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Biochemical and Morphological Responses of Cannabis sativa L. to Postemergent Applications of Paraquat¹

Paraquat (1,1'-dimethyl-4,4'-bipyridinium ion) has been used to control illicit fields of Cannabis sativa L. (marihuana). Little is known concerning the effects of paraquat on the growth and cannabinoid concentrations of C. sativa. Therefore, three greenhouse experiments were conducted to 1) apply paraquat postemergent to C. sativa plants under controlled conditions; 2) evaluate the morphological responses of C. sativa to paraquat applications; and 3) determine the effect of paraquat applications on the concentrations of cannabidiol (CBD) and ⁹tetrahydrocannabinol (⁹THC) in marihuana. Paraquat was applied at 0.07, 0.14, and 0.28 kg/ha. In the first experiment, plants were treated 51 days after seeding (DAS) and harvested 23 days after treatment. CBD and ⁹THC concentrations were reduced 70 and 67% respectively over all herbicide applications. Plants were treated 49 DAS in the second experiment and harvested 0, 1, 2, 4, and 8 hours and 1, 2, 4, 8, and 16 days after treatment. Delta-⁹-THC concentrations were significantly reduced by paraquat applications. Plants were treated with paraquat 25, 35, 65, and 75 DAS in the third experiment and harvested 10 days later. Plant growth, marihuana production, and ⁹THC concentrations were generally significantly lower for the herbicide-treated plants relative to the untreated controls. These experiments revealed that ⁹THC concentrations in marihuana were generally reduced by the application of lethal or sub-lethal rates of paraquat to C. sativa plants of varied chronologic and physiologic ages.

Key Words: Marihuana • Narcotics • Hemp • Herbicides

² Agronomist and plant physiologist, respectively.

Received for publication May 31, 1979.