Gossypium (PROSEA)

Plant Resources of South-East Asia
Introduction

Protologue: Sp. pl.: 693 (1753), Gen. pl., ed. 5: 309 (1754).

Family: Malvaceae

Chromosome number: x= 13; G. arboreum, G. herbaceum: 2n= 26 (diploids); G. barbadense, G. hirsutum: 52 (allotetraploids)

Major species and synonyms

Gossypium arboreum L., Sp. pl.: 693 (1753), synonyms: G. indicum Medik. (1784), G. obtusifolium Roxb. ex G. Don (1831), G. nanking Meyen (1834).

Gossypium barbadense L., Sp. pl.: 693 (1753), synonyms: G. vitifolium Lamk (1786), G. acuminatum Roxb. ex G. Don (1831), G. brasiliense Macfad. (1837).

Gossypium herbaceum L., Sp. pl.: 693 (1753), synonym: G. frutescens Lasteyrie (1808).

Gossypium hirsutum L., Sp. pl., ed. 2: 975 (1763), synonyms: G. religiosum L. (1767), G. latifolium Murray (1776), G. javanicum Blume (1825).

Vernacular names

General: cotton (En). Coton, cotonnier (Fr)
Indonesia: kapas
Malaysia: kapas
Philippines: kapas (Ilokano), bulak (Tagalog), indamey (Bontoc)
Cambodia: krabas
Laos: fay hua
Vietnam: chi bông.

G. arboreum : tree cotton (En). Coton arborescent, cotonnier arborescent (Fr)
Indonesia: kapas merah (general), kapas beureum (Sundanese), kapas jawa (Javanese)
Cambodia: krabas tes
Thailand: faai daeng (Bangkok)
Vietnam: bông cỏ, bong hang niên.

G. barbadense : sea island cotton, Egyptian cotton, Pima cotton (En). Coton des Indes Occidentales, cotonnier d'Egypte (Fr)
Indonesia: kapas rampit, kapas kayu
Philippines: pernambuko (Tagalog, Bikol), bulak-kastila (Tagalog)
Laos: dok foy
Thailand: faai chan (Lampang), faai dok (Chiang Mai), faai thet (north-eastern)
Vietnam: bông hải dảo.

G. herbaceum : Arabian cotton, Levant cotton (En). Cotonnier herbacé, cotonnier d'Asie (Fr)
Thailand: faai (general).

G. hirsutum : upland cotton, American upland cotton (En). Coton velu, cotonnier américain (Fr)
Indonesia: kapas mori (Java), kapas kejerat (Sumatra)
Vietnam: bông luồi.

Origin and geographic distribution

Gossypium comprises 40-50 species distributed in warm temperate to tropical zones. The origin of the genus is unknown, but 3 primary centres of diversity exist: in Australia, in north-east Africa to Arabia, and in west-central to southern Mexico. The 4 cultivated cottons of the world (the Old World diploid "desi-cottons" G. arboreum and G. herbaceum and the New World tetraploids G. barbadense and G. hirsutum) have been domesticated independently in different parts of the world. The 4 species are probably all grown in South-East Asia although some doubt exists about G. herbaceum.

G. arboreum is only known in cultivation. Its origin is uncertain. It may have developed from G. herbaceum, though molecular comparisons support the hypothesis that G. arboreum and G. herbaceum diverged from a common ancestor. It has been cultivated in South and South-East Asia and southern China for many centuries. It is still found there, but has mostly been replaced by G. hirsutum.

G. barbadense probably originated in Peru as a cross between G. herbaceum and G. raimondii Ulbrich or G. gossypioides (Ulbrich) Standley. It grows naturally on the coasts of Peru and Ecuador and perhaps the Galapagos Islands. It was domesticated in north-western South America, is cultivated in South and Central America and has been introduced into Africa, Asia and the Pacific Islands, where it has become established in some regions. It was introduced into the United States in 1785 where it was known as "sea island cotton", as opposed to "upland cotton", the name for G. hirsutum. Cv. group Braziliense has long been cultivated in South-East Asia for local home industry, whereas representatives of cv. group Barbadense were introduced into South-East Asia in the 20th Century, but usually with little success.

G. herbaceum is possibly native to South Africa and reached Asia and America in early prehistoric times. It is cultivated in Asia and Africa, and sometimes planted in the New World. It has possibly been grown in South-East Asia, e.g. in Indonesia and Cambodia.

G. hirsutum probably originated as a cross between G. herbaceum and G. raimondii or G. gossypioides in southern Mexico and was domesticated in Central America or northern South America. It has become the main cotton of commerce and is widely cultivated throughout the warmer parts of the world, including South-East Asia. It has often naturalized.

Cotton has been gathered and cultivated on different continents for thousands of years. The oldest evidence of the use of cotton, probably G. arboreum, as a fibre plant in the Old World was found in Pakistan and is estimated to date from about 2300 BC. In Peru, cotton products from G. barbadense such as yarn, cordage and fishing nets date back to about 2500-3500 BC. For a very long period India was the centre of cotton-cloth production, but developments in the second half of the 18th Century altered this situation. The main factors were the invention of the high capacity saw gin in North America in the 1790s, resulting in a strong increase in cotton production in the United States, and the development of the mechanized power-driven factory system in England in the second half of the 18th Century. Following these developments, a rapid expansion of the cultivation and use of cotton occurred in the 19th Century, and the share of cotton in world textile production increased from only 4% at the end of the 18th Century (versus 78% for wool) to about 80% by the end of the 19th Century. In South-East Asia cotton-cloth was already being imported from India some 2000 years ago, and subsequently cotton cultivation and home ginning, spinning and weaving became established in the region. Cotton cloth production in South-East Asia gradually declined in the 19th century due to the vigorous expansion of the European cotton industry.

