wk 2 - Cultivar Selection

Cultivar Selection: HORT 320

What is a Cultivar?

• A cultivar is a cultivated variety, representing a variation within a plant species that has been developed.

• Variety (or var.): A natural variation within a plant species that developed in nature and was selected by humans.

  • Example: Acer palmatum var. atropurpureum (Japanese maple).

• Cultivar (designated by single quotes ' '): A variation within a plant species developed by human crossing.

  • Example: Solanum lycopersicum 'Sun Gold' (Sun Gold tomato).

What is a Breeding Program?

• A program run by universities, governments, or industry focused on developing and releasing improved cultivars of crops.

• Typical Goals:

  • Improved yield.

  • Enhanced pest resistance.

  • Suitability for specific environmental conditions.

• Emasculation: A process where anthers (pollen-producing parts) are removed from a flower to prevent self-pollination, allowing for controlled cross-pollination. This is a crucial step in plant breeding.

Breeding Program General Process

1. Goal Identification: Define the desired traits for the new cultivar.

2. Select Parents: Choose parent plants with desirable characteristics.

3. Cross Parents: Perform controlled pollination between the selected parents.

4. Grow Out Seeds: Plant the seeds resulting from the cross.

5. Select Breeding Lines: Choose offspring with stable traits that exhibit the desired improvements.

6. Test Breeding Lines: Evaluate the selected lines in multiple locations to assess their performance across different environments and confirm stable traits of interest.

7. Release New Cultivar: Introduce the improved cultivar to the market.

Genotype and Environment Effects (GxE)

• Genotype (G): Refers to the genetic makeup of a cultivar.

  • Some cultivars perform better across all environments due to their strong genetic advantages.

• Environment (E): Refers to external conditions such as soil, climate, and management practices.

  • All cultivars generally perform better in certain environments optimal for their growth.

• Genotype x Environment (GxE) Interaction: Occurs when the relative performance of cultivars changes across different environments.

  • Some cultivars excel in specific environments, while others thrive in different ones.

Plant Breeders Focus on Heritable Traits

• Heritable Traits: Traits that are influenced more by the plant's genotype than by environmental factors.

  • These traits can be effectively modified through breeding efforts and are typically stable across diverse environments.

• Broad-sense heritability ($H^2$): Represents the proportion of the total phenotypic variance in a trait that is attributable to genetic variance ($H^2 = rac{V<em>G}{V</em>P}$; where $V<em>G$ is genetic variance and $V</em>P$ is phenotypic variance). A higher $H^2$ indicates a greater potential for selection in breeding programs.

Day Length (Photoperiod)

• Definition: The number of hours of light per day, influencing plant physiological processes like flowering and bulb/tuber formation.

• Factors affecting day length: Latitude, season, and the use of artificial light.

• Latitude and Day Length:

  • The Earth's tilt causes variations in day length with latitude and season.

  • Equinox: When the sun is directly above the Equator, resulting in roughly equal day and night lengths worldwide.

  • Solstice: When the sun reaches its northernmost or southernmost point (Tropic of Cancer or Tropic of Capricorn), marking the longest or shortest day of the year.

• Photoperiodic Plants: Some plants flower or produce bulbs/tubers in response to day length (technically night length).

  • Examples: Onions, Garlic, Spinach, Lettuce, Potato, Edamame.

  • Onion Varieties:

    • 'Ailsa Craig': A long-day, globe-shaped heirloom onion known for its large size (up to $5$ pounds), pale yellow skin, and mild, sweet flesh suitable for fresh use, not storage. Introduced in $1887$ and adaptable to long-day regions.

    • 'Patterson' (F1): An exceptionally long-storing yellow onion known for firm, blocky bulbs with good yield potential and dark yellow skin. Adapted to $38-55^ ext{o}$ latitude, requiring long days for bulb initiation and development.

What is Pest Resistance?

• Pest: Any organism that interferes with human goals, typically causing damage to crops.

• Resistant Cultivar: A cultivar that produces a higher marketable yield under the same pest pressure compared to susceptible cultivars.

• Tolerances for Pest Injury: While resistance aims to reduce pest impact, some level of pest injury may be tolerated depending on various factors.

  • Factors influencing tolerance: Crop type, specific pest, intended processing, market class, and the final market.

  • Establishment of Tolerances:

    • USDA: For commercial markets and exports.

    • Processors or Retailers: For products destined for specific commercial channels.

    • Farmers or Consumers: For direct market sales where aesthetic standards might be different.

  • Economic Impact: Pest injury exceeding a certain level can lead to lower prices for the crop.

• USDA Sweetpotato Grades & Standards: The USDA defines specific tolerance levels for various defects, including pest damage, for different grades of sweetpotatoes.

  • U.S. Extra No. 1, U.S. No. 1, U.S. No. 1 Petite Grades: Up to $10$% of sweetpotatoes can fail to meet requirements, but no more than $5$% can be seriously damaged, including no more than $2$% affected by soft rot or wet breakdown.

  • U.S. Commercial Grade: Up to $25$% can fail, with no more than $5$% seriously damaged, including no more than $2$% affected by soft rot or wet breakdown.

  • U.S. No. 2 Grade: Up to $10$% can fail, including no more than $2$% affected by soft rot or wet breakdown.

  • **Specific