Phenotypic plasticity influences the success of clonal propagation in industrial pharmaceutical <i>Cannabis sativa</i>

The burgeoning cannabis market requires evidence-based science such that farmers can quickly and efficiently generate new plants. In part, horticultural operations are limited by the success of cloning procedures. Here, we measured the role of environmental conditions and cultivar identity on the success of generating long branch material with many meristems in planting stock (mothers) and in rooting success of stem-derived clones. To evaluate the influence of lighting treatments on the optimal production of branching mothers, four lighting conditions (Fluorescent High Output T5s [T5], Metal halide lamps [MH], Plasma lamps [PL], or Metal halide lamps augmented with far red LED lights [MH+FR]) were applied to two cultivars of container grown plants (Cannabis sativa L. ‘Bubba Kush’, ‘Ghost Train Haze’) grown in peat-based organic substrates in mylar grow tents. To evaluate the influence of lighting, cutting tool (secateurs or scalpels), and stem wounding (present/absent) on optimal rooting of stems, three lighting conditions (Fluorescent T8s, T5, PL) were applied to three cultivars of peat pellet grown plants (C. sativa L. ‘Bubba Kush’, ‘Ghost Train Haze’, ‘Headband’). Mothers grown under T5 and MH (vs MH+FR) produced ~30% more meristems. However, growing mothers under MH+FR were 19% taller than mothers under T5, with ~25% longer internodes on dominant stems than plants under any other lighting condition. Canopies were denser under T5 because petiole length was ~30% shorter under T5 and fan leaves were longer and narrower under MH+FR and MH+FR and PL, respectively, than under other lighting conditions. Cultivar Ghost Train Haze stems rooted most frequently and most quickly. Wounded stems were 162% more likely to root than unwounded stems and rooted 1.5 days earlier. Our results will guide producers attempting to maximize the rate of clone production in licensed facilities; although results may differ among cultivars, where cultivars differed in their average phenotype as mother plants, and their propensity to root from cuttings, and the speed with which they produced those roots.