Uses

Cotton is the most important fibre plant in the world. The main fibres of the cotton plant are the longer seed hairs ("lint"), used for making yarn to be woven into textile fabrics, alone or in combination with other plant, animal or synthetic fibres. Cotton lint is also made into other products including sewing thread, cordage and fishing nets. Cotton textile cuttings and rags serve in the paper industry for the production of the best writing, book and drawing paper. Short fibres ("fuzz" or "linters") are processed into a range of products, including paper, twine, automobile upholstery, explosives, plastics and photographic film. Linter pulp is made into different types of paper, depending on its grade. Linters have also been used for the production of cellulose acetate and viscose. Cotton stalks are processed into paper and paperboard, for instance in China, and into cement-bonded particle board.

Cotton is an important oil plant. The oil serves locally for cooking and frying and industrially in a range of products, including margarine, mayonnaise, salad and cooking oils, salad dressing and shortening. The oil is also made into soap, cosmetics, lubricants, sulphonated oils and protective coatings. The residual seed cake is an important protein concentrate for livestock. Low-grade cake is used as manure. The whole seed can be fed to ruminants, which are less sensitive to the toxic gossypol in the seed than non-ruminants, or is applied as manure. Hulls are a low-grade roughage for livestock or serve as bedding or fuel. Leftover bolls, "burs", leaves and thin twigs are grazed by ruminants. Dry stalks serve as household fuel.

In the Philippines a decoction of Gossypium leaves is applied medicinally against diarrhoea and mild dysentery. In Indo-China entire plants are antifebrile; a decoction is taken against malaria and other fevers; an infusion of flowers and leaves is used as a pectoral; a concentrated maceration is utilized against inflammation; the roots are astringent, antidysenteric, diuretic and emollient, and they are used in fumigations against haemorrhoids.

Production and international trade

The estimated annual world cotton production in 1996-2000 was 53.5 million t seed cotton (unginned cotton, containing seed, lint and fuzz) or 18.7 million t lint, from 33.2 million ha. The main producers were China (4.3 million t lint per year), the United States (3.7 million t), India (2.0 million t), Pakistan (1.7 million t) and Uzbekistan (1.1 million t). The main producing country in South-East Asia in this period was Burma (Myanmar), with about 55 000 t per year, whereas smaller amounts were produced in Vietnam (24 000 t), Thailand (15 000 t), Indonesia (9000 t), Laos (6000 t) and the Philippines (1000 t). More than 90% of the world cotton production comes from G. hirsutum, which is highly productive and shows strong yield responses to improved growing conditions, fertilizers, crop protection and supplementary irrigation. Most of the remainder comes from G. barbadense, which produces longer, finer and more expensive fibre than G. hirsutum but yields less and shows limited adaptation to most cotton-growing areas. G. arboreum and G. herbaceum are grown to some extent in Asia and Africa. In India, for instance, about 10-15% of the cotton area is under G. arboreum and 13% under G. herbaceum.

In 1996-2000 world cotton lint exports amounted to about 5.4 million t per year, with a value of US$ 7.8 million. The major exporting countries were the United States (1 210 000 t per year), Uzbekistan (820 000 t), Australia (610 000 t), Argentina (250 000 t) and Turkmenistan (180 000 t). There was hardly any export from South-East Asian countries. The main importers in 1996-2000 were China (620 000 t), Indonesia (490 000 t), Brazil (390 000 t), Turkey (350 000 t), Italy (320 000 t) and Thailand (310 000 t). Malaysia imported 75 000 t per year, Vietnam 61 000 t per year, and the Philippines 57 000 t per year. South-East Asia uses about 1 million t of cotton lint per year but produces only 100 000 t and must therefore import 900 000 t. The low cotton production in South-East Asia is mainly due to unattractive pricing by buyers, averaging US$ 0.40 per kg seed cotton, whereas imported cotton lint costs US$ 1.80 per kg (equivalent to a seed cotton price of about US$ 0.60 per kg).

The world cottonseed oil production in 1996-2000 averaged 3.7 million t per year. The major producers were China (960 000 t), the United States (490 000 t), India (400 000 t), Pakistan (340 000 t), Uzbekistan (240 000 t) and Turkey (220 000 t). In South-East Asia, Burma (Myanmar) produced about 16 000 t cottonseed oil per year in 1996-2000, Thailand about 5000 t, Indonesia about 3000 t and Laos about 2000 t.

Cotton production systems range from smallholdings in most developing countries to large-scale farms in developed countries.

Properties

Cotton fibres are unicellular extensions of epidermal seed cells. One seed produces 12 000-16 000 fibres. Some hairs remain short and form the 2-7 mm long fuzz covering the ripe seeds, providing the commercial linters. Seeds of some genotypes, particularly modern G. barbadense cultivars, do not produce any fuzz. More important are the long hairs ("lint"), which are more than 25 mm long in modern cultivars. The fibre walls contain many layers of cellulose chains, which run spirally and give dry cotton fibres their characteristic twisting appearance. The hairs are covered with a waxy cuticle, giving unprocessed fibre a greasy feel and making it water-repellent. Lint fibres are smooth-looking, ribbon-like and twisted, with the fibre walls showing longitudinal and spiral striations. Linter fibres are similar in appearance, but shorter, more cylindrical and with thicker walls. Cotton fibre contains 88-96% α-cellulose, 3-6% hemicelluloses and 1-2% lignin. Cotton lint fibres are 10-40(-64) mm long, with a diameter of (12-)18-28(-38) μm and a length:width ratio of 1000-4000. G. barbadense yields the highest quality lint of all cottons, with a fibre length of 30-40(-64) mm. The lint of G. hirsutum is about 20-30 mm long. G. arboreum yields short (normally less than 25 mm long), usually rather coarse lint. The lint of G. herbaceum is (10-)20-25 mm long and mostly coarse, though very fine fibres also occur and are processed into handmade, decorative traditional textiles (the "woven winds of India"). In international trade, the fibre length or "staple length" of cotton is expressed in fractions of inches. Commercial upland cotton (G. hirsutum), a medium staple cotton, generally has a range of staple length from 13/16 to 1 1/4 inch. Commercial pima cotton (G. barbadense), an extralong staple cotton, mostly ranges from 1 5/16 to 1 1/2 inch. Apart from fibre length and its uniformity, the most important properties of cotton are strength, elasticity and fineness (diameter). The combined fineness and maturity (the degree to which the secondary cell wall has developed) of cotton fibre is usually determined by resistance to air flow, and expressed in a value called "micronaire", reflecting the linear density of fibres. Typical values of the tensile strength, elongation at break, and Young's modulus of cotton fibre are 285-595 N/mm², 7.0-8.0% and 5.5-12.6 GPa, respectively. Among the world's major textile fibres cotton has a unique combination of properties, being strong, comfortable, washable, durable and printable. It also blends well with other fibres to give it additional strength, lustre and crease resistance.

