Introduction
Cotton is one of the most important fiber crops in the world, often called “the white gold” due to its immense economic significance. Belonging to the genus Gossypium, it is a leading natural textile fiber and also provides edible oil and protein-rich seed meal. Cotton is grown in both tropical and subtropical regions across the globe and has been a cornerstone of agricultural and industrial development.
The scientific name of upland cotton is Gossypium hirsutum. Cotton belongs to the family Malvaceae. The genus Gossypium comprises both diploid and tetraploid species, with chromosome numbers varying as follows:
- Diploid species: 2n = 2x = 26
- Tetraploid species: 2n = 4x = 52
The domestication and improvement of cotton have been shaped by both natural selection and human interventions, resulting in high-yielding varieties and hybrids with improved fiber quality, resistance to pests, and tolerance to diverse environments.
Centre of Origin
Cotton has multiple centers of origin due to the presence of different species in diverse geographic regions:
- Gossypium hirsutum (Upland cotton): Originated in Central America and Mexico.
- Gossypium barbadense (Egyptian/Sea Island cotton): Originated in Peru and the Galapagos Islands.
- Gossypium arboreum (Tree cotton): Originated in the Indian subcontinent.
- Gossypium herbaceum (Levant cotton): Originated in Africa, particularly around the Arabian Peninsula.
Thus, cotton has two major independent domestication centers: the New World (Americas) and the Old World (Asia and Africa).
Distribution of Species
Cotton is distributed widely across tropical and subtropical regions of over 70 countries. The major cotton-growing countries include India, China, the United States, Brazil, Pakistan, and Uzbekistan. In India, cotton is cultivated in three major zones:
- North Zone – Punjab, Haryana, Rajasthan
- Central Zone – Maharashtra, Gujarat, Madhya Pradesh
- South Zone – Andhra Pradesh, Telangana, Karnataka, Tamil Nadu
Globally, G. hirsutum accounts for nearly 90% of total cotton production, while G. barbadense contributes about 5–8%, and Old World diploid species (G. arboreum and G. herbaceum) contribute the remainder, primarily in India and parts of Africa.
Cultivated Species
Out of more than 50 species of Gossypium, only four are domesticated for commercial cultivation. They are divided into Old World diploids and New World tetraploids:
1. Gossypium hirsutum (Upland cotton)
- Accounts for about 90% of global cotton production.
- Native to Central America and Mexico.
- Medium to long staple length (20–30 mm).
- Widely adapted, high yielding, and responsive to irrigation and fertilizers.
- Extensively used for hybrid cotton development in India.
2. Gossypium barbadense (Egyptian/Sea Island cotton)
- Contributes about 8–10% of world cotton.
- Originated in Peru and widely grown in Egypt, Sudan, and parts of India.
- Known for extra-long staple (ELS) fibers of 30–40 mm.
- Superior spinning quality, luster, and strength.
- Used in premium textile industries.
3. Gossypium arboreum (Tree cotton)
- Native to the Indian subcontinent.
- Cultivated in rainfed and drought-prone areas.
- Staple length: 15–20 mm.
- Resistant to drought, jassids, and diseases like wilt.
- Lower yield and inferior fiber quality compared to G. hirsutum.
4. Gossypium herbaceum (Levant cotton)
- Native to Africa and parts of the Middle East.
- Staple length: 15–20 mm.
- Tolerant to drought and pests.
- Cultivated in parts of Gujarat and Maharashtra in India.
- Used mainly for local consumption and coarse cloth.
Wild Species
Wild relatives of cotton are distributed in Africa, Australia, and the Americas. These species serve as reservoirs of genes for resistance to diseases, pests, drought, and salinity. Some important wild species include:
- G. anomalum – source of drought tolerance.
- G. armourianum – resistant to bollworms and bacterial blight.
- G. tomentosum – used in fiber improvement.
- G. darwinii – native to Galapagos Islands, tolerant to saline soils.
- G. sturtianum – from Australia, source of disease resistance.
Botanical Description
Cotton is an annual or perennial shrub. The important botanical features include:
- Root system: Deep taproot system adapted to drought-prone conditions.
- Stem: Erect, woody at the base, branches monopodial (vegetative) and sympodial (reproductive).
- Leaves: Simple, palmate, lobed, alternate, with stipules.
- Flowers: Large, solitary, actinomorphic, bisexual, pentamerous; yellow or cream-colored with a purple spot.
