Are There Limits to the Range of Possible Cannabinoid Ratios in Plants?: Page 3 of 6

March 12, 2020
Figure 5 (click to enlarge)
Abstract / Synopsis: 

Strikingly similar results have been reported from a wide range of studies on the ratio of tetrahydrocannabinol (THC) to cannabidiol (CBD) concentrations in strains of cannabis. Whether the source has been legalized markets in the west, medical markets in the U.S. and Canada, or collections from law enforcement and researchers, three easily distinguishable types of plant have consistently been found: THC-dominant strains (with less than 1% CBD); CBD-dominant strains (less than 1% THC); and balanced strains with comparable concentrations of both substances. Another consistent finding of these studies, carried out in a variety of laboratory settings, is a positive correlation between THC and CBD levels in those plants that can make substantial quantities (>1%) of each. The correlation between THC and CBD quantities in these varied populations suggests that there is a fundamental property of the plant that makes some combinations impossible, for instance, >15% THC and also >5% CBD. Such results have never shown up in published data sets of carefully, consistently tested samples, but those were all relatively small collections. A much larger data set has been released by the state of Washington (140,000 flower samples), and this has been scrutinized for evidence of consistently propagated strains with higher than a 2-to-1 ratio.

The largest collection of strain testing results came from the state of Washington’s recreational program (9). In this study, published by Jikomes and Zoorob, more than 160,000 test results from 23 independent laboratories were reviewed. This effort produced a wealth of data on consistency and variation in strain identity, as well as on laboratory-to-laboratory consistency. A scatter plot of THC versus CBD concentrations resulted in the three-cluster arrangement that mirrors the results of other studies (Figure 5). These authors used the relative concentrations to classify each strain as being of Chemovar Type I (high THC), Type II (balanced), or Type III (high CBD). (See upper right for Figure 5, click to enlarge.)

The common pattern of three distinct clusters on these scatter plots provides strong evidence that this is a fundamental property of cannabis plants. The commonality of the result is all the more striking when the range of sources, both in material and methods, is considered.  The laboratories that conducted the cited studies were: a single state-run laboratory; a single academic laboratory; commercial laboratories serving a single medical market; and numerous commercial laboratories serving a fast-growing recreational market. The material tested came from highly controlled medical markets, a recreational market that responds to varied market demand, and strain collections that pre-date the current liberalized legal environment.  For all of these studies to produce such similar results strongly suggests that there are very few patterns possible in the ratio of cannabinoids.

THC and CBD Are Present Proportionally in Blended Strains

The clarity of the three distinct clusters in these studies is in part because of the very striking distribution of the data points in the middle of the scatter plots. In each of the scatter plots, the middle cluster, from strains that contain appreciable quantities of both THC and CBD, is tightly bunched and appears centered on an upward-sloping line.

The upward slope of the center cluster might be casually overlooked, or thought of as expected and “normal,” but such a consistent finding, especially across such varied studies, merits some scrutiny. As THC and CBD (more specifically, their acidic forms tetrahydrocannabinolic acid [THCA] and cannabidiolic acid [CBDA]) are synthesized from the same precursor molecule cannabigerol acid (CBGA), a naive assumption might be that as the plant makes more THCA it would make less CBDA, and vice versa. If that were so, the points in the middle of the scatter plots would have a downward slope, a reverse correlation. But the clear result, and one that appears to be common to all cannabis strains, is that in balanced-type plants that can accumulate significant (>1%) concentrations of both THC and CBD, the two substances are present in proportional quantities. Balanced plants with high THC also have high CBD (toward the top right of the scatter plot), and those with lower THC have lower CBD (toward the bottom left of the center cluster).

The clarity of the clustering in the scatter plots also calls attention to the white spaces, and raises the question of whether there are certain combinations of cannabinoids that are impossible to find in a single plant. Note the sectors in these plots do not contain any data points: above a certain concentration of THC, it appears that the CBD level must be miniscule, so that a strain with 15% THC and 5% CBD is not found on these plots. This is most clear in the plots with fewer data points, and less clear in the plot of thousands of data points from the state of Washington. A principal finding, though, of that study on the state of Washington data, was evidence for reporting inflated THC levels. The question remains, then, whether certain combinations of cannabinoids simply have not been reported, or are actually not possible. A closer examination of the Washington data may reveal whether it contains good evidence for some combinations of cannabinoid concentrations that have not been reported elsewhere, in smaller collections, or whether they represent “noise” in a very large and poorly controlled testing environment.

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About the Author

Thomas A. Coogan, PhD, is an Academic and Research Liaison with the New Jersey Cannabis Industry Association. Direct correspondence to: [email protected]


How to Cite this Article

T.A. Coogan, Cannabis Science and Technology 3(2), 32–39 (2020).