6.2. Flowering time control in Cannabis - what we know so far
Cannabis has the potential
to be a sustainable multipurpose crop. For virtually all applications ofCannabis , a better understanding of the genetic factors
controlling flowering time would be highly beneficial (Figure 1). The
reasons are evident when the flowers or seeds are the main agricultural
product, such as hemp oil from seeds or CBD production from flowers. But
flowering time also determines the crop purpose in more general terms,
with later flowering varieties favouring vegetative stem growth thus
suiting fibre production and earlier varieties displaying enhanced
flower/seed yield
(Salentijn et al.,
2019). The interactions between flowering time and fibre quality are
complex (reviewed in
Salentijn et al.,
2019) and the developmental stage at harvest has major implications for
fibre quality. Additionally, a better understanding of flowering time is
important to generate varieties that are adapted to local climatic and
photoperiod conditions.
While various environmental signals including temperature
(Amaducci et al.,
2012, 2008; Cosentino et al., 2012; Nelson, 1944) prompt floral
initiation, Cannabis is particularly sensitive to changes in
photoperiod (Hall et
al., 2012; Salentijn et al., 2019). In as early as 1912 it was observed
that flower induction in Cannabis is influenced by the
photoperiod (Tournois 1912 (cited by
Heslop-Harrison,
1957)). Cannabis is a facultative short-day plant
(Salentijn et al.,
2019). This means that while plants will typically flower eventually
under long-day conditions, flowering occurs faster in short-day
conditions i.e. by experiencing a sequence of days each with a minimum
uninterrupted period of darkness. Cultivar-specific variation for the
photoperiod at which flowering is induced has been reported, with the
optimal photoperiod ranging from 9 to 14 h
(Lisson et al., 2000
and references cited therein).
A related question is how many consecutive short days are required to
induce flowering.
Borthwick and Scully
(1954) reported two weeks of a short photoperiod induced flowering in
3-5-week-old plants, and the greater the plant age at the time of
switching to short-day the faster the floral transition. Furthermore,
Potter (2014) stated
flowers can be visible one week after the reduction in day length.
Clearly, more research is needed in this area, in particular to explore
the variation between cultivars for this trait.
Given that most individuals eventually flower under non-inductive
photoperiodic conditions, a more detailed analysis of the age-related
and the autonomous pathway and their influence on flowering time is
warranted. In addition, it would be especially interesting to observe
stressors that can accelerate flowering under non-inductive
photoperiodic conditions, and detect whether the same signalling
pathways are at play when individuals eventually flower under long days
(Takeno, 2016).