On a hot summer day, you can’t do without a number of comfy cotton shirts. They get stronger when they are wet which makes them ideal in the summer since they need to be washed frequently. Even in winter, nothing keeps your feet and toes warm like a cozy cotton sock that traps in the warmth and keeps you feeling snug amidst the coldest of weathers. All this and more is thanks to one seed, the cotton seed.
Cotton (Gossypium spp.) is one of the earliest and most important domesticated non-food crops in the Old and New Worlds. The term cotton originated from the Arabic word al-qutn, which turned into algodón in Spanish and finally became cotton in English. Cotton is a soft fluffy staple fiber that grows in a protective case known as a boll. The key ingredient of the plant is the fibre which is almost pure cellulose. Cotton accounts for 40% of the raw materials used in the global textile industry producing T-shirts, towels, robes, underwear, bed sheets just to name a few. Moreover, it is used in more than 25 other industries such as manufacturing fishnets, tents, coffee filters, archive paper, book-binding, cotton seed oil and many others. Its vital importance in the field of industry and life in general gave it the nickname “White Gold”.
Cotton is cultivated worldwide over an area of 35.2 million hectares producing a total amount of cotton that is around 69.9 million tonnes. According to values produced by the Food and Agriculture Organization (FAO), with 8,988 million hectares, India ranks first worldwide in terms of acreage of cultivated cotton, and with 5.13 million tonnes of produced cotton, it ranks third in global cotton seed production. The first and second places in global cotton seed production are occupied by China and USA with a total production of 12.64 and 7.437 million tonnes respectively. Other important cotton growing countries include Pakistan and Brazil.
Growth of cotton
Fiber cell initiation on the day of anthesis (A), elongated cotton fibers under electron microscope (B), and mature cotton bolls (C)
Fibre initiation: The fibre determines if the cultivated cotton is either “lint” which is longer and easily detached (used for commercial cotton) or “fuzz”. Lint develops within a day after anthesis in the first wave of fibre initials, while fuzz develops several days later in a second wave.
Fibre density: The density of the fibre can dramatically affect the morphology of the mature seed. Due to later epidermal cell divisions, the ratio of fiber cells to non-fibre cells can change. If fibers of equal length are produced more densely, the total fibre production will be greater.
Fibre elongation: During the 25-30 days after flowering, the process of cotton fibre elongation takes place. Towards the end of this period, a deposition of cellulose occurs which may continue to the 78th day after flowering. The growth of fibre in length and amount of deposited cellulose vary depending on both the variety of the cotton plant and the environmental conditions.
Fibre development: The cotton fibres elongate to 3-6 cm in G. hirsutum and G. barbadense. The G. barbadense variety produces longer, stronger fibres than G. hirsutum. The fibres of G. barbadense are also finer, which means that they have less mass per length. This can either arise from a thinner cell wall or a fibre of a smaller diameter. The identification of all cellular differences and regulatory genes allows for the introduction of key fibre quality genes from G. barbadense to G. hirsutum to enhance the fibre elongation, strength and fineness.
Root: The cotton plant root system is made of a tap root. Secondary roots branch out laterally from the primary root.
Stem: The main stem stands erect and it contains nodes with buds that develop a significant number of branches. Each leaf petiole has two buds at its base; a true axillary bud that develops into a vegetative branch bearing only leaves, and an accessory bud that develops into sympodial or a fruiting branch. Vegetative branch can develop from the axillary bud or an accessory bud. The lower branches tend to be vegetative while the upper ones tend to be fruiting.
Leaves: The cotton plant leaves are spirally arranged on the main stem and vegetative branches. However, on the fruiting branches, they form two alternate rows. The leaves are petiolate with an outline that is cordate with 3-9 lobes. Usually the leaves are green, but in some species like the G. arboreum, they have a purplish tinge.