Cotton seeds remaining after ginning consist of linters (5-10%), oil (15-33%), oilcake (33-45%) and hulls (24-34%). The semi-drying oil is obtained by mechanical and/or solvent oil extraction from the seed. The principal fatty acids in cottonseed oil are linoleic acid (42-55%), palmitic acid (20-27%) and oleic acid (19-25%). The oil as well as other plant parts of Gossypium contain gossypol, a triterpenoid aldehyde, which is toxic to monogastric animals in particular. Gossypol has insecticidal, antimicrobial, antifertility and antitumour properties. Gossypol and related compounds have been implicated in conferring insect tolerance or resistance and antimicrobial properties to cotton plants. Glandless, gossypol-free cultivars exist, but they are more vulnerable to pests. Gossypol can be removed from the oil by solvent extraction, following mechanical and/or solvent oil extraction from the seed. The cake and meal contain over 40% crude protein, but, because of the gossypol present, are not without danger for monogastric animals. The gossypol in the cake can be removed or made harmless by chemical (ferrous salts) or physical (heating) means, but this is more difficult to achieve economically than in case of the oil.

The 1000-seed weight is 100-130 g.

Description

Annual herbs, subshrubs, perennial shrubs or rarely small trees, nearly all parts irregularly dotted with black oil glands.
Leaves spirally arranged, most often palmately lobed or parted, margin entire, palmately veined, usually with 1-5 nectaries on the central basal veins beneath.
Flowers solitary, usually on sympodial branches; pedicel not articulated, below the insertion of the epicalyx segments usually bearing nectaries; epicalyx segments (bracteoles) 3, free or connate at base, usually leaf-like, entire or dentate to deeply gashed into long triangular segments, persistent; calyx cup-shaped, truncate, undulate or 5-dentate or 5-lobed, outside at the base usually bearing 3 nectaries; corolla with 5 imbricate, showy petals, yellow, white, red or purple, often with a purple or purple-spotted centre; stamens numerous, forming a column, lower part of filaments united into a tube, upper part usually free with unicellular anthers; pistil with 3-5-loculed ovary and one short style with clavate, 3-5-sulcate stigma.
Fruit (boll in agronomic literature) a capsule, globular to ovoid, rarely fusiform, opening loculicidally.
Seed ovoid, with a dense covering of long woolly hairs (lint or floss) and sometimes also with a fine, short tomentum (fuzz).
Seedling with epigeal germination.

G. arboreum.

An annual or perennial shrub or small tree, 1-2 m tall, extremely variable, most parts densely covered with minute stellate hairs and patent simple hairs.
Twigs slender, prostrate, terete.
Petiole 1.5-14 cm long; stipules linear to lanceolate, often falcate, 4-15 mm long; leaf-blade ovate to orbicular in outline, 2-12 cm in diameter, palmately lobed or parted with 3-7 segments, frequently with an extra tooth in the sinuses, base cordate and 5-7-veined, 1-3 veins with an oblong nectary.
Pedicel 0.5-2 cm long, usually without apical nectaries; epicalyx segments closely embracing the corolla and fruit, rarely spreading, 1.5-3.5 cm × 1.3-3 cm, united for 1 cm or more, base deeply cordate, margin entire or remotely toothed, apex acute, slightly accrescent in fruit; calyx cupular, about 5 mm tall and 7 mm wide, inconspicuously 5-dentate; petals obovate, 3-4 cm long, at opening usually cream to yellow and after 1-2 days turning red or purple, with or without a purplish centre; staminal column 1.5-2 cm long, filaments 1.5-2 mm long.
Capsule ovoid to globular, 1.5-2.5 cm in diameter, beak 3-5 mm long, after dehiscing and splitting often reflexed, outside densely pitted and glabrous, 3-4-celled.
Seed 5-8 per cell, ovoid to globular, 5-8 mm in diameter, lint copious, fairly long, white or rusty, firmly attached to the seed; fuzz present.

G. barbadense.

An annual subshrub, perennial shrub or small tree.
Twigs first angled, becoming terete, often tinged with purple.
Petiole as long as or slightly longer than the leaf-blade; stipules large, leaf-like, linear, lanceolate or ovate, in flowering shoots often broadly ovate to orbicular and auricled at base; leaf-blade orbicular to broadly ovate in outline, 3-5-palmately parted with central segment largest, superior leaves sometimes not segmented, base cordate, 3-7-veined, 1-3 central veins with an elliptical nectary slightly above the base beneath.
Flowers with pedicel shorter than petiole, sharply trigonous, at apex usually with nectaries; epicalyx segments erect, appressed against corolla or fruit, orbicular to ovate, large, at base cordate and strongly auricled, with 10-15 acuminate teeth; calyx cupular with 5 obtuse short teeth at truncate apex, densely gland dotted, at base with 3 nectaries, splitting after flowering; corolla usually yellow with a dark red spot at the base, petals obovate, 5-8 cm long, truncate and emarginate at apex; staminal column erect, 3-4 cm long, filaments short.
Capsule ovoid to fusiform, beaked, glabrous, densely pitted, black.
Seed ovoid with an acute hilum, black to dark brown, lint long, fine, white and easily removed, fuzz only at the hilum.

