For outdoor farmers, the DN flowering trait offers several advantages over SD crops. At temperate latitudes, successive crops can be cultivated over a single growing season without the need for light-deprivation infrastructure. At more northern latitudes, all crops can be timed to ripen during the peak solar window, thereby increasing the quality and yield, and enabling harvest before elevated seasonal pathogen pressures occur. Regardless of latitude, the DN flowering trait allows for harvests over an extended time period. This enables more efficient use of labour and processing infrastructure and can provide market timing advantages for finished product.
For greenhouse growers using supplemental lighting, DN cultivars can increase the yield and quality of crops grown during low light seasonal conditions relative to SD cultivars. The productivity of plants is based on the total light over a given photoperiod, or the daily light integral, rather than light intensity. Flowering with a 20 hour photoperiod allows for a much higher daily light integral for DN cultivars in these conditions. Morphologically this supports denser flower formations, higher cannabinoid and terpenoid production, and faster cycle times. Cultivating DN crops can also obviate the need for expensive blackout systems to be installed for greenhouse production.
For indoor growers of heritage cultivars, the ability to clonally propagate and initiate flowering at an optimal stage for canopy development will likely favour the continued use of SD over DN cultivars. Many economically important crops with photoperiod obligate origins have been improved and acclimated to wider ranges through the selection and stabilization of DN varietals. It was through the study of a high-yielding DN tobacco variety that the concept of photoperiodism in plants was first elucidated by American scientists 100 years ago.
In Cannabis, the ruderalis subspecies is defined by the DN flowering characteristic. These DN populations evolved at higher latitudes where SD flowering is a disadvantage to the reproductive cycle of cannabis. C. ruderalis populations tend to have very low THC content, as they are wild type (uncultivated), and elevated THC biosynthesis in cannabis is largely the result of human selection.
It is likely that a C. ruderalis plant was the source for the DN gene expression in modern autoflowers. The first named cannabis variety to exhibit stabilized DN flowering characteristics was the ‘Lowryder’ strain. This variety exhibited a short stature and modest THC and terpene production, allowing it to be fearlessly cultivated in the closets of college dorms. From this source, the DN flowering trait has been ingressed into various cannabis germplasms over many generations.
Trait inheritance patterns for DN flowering generally follow Mendelian ratios for recessive inheritance. There are some modifiers to this pattern, as evidenced by the intermediate trait expression present in F1 hybrid (first-generation) crosses between SD and DN varietals. These F1 hybrid cultivars will typically exhibit root-bound flowering characteristics and will initiate flowering under a longer photoperiod than their SD parent. This F1 generation can be termed photoperiod sensitive, and it shares characteristics with other cultivars that do not have a ‘Lowryder’ origin.
Given the relatively simple inheritance patterns of the DN trait, great improvements have been made to autoflower strains through repeated hybridization and backcrossing. Some current autoflower cultivars exhibit cannabinoid and terpenoid profiles that are on par with fine heritage cultivars.
Clonal propagation of DN varietals is impractical, as the “clock” regulating flower initiation continues to tick while cloning. Seed propagation has some drawbacks in production environments. Cannabis is usually dioecious meaning that male and female reproductive parts occur on different organisms. In dioecious cannabis populations, male plants must be culled promptly to prevent the pollination of sinsemilla crops. Currently there is a high level of heterozygosity or variability in many cannabis cultivars propagated from seed. Most of these issues can be negated through the production of quality feminized seeds.
In order to create a gynoecious (all female) population from pistillate (female) plants, pistillate (XX) pollen must be produced. This is achieved by treating the pistillate pollen donor plant with an ethylene blocker like Silver thiosulphate solution (STS) to initiate a monoecious phenotypic state, with both pistillate and staminate (male) flowers. Ethylene is a gaseous phytohormone that is involved with reproductive differentiation, ripening, and senescence in higher plants.
The extent to which male flowers are induced through ethylene blocking can vary greatly among cannabis plants within treated germplasms. Some plants will respond with profuse pollen production. Others are much more resistant and will express fewer pollen sacs within the pistillate floral mass, making pollen extraction difficult. To collect pollen from these donors, the flowers must be dried rapidly in a desiccant chamber and then mechanically tumbled in a pollen extractor.
The variability in resistance to induced monoecity in cannabis plants likely relates to a cultivar’s propensity to express intersex tendencies in response to environmental stressors. As intersex tendencies should not be promoted in drug varietals, this variability provides an opportunity to select for intersex resistance by using the pollen donor parents that express the least amount of pollen. Unfortunately, the hard road is not always taken. It is important to understand that in all cannabis populations propagated from seed, a minimum of two to three plants per thousand should be expected to express intersex tendencies. These plants should be culled, and seeds resulting from unintentional pollination events should be discarded.
In addition to creating feminized seed for flower production, the maintenance of gynoecious cannabis lines through induced monoecity offers additional tools for germplasm improvement. All breeding plants can be selected based on their floral characteristics, rather than by selecting males in each generation based on other criteria. Plants can be self-pollinated (selfed) by pollinating a cultivar with its pollen donor clone. Self-pollination is the norm in higher plants, as approximately 94% of flowering plants are monoecious, and therefore self-pollinate to varying extents.
Selfing plants is the most intensive form of inbreeding. This method has particular value when applied to cannabis. Cannabis is typically an outcrossing plant, and it has not been subject to methodical pre-breeding development due to prohibition. As a result, current cannabis germplasms are generally highly variable, or heterozygotic. The ability to self cannabis plants is useful in creating true-breeding cannabis lines within realistic timeframes. True breeding lines can then be hybridized to restore heterosis (hybrid vigour) while retaining a high degree of phenotypic consistency in the F1 hybrid seed line for flower production.
To assess the impact of breeding gynoecious lines over many generations through induced monoecity, it is informative to examine the breeding of commercial cucumber varietals. Cucumbers are typically monoecious when open pollinated. Gynoecious lines have been selected for and maintained since the early 1980s through disruption of the ethylene signaling system using STS and other agents to produce monoecious phenotypes. Most current commercial cucumbers are hybrid varieties, in which one or both parents are of gynoecious lines. The amount of work required to maintain these gynoecious lines over decades is testament to their value in the marketplace due to substantial increases in yield over monoecious heritage cultivars.
Due to their determinate nature and propensity for root restriction–induced flowering, DN seedlings should be handled differently than their SD peers. Autoflower seeds can be direct sown in the field using technology appropriate to the seed size and scale of production. Seeds can be sown in rooting plugs and transplanted at first true leaf set. Seeds can also be sown in deeper bottomless or plantable formats and transferred to production media at three nodes of development. If DN varietals are subject to even modest root restriction, they can begin flowering prematurely, sometimes to comical effect.
In accordance with their smaller stature, autoflowers are planted at higher densities, and trellis support is often unnecessary. Excessive pruning is detrimental when cultivating autoflower cultivars. A loose, aerated medium and minimal feeding will benefit this quick cycle crop.
The day-neutral flowering trait is useful and valuable. When propagated from feminized seeds produced by capable breeders, autoflowering cultivars have the potential to transform cannabis agriculture.