Cotton

Morphology of different Species

Gossypium arboreum (n = 13, diploid)

• It is a perennial, much branched shrub or an annual sub-urb 1.5 to 2.0 m tall. Its twigs and young leaves are pubescent. Fruit is a boll, trilecular, tapering with prominent oil glands. Seeds are small, numerous, 6 to 17 per loculus. Seeds have short fuzzy hairs deep, well developed root system and drought tolerant. Nearly 29 per cent of the area grown with cotton in India is under arboretums.

Gossypium herbaceum (n = 13, diploid)

• It is a sub-shrub, 1 to 1.5 m tall, its stem thick and rigid and, leaves horizontally placed. Leaves and twigs are sparsely hairy and rarely glabrous. Fruit or boll is rounded, beaked 3 or 4 lobular with 11 to 10 seeds per loculus. Seeds have short fuzz and lint. Nearly 21 per cent of the area in India is under herbaceums.

Gossypium hirsutum (n = 26, tetraploid)

• These are known as upland cotton and about 90 per cent of the current world production use this species. This is a small annual shrub 1 to 1.5 m tall. The stem is usually green or brown with leaves and twigs glabrous to densely heavy. Leaves are large and cut into 3 to 5 lobes. Boll is rounded, 3 to 5 lobular with 5 to 7 seeds per loculus. Seeds bear copious hairs and a thick fuzz.

Gossypium barbodense (n = 26, tetraploid)

• It is a perennial shrub or annual sub-shrub, 1 to 3 m tall. Twigs and leaves are glabrous or densely coated with long grey hairs. Leaves are highly lobed cokro leaf Bolls are large, round and bold 5 to 8 seeds per loculus. Seeds are bald with no fuzz. The lint is of finest quality with extra long staple length (>30 mm) and contribute to 8 per cent of total cotton production. This is used in luxury fabrics.

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Growth stages of Cotton

• Cotton is an indeterminate plant exhibiting overlapping vegetative and reproductive growth phases and these phases cannot be clearly demarcated. The duration of the different phases depends upon the variety, latitude, climate and management practices. These phases can broadly be differentiated into:

Germination phase

• Under optimum ambient conditions, the germination and the emergence are completed in 4-7 days.

Early vegetative phase

• This phase is characterized by a rapid root growth, slow growth of the main shoot and emergence of the first true leaves. This is followed by the growth of main stem and the differentiation of early monopodial branches.

Squaring

• The first square (flower bud) appears between 35 and 70 days after germination depending on the variety location and crop husbandry. The squaring continues till maturity.

Flowering

• The first flower appears about 20-35 days after appearance of the first square and flowering continues for about 60-80 days or more depending upon the availability of soil moisture. However, the peak flowering period is from 70-100 days after germination.

Boll development

• This period commences from flowering and continues till maturity. During the first 15-18 days after the fertilization of the flower, bolls attain 90 per cent of their final size. The elongation of lint fibres in the boll is completed in 21-24 days. During the subsequent phase of approximately the same direction, the cell wall thickens due to the deposition of cellulose. The boll cracks on maturity along the locules and the fibres ultimately dry out and become ready for picking.

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Roots

• Cotton has a taproot grows quickly and it can reach a depth of 20-25 cm before the seedling has even emerged above ground. After emergence and unfolding of cotyledons, lateral roots begin to develop; they first grow side ways and then down wards. The taproot continues to grow rapidly.
• Final depth of root system depends on soil moisture, aeration, temperature and variety but is usually about 180-200 cm.
• Under dry growing conditions, cotton roots have been known to reach a depth of 3-4 m.
• When soil moisture is adequate, most of the lateral roots are concentrated in the upper soil layer to a depth of 30-35 cm and may extend laterally to 100 cm and more. Under dry growing conditions lateral roots penetrate much more deeply.
• Factors affecting vegetative growth also influenced root growth of cotton plant.
• The growth of tap root as well as lateral roots are affected by excessive, moisture, hard dry soil layer and degree of soil alkalinity. Lateral roots adjust their quantum to the plant spacing and soil moisture regimes.
• Under conditions of water saturation and submersion, the laterals get asphyxiated and die but are regenerated when the optimum conditions reappear.

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Stem

• Cotton plant consists of an erect main stem and a number of lateral branches. The stem has a growing point at its apex, with an apical bud. As long as this bd remains active, lateral buds situated below the apical bud, remains dormant. The main stem carries branches and leaves but no flowers.
• Length and number of internodes determine the final height of the plant. As a rule plant with short internodes are early maturing.
• Length of internodes is determined mainly by the moisture supply while the number of internodes is usually a function of nitrogen supply to the plant.
• At the axil of each leaf are two buds, the axillary bud from which most vegetative and fruiting branches develop and a lateral bud on one side of axillary bud normally remains dormant; but if the axillary aborts, the lateral and may develop into a branch.
• Vegetative branches are morphologically similar to the main stem. They do not bear flowers or fruits directly, but carry secondary branches (fruiting branches), that are characterized by their sympodial growth habit.
• The fall in the growth rates noticed in the later half of the season is attributed to the deflection of carbohydrates from apex of the plant and the lateral apices of branches, to the developing bolls.

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Leaves

• Leaves are generally hairy, some varieties may have glabrous leaves. Hairy leaves cause fewer difficulties in mechanical harvesting but more tolerant to Jassids, but bear larger proportions of white fly which apparently find more sheltered conditions among the leaf hairs.

