Introduction
Groundnut, commonly called peanut, earthnut or monkey nut, is an important oilseed and legume crop cultivated widely across tropical and subtropical regions. It is valued both as an edible seed rich in oil and protein and as a cash crop supporting millions of farmers. Groundnut has distinctive biology — its fertilized ovary (peg) grows into the soil and forms pods underground — and important ecological benefits through biological nitrogen fixation.
Scientific name: Arachis hypogaea L.
Family: Fabaceae (Leguminosae)
Chromosome number: 2n = 4x = 40 (tetraploid)
Centre of Origin
Groundnut was domesticated in South America, primarily in the region now encompassing parts of Brazil, Bolivia, Paraguay and northern Argentina. Archaeological and genetic evidence point to this area as the primary center of origin and early cultivation. From South America the crop spread across the world during the European Age of Exploration, establishing secondary centres of diversity — notably in parts of Africa and Asia.
Distribution of Species
Today groundnut is grown across tropical, subtropical and warm-temperate regions. Major producing countries include India, China, Nigeria, the United States, Sudan and Senegal. In India, principal states producing groundnut are Gujarat, Andhra Pradesh, Tamil Nadu, Karnataka, Rajasthan and Maharashtra. The crop is suited to both rainfed and irrigated conditions and is grown in kharif and rabi seasons depending on the region.
Cultivated Species (More descriptive)
The single cultivated species is Arachis hypogaea L., which is tetraploid and believed to have formed from hybridization between two wild diploid progenitors (A. duranensis and A. ipaensis). Cultivated groundnut exhibits wide morphological diversity that has been grouped into two subspecies and four botanical varieties.
Subspecies and Botanical Types
- Subspecies hypogaea — includes Virginia and Runner types. Plants often have a spreading habit, larger pods and are generally later maturing (120–150 days). These types are important for confectionery and some oil markets.
- Subspecies fastigiata — includes Spanish and Valencia types. Plants are more erect, often earlier maturing (90–110 days) with smaller pods and are preferred under short-season and rainfed systems.
Botanical varieties commonly referenced are var. hypogaea (Virginia), var. fastigiata (Valencia), var. vulgaris (Spanish) and var. peruviana. Each group differs in pod size, number of seeds per pod, growth habit and adaptation to environments.
Wild Species
The genus Arachis contains over 80 wild species concentrated in South America. These wild relatives are critically important reservoirs of genes for biotic and abiotic stress resistance, quality traits and adaptation. Because the cultivated species has a narrow genetic base, breeders rely on wild germplasm and interspecific approaches to broaden variation.
| Wild species | Ploidy | Notable traits |
|---|---|---|
| Arachis duranensis | 2n = 20 | One of the diploid progenitors; source of resistance genes |
| Arachis ipaensis | 2n = 20 | Other diploid progenitor; contributes quality traits |
| Arachis cardenasii | 2n = 20 | Resistance to late leaf spot, rust and rosette disease |
| Arachis stenosperma | 2n = 20 | Tolerance to nematodes and some pests |
Botanical Description
Groundnut is an annual herb with the following salient features:
- Roots: Taproot with lateral roots and nodules formed by Rhizobium spp., enabling biological nitrogen fixation.
- Stem: Erect or prostrate stems, often hairy.
- Leaves: Pinnate leaves with two pairs of opposite leaflets; leaves are sensitive to light intensity.
- Flowers: Yellow, papilionaceous, borne on axillary racemes. Flowers are predominantly self-pollinated.
- Pegs and pods: After fertilization, the ovary elongates into a peg (gynophore) that penetrates the soil; pod formation and maturation occur underground — a unique feature among crop plants.
- Seeds: Pods contain 1–4 seeds; kernel shape, size and seed coat colour are variable.
Economic Importance
Groundnut contributes to food, nutrition, industry and livelihoods in many countries.
- Oil production: Groundnut seed contains ~45–55% oil used for edible oil, margarine and industrial products (soaps, lubricants).
- Protein and food: Seeds have ~25–30% high-quality protein and are consumed raw, roasted, boiled, or processed into peanut butter, confections and bakery ingredients.
- Animal feed: Haulms (crop residues) are widely used as nutritious livestock fodder.
- Soil fertility: Biological nitrogen fixation enriches soil N (often 60–200 kg N/ha equivalent), benefitting subsequent crops.
- Income: Groundnut is a cash crop for smallholder farmers and an export commodity in many countries.
Breeding Objectives
Major objectives guiding groundnut breeding programs include:
- Enhanced yield potential and improved harvest index.
