Cotton Fibers – the king of fibers

Development of Cotton Plant

To optimize fiber production, one must optimize reproductive growth and development in the cotton plant. Cotton plant development occurs in a very specific sequence of events. Each developmental stage of the plant impinges on fiber development. A better understanding of cotton plant growth and development and of the biological mechanisms that control flower production, fruit set, and boll retention is essential for the continued profitability of the cotton industry. This section summarizes the growth and development of the cotton plant as it relates to fiber production.

Seasonal patterns of plant Development Cotton is a perennial plant, capable of the growing year after year, producing flowers and fruit each year. Being a perennial, the cotton plant evolved

Indeterminate growth is the ability to continuously develop and grow new organs as long as the plant lives. This means that throughout its life the cotton plant will continuously produce flowers and fruit (bolls). Agriculturally, cotton is grown as an annual, being planted and harvested on a yearly basis. This combination of a perennial growth pattern and agricultural management as an annual result in tremendous variability in boll maturity at the time of harvest.

Plant development in cotton proceeds through five main growth stages: germination and emergence, seedling establishment, leaf area-canopy development, flowering and boll development, and maturation. The transitions between these successive stages are not clearly distinguishable. Furthermore, each stage may have different growth processes operating with specific environmental and nutritional requirements.

Seed Germination and Emergence

Seed with all fibers removed is an ovoid, somewhat pointed, dark brown structure. The seed consists of a seed coat surrounding an embryo with two well-developed cotyledons. The embryo axis consists of a radicle (embryonic root), a hypocotyl, two cotyledons,

And a poorly developed epicotyl (embryonic stem). The cotyledons, or seed leaves, form the first green leaves after emergence. Initially, cotyledons contain stored food that supplies the energy for germination and early development. There are usually about 3,500-4,000 delinted seeds per pound.

Germination begins within the first few hours of the entry of moisture into the seed. This results in increased oxygen uptake and the utilization of stored energy reserves to build new cells and tissues during embryonic growth. The seed/embryo swells as water is absorbed causing the seed coat to split. Under favorable conditions for germination, the radicle emerges through the pointed micropylar end of the seed in two to three days. The radicle becomes the primary root that grows downward into the soil. The tissues between the radicle and cotyledons (i.e. The hypocotyl)

Grow rapidly, arching near the cotyledons. With continued elongation of the hypocotyl, the cotyledons and embryonic shoot are pulled up through the soil surface. This is called emergence. Typically, the seed coat is shed and remains in the soil. Soil crusting due to surface compaction or high clay content may hinder the emergence of the cotyledons and embryonic shoot. When the cotyledons are free of the soil, they unfold and expand. After emergence and exposure to light, the cotyledons develop chlorophyll and are capable of synthesizing food via photosynthesis.

Root Development

The function of roots is to absorb nutrients and water from the surrounding environment and transport these materials to the above-ground portions of the plant. Much of the early development of the cotton plant is focused on growing a substantial root system. Growth of the above-ground portion is relatively slow prior to canopy development. The primary root, or taproot, penetrates the soil rapidly and may reach a depth of up to 10 inches or more by the time the cotyledons unfold. Root development may proceed at the rate of 0.5 to 2.0 inches per day, depend- ing on conditions, such that the roots may be 3 feet deep when the above-ground portion of the plant is only about 1 foot.

Numerous lateral roots spread outward from the taproot, forming a mat of roots extending several feet. The largest portion of the root system is located within three feet of the soil surface. Root distribution within the soil (root length density) is usually about 24 inches of root per cubic inch of soil but can vary considerably with soil and plant conditions. The total root weight comprises approximately 20% of the total dry weight produced by the plant during the growing season. However, the total root length produced during the same time may be several hundred yards. The total root length continues to increase as the plant develops until the maximum plant height is reached and fruit begins to form. Root length then begins to decline as older roots die.

Shoot Development

The cotton plant has a very prominent main stem that consists of a series of nodes (points of leaf and bud development) and internodes (length of stem between nodes). The main stem will continue to grow, producing new nodes and internodes indefinitely, consistent with an indeterminate growth habit. The main stem is erect and supports a spiral arrangement of leaves and branches. Branches develop from a bud located at a node in a location immediately above where the leaf joins to the main stem. Two types of branches are produced — vegetative and fruit-.

Vegetative branches are structurally similar to the main stem. They normally arise from the main stem near the ground and grow in an upright position. The number of vegetative branches produced depends primarily on the environment and plant spacing.

Fruiting branches develop from buds on the main stem or from vegetative branches and are defined by the presence of floral buds (squares), flowers, and fruit. Once fruiting has begun, fruiting branches tend to be produced at each successive main-stem node. The first fruiting branch is normally produced at the sixth or seventh node above the location of the cotyledons on the main stem.