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Branching

• Lateral branches arise from the axils of the leaves of main stem.
• Lateral branches consist of two types viz., Vegetative and fruiting.
• Vegetative branches are more vertical and ascending.
• Fruiting branches are nearly horizontal.
• Fruiting branches the internodes are not straight as in main stem but have a zig zag appearance with the leaves alternately placed.
• Economic importance of sympodial branching is great. The flowering and fruiting are dependent on the initiation of such branches and the timing or the crop for harvest is determined by the early or late production of such sympodial on the plant body. Very early varieties have their fruiting branches even at first or second node to the total exclusion of vegetative branching from leaf axils, similarly very late varieties go on producing a very large number of monopodial before sympodial divergence appear. In such cases, the late sympodial on the main stem and the secondary sympodial arising on the monopodial will contribute to the harvests.
• As a rule, the vegetative branches are located near the base of the plant and above these are fruiting branches. In most upland varieties, the first fruiting branch generally develops on the 5th to 7th node whereas in ‘Egyptian varieties’ it is located on 8th or 9th node. In dense stands, the first fruiting branch generally develops at a higher level than in more open stands.
• Relative proportion of vegetative and fruiting branches is dependent on temperature, day-length, plant density and the rate of boll shedding.

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Floral Bud

• Within the corolla is a tube formed of numerous stamen filaments, surrounding the pistil.
• The ovary at the base of the pistil consists of from two to six carpels, containing a many locales or "Locks". Each lock contains from 8-12 ovules.

• When the flower opens it is white or creamy white in the American Varieties, changing to pink towards the end of the day and becoming red the following morning; on the third day the petals wither and fall.

• Cross pollination is more in early flowers than in later-appearing flowers.
• Many insects are attracted to the cotton flowers, and they are active in cross-pollination.
• Some of the ovules may fail to develop normally, the aborted seeds are called "motes". Bolls in which the majority of ovules fall to develop are usually shed within ten days after flowering.

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Fruit

• Bolls of Hirisutum are large (5-8 g), pale green, smooth – skinned and with few oil glands. Bu contrast bolls of arboreum are much smaller (3 g) dark green, covered with numerous glands.
• Cotton plants by its remarkable auto-regulatory mechanism sheds the bolls that are in excess of the load capacity of the plant under given environmental conditions. As a result, the ratio of bolls to total vegetative growth is fairly constant.
• In general, varieties or strains with large bolls do not adjust so well to change in environment and to stress, as do types with smaller bolls. Hence, shedding will occur more readily and to a large extent in the former than in the latter case.
• The development of fruit (boll) begins with the fertilization, and shedding of withered floral organs enclosing it.
• Bolls developing under falling temperature will lead more days to mature than those growing under rising temperature. The big – bolled American types in India take about 55 days while the Asiatic cottons require only 45 days which may further reduced to 35 days under higher soil and atmospheric temperatures. It may however, be stated that the first half period of maturation of a boll is spent in growth and the second half in internal development without any changes in the boll size.
• The boll consists of four to six locales each of which contains number of seeds. Majority of Asiatic cottons has only upto 7seeds per locales. A fair percentage of the seeds remain undeveloped due to non-fertilization, heredity and environment. These are called "motes".
• The size and shape of the bolls differ and are varietal characteristics.

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Seed

• The full-grown seed is irregularly pear – shaped, varying in size depending on the variety and conditions of growing.

• The lint is removed by gins while the fuzz remains attached. The colour of fibres may be white, brown or green and that of the seed is usually grey, brownish or black.
• The mature seed has two cotyledons folded up and occupying the entire portion of its cavity.
• They are broad and kidney – shaped. Delayed germination in some of the species and varieties may be due to hard seed coat, closed micropyle and partially filled cotyledonary – cum – embryonic contents.
• The first two retarded the passage of water required for germination while the ill – developed contents were unable to swell rapidly and exert the requisite pressure for the early rupture of the hard coat necessary for the proper emergence of the plumule.
• The germination increased when the seed coat thickness was reduced by treatment with sulphuric acid or by abrasives or by partial removal .
• The seed account for about 65 to 70 per cent of the total yield by weight.
• The kernels are rich in protein (10-20%) and oil (upto 25%). Egyptian cottons usually have a higher oil content than hirsutum cottons.

Seed Hairs

• Lint and fuzz represent the outgrowths of epidermal cells on seeds. Some cells continue to lengthen while other stops growing after at time. The former are the lint and the latter the fuzz.
• The lint hair is unicellular and its development is phased in two stages, the first is a period of elongation and the second in thickness.
• A lint cell bulges first, the protoplasm inside turns granular, and the nucleus moves towards the bulge. The swelling enlarges until it is twice the diameter of the original cell and the nucleus moves to or near the tip.
• The elongation of cell may take about 24 days, thereafter ceases. There is no change in thickness. The growth is not regular; slow at first but fast from about the 15th day The rate slackens during days and quickens during nights.
• The cell wall thickens in the second half of boll maturation. Deposits of cellulose are formed on the inside of primary wall.
• They are laid in layers as seen from some fibres showing as many as 25 concentric layers.
• As soon as the boll dehisces, the hairs dry, collapse and flatten the cylindrical from, assuming ribbon like shape and go into spirals.
• The mature hair is uniform in diameter upto 3/4th length and then gradually tapers to a point.
• Lint at maturity contains three types ripe, half ripe and unripe fibres known as dead fibres, have thin walls, lack twist and are weak, with a tendency to break up during manufacture.
• The length of lint is a varietal character and varies from 5-50 mm.
• The fuzz may either cover the entire seed coat as in hirsutums or be concentrated in a single tuft at the hilum end of the seed as in barbadense.

Glands

• On all aerial parts of cotton plants are found internal glands which in different species vary in size, number, distribution and pigmentation.
• These glands secrete a volatile oil (gossypol) and related compounds. Gossypol is a poly phenolic yellow pigment and is toxic and hence, gladness varieties have been bred in recent years.
• However, it has been formed that gladness varieties are susceptible to a wide range of pests over the glanced varieties.

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