G. herbaceum.

A perennial or annual shrub or subshrub, usually 1-1.5 m tall with few branches. Stem thick and rigid, twigs and young leaves usually sparsely hairy, fruiting branches mainly jointed.
Stipules small, linear, caducous; leaf-blade 3-7-lobed, cut less than halfway; lobes ovate to rounded, only slightly constricted at the base, without accessory lobes between the main lobes, glands present on midrib.
Pedicel glandless; epicalyx flaring widely from the flower and the fruit, rounded or broadly triangular, usually broader than long, cordate at base, margin with 5-13 triangular teeth; corolla 3.5-5 cm long, yellow or white with a dark centre; staminal column antheriferous throughout, filaments short; style short, stigma entire, rarely cleft at the top.
Capsule rounded, 2-3.5 cm long, beaked, surface smooth or very shallowly dented, with few oil glands, 3-4-valved, opening slightly when ripe.
Seed up to 11 per valve, with pure white, long lint and short fuzz, strongly attached to the seed.

G. hirsutum.

An annual herb or perennial shrub, 1-3 m tall.
Petiole 2-10 cm long; stipules ovate to lanceolate, often falcate, 6-13 mm × 2-5 mm; leaf-blade orbicular in outline, 3-15 cm in diameter, mostly palmately 3-lobed, rarely palmatifid, lower ones sometimes 5-lobed, upper ones occasionally ovate and entire, base cordate, 5-7-veined; segments broadly ovate to triangular, acuminate, sinuses acute to rounded, midrib nectary oblong.
Flowers with pedicel 1-2.5 cm long, at apex with 3 nectaries; epicalyx segments free, closely enveloping the flower and fruit, widely ovate to triangular, 2-6.5 cm × 1.5-4 cm, at base deeply cordate and auricled, margin with 7-12 acuminate teeth; calyx campanulate to cupular, 6-7 mm tall and 6 mm in diameter, with 5 rounded (rarely acuminate) segments, outside with 3 inconspicuous nectaries, ruptured after flowering; corolla usually pale yellow to white, rarely with a purplish centre; petals obovate, 4-5.5 cm long; staminal column 1-2 cm long, filaments 3-4 mm long, higher ones longest.
Capsule ovoid or globular, 2-5 cm × 1-1.5 cm, rostrate at apex, coarsely pitted, 3-5-celled.
Seed ovoid, 3.5-5 mm long, acute at the hilum, black or brown with white or rusty, easy or difficult to remove lint and with fuzz everywhere or only at the hilum.

Growth and development

Cotton is normally a perennial plant with an indeterminate growth habit, but is usually grown as an annual, with the formation of nodes on the main stem stopped by fruit load, temperature, soil moisture, photoperiod, or a combination of these factors. The crop cycle ranges from 120-220 days. Seedlings emerge 5-15(-20) days after sowing and the first true leaf unfolds 7-9 days later, but these processes vary with temperature. The plant remains unbranched for about 1 month. Upon germination, seedlings initiate a long taproot, which can reach a depth of more than 25 cm by the time the cotyledons unfold and may reach a depth of 3 m by mid-season. The shoot system is dimorphic, with the main axis and lower branches (emerging from axillary buds) being monopodial and vegetative, whereas the fruiting branches (emerging from extra-axillary buds) are sympodial. Fruiting branches develop as primary branches higher on the main stem and as secondary branches on vegetative branches. Generally only one sympodial branch develops at each main stem node, with 3-5 fruits per branch. The first sympodial branch appears at node 4-10 of the main stem, between 1 and 2 months after sowing, and visible flower buds appear as small, green, pyramidal structures, known as "squares". They need 20-35 days to develop into open flowers. The flowers mostly open near dawn and pollination normally occurs within a few hours. Self-pollination is the predominant mating system, but visiting insects can cause considerable outcrossing (usually 5-30%). The flowers turn pink in the afternoon and red by the following day. They start withering late on the second day and die on the third day. Flowering peaks at 3(-6) weeks after the onset and may continue for about 6 weeks. The fruits grow very quickly after pollination, reaching their final size at about 20-25 days. After a further (20-)25-45(-60) days, depending on genotype and environmental conditions, they are ripe. Usually the dry fruits open at their sutures and the white, fluffy fibre-mass emerges. The seeds remain attached to the placenta and are only separated by picking or by very strong rain or wind. During the first 2-4 weeks after flowering (the elongation phase) the fibres grow rapidly, reaching their full length, and during the next 4-6 weeks (the secondary thickening phase) the secondary cell walls of the fibres thicken through deposition of cellulose in consecutive layers. The fibre cell wall thickness, or the degree to which the fibre has been filled with cellulose, mainly depends on plant vigour during ripening. Upon boll opening the fibre dries and the lumen collapses, resulting in the characteristic twisted, ribbon-like appearance of the fibre that makes it spinnable.

Shedding of squares and young bolls is common in cotton. It is aggravated by adverse conditions such as prolonged overcast weather, extreme temperatures, water stress, water logging, nutrient deficiencies, diseases and insect damage. Commonly 60% of the squares and young bolls are shed, but flowers are rarely shed. Characteristic for cotton is its ability to overcome adverse events by compensatory growth. However, recovery is only partial and insignificant when severe insect damage occurs late in the season.

Other botanical information

The taxonomy of Gossypium is complicated, partly due to the domestication of 4 distinct species and extensive interspecific hybridization. The literature is confusing and authors disagree on the identity of many species, subspecies, sections, varieties, forms, races and cultivars that have been distinguished. Currently the taxonomic system of P.A. Fryxell, with about 50 species grouped into 4 subgenera and 8 sections, is the most generally accepted one. It is mainly based on morphological and geographical data, but is confirmed by cytogenetic and molecular evidence. Cytological research has led to the recognition of 8 basic diploid "genomic groups", designated A through G, plus K. In general, species within a genomic group can form fertile interspecific hybrids. G. arboreum and G. herbaceum are included in the subgenus Gossypium : diploid Old World species with A-genome, whereas G. barbadense and G. hirsutum belong to the subgenus Karpas : tetraploid New World species with AD-genome. Most probably the 4 cultivated cottons all occur in South-East Asia, but it is difficult to obtain confirmation about the exact cultivar groups that are grown from the literature alone. A brief survey of the major groups distinguished within the 4 species is given below.

