Effect of far-red and blue light on rooting in medicinal cannabis cuttings and related changes in endogenous auxin and carbohydrates

Stem cuttings are used in the commercial cultivation of many crops, including medicinal cannabis, to produce large numbers of uniform and genetically identical plants. Light is an important environmental factor determining the success of the rooting of stem cuttings. The aim of this study was to investigate the influence of different fractions of far-red and blue during the adventitious rooting phase of medicinal cannabis stem cuttings on rooting and whether these effects are related to changes in endogenous auxin and/or carbohydrates. Two experiments were conducted in climate chambers with sole LED lighting (blue, red, far-red) using two cannabis cultivars. In Experiment 1, four light treatments were applied: 100 µmol m⁻²s⁻¹ red:blue (88:12) with additional 0, 50 or 100 µmol m⁻²s⁻¹ far-red and a fourth treatment with 100 µmol m⁻²s⁻¹ sole red with additional 50 µmol m⁻²s⁻¹ far-red. In Experiment 2, the following four light treatments were applied: 90 µmol m⁻²s⁻¹ red:blue (45:45) with additional 0 or 45 µmol m⁻²s⁻¹ far-red, a third treatment with 45 µmol m⁻²s⁻¹ sole red with additional 45 µmol m⁻²s⁻¹ far-red, and a fourth dynamic treatment which was 45 µmol m⁻²s⁻¹ sole red with additional 45 µmol m⁻²s⁻¹ far-red for 7 days followed by 90 µmol m⁻²s⁻¹ red:blue (45:45) for 14 days. The effects on rooting in both experiments were measured after 21 days of light treatments. In Experiment 2, periodic measurements of auxin and carbohydrates were performed. Far-red improved adventitious rooting only in Experiment 2, where both cultivars responded similarly. Adding far-red only during the initial stage (7 days) of rooting was sufficient to improve rooting, while it did not result in excessive stem elongation. The presence or absence of blue did not significantly affect rooting. Although the positive effects of far-red on auxin and carbohydrate concentrations in stem cuttings are a likely explanation for the observed effects of far-red on rooting, we did not find a correlation between auxin or carbohydrates and rooting.