawford, research support specialist in Viand’s lab; Christine Smart, professor of plant pathology and plant-microbe biology; Ali Cala, a graduate student in Christine Smart’s lab; Jocelyn K.C. Rose, professor of plant biology; and Glenn Philippe, a postdoctoral researcher in Rose’s lab. 

The research was funded primarily by New York State Department of Agriculture and Markets, through a grant from Empire State Development Corporation.

Krisy Gashler is a freelance writer for the College of Agriculture and Life Sciences.