The researchers examined different types of cannabis, including drug-type varieties that produce large amounts of psychoactive tetrahydrocannabinol (THC). Plant breeding has increased the THC level to the point where it can account for up to a quarter of a plant’s total weight. The scientists found that breeding high-THC plants with hemp-type plants can swap a few genes and make a new plant that produces high levels of CBD instead.
Researchers say this means that when high-CBD plants are grown to full maturity, farmers interested in producing hemp for CBD run the risk of their crop crossing over the federal, legal limit of 0.3% THC.
“This poses a challenge, though,” said study co-author and CBS graduate CJ Schwartz of Sunrise Genetics. “The genes that allow for the production of CBD are also a bit ‘leaky.’ This can result in about 5% of the product ending up as THC instead of 100% CBD.”
Published in the journal New Phytologist, the researchers found that high-CBD plants inherit about 90% of their genes from drug-type cannabis and the rest from hemp. Traditionally, cannabis plants are split into two varieties: a drug-type grown for psychoactive or intoxicating properties (i.e., marijuana) and a hemp-type used to make industrial products like fiber.
After assembling a complete genome from a new breed of cannabis, University of Minnesota and Sunrise Genetics researchers found that high-CBD “hemp” plants have a mostly marijuana genome.
“Over the past decade, we’ve seen a surge in demand for CBD and we wanted to better understand how plant breeders created high-CBD plants to meet rising demand,” said study co-author George Weiblen, who is a professor in the College of Biological Sciences (CBS) and the science director at the Bell Museum.
“These high-CBD plants are genetically marijuana for the most part and they can’t be expected to meet the legal definition of industrial hemp in every situation,” said Weiblen. “This means that CBD products — such as flowers, extracts and edibles — that are labeled ‘hemp’ could be incorrectly labeled and falsely branded. Fiber hemp and products made from hemp seeds, however, are drug-free.”
They then tested THC and CBD content over a four-week period when the flowers matured. One issue for farmers is that CBD and THC levels are linked, and both rise in the flowers at harvesttime, creating a precarious calculation to reap the highest possible CBD levels and value of the crop, without losing everything by surpassing THC thresholds.
The research from Smart’s lab has assisted USDA in developing hemp regulations that focus more on genetics rather than environmental stresses that lead to noncompliant THC levels.
In previous work, Toth developed a molecular marker to identify genes that produce CBD in hemp and THC in cannabis. He was able to show that some hemp varieties included plants with different genetic arrangements — some with genes for mainly CBD production, others for mainly THC and some with a combination of both. In this study, all the plants were predominately CBD producers.
Now, a new Cornell study, published July 28 in the journal Global Change Biology-Bioenergy, finds no evidence that stress on hemp plants increases THC concentrations or ratios of CBD to THC.
Justin Muir, Cornell University
The lab’s research indicates that USDA’s decision to raise the THC limit for what is considered a “negligent crop” from 0.5% to 1% THC in January will dramatically lower the legal risks for growers. Too many negligent violations can ban a grower from producing hemp for five years.
Krishna Ramanujan | Aug 06, 2021