G. arboreum is only known from cultivation. Its large variation has been classified into 6 geographical groups, usually indicated as races, but probably mostly cultivar groups:

cv. group Bengalense: annual cottons, cultivated in eastern Bengal, western Bangladesh and Assam in regions that are subject to frost. The lint is coarse and short.

cv. group Burmanicum: perennial cottons, cultivated in north-eastern India and South-East Asia, particularly Burma (Myanmar). The lint is variable.

cv. group Cernuum: annual, long-bolled cottons, grown in northern India (Assam). Included by some in cv. group Burmanicum.

cv. group Indicum: mostly perennial shrubs, although in India some annual cultivars ("Rozi" cottons) exist. It is the most primitive cultivated group within G. arboreum , and is mostly grown along the eastern coast of Africa and in Madagascar. The lint is scanty, course and frequently coloured.

cv. group Sinense: annual cottons, early maturing. Cultivated in China, Korea and Japan. It provides the original Asiatic cotton.

cv. group Soudanense: the "Senaar" tree cottons, large perennial shrubs or small trees. It is grown in north-western Africa and the Sudan. The lint varies from coarse to moderately fine.

G. herbaceum is primarily known from cultivation but wild forms exist as well. It is usually subdivided into 5 races or cultivar groups:

cv. group Acerifolium: cultivated perennial cottons, large, rounded, many-branched shrubs with small bolls, in which the seeds are sparsely covered with coarse lint. It is the most primitive cultivated form of G. herbaceum and is grown in northern Africa and Arabia. Some authors believe that this group is or has been cultivated in South-East Asia. However, others state that G. herbaceum is not grown at all in South-East Asia.

cv. group Africanum: the group of wild forms in G. herbaceum, perennial bushy shrubs, many-branched, with small bolls. They are found in southern Africa, and is classified by some as a subspecies. In cotton production it plays a minor role but is considered the common ancestor of the cultivated diploid cottons.

cv. group Kuljianum: annual, very small, slender, sparsely branched subshrubs, with small bolls and scanty lint of low quality. It is early maturing, selected for regions with short, hot summers and long, cold winters. Mainly cultivated in Russia and western China.

cv. group Persicum: annual, small, sparsely branched, stout subshrubs, with large, round bolls and copious lint of moderate quality, adapted to regions with a relatively cool winter season. Initially spread around the Mediterranean, this was the first cotton cultivated in the Nile delta. Later it also spread to Afghanistan and Turkestan.

cv. group Wightianum: annual, large, stout, moderately branched shrubs, with large bolls and copious lint of high quality. They are cultivated in western Peninsular India.

G. hirsutum is primarily known from cultivation, but wild forms exist as well. Several subdivisions have been proposed, but none is used widely. Some better known races or cultivar groups are:

cv. group Latifolium: annual subshrubs with medium large to very large bolls, early maturing but with a wide range of day-length sensitivity. They are cultivated throughout Central America, extending pantropically. This is commercially the most important cotton group of the world, collectively also called upland cotton. Modern cultivars are difficult to identify on the basis of vegetative characteristics alone, making reliable seed propagation methods very important.

cv. group Marie-Galante: perennial, strongly arborescent (dominant stem up to 14 cm in diameter) group, cultivated in northern and north-eastern South America and the Caribbean. Cultivars are strongly day-length sensitive.

cv. group Morrilli: stout, upright, profusely branched shrubs with a rounded appearance, with small, round bolls. Grown in central Mexico.

cv. group Palmeri: pyramidally shaped, glabrous shrubs with distinctive laciniate leaves, flowering prolifically and bearing numerous small bolls. They are cultivated in western Mexico.

cv. group Punctatum: perennial, slender stemmed shrubs, characterized by leaves with 3 lobes and shallow sinuses, bearing a large number of small bolls having seed with short fibres. Mainly occurring in Central America. Agronomically it is a very primitive group.

cv. group Richmondi: large, profusely branched, sprawling shrubs with bolls of moderate size. It is grown on the Pacific side of the Isthmus of Tehuantepec (southern Mexico and Guatemala).

group Yucatanense: small, highly branched, procumbent or prostrate subshrubs, only known wild in undisturbed beach vegetation on the northern coast of the Yucatan Peninsula. Bolls are small, widely flaring, and the lint of the small seed is sparse, coarse, brown. Agronomically it is the most primitive group, the only one known to grow wild, intergrading morphologically with cv. group Punctatum.

G. barbadense has been subdivided into 3 races, varieties or cultivar groups:

cv. group Barbadense: annual shrubs, cultivated mainly in the West Indies, Central America and the southern parts of the United States, but also in Egypt and Sudan.

cv. group Braziliense: the "kidney cottons" of the Amazon Basin. The seeds in each locule of the capsule are fused into a solid kidney-shaped mass. Under primitive agricultural conditions the kidney seed mass facilitates hand ginning.

cv. group Darwinii: perennial shrubs with fine brown lint. The group comprises the allotetraploid cottons endemic to the Galapagos Islands. Some consider this group a separate species (G. darwinii Watt), which is supported by evidence from molecular data.

Ecology

Nowadays cotton is found from 47°N to 32°S. The optimum temperature for germination is 29-30°C and the minimum temperature for germination of most cultivars is about 14-15°C, though some germinate at temperatures as low as 12°C. For optimum growth and development an average growing season temperature of 25-30°C is needed. Cool weather slows growth and development, leading to retarded and sometimes insufficient ripening. Cotton is extremely frost-sensitive. Ample sunshine promotes flowering and fruit set and the highest yields are obtained in dry areas under irrigation, for example in Arizona, United States. Cotton does not tolerate shade. The average rainfall during the growing season should be 500-1500 mm, with dry weather during ripening, because rainfall after fruit opening leads to a decreased fibre quality. When the rainfall during the growing season is less than about 500 mm, irrigation is necessary. Because of its deep rooting system, cotton is drought tolerant, but prolonged drought during flowering and fruiting causes yield reduction. Strong winds can damage the seedlings and the open bolls. Primitive cottons are usually photoperiod-sensitive, becoming reproductive at short to medium photoperiods, but modern cultivars are generally photoperiod-insensitive and can be grown at a wide range of latitudes provided there are about 6 months of hot rainy season.

