Abstract: Pigeon pea (also known as red gram, arhar, toor) is a key pulse crop in tropical & subtropical agriculture. Hybrid seed technology in pigeon pea (especially using cytoplasmic-genetic male sterility, CGMS) has proven yield potential. This chapter describes the genetic systems, parental lines, region-specific methods (India), contemporary research on isolation distances, field layout, cultural practices, and quality assurance in hybrid seed production.
1. Introduction
Pigeon pea is valued for its protein content (~20-22%), nitrogen fixation, drought tolerance and importance in cropping systems. The release of commercial hybrid ICPH-2671 (A × R crosses) marked a milestone in pigeon pea breeding in India. Hybrid pigeon pea promises higher yield and improved resistance to pests & diseases when managed properly. Recent research is refining protocols for isolation, field design and seed purity. (Saxena 2013) :contentReference[oaicite:0]{index=0}
2. Genetic Basis: A, B, R Lines & CGMS System
- A-line (Male-sterile female parent): e.g. ICPA 2043A in ICPH-2671 hybrid. Does not produce viable pollen due to sterile cytoplasm plus nuclear background. :contentReference[oaicite:1]{index=1}
- B-line (Maintainer): Genetically similar to A-line but fertile; used to perpetuate the male-sterile (A) line. :contentReference[oaicite:2]{index=2}
- R-line (Restorer / Pollen donor): Restores fertility when crossed with A-line; produces the commercial hybrid (A × R). Example: ICPR-2671. :contentReference[oaicite:3]{index=3}
3. Region-Specific Practices (India)
3.1 Isolation Distance
Isolation is essential to maintain genetic purity (avoid unwanted pollen contamination). Recent studies suggest:
- Traditionally, standards were **250 m** for foundation seed and **100 m** for certified seed production. :contentReference[oaicite:4]{index=4}
- A recent empirical study (Kona et al., 2023) recommends increasing isolation to **~350-400 m** between female (A-line) and male (R/B) parents to reduce genetic contamination. :contentReference[oaicite:5]{index=5}
- ICRISAT has, in some contexts, recommended up to **500 m** isolation for both certified and breeder seed of pigeon pea hybrids. :contentReference[oaicite:6]{index=6}
3.2 Row Ratio, Planting Layout, Spacing & Seed Rate
Some standard and research-validated practices in India:
- A : R line row ratio of **4 : 1** is commonly used for both maintaining A-line (with B-line) and producing hybrid (A × R) seed. :contentReference[oaicite:7]{index=7}
- Row spacing: ranges from **60 to 75 cm** between rows; plant-to-plant spacing around **25-30 cm** in some practices. :contentReference[oaicite:8]{index=8}
- Seed rate: For example, when using a 4:1 female:male ratio, seed required could be ~12-15 kg/ha for female parent, ~5 kg for male parent (R/B) in certain hybrids. :contentReference[oaicite:9]{index=9}
- Sowing time: In many parts of India, hybrid seed plots are sown in early June (Kharif season). Some are also done in December (depending on region). :contentReference[oaicite:10]{index=10}
3.3 Field Selection and Cultural Practices
- Select fields with good fertility, adequate irrigation or moisture, and devoid of volunteer pigeon pea plants (from past seasons) or stray pollen sources. :contentReference[oaicite:11]{index=11}
- Fertilization: Basal doses of nitrogen (N) and phosphorus (P) are applied (e.g. 25 kg N + 50 kg P) along with organic matter/manure. :contentReference[oaicite:12]{index=12}
- Irrigation: Light irrigations during flowering and pod filling stages significantly improve seed set. :contentReference[oaicite:13]{index=13}
- Row arrangement: Border rows (of male parent) or pollinator attractants (e.g. companion plants) are used to increase pollinator activity. Roguing of male fertile plants in the female (A-line) block before flowering is essential. :contentReference[oaicite:14]{index=14}
4. Latest Empirical Findings
| Aspect | Findings |
|---|---|
| Seed Yield vs Row Ratio & Spacing | A study showed that with row ratio 4:1 and spacing of 75 x 30 cm, supplied with 2-3 irrigations during flowering till pod development, seed yield reached ~1,306 kg/ha. Wider spacing improved individual plant traits but did not always increase total seed yield due to lower plant density. :contentReference[oaicite:15]{index=15} |
| Isolation Distance Effect | At tested distances of 100, 150, 200, 250, 300, and 400 m between A and R lines, contamination (off-type / unwanted cross-pollination) reduced substantially at 300-400 m. The research suggests redefining of standards to ~400 m for ensuring genetic purity. :contentReference[oaicite:16]{index=16} |
5. Practical Protocol for Hybrid Seed Production (India example)
- Site Selection: Field with fertile soil, good drainage, and no pigeon pea grown in preceding season to avoid volunteers. Field should be isolated from other pigeon pea plots by at least 300-400 m. Use barrier rows if full isolation not possible.
