Unit I of Commercial Plant Breeding. | Commercial Plant Breeding Notes

Topic Covered! Types of crops and modes of plant reproduction. Line development and maintenance breeding in self and cross pollinated crops (A/B/R and two line system) for development of hybrids and seed production.

1. Types of Crops Based on Agricultural Classification

1.1 Based on Season

Kharif Crops: Sown in June-July and harvested in September-October. These crops require warm, wet weather for growth and maturation. Examples include rice, maize, cotton, sorghum, pearl millet, and groundnut.

Rabi Crops: Sown in October-November and harvested in March-April. These crops require cool weather for growth and warm weather for maturation. Examples include wheat, barley, gram, mustard, and pea.

Zaid Crops: Short duration crops grown between Kharif and Rabi seasons (March-June). Examples include watermelon, muskmelon, cucumber, and vegetables.

1.2 Based on Economic Importance

• Food Crops: Cereals (rice, wheat, maize), pulses (chickpea, lentil, pigeon pea)
• Cash Crops: Cotton, sugarcane, tobacco, jute
• Plantation Crops: Tea, coffee, rubber, coconut
• Horticulture Crops: Fruits, vegetables, flowers, spices
• Fodder Crops: Berseem, lucerne, sorghum (for animal feed)

Remember the classification based on season is frequently asked. Associate Kharif with monsoon and Rabi with winter for easy recall.

2. Modes of Plant Reproduction

2.1 Sexual Reproduction

Involves fusion of male and female gametes resulting in genetic recombination. This is the basis for plant breeding programs.

A. Self-Pollination (Autogamy)

Transfer of pollen from anther to stigma of the same flower or another flower on the same plant. Common in crops like wheat, rice, barley, pea, and tomato.

Characteristics of Self-Pollinated Crops:

• High degree of homozygosity (>95%)
• Natural cross-pollination usually <3%
• True breeding populations
• Flower structure favors self-pollination (cleistogamy)

B. Cross-Pollination (Allogamy)

Transfer of pollen from anther of one plant to stigma of flower on another plant. Common in maize, pearl millet, sunflower, carrot, and onion.

Characteristics of Cross-Pollinated Crops:

• High degree of heterozygosity
• Natural cross-pollination >50%
• Heterogeneous populations
• Self-incompatibility or monoecious/dioecious nature
• Show inbreeding depression and heterosis

C. Often Cross-Pollinated Crops

Crops with 5-50% natural cross-pollination like cotton, pigeon pea, and sorghum. These show intermediate characteristics.

2.2 Asexual Reproduction

Vegetative propagation without gamete fusion. Important in crops like potato (tubers), sugarcane (stem cuttings), banana (suckers), and cassava (stem cuttings). These crops maintain genetic uniformity but can also be improved through breeding programs using sexual reproduction.

Understand the relationship between pollination mode and breeding methods. Self-pollinated crops use pure line selection; cross-pollinated crops require hybrid breeding or population improvement.

3. Line Development and Maintenance Breeding

3.1 In Self-Pollinated Crops

Pure Line Selection

Isolation of single homozygous plant from a mixed population. Steps include: (1) Selection of individual plants, (2) Progeny row testing, (3) Evaluation in replicated trials, (4) Multiplication and release.

Pedigree Method

Most popular method involving crossing, selfing, selection, and maintaining records of plant ancestry. Key steps:

1. F₁ generation: Grow hybrid seeds, usually bulk harvested
2. F₂ generation: Grow large population (5,000-10,000 plants), select superior plants
3. F₃-F₅ generations: Progeny row testing with selection within and between families
4. F₆ onwards: Preliminary yield trials, advanced trials

Bulk Method

F₂ to F₅ generations grown in bulk without selection. Natural selection operates. Individual plant selection begins in F₅ or F₆ when homozygosity is high.