Cotton is not very demanding with respect to soil quality, but it needs deep soils (permeable to water and roots down to at least 1.2 m), with sufficient drainage and a pH between 5.5-8.5. Very fertile soils stimulate vegetative growth and may cause an excessively long vegetative period. Cotton is relatively salt tolerant, with a salt content of 0.5-0.6% normally not causing damage, but cultivars differ considerably in this respect.

Propagation and planting

Cotton is propagated by seed. Organized seed multiplication and distribution is important to guarantee seed quality and purity. Where smallholder cultivation predominates, ginners are usually required to set aside seed for planting. It is usually recommended that seed be delinted by mechanical or chemical means, with the former being preferred for economic and environmental reasons. For mechanical sowing, removal of the fuzz is obligatory. Seeds lose their viability rapidly if their moisture content exceeds 10%, but seeds with a moisture content of 7% can be stored in sealed jars for up to 15 years. It is possible to propagate cotton vegetatively by cuttings, budding or grafting. When grown as a perennial, cotton can be cut back and ratooned, but this is not advisable. The development of rapid, reproducible and genotype-independent systems of in vitro propagation of Gossypium has been difficult, though methods have now been developed to produce large numbers of somatic embryos from callus from hypocotyl or cotyledon explants of G. arboreum, G. barbadense, G. herbaceum and G. hirsutum.

In many countries sowing is done by hand. The planting density varies widely with cultivar, climate and soil characteristics, and cultivation as well as harvest methods. Spacings are 50-120 cm between rows and 20-60 cm within the row. About 10-20 kg/ha delinted seed is necessary for sowing, for instance at 80 × 30 cm spacing (41 700 hills/ha) with 2-4 seeds per hill. The seed should not be sown deeper than 5 cm. Good soil preparation before sowing is very important, because the seedlings do not penetrate hard or crusted soils easily and do not compete well with weeds until they are 3 weeks old. Dry season weeds can be killed by harrowing or hoeing. This greatly reduces weed competition after planting. Early soil cultivation, allowing early planting, extends the growing season. Ridges are necessary on poorly drained soils. Cotton is usually grown in rotation with other crops to control pests and soil-borne diseases.

Husbandry

Within 1-2 weeks after sowing gaps in the stand of cotton are resown, followed by rapid between-row weeding. Two weeks later the hills are thinned to 1 plant by sideways pulling, and within-row weeding is carried out. A month later the third weeding follows. Cotton has a very good nutrient uptake ability, and nutrient requirements are therefore moderate. The uptake of a crop producing 1680 kg/ha seed cotton has been estimated at 105 kg N, 18 kg P and 66 kg K per ha. With the seed cotton 40 kg N, 7 kg P and 14 kg K per ha are removed; with the crop residues (especially the leaves) 65 kg N, 11 kg P and 52 kg K per ha are removed. Half the N and all P and K should be applied before sowing in a fertilizer furrow close to the intended contour furrows for sowing. This application is mainly for rapid and firm development of roots, stem, leaves and branches. The 2nd application of 60 kg/ha N, about 2 months later, is meant for rapid fruiting and filling of the bolls. Too much N stimulates vegetative growth and extends the vegetative period, whereas N-shortage leads to chlorosis, reduced growth and boll-shedding. Sufficient K is important for fibre quality and disease resistance and shortage leads to mottled yellowish leaves with brown concentric spots, with the leaf margins drying and curling inwards, and the leaves eventually drying out and being shed. P-deficiency results in dark green leaves and delayed flowering and fruiting. G. hirsutum is mostly grown under rainfed conditions and G. barbadense with irrigation. Under rainfed conditions supplemental irrigation may help early planting and alleviate dry spells in mid season. To stop vegetative development, irrigation should cease shortly after boll opening has started.

Diseases and pests

Diseases are less important in cotton than pests. The most widespread diseases are bacterial blight, leaf spot, blackarm or boll rot caused by Xanthomonas campestris pv. malvacearum, anthracnose caused by Glomerella gossypii, Fusarium wilt caused by Fusarium oxysporum, and Verticillium wilt caused by Verticillium dahliae. Bacterial blight is controlled by growing cotton only once every 3 or more years on the same field, removing the harvest remains and seed treatment. G. hirsutum cultivars with adequate host resistance are available. Anthracnose can be controlled by the same measures, but resistant cultivars are not available. G. hirsutum and G. arboreum cultivars with resistance to Fusarium wilt and G. barbadense cultivars with adequate resistance against Verticillium wilt are available. Cropping methods to control wilt diseases include crop rotation, sufficient K-fertilization and the control of nematodes.

Cotton suffers from a wide spectrum of pests. Bollworms are among the most serious. They feed inside the bolls, damaging lint and seed and so causing considerable reduction in yield and quality. The main bollworms are American bollworm (Helicoverpa armigera), pink bollworm (Pectinophora gossypiella) and spiny bollworm (Earias spp.). Spiny bollworm, to which G. arboreum and G. barbadense seem less susceptible than G. hirsutum, has a serious early effect of tipboring in the main stem leading to excessive formation of vegetative branches and delaying the setting of bolls, which makes them vulnerable to mid-season American bollworm and stainers. Resistance to bollworms has not been achieved to the desired extent, and their control has long relied heavily on insecticides. Leaf, stem and bud-sucking bugs can cause considerable damage. Jassids (Amrasca, Empoasca, Erythroneura, Jacobiella and Jacobisca spp.) are the first pests to appear, but a dense coating of long hairs on leaves and stems provides good protection. Whitefly (Bemisia tabaci) and cotton aphid (Aphis gossypii) are pests later in the season. Early sowing, weeding and harvesting and the use of short-season cultivars can reduce their damage. Cotton stainers (Dysdercus spp.) occur in all cotton-growing countries. About 4 alternating sprays of organophosphates and pyrethroids can overcome this pest. Fairly effective preventive control can be obtained by strict phyto-sanitation, early cropping of maize or sorghum followed by early ploughing and close planting of cotton using an early maturing cultivar. The boll weevil (Anthonomus grandis) is economically the most serious cotton pest in the United States, causing estimated annual yield losses of 8%. Close relatives of cotton, such as Abutilon spp., growing nearby are alternative hosts of cotton pests, especially stainers.

