As the cannabis revolution continues, and more and more licensed producers enter the market, there is an increasing need to introduce standardised protocols and growing plans to optimise the plant’s output for medical uses. With the more reluctant countries and regions finally beginning to open the door to medical cannabis products, there is an increasing demand for standards which, although common in many other crops and horticultural plants, are lacking in medical cannabis. In one respect, this is due to a lack of maturity in the breeding space due to decades of prohibition, which has left the industry well behind its counterparts in terms of established homozygotic lines and homogenous plant products/output. This means that not only are the genetics not always entirely stable, but that the emerging best practises are often cultivar specific (Clarke & Merlin 2016). This cultivar-by-cultivar variation can cause dramatically different outcomes, and variation of secondary metabolites (e.g. cannabinoids) throughout individual plants is not uncommon (Andre et al 2016). One of the biggest problems in this regard is the plant’s natural morphology (Bernstein et al 2019).
Training plants, shaping outcomes
Each cultivar has its own particulars in terms of architecture, but generally the way most cultivars grow means if cultivators/growers do not interfere with the plant’s structural design, there tends to be major differences in the accumulation of cannabinoids in different parts of the plant (top to bottom, for example) (Janatova et al 2018).
Many indoor cultivators have developed their own preferred methods of training/shaping plants to maximise both yield and consistency within individual plants – some will adapt their techniques to suit specific cultivars, but some will revert to tried and tested methods irrespective of the cultivar. This can be dangerous, as some cultivars are more likely to thrive under plant-touching techniques, and some will show a drop-off in output with one technique over another, and both yield and potency of cannabinoids can be affected (Pal & Mahajan 2017).
The reasoning behind such intervention makes for compelling discussion. In the first instance, pruning, topping, cropping, defoliation et cetera can help to improve air flow, light penetration, and generally help manage the microenvironment the plant over (Wen et al 2019). With indoor grows, some of these techniques will also help growers manage the distance from the light source to each canopy layer, which means the top of the plant isn’t getting a hugely different intensity of light to that of the sub layers. In outdoor-grown plants where intervention is less common (or at least less intense) the apical bud will often stretch higher than the rest of the plant, which tends to lead to better-ventilated, more direct light, and often the apical bud is genetically programmed to mature first (Canadian Hemp Trade Alliance 2019). When compared to secondary and tertiary buds, the cannabinoid production in the under growth will typically be substantially less than that found at the aforementioned apical buds (Wen et al 2019).
Where is the science?
There has been a severe lack of peer-reviewed papers on how plant training effects cannabinoid production. It is highly likely, for example, that defoliation – reducing the leaf biomass – will at least influence the photosynthetic output, change transpiration rates, and potentially trigger a stress response (Poni et al 2006, Quentin et al 2011, Oesterheld et al 1992).
Given the roles of the shoot apical meristem on both signalling and developmental cues, any changes to the plant architecture (such as topping) will cause changes in the plant’s secondary metabolite production (Kocjan Acko et al 2019, Pal & Mahajan 2017). This statement can be applied both holistically and regionally within a plant where there are fluctuations in the microenvironment, and this short example highlights the need for more research into this component of cannabis cultivation.
Plant architecture and secondary metabolite production
In a 2021 study from the Nirit Bernstein lab, the first of its kind in this subject/plant, Danziger & Bernstein address the issue of the effect that plant architecture manipulation has on cannabinoid production and standardisation. In the study, the authors tested the following hypothesis; ‘reproductive development and metabolism in cannabis will be altered by plant architecture modulation treatments. If the new generated plant architecture will decrease the variability of the microclimate in and around the plant, it is possible that the secondary metabolites profile throughout the plant will be more uniform.’.
The authors set up a series of straightforward experiments to check this theory, taking two cultivars and applying seven different techniques (Defoliation, Single Prune (topped), Double Prune (topped twice), Primary Branches Removed, Secondary Branches Removed, Bottom 3rd Stripped (all leaves and secondary branches removed), and Bottom 3rd Stripped + Defoliation) to compare to the untouched control plants and measuring the cannabinoid output from each.
The results bring a mix of expected and unexpected outcomes. In line with commonly recited growing rhetoric, all the architectural manipulations resulted in less varied cannabinoid concentrations in the lower parts of the plants, meaning there is improved uniformity over the plant. Findings which may be more surprising to some readers include cannabinoid-specific changes dependant on the cultivar and technique dependant, and, that putting variation aside, the overall changes in total concentrations were very subtle with one exception – primary branch removal which reduced both the yield and cannabinoid concentrations.
The authors scored uniformity for each of the measured cannabinoids and marked the variation from the plant average. This again showed that all techniques except primary branch removal improved uniformity, but to different degrees for each cannabinoid – and again, this was cultivar-specific. This is best summarised by the following statement: increases to improve the standardisation of cannabinoids is achieved by managing the microclimate to allow increased cannabinoid production at the lower parts of the plant.
The publication also showed that individual cannabinoids varied in concentration depending on how the plant was manipulated. The wider implications are that fine-tuning of the cultivation technique may be needed to reach the genetic potential of cannabinoid production. However, much more investigation is needed to understand this on a larger scale and with other cultivars, including CBG, CBD, THCV and balanced ratios etc.
If the manipulation technique redirects the secondary metabolism in measurable ways, it is possible that other minor cannabinoid-dominant cultivars will respond in an equal or greater way.
Of course, if cultivators achieve good results contrary to the above information, this is by no means a call for them to change – this is simply a part of the cannabis cultivation revolution, which, although has been going on for many years, is now starting to build great momentum..
For some time, the Frontiers in Plant Science journal has allowed scientists such as Dr Nirit Bernstein to publish research on the cannabis plant. As more and more publications come through, and more scientists receive funding and exposure, it is fair to expect a great deal of change in both the current understanding of the plant, and the directions we predict the industry will go in.
This will help shape the industry and the businesses which hold it up. Knowledge – and access to knowledge – is vital for sustaining a health industry, in any sector, not to mention one which traverses agriculture, medical, pharmaceutical, food and drink, and many more.