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).