- Fruit: Capsule (loculicidal), called boll; contains 3–5 locules.
- Seed: Kidney-shaped, covered with lint and fuzz hairs (fibers).
- Fiber: Elongated single-celled trichomes of the seed epidermis; primary source of natural textile fibers.
Economic Importance
Cotton plays a critical role in global agriculture and industry:
- Fiber: Principal source of raw material for the textile industry.
- Oil: Cottonseed oil is edible and used in cooking, margarine, and soap industries.
- By-products: Cottonseed cake is rich in protein (20–24%) and used as animal feed.
- Employment: Provides livelihood to millions of farmers, traders, and workers in textile and ginning industries.
- Export: Cotton and cotton textiles contribute significantly to the foreign exchange earnings of countries like India, China, and the USA.
- Industrial uses: Used in paper, explosives, surgical dressings, and high-quality cellulose derivatives.
Breeding Objectives
The main objectives of cotton breeding programs are:
- High yield potential with stability across environments.
- Improved fiber quality (length, strength, fineness, uniformity, luster).
- Early maturity to escape pests and drought.
- Resistance to major pests (bollworms, jassids, whitefly) and diseases (wilt, bacterial blight).
- Tolerance to abiotic stresses like drought, salinity, and heat.
- Development of varieties suitable for rainfed and irrigated conditions.
- High oil and protein content in seeds.
- Suitability for mechanized harvesting.
Important Breeding Methods
Cotton improvement has been achieved through both conventional breeding and modern biotechnological tools. The major approaches are described below:
1. Introduction and Selection
- Exotic and indigenous germplasm introductions form the base of cotton improvement.
- Selection is carried out for desirable traits like early maturity, fiber quality, and pest resistance.
- Mass selection is common in diploid cottons, while pedigree and bulk methods are used in tetraploids.
2. Hybridization
- Interspecific and intraspecific hybridization is used to combine desirable traits.
- Hybridization between G. hirsutum × G. barbadense has led to superior fiber quality and yield.
- Cytoplasmic male sterility (CMS) and Genetic Male Sterility (GMS) systems are exploited in hybrid seed production.
- India pioneered commercial hybrid cotton (1968 – “H-4” in Gujarat).
3. Mutation Breeding
- Induced mutations using physical (gamma rays, X-rays) and chemical mutagens (EMS, DES) have produced variability.
- Mutants with early maturity, improved yield, and disease resistance have been developed.
- Example: “Indore-2” mutant cotton.
4. Polyploidy Breeding
- Amphidiploids are synthesized to transfer traits from diploid to tetraploid cottons.
- Example: Transfer of jassid resistance from G. arboreum to G. hirsutum.
5. Biotechnological Approaches
Modern tools have revolutionized cotton improvement:
- Genetic Engineering: Bt cotton (carrying Cry genes from Bacillus thuringiensis) provides resistance to bollworms and is widely cultivated in India, USA, and China.
- Marker-Assisted Selection (MAS): Used for incorporating fiber quality and stress resistance traits.
- Genome Editing: CRISPR-Cas9 being explored for precise trait modification.
- Tissue Culture: Somaclonal variation and haploid production used for creating variability.
6. Development of Hybrids
- Hybrid breeding is a major success in cotton.
- India is the first country to release commercial cotton hybrids (H-4, Gujarat).
- Hybrids offer 30–50% yield advantage over varieties.
- Traits improved in hybrids: yield, adaptability, fiber quality, and resistance.
- Bt hybrids dominate Indian cotton cultivation today.
7. Breeding for Abiotic and Biotic Stress Resistance
- Breeding efforts target drought and salinity tolerance for rainfed cotton areas.
- Insect resistance is improved by incorporating genes from wild species and by Bt technology.
- Resistance to bacterial blight (Xanthomonas citri pv. malvacearum) and fusarium wilt is a major goal.
Conclusion
Cotton, as the world’s foremost fiber crop, continues to be central to agricultural and industrial economies. Its crop improvement programs have evolved from traditional selection to modern biotechnological approaches, ensuring yield stability, fiber quality, and resistance to pests and stresses. The development of hybrid cotton, particularly Bt hybrids, revolutionized cotton production in India and other countries. With continuous advancements in genomics, molecular breeding, and genome editing, the future of cotton improvement will likely bring varieties that are not only high-yielding and superior in fiber quality but also climate-smart and sustainable for global agriculture.
