UConn researchers working with cannabis in greenhouse. Credit: Jason Sheldon/UConn Photo
Cannabis cultivators typically use stem cuttings from “mother” plants in order to grow new, identical plants. However, these mother plants take up significant cultivation space, can accumulate diseases and lose vigor due to multiple cuttings, which typically requires them to be replaced at least every few months. Micropropagation, in which plants are grown from microcuttings taken from plant tissue culture grown in a lab, can overcome some of the limitations of stem cutting, but often faces the issue of relatively-low rooting success. Researchers from the University of Connecticut (UConn) recently tested a newer method called “retipping,” and showed that the method produced high quality cannabis plants with greater rooting success and in less space than traditional methods.
Retipping involves the rooting of shoots from recently micropropagated cannabis plants; the recent study builds on methodology previously developed and published by the UConn team in 2021. The 5- to 7-cm-long shoots were collected from 17-day-old micropropagated plants in the lab, rooted in rockwool cubes and acclimated to greenhouse conditions before being potted in a peat-based medium and allowed to grow for up to 35 days. Microcuttings from tissue culture and traditional stem cuttings were handled and grown in a similar manner to compare the results of the three methods. Two different cultivars of Cannabis sativa – “Abacus” and “Wife” – were used in the study.
The researchers found that the retip cuttings had a rooting success rate of 76-81% for both cultivars without the use of any rooting hormone, and 85-88% when treated with 1000 ppm indole-3-butyric acid (IBA). By comparison, previous research showed an 84% rooting success rate for cannabis stem cuttings treated with 2000 ppm IBA. For the “Wife” cultivar, microcuttings from 6-, 9- and 12-week-old cultures rooted at rates at or above 80%, but this declined to 50% for 15-week-old cultures and 30% for 18-week-old cultures. Microcuttings from 6- to 18-week-old “Abacus'' cultures rooted at rates between 47% and 70%. Parameters such as shoot length and flower dry weight were similar between “Abacus” plants grown using the three different methods, while “Wife” plants grown from culture microcuttings had a higher dry weight than their two counterparts. High performance liquid chromatography (HPLC) analysis showed the cannabinoid content for all the plants was equivalent. This research was published in HortScience.
A major benefit of the retipping method is the possibility to obtain many retips from one micropropagated mother plant; in the 2021 study, the researchers collected retip cuttings once a week for 10 weeks, obtaining a total of about 300 cuttings total from 10 micropropagated plants. Due to this advantage over traditional stem cutting, and the high rooting success rate of retips, the researchers estimate cultivators could grow up to nine times more plants in the same amount of floor space by using the retipping method, stated principal investigator Jessica Lubell-Brand.
“The legal cannabis industry is forging ahead of the science,” said first author Lauren Kurtz. “Our lab is helping to bridge the gap and provide evidence-based strategies to improve cultivation.”
While not all cannabis cultivation facilities possess their own labs to grow micropropagated plants, these plants could be provided by plant nurseries, in a similar supply chain strategy to that used in the ornamental plant industry, Lubell-Brand noted.