6.1. The evolutionary and developmental importance of flowering
time
Flowering time control is essential for reproductive success
(Shim et al., 2017).
Mechanisms to control the time of flowering have evolved given the
negative consequences of spontaneous floral initiation: Premature
flowering could coincide with the absence of pollinators or dispersers,
subsequently causing reduced fertilisation rates and deficient seed
dispersal. Conversely, if flowering occurs too late the plant may fail
to set seed before harsh conditions hit at the end of the growing season
(Gaudinier and
Blackman, 2020). Moreover, in dioecious species such asCannabis , the timing of flower emergence is particularly crucial,
because if male and female plants do not flower concurrently,
pollination cannot occur
(Hall et al., 2012).
Hence it is evolutionarily beneficial for plants to possess mechanisms
to fine-tune their floral initiation
(Gaudinier and
Blackman, 2020). Analyses in various plant species have demonstrated
that flowering time in angiosperms is controlled by internal timekeeping
mechanisms as well as environmental signals. Among the major factors
controlling flowering time are the plant age, the photoperiod, the
circadian clock, ambient temperature, the phytohormone gibberellin and
the autonomous pathway (Fornara et al., 2010; Hill and Li, 2016).
The fine-tuning of flowering time is a major goal for plant breeding and
crop improvement efforts. Floral transition represents the developmental
shift from vegetative to reproductive growth and is a major determinant
of yield potential
(Jung and Müller,
2009). Alterations in key flowering time genes have been crucial to
crop domestication, facilitating the adaptation of crops to local
climatic conditions
(Gaudinier and
Blackman, 2020; Schilling et al., 2018). The success and worldwide
expansion of staple crops like wheat and rice can partly be attributed
to natural variation in flowering time genes, which enabled local
adaptation for cultivation at a wide range of latitudes
(Hill and Li, 2016;
Langer et al., 2014). As a quantitative short-day plant, Cannabis flowering time is particularly determined by the
photoperiod. Under long-days Cannabis remains vegetative and
flowering is only induced when a number of short-day photoperiods have
passed. Therefore, in order to cultivate Cannabis at new lines of
latitude (for example in Ireland where summer daylengths can be over
17h), the adjustment of flowering time genes can be advantageous.
Consequently, comprehensive characterisation of the Cannabis flowering time pathways is crucial to the integration of this crop
species into modern agriculture.