The DNA of Cannabis Cultivar Selection

Cannabis cultivar Cloning phenotypingThe importance of genetic suitability between crop and environment cannot be overstated. A cannabis plant’s structure, cannabinoid synthesis characteristics, yield, and pest resistance are highly influenced by inherited potential. Understanding the effect of the underlying genotype on phenotypic expression enables commercial growers to select cultivars that are likely to exhibit superior performance within their specific production environments.

Co-Evolution With Human Pollinators

Cannabis is among the most ancient of cultivated crops. In its ruderal state, cannabis exhibits a very limited propensity for THC production. Several millennia of co-evolution and prehistoric human selection have provided the diversity of THC-rich, landrace varietals from which contemporary strains have been developed. During this period, the cannabis plant has leveraged its relationship with humans to expand worldwide from its origins in central Asia.

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From a germplasm development perspective, cannabis is somewhat of an outlier among horticultural crops. Prohibition effectively sidelined cannabis from the incredible advancements in plant breeding achieved during the 20th century. Clandestine cultivation and trade, and the outcrossing nature of this plastic species, have resulted in the diversity of poly hybrid cultivars currently in production. This germplasm presents a wealth of genetic diversity and the opportunity for significant gain of function.

Commercial Developments

Currently, the flood of economic interest in this species has resulted in breeding developments that belie the nascent stage of cannabis germplasm development. While there are very few registered, or true-breeding drug varietals available, genetic research has illuminated potential advancements for cannabis productivity in the very near future.

Triploid cannabis plants have been created that are resistant to pollination by typical diploid cannabis crops. This is an important development for outdoor and greenhouse cannabinoid farmers who may be exposed to the pollen pollution that is increasingly present due to the expansion of cannabis fiber and oil production.

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Researchers in Oregon have recently identified a single gene which (reportedly) confers total resistance to the G. ambrosiae strain responsible for many powdery mildew (PM) infections. This gene is dominantly inherited and can be identified by an associated single nucleotide protein (SNP). This SNP serves as a genetic marker that enables plant material to be assessed genomically, at any growth stage, to determine susceptibility to PM disease.

The finding that PM resistance can be controlled by a single, dominantly inherited gene is significant. Many resistance traits are horizontally inherited across many gene loci, and breeding for and intensifying these gene expressions can require generations of work. Dominant inheritance enables a plant with both copies of the PM resistance gene to be hybridized with a susceptible plant, with all F1 progeny exhibiting resistance to PM disease. These inheritance characteristics have significant impact on the time that will be required for these improvements to be available in commercially viable F1 cultivars.

Phenotyping

Phenotyping is the process of selecting organisms based on their physical characteristics exhibited within a given environment. This process requires careful trialing and data analysis, as informed by experience and intuition. Intellectual and facility investments, and a clear plan with predetermined scoring metrics and objectives, are necessary for success.

The largest possible number of candidate plants should be assessed during several trial repetitions to select the most valuable production cultivars. An important component of efficient trialing is the elimination of many individuals early in the process. Recognizing traits of value during initial developmental stages enables timely culls. Early selection criteria that can be employed include chromosomal analysis to eliminate male plants immediately after germination and assessment of seedling vigor.

Several vegetative characteristics can generally be assumed to be beneficial for greenhouse cultivators. These may be different criteria than would be desired for indoor production environments. Plants with narrow leaf blades, intermediate internode length, and strong stems will enable airflow and inflorescence support to the benefit of labor efficiency. For many cultural systems, plants that exhibit low apical dominance (branchy plants) will enable efficient canopy and height management.

To assess the relative fitness of individual plants, it is necessary to minimize the environmental differences that should be assumed to exist spatially within all cultivation environments. This can be achieved through efficient airflow design, and random distribution of plants during trials.

Growers who invest the resources necessary to select the best possible cultivars for their climate and market will realize dividends in plant health, quality, and yield. Cannabis aficionados should be encouraged by the diversity of complex floral traits available for discovery. The future is so bright, we gotta wear shades!

Trialing Parameters

Cultivar selection is critical to horticulture success. The following is a design example for a typical phenotyping trial to enable cultivar selection. This example is based on a facility with 1,000 square feet of flowering space available for phenotype trials, with all plants grown from seed.

Trial 1 (Germination of 2,000 Seeds)

  • 1,800 plants germinate with cotyledons submitted for chromosomal assessment
  • Males culled, 900 pistillate plants assessed for structure and vigor at four weeks
  • 500 pistillate plants selected for transplant to flowering benches
  • 50 plants eliminated from consideration based on vegetative structure metrics
  • 50 plants eliminated based on flower structure/distribution and phyllotaxy
  • 50 plants eliminated for elevated susceptibility to pests within the production environment
  • 50 cultivars selected from 350 candidate plants based on post-harvest potency testing

Trial 2 (10 clones Each of 50 Selected Cultivars Propagated)

  • Cultivars are randomly distributed within the facility to minimize environmental vagaries
  • Plants are scored on four to eight metrics during production
  • Five cultivars eliminated based on structure
  • Five cultivars eliminated based on susceptibility
  • 10 of 40 candidates selected based on yield

The ten remaining cultivars represent .5% of the original germplasm and can be further assessed for horticultural and market value. Quantitative selection during Trial 1 for potency (cannabinoid and terpene) metrics should ensure that these cultivars are viable candidates for market acceptance. Observant grow personnel will be familiar with the characteristics of the remaining candidates and can provide valuable anecdotal insight into their fitness for production.

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