Phenotypic plasticity influences the success of clonal propagation in industrial pharmaceutical Cannabis sativa

Abstract

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.

The Past, Present and Future of Cannabis sativa Tissue Culture

“…Traditionally, hemp has been cultivated by seed using large-scale, highly mechanized, production practices similar to other grain crops [ 54 ]. In contrast, drug/recreational Cannabis is generally propagated using clonal methods and treated as a horticultural crop [ 51 , 52 , 55 ]. This is done to mitigate the high level of phenotypic diversity displayed within seedling populations and to consistently produce high quality, uniform crops that meet consumer preferences and comply with government regulations [ 56 ].…”

“…Clonal propagation can take many forms, but traditionally, Cannabis has been propagated through stem cuttings. In general, Cannabis is relatively easy to root, and large numbers of plants can be produced from a single mother plant [ 55 ]. While more expensive than seed, this approach can be efficiently used to mass-produce genetically and phenotypically uniform plants at a commercial scale to produce a more uniform crop.…”

“…Strong cultivar-specific responses to treatments have been noted in Cannabis tissue culture and this likely contributes to the lack of reproducibility in both shoot proliferation and regeneration-based systems [55,74,77,79,87,89]. As Cannabis research becomes more accessible, the implications of drawing sweeping conclusions based on single-cultivar studies have come under scrutiny [12,89], and evidence has called into question the genotypeindependent responses implicit in many of these single-cultivar studies [77].…”

The Past, Present and Future of Cannabis sativa Tissue Culture

“…Traditionally, hemp has been cultivated by seed using large-scale, highly mechanized, production practices similar to other grain crops [54]. In contrast, drug/recreational Cannabis is generally propagated using clonal methods and treated as a horticultural crop [51,52,55]. This is done to mitigate the high level of phenotypic diversity displayed within seedling populations and to consistently produce high quality, uniform crops that meet consumer preferences and comply with government regulations [56].…”

“…Strong cultivar-specific responses to treatments have been noted in Cannabis tissue culture and this likely contributes to the lack of reproducibility in both shoot proliferation and regeneration-based systems [55,74,77,79,87,89]. As Cannabis research becomes more accessible, the implications of drawing sweeping conclusions based on single-cultivar studies have come under scrutiny [12,89] and evidence has called into question the genotype-independent responses implicit in many of these single-cultivar studies [77].…”