Evaluation of Masculinization Treatments to Produce Feminized Hemp Seed (Cannabis sativa L.)

Cannabis sativa L. (hemp) develops plants with either male or female flowers, and growers of hemp greatly prefer female flowers which bear the glandular trichomes that contain cannabinoids. Feminized (all female) seeds are highly desired, which are produced by crossing a female plant with a masculinized female plant. Masculinization is achieved through the inhibition of ethylene and/or addition of gibberellins before flower initiation in female plants. The hemp industry uses silver thiosulfate (STS) to masculinize hemp, but spraying silver poses environmental concerns. This study compared STS to three other ethylene-inhibiting agents: aminoethoxyvinylglycine (AVG), cobalt nitrate (CBN), and 1-methylcyclopropene (1-MCP). Treatments of STS and CBN also included gibberellic acid as a synergist. Plants treated with STS exhibited superior masculinization and pollen dispersal compared to plants treated with AVG, CBN or 1-MCP. Only plants treated with STS or AVG produced pollen in sufficient quantities for collection. This pollen was assayed for germination potential initially and after storage for up to five weeks at 22.2, 7.2, or 1.1°C. Pollen from plants treated with AVG remained viable for four weeks at 1.1°C, whereas STS-treated plants produced pollen that was viable for three weeks at 1.1°C. Due to phytotoxicity problems with AVG, STS remains the best treatment to masculinize female hemp plants when breeding for feminized seeds. In a separate study, flower tissues of hemp had considerably higher total cannabinoid concentrations compared to leaf tissues but significantly lower ratios of cannabidivarin (CBDV) to cannabidiol (CBD). To reduce variability, at least 1 g samples of fresh leaf or flower tissue should be extracted with 10 mL of methanol. Rapid throughput testing of cannabinoids as part of a breeding program should use flower tissue, preferably at the time typical of harvest