Flowers: The cotton plant flowers develop in fruiting branches only at the node opposite to a leaf. They are typical of the Malvaceae family and their buds are small pyramidal shaped structures referred to as ‘squares’. 18-24 days after the squares are formed, the flowers start to open. The flower is made of the pistil, and stamens that are arranged in a tube-like stamina column around the style of the pistil, 5 petals and 5 sepals, all joined together to form the calyx.
Fruit: The enlarged ovary develops into 3-5 loculed capsule or boll, which is the fruit. When the boll ripens, the capsule cracks along the lines where the carpels meet and the cotton within starts to expand in a soft white fluffy mass. The number of seeds in the boll ranges from 24 to 50.
Cotton is a warm season shrub that is grown under a wide range of climate and has the ability to withstand different degrees of drought. The temperature of germination ranges from a minimum of 15.5oC to a maximum of 18-38oC. In the growing season, the average humidity should be more than 50%, but it can still tolerate high temperatures up to 43oC. To obtain good quality cotton, abundant sunshine is essential during the period of boll maturity and harvesting.
Genetic species with genome designations
The four commonly cultivated species of cotton are the two diploids Gossypium herbaceum and Gossipium arboretum and the two haploids, Gossypium Barbadense and Gossypium Hirsutum. However, other species both cultivated and wild were studied and their genome designations were recorded (Endrizzi et al. 1985; Stewart 1995), some of which are displayed in the table below.
|A2||G. arboreum L.||Sudan|
|A1||G. herbaceum subsp. Africanum||Southern Africa|
|AD1||G. hirsutum||United States|
By panel, mature seeds A) of cultivated Gossypium hirsutum B) of wild Gossypium hirsutum C) wild Gossypium tomentosum D) cultivated Gossypium arboreum E) wild Gossypium herbaceum F) wild diploid species Gossypium raimondii G) wild Gossypium davidsonii H) Gossypium longicalyx I) Gossypium anomalum J) Gossypium sturtianum K) variations in seed size and fibre characteristics of species A to J
Origin of cotton
About 7,000 years ago cotton was first domesticated in the Old World in the sixth millennium BC. The cultivation of G. arboreum began in India and Pakistan and the species, G. herbaceum was first cultivated in Arabia. However, specialists agreed that the wild ancestor of G. herbaceum was from Africa, while the ancestor of G. arboreum is either from East Africa or India, since these were the locations where the most ancient archaeobotanical evidence of cotton cultivation has been found.
In the diagram above, Hutchinson illustrates his theory of the distribution and diversification of the old-world cotton varieties. They were initiated from G.herbaceum subsp. africanum which first appeared in South Africa (1), then it moved to several areas where new sub species started to appear starting from Arabia. From this point it started to spread to central Africa (2) and Asia (3, 4 and 5).
Until the discovery of the New World (the Americas), the origin of the cotton used in the old world was Sudan and Southern Africa. The New World varieties were independently domesticated from wild species. Later, the haploid species G.Hirsutum and G. Barbadense started to evolve in the New World.
G.herbaceum and G. arboeum (the diploid species among the four cultivated species) are usually cultivated in Asian drought-prone environments due to their ability to survive in low humidity levels. They also have the advantage of resisting plant disease and pests such as whiteflies, thrips, hoppers, aphids and leaf curl virus. On the downside though, diploid cotton species have short, coarse and week fibre and undesirable boll along with other disagreeable features. Thus, it was gradually replaced by the G.hirsutum. However, diploid species are still cultivated in traditional cotton growing areas in India, Pakistan, Bangladesh, China and Iran.
- Yafa, Stephen (2004). Cotton: The Biography of a Revolutionary Fiber. Penguin (Non-Classics). ISBN 0-14-303722-6.
- Rajpal, Vijay Rani (2016). Gene Pool Diversity and Crop Improvement, Volume 1. Springer. p. 117. ISBN 978-3-319-27096-8.
- Smith, C. Wayne; Cothren, J. Tom (1999). Cotton: Origin, History, Technology, and Production. John Wiley & Sons. ISBN 978-0471180456.