Insect pests in cotton have been effectively controlled since 1945 with the use of insecticides. The repeated development of resistance of insect pests (especially the American bollworm) to new insecticides has in some countries led to excessive spraying, up to 15 sprays per season, killing all natural enemies. This may also induce outbreaks of previously minor pests, requiring additional spraying. In the early 1990s, it was estimated that cotton used 53% of the pesticides applied in Indian agriculture, while occupying only 5% of the cultivated area. To reduce the use of pesticides, the application of Integrated Pest Management (IPM) or Integrated Weed and Pest Management (IWPM) is advocated. Very early field preparation including repeated weeding, fertilizer application, early planting of jassid-resistant cultivars, gapping, thinning and judicious use of pesticides on the basis of insect monitoring and damage thresholds, form the basis of interacting IWPM farming practices. Preventive weed control by ploughing or hoeing promptly after clearing the preceding crop stores moisture from unexpected storms in the subsoil which makes it possible to plant early. This encourages early fruiting well ahead of the main pest, American bollworm, and provides ample time for compensatory fruiting in case of early fruit damage. As insecticides against jassids are no longer needed they do not kill the natural enemies which control American bollworm in its vulnerable young stage. By the time the later pink bollworms, stainers and whitefly occur, the main crop will be safe. Recommended IPM practices to control American bollworm in the Philippines include the use of specific cultivars, a short planting period, adequate fertilizer application, planting of trap crops, weekly pest monitoring, release of natural enemies (Trichogramma chilonis), spraying with Bacillus thuringiensis at an early growth stage and the use of synthetic insecticides when the American bollworm population reaches a critical level.

The most widely distributed economically important nematode in cotton is the root-knot nematode (Meloidogyne spp.), whereas the reniform nematode (Rotylenchus spp.) is more restricted to tropical and subtropical environments. They can be controlled by rotation and chemicals, whereas cotton genotypes have been developed with some tolerance to the reniform nematode.

Harvesting

Cotton in South-East Asia is picked by hand 4-6 months after sowing. Handpicking gives the cleanest cotton and the highest recovery. Picking should be done regularly, because leaving open bolls in the field may result in deterioration of lint quality. Usually 3-4 pickings are carried out in a field. Mechanical harvesting is practised in the United States and Australia, where hand-picking is too expensive; here low-growing, weakly-branching cultivars are sown at narrow spacings, and plants are sprayed with defoliants before harvesting. Mechanically harvested fibre needs heating and shaking to remove impurities, and this operation makes the fibre more brittle. Mechanical harvesting is not feasible for the tiny plots of South-East Asia, where the hairy, jassid-resistance cultivars are grown, and hand-picking is much cheaper.

After picking, the seed cotton is cleaned and transported in bags or sheets to the ginnery (privately or co-operatively owned) for checking, weighing, sorting and payment. The seed from the first 1/3 of the crop picked is usually the most viable and may be kept separately in clearly marked bags. The lint from the first 2/3 of the crop is the most mature and strong.

After the cotton has been harvested, the field is often grazed by cattle, sheep or goats. The stalks may be lifted, roots and all, and stacked to dry and provide household fuel. The field can then be hoed or ploughed to stop weeds from seeding or rooting as an early IWPM measure. In many countries the destruction of harvested stalks was prescribed to control pests and soil-borne diseases. At present, the plants are often pulled up with their roots and used as household fuel.

Yield

A seed cotton yield of up to 4 t/ha is possible under optimal conditions, but in practice it is seldom over 2.5 t/ha and the average world yield is only about 1.6 t/ha. In most African countries the yield is well below 1 t/ha. Seed cotton of primitive cultivars yields 20-25% fibre after ginning, whereas good upland cultivars have a "ginning-outturn" of at least 35% and sometimes over 40%. Some modern G. arboreum and G. herbaceum cultivars grown in India and China also yield up to 40% lint. On average, 1 t seed cotton of G. hirsutum yields 350 kg fibre, 120 kg oil, 300 kg cake, 160 kg hulls and 60 kg fuzz.

Handling after harvest

Cotton lint is removed from the seeds by ginning, which can be done with a hand gin (capacity of 2-3 kg lint/hour) or mechanically with a saw gin (capacity of 300 kg lint/hour) for the shorterstapled cottons or with the more gentle roller gin (capacity of 30 kg lint/hour) for the longerstapled fine types. The ginner is an indispensable link between farmers and the complex textile chain. If he pays low prices, farmers will in response lower inputs, neglect the crop and eventually reduce their area under cotton. They may even decide to opt out completely as witnessed by deserted ginneries in Thailand and the Philippines.