- Earliness — short-duration varieties suitable for rainfed cropping and multiple cropping systems.
- Disease resistance — to foliar diseases (early and late leaf spot), rust, rosette, stem rot and soil-borne pathogens.
- Pest resistance — resistance to key insect pests (e.g., thrips, aphids, pod borers).
- Drought and heat tolerance for adaptation to semi-arid environments.
- Improved oil quality — high-oleic acid lines for extended shelf life and nutritional benefits.
- Seed quality — appropriate seed size, attractive kernel colour, high shelling percentage and reduced aflatoxin contamination.
- Stability and broad adaptation across environments and cropping systems.
Important Breeding Methods
Groundnut improvement has progressed through a combination of conventional breeding and modern biotechnological approaches. Below are key methods used for hybrid and variety development, with notes on their purpose and limitations.
1. Germplasm Collection and Introduction
Collection and evaluation of landraces, improved varieties and wild relatives form the foundation of any breeding program. Introductions increase available variability and often bring novel resistance or quality traits into local breeding pools.
2. Selection Methods
Pure line selection from landraces and populations has been a classical method to extract stable, uniform varieties. This approach is simple and effective when sufficient variability exists within a population.
3. Hybridization and Pedigree Selection
Controlled crosses combine desirable traits from two parents. After F1 production, segregation and recombination in F2–F6 generations are managed by pedigree selection to develop superior inbred lines. Hybridization is used to combine yield potential, disease resistance and quality traits.
4. Backcross Breeding
Backcrossing is employed to transfer one or a few target genes (e.g., disease resistance) into an elite background while maintaining most of the recurrent parent genome. It is useful when the donor contains a single valuable trait but is otherwise agronomically inferior.
5. Mutation Breeding
Physical (gamma rays) or chemical (EMS) mutagenesis create novel variation for traits such as maturity, plant architecture and oil content. Mutants are screened and selected for desirable characteristics; some released cultivars have mutation-derived improvements.
6. Interspecific Hybridization and Synthetic Amphidiploids
To access the diversity of diploid wild Arachis species, breeders make interspecific crosses followed by chromosome doubling to produce synthetic amphidiploids that can hybridize with cultivated tetraploid groundnut. This strategy has been successful in transferring resistance genes (e.g., from A. cardenasii).
7. Recurrent Selection and Population Improvement
Recurrent selection aims to accumulate favorable alleles for complex traits such as yield and drought tolerance by repeated cycles of selection and recombination across populations.
8. Marker-Assisted Selection (MAS) and QTL Mapping
Molecular markers linked to traits (disease resistance, oil quality, drought-related traits) enable early and precise selection. QTL mapping identifies genomic regions controlling important quantitative traits and facilitates MAS and genomic selection strategies.
9. Genomic Selection and High-Throughput Phenotyping
Genomic selection uses genome-wide marker information to predict breeding values and accelerate selection cycles. Coupling this with high-throughput field phenotyping (drone imagery, spectral indices) speeds up evaluation for traits like drought tolerance and canopy health.
10. Transgenic and Genome Editing Approaches
Transgenic techniques have been explored to introduce insect resistance and virus resistance genes. Genome editing (e.g., CRISPR-Cas) holds promise for precise edits to oil biosynthesis genes (such as FAD2 homologs for high oleic acid) and for modifying susceptibility genes to improve disease resistance.
11. Participatory Plant Breeding
Involving farmers in selection and variety testing ensures that released cultivars meet local needs (taste, cooking quality, maturity) and are acceptable for production practices. Farmer participatory approaches help in adoption and spread of improved varieties.
Examples of Improved Varieties and Traits
Recent breeding successes include varieties with high-oleic oil composition, improved resistance to foliar diseases (late leaf spot, rust), and genotypes adapted to water-limited environments. Notable examples (country and program-specific names vary) include high-oleic lines and drought-adapted cultivars widely adopted in rainfed regions.
Challenges and Future Directions
Challenges in groundnut breeding include the crop's narrow cultivated genetic base, complex quantitative inheritance of many target traits, and constraints in phenotyping for stress tolerance. Future strategies should integrate:
- Genomics-driven pre-breeding to harness wild species.
- Genomic selection to shorten breeding cycles and improve complex traits.
- Precision phenotyping and environment characterization to breed for stability and adaptation.
- Biofortification and quality improvement (e.g., higher oleic acid, reduced aflatoxin contamination).
- Stronger farmer participation and seed systems to ensure timely delivery of improved varieties.
