Grafting Cultivation Method Can Increase THCA Yield in Medical Cannabis, Study Finds
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Researchers from Southern Cross University and the New South Wales Department of Primary Industries have developed a new one-step horticultural grafting methodology specifically designed for medicinal cannabis cultivation.
Published in the journal Agronomy, the one-step grafting method combines clonally propagated tissues from different cannabis strains to produce new plants that bypass previous cultivation limitations. By experimenting with the combination of these varieties, the researchers were able to create grafted plants with different stem morphologies capable of producing increased biomass and an increased yield of THCA per plant.
The problem with traditional cannabis cultivation
Grafting is an ancient propagation technique whereby crops are fused together to improve the overall properties or resilience of the plant. Generally, this involves at least two plants, one to provide the rooting part (the “rootstock”) and another the above-ground growth (the “scion”).
Normally, the rootstock and scion would be produced by germinating the seed for both plants, then grafting them together when each part reaches an optimal size for grafting. The grafting process connects the vascular systems of both plants and has been used to increase crop yields, raise abiotic stress tolerance, and improve disease resistance in vulnerable crops.
“Medicinal cannabis differs from most annual crops cultivated under protected cropping because plants are usually clonally propagated rather than grown from seed,” the researchers wrote.
“In order to apply grafting to medicinal cannabis without adapting commercial propagation procedures, both the rootstock and the scion would need to be clonally propagated from vegetative tissue rather than heterozygous seeds.”
This clonal propagation is done to ensure that cannabis plant crops are genetically uniform, and so will contain a predictable amount of the terpenes and cannabinoids that they are harvested for.
In order to apply grafting techniques to clonally propagated plants without dramatically increasing the labor costs associated with cultivation, the New South Wales researchers have developed a new one-step grafting methodology specifically tailored for medicinal cannabis cultivation.
Grafting cannabis improves THCA yield
The researchers focused on two locally-available medical cannabis strains: “CBD1”, a high-CBDA strain but which yields a relatively low biomass, and “THC2”, a high-THCA and high-yield strain that suffers from a variable root structure that frequently leads to unpredictable yield performance and clonal failure. These two scion options were grafted onto a range of other THC-dominant strain rootstocks and examined. In addition to experimenting with rootstock selection, the researchers also repeated the grafting experiments with different rootstock ages to best optimize the process.
Relatively few changes in cannabinoid concentration were seen in the grafted plant combinations. CBD1 scions grafted onto one of the high-THC rootstocks (THC8) did display significantly higher levels of CBDA and CBN after grafting. However, these plants also yielded less biomass overall.
In the THC2 grafts, there was a trend towards higher THCA concentrations after grafting, but the only significant cannabinoid concentration change was an increase in cannabicyclolic acid (CBLA) levels.
However, biomass yields in the THC2 scions grafted to the THC9 high-THC rootstock produced 20 percent greater biomass yields than the non-grafted plants. This, combined with the slight trend towards higher THCA levels, was found to increase the yield of THCA per plant from 8 grams to 13 grams.
Best growing method for yield
Following this new method, it is possible to propagate freshly cut scions and stems that will root in just fourteen days. While some extra time is needed to make the grafts compared to standard cloning, the method does not require any additional days of cultivation away from the normal cultivation schedule. The method may also open the door to new scientific research into root-stem interactions and signaling pathways in the cannabis plant, the researchers say.
“We estimate that grafting is three-fold slower than standard cloning. Another consideration is that mother plants for both the rootstock and scion would need to be maintained which increases labour demands and spacing requirements within the cultivation facility,” the researchers wrote.
“However, the potential benefits of grafting in medicinal cannabis may extend beyond immediate yield advantages, as this method has been used in other crops to identify hormones, proteins, and RNAs that result in long-distance communication systems within the plant.”
In an interesting twist, some modern cannabis cultivators are moving away from the traditional propagation of plant cuttings altogether. Tissue culture propagation comes in many forms, but it generally starts with very small samples of viable plant tissue that can be cultivated in test tubes containing ideal culturing media. These eventually grow into new young plants, which can be transferred out and cultivated in a normal grow operation.
“If we compare classical breeding with tissue culture breeding, the first step is the same: choose the best plant, normally based on the phenotype,” Dr Veronica Codesido, co-founder and manager of breeding and cultivation at MIFCO Biosciences, told audiences at the Analytical Cannabis Expo Europe Online 2021.
“But then you need to make clones. And what happens when you need to make clones [traditionally] is that you need a very enormous surface to maintain those clones. Compared with in vitro culture, you only need small tubes.”