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Most tetrahydrocannabinol (THC) of Cannabis sativa is located in the resin heads of capitate-stalked glandular trichomes. We found that after harvest the resin heads shrink in diameter in exponential decay fashion under ambient room conditions, losing about 15 % in the first month, rising to 24 % over the first year, 32 % by 50 years, and 34 % after a century. An equation accounting for the asymptotic curve descriptive of the progression of shrinkage was determined [original gland head diameter in microns = observed diameter divided by (0.5255 + 0.4745 multiplied by time in days to the power −0.1185)], so that if the age of a specimen is known, the original diameter of the gland heads in the fresh state can be extrapolated. This equation was employed to compare gland head size in samples of different ages. A sample of high-THC medical marijuana strains marketed under license possessed resin head diameters averaging 129 μm, while a sample of low-THC industrial hemp cultivars possessed gland head diameters averaging 80 μm. The mean volume of the resin heads of the narcotic strains was more than four times larger than that of the industrial hemp strains. This is the first documented report of a consistent morphological separator of elite narcotic strains and non-narcotic plants. Most recognized strains of marijuana were bred clandestinely and illicitly during the last half century. The occurrence of large resin gland heads in a sample of officially marketed pharmaceutical strains is an obvious correlate of selection for higher quantity of resin production.
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Affiliation Department of Plant and Animal Sciences, Faculty of Agriculture, Dalhousie University, Truro, Nova Scotia, B2N 5E3, Canada
We would like to thank Greenhouse Seeds, Kannabia Seeds, Paradise Seeds, Marco Renda and Kenneth Langford for providing seeds; Rory Hudson and The Peace Naturals Project, who grew the marijuana plants under license from Health Canada; Julie Bernier, Susan Koziel and Jan Slaski (Alberta Innovates—Technology Futures) for providing hemp samples; and Patrick J. Brown and Adam R. Boyko for useful comments on this manuscript. This article was written, in part, thanks to funding from the Canada Research Chairs program and the National Sciences and Engineering Research Council of Canada.
Results and Discussion
Affiliations Department of Plant and Animal Sciences, Faculty of Agriculture, Dalhousie University, Truro, Nova Scotia, B2N 5E3, Canada, Anandia Labs, 2259 Lower Mall, Vancouver, British Columbia, V6T 1Z4, Canada
Affiliation Alberta Innovates-Technology Futures, P.O. Bag 4000, Vegreville, Alberta, T9C 1T4, Canada
We observe a putative C. indica marijuana strain from Pakistan that is genetically more similar to hemp than it is to other marijuana strains (Fig 1a). Similarly, the hemp sample CAN 37/97 clusters more closely with marijuana strains (Fig 1a). These outliers may be due to sample mix-up or their classification as hemp or marijuana may be incorrect. The sample of CAN 37/97 that we genotyped was from a Canadian hemp germplasm collection, which obtained this accession from the IPK Genebank (Gatersleben, Germany). The original source country is France but there is limited information to indicate the cultivation of CAN 37/97 as hemp. Alternatively, these samples may be true outliers and represent exceptional strains that are genetically unlike others in their group. Using the current data set, the unambiguous identification of a sample as either hemp or marijuana would be possible in the former case, but not in the latter. In any case, we find that the primary axis of genetic variation in Cannabis differentiates hemp from marijuana.