5.3. Hormonal and environmental factors affection sex
determination
Beyond genetic consideration, the sex expression in Cannabis can
be shaped by environmental factors. It is well established that silver
can be used to induce the formation of male flowers on female plants
(Mohan Ram and Sett,
1982). This treatment is used by breeders to self female dioecious
plants, which results in offspring with exclusively XX sex chromosomes
(Clarke and Merlin,
2016). Because all of the progeny from such a selfed plant will be
female, the ‘feminized’ seeds produced that way are usually much more
valuable than conventionally produced seeds
(Small, 2015). Silver
is a known ethylene inhibitor, and there is also evidence that ethylene
induces the development of female flowers on male plants
(McDaniel and Binder,
2012; Ram and Jaiswal, 1970). Together, this therefore suggests that
ethylene is involved in controlling the sex expression inCannabis .
In addition to ethylene, also other phytohormones have been shown to be
capable of altering the sex expression in Cannabis(Ainsworth, 2000) .
Auxin, for example, can lead to the development of female flowers on
male plants. Indeed, male flower development can be entirely repressed
by auxin treatment
(Heslop-Harrison,
1956). Cytokinin also has a feminizing effect
(Chailakhyan and
Timiriazev, 1979), whereas gibberellic acid triggers the formation of
male flowers on female plants
(Ram and Jaiswal,
1972). Together, the hormonal effects are very similar to what is
observed for sex expression in the well analysed Cucurbitaceae family
(Li et al., 2019;
Pawełkowicz et al., 2019; Schilling et al., 2020a). Cucurbitaceae are
only distantly related to Cannabaceae, the two lineages separated more
than 100 million years ago (Figure 3)
(Magallón et al.,
2015). Also, many of the studied Cucurbitaceae are monoecious and thus
do not possess sex chromosomes
(Boualem et al.,
2015). Nevertheless, we hypothesize that similar developmental genetic
pathways may have been co-opted to control sex expression in Cannabis and the Cucurbits. Melon, cucumber, pumpkin, and their
relatives might thus serve as an excellent model to unravel the
molecular intricacies of sex expression and sex determination in Cannabis .
Cannabis is a short-day plant, and flowering is initiated if day
length is below 14 h (see below), but it is well established that sex
expression is also affected by day length. It was reported some 100
years ago already that dioecious plants grown under short day conditions
only (i.e. without an initial period of long day growth) can develop
male as well as female flowers ((Tournois 1911, 1912, as cited by
(Heslop-Harrison,
1957), (Schaffner,
1923)). A lot of other environmental factors like nitrogen availability
or carbon monoxide also seem to have an influence on sex expression
(Freeman et al., 1980;
Heslop-Harrison, 1957; Small, 2015). Together, this yields a very
complex picture of different environmental factors influencing sex
expression in different directions that is far from being completely
understood.
In summary, the picture emerges that, although sex determination is
genetic, hormonal and environmental influences have a significant effect
on sex expression. Because the flowers of female plants are the main
source of phytocannabinoids, a more detailed study of the sex
determination and sex expression mechanism of Cannabis is one of
the main areas of future research. For example, creating male sterility
would be very beneficial, as phytocannabinoid production is highest in
unpollinated female plants. Studying both sex chromosomes, their gene
content as well as the molecular intricacies of the sex determination
mechanisms will certainly provide valuable insights for breeders as well
as researchers.
6. Timing is everything - the complex network of floral
initiation and hints for Cannabis