Cotton fibre quality is defined in terms of its length and uniformity, strength, elasticity and maturity, whereas whiteness and trash content also play a role. These properties can all be measured rapidly by the "High Volume Instrument" (HVI). The lint is baled under pressure and covered with hessian or another material. Modern processing of cotton fibre into fabric consists of the following steps:

blending: several bales of cotton are thoroughly mixed to ensure a uniform product of the desired quality;

cleaning: the lint is passed through a series of small-toothed saws that pick the fibres apart and remove the trash;

picking: continuation of the cleaning process by picking or fluffing the fibres so that they barely hold together in a continuous sheet ("picker lap");

carding: the picker lap is converted by a card machine into a thin sheet that is slightly twisted to produce a loose rope ("sliver");

drawing: several slivers together are drawn out or stretched to form a more uniform single sliver with more parallel fibres;

roving: further drafting and twisting of the sliver to produce roving, which is wound on to a bobbin;

spinning: further drafting of the roving into small fibre strands that are twisted into yarn;

warping: thousands of yarns are wound parallel on a giant spool ("warper beam") to form "warp" yarns, which will later run the length of the fabric;

slashing: coating of the yarns with starch to temporarily give the extra strength and resistance to abrasion which are needed for weaving;

weaving: warp yarns are interlaced with "weft" yarns (filling or cross yarns) on a loom to produce fabric.

Fuzz is removed by additional saw ginning at the oil mill before seeds are crushed. Linters are pulped by caustic solutions and bleached. Cotton textile cuttings and rags are pulped using the soda process. Dyed textiles and the presence of synthetic fibres and elastic materials in rags can cause problems in paper making.

Genetic resources

The largest cotton germplasm collection is the Cotton Germplasm Collection of the United States Department of Agriculture, Agricultural Research Service (USDA/ARS) at College Station in Texas, United States, which contains about 9000 accessions of 41 species (including about 4600 of G. hirsutum, 2500 of G. arboreum, 1200 of G. barbadense and 200 of G. herbaceum). The Central Institute for Cotton Research at Nagpur, India, also has a large collection (including about 4000 accessions of G. hirsutum, 1700 of G. arboreum and 400 each of G. barbadense and G. herbaceum). The genebank of CIRAD (Centre de Coopération Internationale en Recherche Agronomique pour le Développement) at Montpellier, France, contains about 3600 accessions of 35 species. Cotton germplasm collections are also maintained in China, Vietnam and other cotton-producing countries.

Breeding

Until the 1930s cotton breeding was limited to crossing within the diploid and tetraploid groups. After this, polyploidization of the diploids greatly increased breeding opportunities. Crosses between Old World and New World genotypes have become important, especially for resistance breeding and the breeding of better cultivars for regions in Asia where G. arboreum and G. herbaceum grow well, but G. hirsutum does not. F₁hybrid cultivars with considerable hybrid vigour for yield have been successfully developed. However, the available systems of cytoplasmic male sterility have been inadequate for large-scale production of hybrid seed, mainly due to incomplete expression of fertility restorer genes in the male parents. Current use of cotton hybrids is limited to South Asia and China, where seed production by manual emasculation and pollination is economically feasible because of low labour costs.

The main objectives in cotton breeding, apart from higher yields, are photoperiod-insensitivity, early maturity, adaptation to mechanical harvesting (through low growth, little branching, short flowering time, loosely attached seeds, less hairy leaves), fibre quality (length, fineness, strength and elasticity), seed quality (oil content and low gossypol content by glandless plants, increased suitability of the presscake as a source of protein for humans and animals), resistance to diseases (e.g. bacterial blight and Fusarium wilt) and pests (e.g. bollworms, jassids) and tolerance to drought, cold and salinity. There has been little progress in breeding for pest resistance (except for resistance to jassids), but much success has been obtained in resistance and tolerance to Fusarium and Verticillium wilts, bacterial blight and nematodes.

Molecular breeding has been applied to cotton with considerable success. Bt-cotton, genetically modified (GM) cotton cultivars with resistance to bollworms based on Bt genes derived from Bacillus thuringiensis, is already grown on 4.3 million ha, including 1.5 million ha in China alone. Bt-cotton was first released in Indonesia in 2001. Work is in progress to develop wide-spectrum insect resistance based on a combination of several Bt and proteinase-inhibitor genes. Cotton has also been genetically modified to express resistance to the herbicides bromoxynil ("Bromoxynil-resistant cotton") or glyphosate ("Glyphosate-tolerant cotton"). Some GM cotton cultivars have combined insect and herbicide resistance. The impact on reduced insecticide use in Bt-cotton should become manifest in the coming years, as the area of such cultivars is rapidly increasing. So far, there has been little proven evidence of negative effects of GM cotton on the environment.

Prospects

Because of its excellent fibre properties and low price, cotton will remain very important on a worldwide scale. In South-East Asia, however, less than 1% of the arable land is used for cotton. The small acreage and low yields are mainly due to the generally low price paid for seed cotton in the region and the high damage caused by pests. So, farmers tend to only grow small fields of cotton for their minimum cash needs, whereas they would probably grow more if they were paid more. Furthermore, a higher price would lead to better crop management and increased input use by farmers, and thus to higher yields. Most of South-East Asian cotton production is of a quality aimed at its own low-cost garment market. As a result, it is not easily exported to important markets such the European Union and Japan. The quality can be much improved as happened in Laos after massive and repeated reselection and progeny testing over a period of 3-4 years.

Ginned seed is an excellent concentrate for young non-lactating ruminants (cattle, buffaloes, goat and sheep). In South-East Asia over 5 million head of cattle are slaughtered per year, with an average live weight of 360 kg. A young animal of 270 kg can be fed with ginnery seed at a daily rate of 1% of its live weight, reaching 360 kg in about 6 months, requiring a total of about 540 kg ginnery seed. To fatten 5 million head of cattle would require 2.7 million t ginnery seed. The present 300 000 t seed cotton harvested yearly in South-East Asia produces about 200 000 t ginnery seed. With a cotton lint production equal to the internal consumption in the region (about 1 million t), about 2 million t of ginnery seed would be available. The high susceptibility of cotton to pests is another major factor responsible for the low cotton production in South-East Asia. Integrated Pest Management, especially but not exclusively for cotton, should be further developed in close cooperation with cotton farmers. The use of genetically-modified cotton such as Bt-cotton and herbicide-resistant cotton is another promising option, though its use is still meeting public resistance in some countries.

Literature

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Authors

G.J. Kerkhoven & H.J.W. Mutsaers