Cannabis is primarily dioecious (Moliterni et al., 2004). The male Cannabis flower is green-yellow in appearance and has a perianth of five sepals, while petals are completely absent. Further, an individual male flower contains five free stamens, and no female reproductive organs (Figure 6a and b) (Leme et al., 2020; Spitzer-Rimon et al., 2019).
On the other hand, the female flower is enclosed within a green leaflike perigonal bract. The perigonal bract is sometimes also described as a sepal, but morphological studies agree that it is a bract (Leme et al., 2020; Spitzer-Rimon et al., 2019). As such, it is not strictly a part of the flower. Between the perigonal bract and the carpel is a membranous and hyaline perianth which tightly embraces the ovary (Leme et al., 2020; Reed, 1914; Spitzer-Rimon et al., 2019). It is worth noting that this inconspicuous perianth sometimes is not mentioned in the structure of female Cannabis flowers or is considered missing as it is not visible from the outside of the flower. Most likely, these membranous structures are homologous to sepals (Leme et al., 2020). At the top of the ovary are two filamentous styles. The stigma is brush-like and has epidermal cells elongated into hair-like projections (Reed, 1914; Lemeet al., 2020) (Figure 6c and d).
The commercially interesting phytocannabinoids and terpenes are predominantly produced on the perigonal bracts of female flowers, more specifically in glandular trichomes that cover those bracts. Glandular trichomes can be categorized into sessile, stalked and bulbous trichomes (Hammond and Mahlberg, 1973), with bulbous trichomes being metabolically less active (Livingston et al., 2020). Cannabis plants also have non-glandular trichomes: hair-like uni- or multicellular trichomes which protect them from biotic and abiotic stresses (Andre et al., 2016; Dayanandan and Kaufman, 1976). However, glandular trichomes are the main site of phytocannabinoid synthesis (Furr and Mahlberg, 1981).
Because phytocannabinoids are cytotoxic in higher concentrations, they have to be secreted and are not stored within cellular compartments. Phytocannabinoids along with other secondary metabolites are secreted from glandular trichomes with a globose head-like structure (Figure 7). This head is formed by an enlarged secretory cavity which is surrounded by a culticule that encapsulates the secreted secondary metabolites (Hammond and Mahlberg, 1973). At the base of the head is a layer of secretory cells (Kim and Mahlberg, 1991; Livingston et al., 2020). The head can be sessile, directly on the epidermis and often be found on vegetative leaves (sessile trichomes), or pre-stalked or stalked with the head being elevated above the epidermis (pre-stalked and stalked trichomes), which are mainly found on female inflorescences (Kim and Mahlberg, 1991; Livingston et al., 2020). Additionally, these structures can be distinguished by different levels of autofluorescence, cell numbers as well as phytocannabinoid and terpene profiles (Livingston et al., 2020; Turner et al., 1978). Stalked trichomes seem to be developing from pre-stalked trichomes and contain a terpene profile distinct from true sessile trichomes (Livingston et al., 2020). Transcriptome analysis of floral trichomes of a CBD hemp (‘Finola’) confirmed high expression levels of genes involved in the synthesis of phytocannabinoids, terpenes and their respective precursor molecules in glandular trichomes, with expression differences between bulbous, sessile, and (pre-)stalked trichomes (Livingston et al., 2020).