Leaves Formation

There are three main types of leaves: cotyledons, prophylls, and true leaves. The kidney-shaped cotyledons from the originally planted seed are usually about two inches wide. The prophylls are the first leaves that develop on a branch and are inconspicuous, usually about 0.2 inches long. The true leaves vary in shape from entire to deeply lobed, depending on the developmental stage and variety. The first true leaves formed on the cotton seedling are usually heart-shaped, whereas subsequently formed leaves are lobed. The leaves of U.S. cotton cultivars are usually three to five-lobed and about four to six inches wide. Cotton leaves generally have a thick waxy outer covering for protection. This layer contains numerous small pores (stomates) for the entry of carbon dioxide for photosynthesis as well as the exit of water vapor for evaporative cooling. Epidermal and glandular hairs are also located on the surface of the leaf.

The growth of true leaves is relatively slow at first compared to root growth, such that at one month after planting only about four or five true leaves may be unfolded and visible. During the later vegetative period, the emphasis changes to square and flower development. The average life span of a leaf is about 70 days. The large petiole (stalk joining leaf to stem) at the base of the leaf is often analyzed to estimate plant nutrient status. Total leaf area development continues to increase, reaching approximately three to four square yards of leaf per square yard of the soil surface in a mature crop.

Reproductive Development

The cotton plant, due to its indeterminate growth habit, continues both vegetative and reproductive development throughout the remainder of the season. Reproductive growth commences with the formation of the floral buds in the apical part of the plant which gives rise to the flowers and subsequent bolls

Cotton has a distinctive and predictable flowering pattern. The first flowers to open are low on the plant, usually on main-stem nodes six or seven, and on the first position along a fruiting branch. About three days elapse between the opening of a flower on a given fruiting branch and the opening of a flower at the same relative position on the next higher fruiting branch. On the other hand, the time interval for the development of two successive flowers on the same branch is about six days. The order is thus spirally outward and upward. Flowers continue to be produced (indeterminant growth) as long as the plant is actively growing. In an agricultural setting, active plant growth is stopped by defoliation or frost.

Cotton is genetically programmed to produce seeds for sexual reproduction. For the plant, the fibers that coat the seed evolved to facilitate seed dispersal, probably functioning to entangle in fur and feathers so that seeds were carried away. Wild cotton plants produce much smaller seeds with far fewer and shorter fibers. Humans dramatically influenced the evolution of domestic cotton by selecting plants that produce large quantities of fiber. Thus the original function of the fiber on the seed coat, to ensure the propagation of the species by spreading the seed to new locations, has been superseded by a new function. Humans select seeds for propagation based on fiber production. The end result is the same continuation of the species. The seed contains the embryonic plant for the next generation, composed of cotyledons, a root, a stem, and leaves.

Seed and Boll Development

Initially, the embryo is very small and the endosperm comprises most of the embryo sac. The endosperm swells as it fills with nutrients from the parent plant. As the ovule matures into the seed, the stored nutrients are transferred from the endosperm to the developing cotyledons of the embryo. During seed germination and early seedling growth, these nutrients are used to support the plant until it can photosynthesize and make its own food. By the time the seed is ready to germinate, the endosperm is not detectable and the embryo consists of two large cotyledons and the embryonic axis, including radicle, hypocotyl, and epicotyl.

Seeds attain their full size about three weeks after fertilization but do not reach maturity until the boll opens. Only ovules that are fertilized and develop an embryo reach maturity. Immature or aborted ovules, called motes, are often found in mature bolls. Fertilization and fiber production are linked processes, in that fertilization is required for optimum fiber growth and development on the plant. Unfertilized ovules will develop fibers; however, the extent of fiber development is severely limited and is one of the causes of textile motes (undeveloped seeds with immature fibers).

The boll, or fruit, of the cotton plant, varies in form and size but is generally a spherical or ovoid leathery capsule, light green in color, and with a few pigment glands. The boll grows rapidly after fertilization, especially between 7-18 days, and full size is reached in about 20 to 25 days. Maturation of the boll, from anthesis to the time of boll opening, usually takes about 50 days but this varies with genotype and environmental conditions. The boll is composed of three to five locules (compartments of the ovary) each with eight or nine seeds attached to the central column. At maturity, the boll splits along suture lines in the ovary wall.

The mature white seed-cotton within expands greatly, pushing out beyond the capsule, forming a white fluffy mass divided into lots. About 300 bolls are required to produce a pound of lint and there are about 145,000 bolls per bale of lint. The fertilized ovule de- envelops into a seed if the young boll is not shed. Shedding occurs either before anthesis (squares) or after fertilization (developing bolls). Flowers are not shed. The shedding of squares and young bolls is a natural occurrence in cotton that is accentuated by adverse environmental conditions including extended overcast weather, extremely high temperatures, water stress, and insect damage. A cotton plant in a typical field commonly sheds about 60% of its squares and young bolls, mostly in the younger regions of the plant (i.e., ends of branches and main stem).