Maintenance Breeding

Ensures genetic purity of released varieties through:

• Nucleus Seed: Maintained by breeder (200-300 plants)
• Breeder Seed: Produced under breeder's supervision
• Foundation Seed: Progeny of breeder seed
• Certified Seed: Progeny of foundation seed for farmers

3.2 In Cross-Pollinated Crops

Mass Selection

Selection of desirable plants from population, harvesting bulk, and using as seed for next generation. Suitable for improving highly heritable traits.

Recurrent Selection

Cyclical selection procedure to increase frequency of favorable alleles while maintaining genetic variability. Types include: (1) Simple recurrent selection, (2) Recurrent selection for general combining ability, (3) Recurrent selection for specific combining ability.

Synthetic Varieties

Developed by crossing several selected lines or clones and allowing random mating. Maintains genetic variability while capturing heterosis.

Maintenance Breeding in Cross-Pollinated Crops

More complex due to heterozygosity. Methods include:

• For Synthetics: Maintain parent lines separately and remake synthetic regularly
• For Composites: Allow random mating in isolation
• For Hybrids: Maintain inbred parental lines through controlled selfing

Compare pedigree vs bulk methods. Pedigree maintains plant-to-row records and allows early generation selection; bulk postpones selection but requires less labor.

4. Hybrid Development and Seed Production Systems

4.1 A-Line, B-Line, and R-Line System (Three-Line System)

Used in crops like rice, sorghum, pearl millet, and pigeonpea utilizing cytoplasmic male sterility (CMS).

Components:

Line Type	Genotype	Characteristics	Role
A-Line (CMS Line)	S(cms) rfrf	Male sterile, cytoplasm from male sterile source	Female parent for hybrid seed production
B-Line (Maintainer)	N rfrf	Male fertile, normal cytoplasm, restorer genes recessive	Maintains A-line through crosses (A×B→A)
R-Line (Restorer)	N RfRf or Rfrf	Male fertile, contains fertility restorer genes	Male parent; restores fertility in F₁ hybrid

Hybrid Seed Production Process:

1. Maintenance: A × B → All progeny are male sterile (A-line maintained)
2. Hybrid Production: A × R → F₁ hybrid (male fertile due to restorer genes)
3. Field Layout: Rows of A-line alternated with R-line (ratio 6A:2R or 4A:2R)
4. Roguing: Remove off-types, diseased, and fertile plants from A-line
5. Harvest: Seed collected only from A-line rows (F₁ hybrid)

Advantages of CMS System:

• No need for manual emasculation
• 100% hybrid seed purity
• Economical large-scale production
• Stable male sterility

4.2 Two-Line System (TGMS/EGMS)

Uses environment-sensitive genetic male sterility. Male sterility is controlled by nuclear genes influenced by temperature or photoperiod.

Types:

• TGMS (Temperature-sensitive Genetic Male Sterility): Sterile at high temperature (>28°C), fertile at low temperature (<24°C)
• PGMS (Photoperiod-sensitive Genetic Male Sterility): Sterile under long-day conditions, fertile under short-day conditions

Components:

Line Type	Description
Male Sterile Line (MS)	Shows male sterility under specific conditions; serves as female parent
Pollen Parent (PP)	Any superior genotype used as male parent

Seed Production Process:

1. Multiplication Phase: MS line grown under fertility-inducing conditions (low temp/short day) and self-pollinated
2. Hybrid Production Phase: MS line grown under sterility-inducing conditions (high temp/long day) and crossed with pollen parent
3. No Restorer Required: F₁ hybrid is naturally fertile

Advantages over Three-Line System:

• Only two lines to maintain (simpler)
• No restorer gene required
• Wider choice of pollen parents
• No cytoplasmic effects in hybrid

Limitations:

• Dependent on environmental conditions
• Risk of fertility restoration at wrong times
• Region-specific application

4.3 Comparison of Hybrid Systems

Feature	Three-Line (A/B/R)	Two-Line (TGMS/PGMS)
Number of lines	Three (A, B, R)	Two (MS, PP)
Sterility basis	Cytoplasmic-genetic	Environmental (temp/photoperiod)
Restorer needed	Yes	No
F₁ fertility	Restored by Rf genes	Naturally fertile
Pollen parent choice	Must have restorer genes	Any genotype
Complexity	Higher	Lower
Environmental dependency	Low	High

For exams, memorize the genotypes (S rfrf for A-line, N rfrf for B-line, N RfRf for R-line) and understand why B-line maintains A-line while R-line restores fertility. Draw diagrams showing crosses.

