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Hot Hemp is Down to Genetics, Not Environmental Stresses, Study Claims

Jul 30, 2021

Hot Hemp is Down to Genetics, Not Environmental Stresses, Study Claims

Leo Bear-McGuinness
Science Writer & Editor

Since it was effectively legalized in the US at the end of 2018, hemp has become a lucrative crop for thousands of farmers across the country. But this profit and livelihood is all put at risk the moment a hemp plant breaches the 0.3 percent limit of THC. From then on, the crop is considered federally illegal marijuana, and farmers can be forced to destroy their fields rather than reap them.

So what causes a hemp crop to raise its THC levels? Well, anecdotally among growers, long flowering periods and poor soils have been to blame.

But a new study disagrees with this conventional wisdom. Published in the journal Global Change Biology-Bioenergy, the study found that environmental factors have little effect on whether hemp exceeds its THC limits. Instead, the researchers say, hot hemp plants (as they’re known in the industry) are a result of genetics.


Some don’t like it hot

To make their conclusions, the researchers from Cornell University exposed different hemp crops to five different environmental factors and observed if the plants’ THC levels changed.

These stresses included flooding, wounding, and exposure to a plant growth regulator, a herbicide, and the fungal pathogen powdery mildew. Samples were taken from the plants for analysis before the stresses were applied and then once every week during the tests for three weeks. A control group of unadulterated hemp crops was also sampled.

After analyzing the crops’ cannabinoid concentrations, the researchers found that most stressed crops barely differed to the control plants. Those sprayed with herbicides were the only ones left with a significantly different amount of a cannabinoid, CBD specifically.

But aside from these herbicide-covered crops (which were very nearly dead), all the plants, even those in the control group, exceeded the 0.3 percent THC limit at the end of the experiment. Thus, the researchers say, stress factors alone are unlikely to be behind the hot hemp problem.

“What we found over the weeks that we were sampling, the amounts of CBD and THC went up proportionately in all of these different cultivars for all of these different stresses,” Jacob Toth, a graduate student at Cornell University and lead author of the study, said in a statement.

So, if not the environmental factors, what did cause the crops to multiply their THC levels beyond the legal limit? Well, Toth believes the hemp’s genes may be to blame.

A previous study of his demonstrated that some hemp varieties contain genes that largely code for CBD production, while others contain more genes that code for THC. So, given this known genetic influence and the seemingly negligible impact of environmental stresses, Toth concludes that the balance of genes within the hemp plants is likely the cause of the high THC levels.

“There was no effect of stress treatment on the total CBD to total THC ratio at harvest,” his current paper concludes, “supporting the hypothesis that this ratio is genetic and not strongly influenced by environmental stress.”

However, further studies will be needed to rigorously test this genetic hypothesis.

Toth and his colleagues also noted that their own study included several limitations. For instance, only shoot tips of growing crops were taken for analysis. It is possible, the researchers admit, that the environmental stresses would have affected the total yield of a fully grown crop. Also, the stress treatments used in the study were chosen to be representative of growing conditions in a wet, northeast US climate and didn’t include factors that are typical of other areas, such as drought, extreme heat, or high salinity.


More hot hemp research

Given the scale of the hot hemp issue and how much it costs hemp farmers per year, several research efforts have been launched to better understand the phenomenon.

Last year, a research team at West Virginia University were awarded a $200,000 grant from the National Institute of Food and Agriculture to study how different environmental factors may affect the THC levels of hemp crops.

“We will grow a few selected industrial hemp varieties in our facilities here at West Virginia University,” Michael Gutensohn, an assistant professor of horticulture at West Virginia University, told Analytical Cannabis at the time. “We want to test the effect of different individual abiotic and biotic stressors on the accumulation of terpenes and cannabinoids in hemp. Thus, it is absolutely essential to grow the hemp plants under controlled environment conditions.”

“Once we understand how individual factors affect the terpene and cannabinoid metabolism and its genetic regulation in hemp, then we can start to study the effect of combining several factors, which will be closer to the scenario a plant faces when grown under field conditions.”

The hope is that if Gutensohn and his colleagues identify any hot hemp risk factors, this knowledge could be used to develop a genetically divergent, hardier crop system that can keep its THC content low even under extreme conditions.

“It is not our goal to develop new hemp varieties,” Gutensohn clarified. “However, we hope that the knowledge gained in our research project will help and guide in the development of new improved hemp varieties.”

And back in December 2019, researchers at Universidad Politècnica de Valencia and the Hemp Trading company in Spain announced the creation of a new hemp strain: Panakeia, “the first THC-free and high CBG content hemp variety.”

In partnership with the US companies Tesoro Genetics and Front Range Biosciences, Hemp Trading planned to distribute Panakeia, and rid the “hot hemp” problem for good.

“The American market is key to the commercialization of Panakeia as hemp is grown here on a large scale, and the agreement with these three companies represents a big opportunity for launching Panakeia in the USA,” Ernesto Llosá, CEO of Hemp Trading, said in a statement at the time. 

 

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