High Science - Drought stress for better yields?
Publication: American Society for Horticultural Science
Peer Reviewed: Yes
Authors: Deron Caplan, Mike Dixon, Youbin Zheng
Published online: May 2019
Title: Increasing Inflorescence Dry Weight and Cannabinoid Content in Medical Cannabis Using Controlled Drought Stress
Will drought stressing your plant produce larger yields? This study suggests, in part, the answer is yes. Having said that, there may be a few reasons why this particular example is not likely to apply to your grow, and thus potentially returns this topic back to Bro Science territory.
To be clear, we are talking about an increase in cannabinoids, not the dry weight of the flower. The authors of this study are not aware of any other studies of this nature conducted on Cannabis, which makes this study the primary word until a more current test can be conducted.
Why people think it will work;
Drought stress is a major stimulator of secondary metabolites in certain plants. Secondary metabolite accumulation caused by drought stress has been documented in a number of herbaceous species. Drought stress is used to increase oil production in a variety of plants like lemon balm, summer savory, and lemon catmint, to name a few.
Why people argue against it;
Drought stress has been well documented to cause negative impacts on plant growth which in-turn can reduce harvestable plant mass. Drought reduces rates of carbon assimilation (photosynthesis) as a result of both stomatal and metabolic limitations. Under drought conditions, if the soil has salt based nutrients in the root zone, these can become concentrated and create a high soil EC, burning the plants or leading to nutrient lockout.
Specifics of the study
Because science doesn’t view Cannabis as Indica or Sativa, the classification of Drug-Type and Non Drug-Type Cannabis have been used to group different chemovars. Cultivar is the kind of plant (strain), chemovar refers to the chemical make-up (cannabinoids, terpenes, flavonoids etc).
- Plant type
- Soil type
- Drought onset and Harvest date
- How drought stress was defined and measured
The selected drought group reached the desired level of drought stress when the WP was approximately 1.5 MPa (drought stress threshold)
***These tools aren’t common in the home growers garden and on a large production scale, not commercially viable. Sensors are costly and require significant technical training.
This was achieved without reducing the dry weight of the harvested drought crop as compared to the control group. This points to the researchers finding the right period of drought insufficient to impede growth, but still crucial to increasing secondary metabolite yield. When measured on a “yield per unit growing area”, overall THCa increased 43%, CBDa 47%, THC 50% higher and CBD 67% higher. This would have major implications for commercial grow facilities.
But please, let me rain on the parade a little;
Where this may differ from your grow
• Plant type
Home growers are generally using Drug-Type chemotype I plants, which are classified as a THCa:CBDa ratio of >1-0 (high THC, very low CBD). The authors noted “because rates of cannabinoid accumulation vary by chemovar, the effect of drought on other chemovars should be explored”. They also state, “this study should be applicable for similar varieties of chemovar II cannabis; however, other chemovars or varieties might respond differently”.
- Soil type
- Harvest and drought schedule
• How drought stress was defined and measured
While home growers don’t have those fancy gadgets to measure WP and MPa, there are alternative methods described in the study. Provided there are ideal environmental conditions, leaf angle measurements were an effective indicator of plant stress.
Measurements were made using a protractor or angle finder. At the drought threshold, plants were visibly wilted and the indicator leaf angle increased by 50% from the turgid leaf angle. Measurement was made by measuring the angle between the middle leaflet and the stem from which it originates.
This begs the question, do I drought stress the last 11 days, or do I flush in the final weeks? How would this effect the overall taste and burn-ability of the final harvest? If my Cannabis was 20%THC, an increase of 12% would make it 22.5% THC. How significant is that?
Although there was no marginal difference in dry weight between the two groups, photosynthetic activity significantly dropped in the drought group. Would trying a drought period earlier in flower to allow a flush at the end result in less plant yield because the plant has lower photosynthetic activity during the peak plumping periods of week five and week six? This was not tried in the study.
Is the strain they used and the chemovar type representative of all Cannabis plants responses to drought stress? The study specifically points out, that without further research, these results should not be applied to all Cannabis in general. So where does that leave us? We have some support for the idea, but it also raises more questions.
The idea still lands in the zone of bro science to me, because we still only have anecdotal evidence this may work on the types of chemovars most commonly grown by home cultivators and it doesn’t provide any research on how a synthetically fed plant would react to the same stresses. What do you think?
Link to study - https://journals.ashs.org/hortsci/view/journals/hortsci/54/5/article-p964.xml
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