Medicinal interest in hemp is attributable to its production of unique terpenophenolic substances, called cannabinoids, which are concentrated in the glandular trichomes of the plant (Mechoulam, 1970; Potter, 2014). More than 100 cannabinoids have been identified; however, the most abundant and medicinally important cannabinoids are CBD and THC (de Meijer et al., 2003). Recently, interest in the medicinal effects of CBD has increased because, unlike THC, CBD is nonpsychoactive (Russo, 2011; Small, 2015). In the United States, 25 states and the District of Columbia have enacted laws allowing the medicinal use of hemp, and another 17 states allow products that are high in CBD and low in THC (Mead, 2017).
Hemp is an annual dioecious species with females containing homogametic chromosomes XX and males containing heterogametic chromosomes XY (Moliterni et al., 2004). Male plants are taller, more slender, and have a shorter life cycle than female plants. Sex morphology in hemp is believed to be controlled by an X:autosome balance, where the ratio of X:A (autosomal) chromosomes determines sex by an X chromosome counting system, and the Y chromosome is inactive (Negrutiu et al., 2001; Parker, 1990; Shephard et al., 2000; Vyskot and Hobza, 2004). Exogenous application of plant growth regulators can modify or reverse sex morphology in plants (Galoch, 1978). In hemp, auxins, ethylene, and cytokinins promote the formation of female flowers on male plants; gibberellins promote the formation of male flowers on female plants (Galoch, 1978; Mohan Ram and Jaiswal, 1972; Thomas and Vince-Prue, 1997). Mohan Ram and Sett (1982) induced the formation of male flowers on female hemp plants using silver nitrate and STS, which inhibit the action of ethylene.
Monoecious plants occur occasionally in hemp, and cultivars have been selected for use in the production of industrial hemp fiber and seed (Moliterni et al., 2004). Industrial hemp is defined by the 2014 “Farm Bill,” Section 7606 of the Agricultural Act 2014 (7 USC 5904) as hemp having a THC concentration of ≤0.3% on a dry weight basis (Mead, 2017). Monoecious hemp cultivars demonstrate greater seed yields and greater crop homogeneity, and are easier to harvest mechanically than dioecious plants (Hall et al., 2012). Sex expression in monoecious hemp is not as well understood as for dioecious plants, but it is believed to be a heritable trait present on the X chromosome or autosomes (Faux et al., 2014). It has also been suggested that the monoecious state originated from a small translocation from the Y chromosome, or by one or several mutated genes (Razumova et al., 2016).
In the United States, at least 15 states have enacted legislation authorizing research and agricultural pilot programs related to industrial hemp (National Conference of State Legislatures, 2018). Increased interest in CBD has led to increased cultivation of hemp, but because hemp has been selected for fiber and seed production, it is generally low in CBD content (2% to 4%) (Mead, 2017). Therefore, the development of hemp cultivars with increased CBD content is a current breeding objective (Caplan et al., 2017; Soler et al., 2017).
Female plants are preferred for cannabinoid production because females accumulate, predominantly in the inflorescences, significantly greater cannabinoid content than male plants (Small, 2015). Rahn et al. (2016) and Soler et al. (2017) report that hemp growers are using hormones or chemicals like STS to make feminized seed, in which 100% of the seed is female. Mohan Ram and Sett (1982) suggest STS can be used to make male flowers with viable pollen on female hemp plants, but their methods are not directly translatable to commercial applications. Male flowers induced on genetically female plants will produce pollen containing only X gametes, which when crossed with eggs from female plants result in all-female seed (Mohan Ram and Sett, 1982). Foliar sprays of STS have been effective at blocking ethylene production and extending the flowering time for several different species of ornamental plants (Cameron and Reid, 1981, 1983). Green (2015) describes using a single foliar spray of STS to produce feminized hemp seed. This method has been shared among growers online, but this information is not based on scientific research. There are no published reports about using foliar sprays of STS to produce feminized hemp seed. Therefore, the objective of this work was to evaluate the efficacy and rate of foliar sprays of STS for inducing male flowers and producing female seed for different strains of hemp.
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