5. Seed Production in Self and Cross-Pollinated Crops

5.1 Seed Production in Self-Pollinated Crops

Relatively simple due to natural self-pollination maintaining genetic purity.

Key Requirements:

• Isolation Distance: Minimum 3 meters from other varieties to prevent mechanical mixing
• Field Inspection: 2-3 inspections to remove off-types and diseased plants
• Genetic Purity: Should maintain 99% purity for certified seed
• Seed Class Multiplication: Nucleus → Breeder → Foundation → Certified

Steps:

1. Land selection with no volunteer plants
2. Certified seed of previous class as source
3. Proper spacing and agronomic practices
4. Roguing at flowering and pre-harvest stages
5. Proper harvesting, threshing, and storage

5.2 Seed Production in Cross-Pollinated Crops

For Open-Pollinated Varieties:

• Isolation Distance: 200-400 meters from other varieties
• Synchronization: Ensure flowering synchrony between rows
• Border Rows: Male rows removed after pollination to prevent selfing
• Roguing: More critical to remove off-types

For Hybrids (Three-Line System):

1. Planting Ratio: 6 rows of A-line: 2 rows of R-line (or 4:2)
2. Staggered Planting: Adjust planting dates to synchronize flowering
3. Isolation: 200-400 meters from other varieties
4. Roguing A-Line: Remove fertile plants from A-line before flowering
5. Border Rows: Plant R-line around field perimeter
6. Removal of R-Line: Cut male (R-line) rows after pollination in some crops
7. Harvest: Harvest only A-line rows (contains F₁ hybrid seed)

For Two-Line Hybrids:

1. Grow MS line under sterility-inducing conditions during flowering
2. Plant pollen parent in specific ratio (4:2 or 6:2)
3. Monitor environmental conditions carefully
4. Inspect and remove any fertile plants from MS line
5. Harvest only MS line rows

Quality Control Measures:

• Field inspection by seed certification agency
• Post-harvest seed testing (purity, germination, moisture)
• Grow-out tests to verify genetic purity
• Proper labeling and documentation

Remember isolation distances: 3m for self-pollinated, 200-400m for cross-pollinated. Understand the logic: cross-pollinated crops need more isolation to prevent unwanted pollen contamination.

6. Quick Revision Points for Exams

Key Definitions:

• Heterosis: Superiority of F₁ hybrid over parents
• Inbreeding Depression: Reduction in vigor due to selfing in cross-pollinated crops
• CMS: Male sterility governed by cytoplasmic genes and nuclear restorer genes
• Combining Ability: Capacity of a line to transmit desirable performance to hybrids
• Pure Line: Progeny of single homozygous plant

Common Exam Questions:

• Distinguish between self and cross-pollinated crops with examples
• Explain CMS system with genotypes
• Compare pedigree and bulk methods
• Describe steps in hybrid seed production
• Explain difference between A/B/R and two-line systems
• Seed production techniques for specific crops

Important Crops to Remember:

Self-Pollinated: Rice, wheat, barley, oat, chickpea, lentil, pea, soybean, tomato, brinjal

Cross-Pollinated: Maize, pearl millet, sorghum, rye, sunflower, sugarbeet, carrot, onion, cabbage, radish, coconut

Often Cross-Pollinated: Cotton, pigeonpea, safflower

Plant Breeding & Crop Production

Comprehensive Exam-Oriented Study Material

Review regularly and practice diagram-based questions for better retention