Suppressing a plant growth hormone pathway caused cannabis plants to grow substantially shorter while increasing cannabinoid concentrations and the proportion of biomass devoted to flowers, according to a study published in Plant and Cell Physiology.
Researchers from Southern Cross University and the Yunnan Academy of Agricultural Sciences examined how manipulating the gibberellin pathway affected female cannabis plants during flowering. Gibberellins are hormones involved in plant height, flowering and the allocation of resources between vegetative and reproductive growth.
Researchers treated cloned plants with either gibberellic acid, which stimulates the pathway, or paclobutrazol, which inhibits gibberellin production. A third group was left untreated as a control.
Plants treated with paclobutrazol were 66.8% shorter than control plants, while those receiving gibberellic acid were 31.2% taller.
Paclobutrazol-treated plants had a mean harvest index of 53.5%, compared with 31.3% for control plants and 10.7% for those treated with gibberellic acid. Harvest index measured the percentage of above-ground plant biomass made up of fresh flowers.
Total cannabinoid concentrations averaged 84.9 milligrams per gram in paclobutrazol-treated flowers, a 78.6% increase over the 47.5 milligrams per gram recorded in the control group. Paclobutrazol significantly increased every measured cannabinoid except cannabigerolic acid, or CBGA.
Gibberellic acid did not significantly affect cannabinoid concentrations, and neither treatment significantly changed total terpenoid levels.
Although paclobutrazol-treated plants had a higher average fresh flower weight, the difference from untreated plants was not statistically significant. The higher harvest index appeared to result from reduced vegetative growth and the production of more flowers rather than larger individual flowers.
Genetic and protein analyses indicated that suppressing gibberellin activity changed how sugars and other resources were transported to flowers and trichomes. The cannabinoid increase appeared to be associated with greater availability of fatty-acid-derived cannabinoid precursors rather than increased activity among the enzymes that produce individual cannabinoids.
Researchers identified the gene GA20ox2 as a potentially promising breeding target for developing semi-dwarf cannabis varieties that direct more resources toward flower and cannabinoid production.
However, the study involved just one CBD-dominant landrace and six plants in each treatment group. Paclobutrazol also left residues in the dried flowers at levels exceeding Australian food standards, limiting its usefulness in commercial production. Researchers said genetic approaches or alternative inhibitors could potentially reproduce the effects without the residue concerns.
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