Unit 5: Plant Breeding

Syllabus Reference: Objectives; Breeding systems; Centers of origin; Acclimatization; Selection methods; Hybridization; Heterosis; [cite_start]Inbreeding depression [cite: 519-523].

1. Introduction & Objectives

Plant Breeding is the art and science of changing the traits of plants to produce desired characteristics.

• Objectives: Higher yield, improved quality, disease resistance, early maturity, photo-insensitivity, and stress tolerance.

2. Domestication & Centers of Origin

• Domestication: Bringing wild species under human management.
• Centers of Origin (Vavilov): N.I. Vavilov proposed 8 main centers where crop plants originated (e.g., Rice in India/Burma, Wheat in Middle East).
  Primary Center: High diversity, wild relatives present.
  Secondary Center: Diversity due to migration/breeding.
• Acclimatization: Adaptation of an introduced variety to a new environment.

3. Selection Methods

1. Mass Selection: Best plants selected from a population, seeds mixed and sown. Simple but less uniform. Used in cross-pollinated crops.
2. Pure Line Selection: Progeny of a single homozygous self-pollinated plant. Highly uniform. Used in self-pollinated crops (Rice, Wheat).
3. Clonal Selection: For vegetatively propagated plants (Sugarcane, Potato). Progeny of a single plant obtained asexually.

4. Hybridization

Mating of two genetically dissimilar plants to combine good traits.

Steps:

1. Selection of Parents: Choosing plants with desired traits.
2. Emasculation: Removal of anthers from the female parent (bisexual flower) before pollen maturation to prevent selfing.
3. Bagging: Covering flower with paper bag to prevent unwanted cross-pollination.
4. Pollination: Dusting pollen from male parent onto stigma of female.
5. Tagging & Harvesting: Labeling and collecting seeds (F1 generation).

5. Heterosis & Inbreeding Depression

• Heterosis (Hybrid Vigor): The phenomenon where the F1 hybrid shows superior performance (yield, size, vigor) compared to both parents.
  Dominance Hypothesis: Dominant alleles mask deleterious recessive ones.
  Overdominance Hypothesis: Heterozygote (Aa) is superior to homozygotes (AA or aa).
• Inbreeding Depression: Loss of vigor and fertility due to continuous self-pollination (inbreeding), leading to the expression of harmful recessive genes.