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).