Triggering flowering in cannabis: a consequence of photoperiodism

By: Abdul Rehman Mohammad

The practice of indoor cultivation of cannabis is advantageous for accelerating the growth of the plant. Annual harvest capability can be maximized by increasing the annual production cycles of cannabis production. The phenomenon which is exploited to artificially flower the cannabis plant is the photoperiodic control: the innate biological light sensory system.

Many flowering plants sense seasonal changes, particularly the changes in night length, as signals to initiate flowering. Some plants flower when the night length starts to decrease and fall below their critical photoperiod, and these plants are classified as long day plants. Long day plants initiate their flowering during late spring or early summer. Examples of long day plants include oat (Avena), pea(Pisum sativum) and barley (Hordeum vulgare).

Other plants work in an entirely opposite manner. They initiate flowering when the night length starts to increase, and they are classified as short-day plants. A period of undisturbed darkness, above the critical photoperiod, is required for floral development. Examples of short-day plants include cotton (Gossypium), rice (Oryza), soybeans (Glycine max) and cannabis (Cannabis sativa and Cannabis indica).

There are some plants which are classified as day-neutral plants, in which floral development is independent of the photoperiod. Plants such as cucumbers(Cucumis sativus), corn (Z. mays subsp. mays) and tomatoes (Solanum lycopersicum) are examples of day neutral plants.

The Phytochrome light sensors

Phytochromes are proteins which are present in the cannabis plant and sensitive to light in the red and far-red region of the visible spectrum. Phytochromes exist in either the ground (Pr) or excited state (Pfr). In the Pr state, phytochromes can absorb red light (wavelength of 650–670 nm)  and consequently transform into the excited state Pfr. The excited state only absorbs light near the infra-red region (wavelength 705–740 nm), converting it back to the ground state. Infra-red light is commonly used in production facilities to promote the flowering of cannabis.

Phytochromes have two photo-inter convertible forms: Pr and Pfr

Since Pfr reverts to Pr during darkness, there will be no Pfr remaining at sunrise if the night is long (winter) and some Pfr remaining if the night is short (summer). The amount of Pfr present controls flowering, setting of winter buds, and vegetative growth according to the seasons. Consequently the phytochrome system acts as a biological light switch, as it is involved in the regulation of the circadian clock of the plant.

Signalling Cascade to trigger flowering

The phytochrome system is interlined to a downstream signalling cascade which leads to the production of flowering hormones or “florigens”. There is currently a large absence of knowledge of the molecular biology and genetic control of flowering in cannabis plants.  The photoperiodic genetic interplay of Arabidopsis thaliana, the model organism in the plant world, has been studied in depth. However, as it is a long day plant, it would have a considerably different molecular interplay of flowering as compared to cannabis.

Rice is a more accurate model for analogy as it is a short day plant (henceforth would exhibit more homology with cannabis), and has been studied extensively. In rice, the phytochrome system plays an important role in day-length recognition, partly through the control of transcriptional factors which influence the production of flowering hormones. One example is the transcriptional factor SE1, and interaction between activated phytochromes (Pfr) and SE1 repressed the activation of flowering genes. A key protein involved in the flowering of rice, Hd3a, has it expression decreased by red light, and reversed by far red light. This emphasizes the involvement of the phytochrome system in the production of flowering hormones.

In indoor cultivation practices of cannabis, the plant is exposed to extended periods of artificial light in the vegetative state (first 8 weeks) expedites growth, particularly height. However in the subsequent four weeks (the flowering state), equal periods of light and darkness are important for the plant to flower.  It is hypothesized that the period of darkness is required to inactivate phytochromes, and inhibit any repression of flowering genes caused by activated phytochromes.

Autoflowering genetics

Cannabis ruderalis is a species of cannabis present Eastern Europe, Russia, China and elsewhere in central and northern Asia. Typically it is present as a wild cannabis plant outdoors in these regions. This species of cannabis is day neutral or ‘auto-flowering’, and can begin flowering within three weeks of germination.

The “wild” cannabis species

Given that the day-neutral or auto-flowering genetic characteristic is recessive, it indicates that the genes involved have a loss of function mutation. Due to a lack of genetic research, it is unknown which pinpointed genes are responsible for lack of photoperiodism in Cannabis ruderalis.  Studies from Arabidopsis thaliana showed that LHY and CCA1 are critical for day-length sensitive flowering, and loss of function of these genes caused the plant to attain day-neutral characteristics. A similar consequence could be occurring in Cannabis ruderalis, where critical interacting genes are mutated.

Harnessing the flowering mechanism

Through various breeding exercises, the auto-flowering genetics of Cannabis ruderalis have been used to create auto-flowering strains of the plant. These strains switch from the vegetative stage to the flowering stage of the lifecycle with age, independent of the photoperiod. These plants can be grown outdoors without the need for close control of lighting periods, while maintaining a short harvest period. This is one example of how the flowering mechanism has been harnessed for commercial production.

There is a need to understand the genetic mechanisms which are involved in the flowering process of cannabis, and to pinpoint the key genes which are responsible. This can lead to the better selection of genetics strains for auto-flowering plants – looking for target genes through the analysis of DNA. The flowering molecular biology and the genetics controlling it, could be targeted with various transcription factors. This could be done so to accelerate production time of cannabis and be influenced to flower earlier, consequently increasing the number of production cycles in a year and maximising yield.

Permission to use article granted by UTT biopharma