- Parental Lines Setup: Female A-line and male R-line (or B-line for A-line maintenance) chosen. Maintain A/B or A/R ratio 4 : 1.
- Sowing Time: In Kharif (rainy) season – early June in many Indian agro-ecologies. Adjust date of male parent sowing slightly (often 5-7 days earlier or later) to achieve flowering synchrony with female parent.
- Spacing: Row to row: 60-75 cm; inner plant spacing 25-30 cm. If plots are larger, maintain consistent spacing throughout.
- Fertilizer & Water Management: Apply basal fertilizer at or before sowing (N and P), ensure water supply especially during flowering and pod formation. Manage weeds. Avoid insecticide sprays during pollinator activity periods.
- Roguing & Bordering: Remove any fertile plants from male-sterile (A) rows before flowering. Plant pollinator or male parent border rows around seed plot. Use flowering attractants if needed.
- Harvesting & Seed Handling: Harvest when pods mature physiologically; thresh with care; dry to ~12-13% moisture; clean and grade seed, test germination, purity, moisture etc.
6. Certification and Quality Control
- Seed classes: Breeder seed (A, B, R lines), Foundation seed, Certified hybrid seed. Follow government or national seed certification standards.
- Genetic purity: Ensure high true F₁ hybrid proportion, minimal off-types; verify via field inspection and lab tests.
- Germination & other seed quality parameters: test for germination, moisture, seed health, physical purity.
7. Summary of Key Recommendations
- Use an isolation distance of **~350-400 metres** for hybrid pigeon pea seed production in India to ensure genetic purity.
- Adopt A : R row ratio of **4 : 1**, with border rows and good pollinator support.
- Maintain row spacing of about **60-75 cm** and in-row spacing ~25-30 cm.
- Sow in early June (Kharif) in most Indian environments; adjust male parent sowing to synchronize flowering.
- Rogue male-fertile plants in A-line well in advance of flowering. Use barrier isolation or physical borders if complete spatial isolation not feasible.
8. References & Further Reading
- Kona, P., Jagan Mohana Rao, R.G.S Rao & V. Swarna Latha (2023). “Redefining Isolation Distance Standards for Quality Seed Production in Hybrid Pigeonpea” Legume Research, 47(9):1588-1591. :contentReference[oaicite:17]{index=17}
- Saxena, K.B. (2013). “ICPH-2671 – the world’s first commercial food-legume hybrid” Plant Breeding Reviews. :contentReference[oaicite:18]{index=18}
- FAO / Indian Minimum Seed Certification Standards (various years), ICRISAT technical bulletins, etc. :contentReference[oaicite:19]{index=19}
- Field trial data: “Yield and yield attributes of hybrid pigeonpea (ICPH-2671) grown for seed purpose as influenced by plant density and irrigation” (Journal of Food Legumes) :contentReference[oaicite:20]{index=20}
