In this article we will discuss about the growth and development of cotton.
Germination and Emergence:
Poor germination and seedling establishment often limit crop stand and yield. An impermeable seed coat and impenetrability of seed coat fibres by water are the two major factors inhibiting cotton seed germination. It is, generally, recommended that fuzzy seeds should be delinted by machine, flame or dilute sulfuric acid. Soil crusting also affects cotton seed germination.
Under favourable conditions, seedlings emerge in 5-15 days after sowing. Temperature limits for emergence are 14° to 42°C with an optimum of 34°C. At optimum temperature, seedlings can emerge in 5 days at 0.03 MPa, 7 days at -0.3 MPa and will not emerge in 13 days at -1.0 MPa.
Root, Stem and Leaves:
Depth of primary root penetration depends on soil, climate and genotype. Maximum root depth varies from 150 to 300 cm under favourable conditions. Depletion of water in upper profile leads to proliferation of roots at depth resulting in greater extraction of water from deeper profile.
The main stem of the cotton plant is monopodial with leaves and branches but no flowers. There are usually two axillary buds at each main stem node, the second branching off the first. Normally, only one bud develops. At lower node, the first bud remains vegetative and may develop into a vegetative branch of monopodium, which is a replica of the main stem.
Usually, the vegetative branches occur in a definite zone near the base of the plant and the fruiting branches occur further up the stem. Number of nodes from the base of main stalk to the first fruiting branch varies considerably among cotton species and is affected by cultural practices.
The fruiting limb or sympodium, having flower buds called squares, will occur at the fifth through the ninth main stem node about 30 DAS. Cotton plant fruits vertically via the main stem and horizontally via extension of fruiting limbs.
Flowering and Fruit Development:
Though, the rate of development of cotton plant is conditioned by the variety, cultural practices and climate, the cotton plant follows broadly the following time schedule:
Seedling emergence:
About 4-8 days after planting.
Development of flower bud into flower:
It varies from 17 to 25 days and is usually constant for a variety.
Appearance of blooms:
The interval between blooms in corresponding nodes of successive fruiting branches is about 3 days, while that between successive nodes of the same branches is about 6 days.
Flowering to boll opening:
It takes 40 to 80 days depending on the variety and temperature.
Pollination of cotton flower, generally, takes place on the first day the flower opens. After anthesis, the fruit called a boll develops from a superior ovary consisting of 3-5 united carpels into a tough walled capsule. Internally, it is divided in to 3 to 5 loculi, each of which initially contains 10-12 ovules, 5-11 of which develop in to coat, seeds covered with fibre. The contents of one loculus when mature forms a lock of seed cotton.
Cotton fibres are unicellular extensions of epidermal seed cells that begin to elongate early on the day of anthesis, a phenomenon independent of fertilisation. Twelve to 20,000 of these cells per seed, approximately 20 per cent of its epidermal cells will elongate sufficiently to be spun into yarn.
Other epidermal seed cells will begin to elongate enough to be broken away from the seed during ginning. These short thicker fibres left after ginning will be removed at the oil mill and used in several ways such as rayon, paper etc. The cotton fibre contains approximately 94 per cent cellulose.
The elongating seed epidermal cell, now called a fiber, grows rapidly in length for 16 to 19 days after anthesis. At this point in development, the fibre consists of a primary wall, a layer of protoplasm and a central vacuole called the lumen. Rate of secondary wall development increase at about 19 days after the anthesis and development is rapid until about 40 days after anthesis. At maturity, the fibre dries and the lumen collapses, giving the fibre its characteristic twisted, ribbon-like appearance.
Bud and Boll Shedding:
It is natural for some squares, blooms and small bolls to be shed. Such factors as moisture deficiency or excess, inadequate number of fertilised ovules, inadequate nutrient supply, excessive heat or cold and damage from insects and diseases increases sheading.
Several theories have been proposed to explain flower and boll shedding. The nitrogen theory indicates that a normal leaf has 5 per cent nitrogen and drops to 2.5 per cent as the crop progress towards reproductive stage. Internal retranslocation of nitrogen sometimes leads to boll shedding. Nutritional theory suggests that plant growth is limited by carbohydrate and mineral supply.
Heavy boll load followed by limited carbohydrate production leads to boll shedding. Nutrient diversion theory indicates that hormones and inhibitors interact with the supply of photosynthates, thus controlling growth and fruit development. Spray application of hormones like NAA improves bole setting and seed cotton yield.
The malady of bad boll openings, commonly known as tirak has been reported in hirsutam cotton. It has been attribute to soil salinity or alkalinity in subsoil, nitrogen deficiency in light soils or abnormally high temperature during boiling period.
Reddeingning of leaf has been attributed to magnesium deficiency, low nitrogen content of leaves, low available soil nitrogen and sudden fall in night temperature High wind velocity and low minimum temperature may also cause reddening of leaf. Urea or diammonium phosphate (2%) at peak flowering minimises leaf reddening and help in boll retention.
Fruit retention is supposed to be hormone regulated. The balance between auxin and anti- auxin compounds controls it. Auxins inhibit abscisic acid (ABA) and ethylene promotes abscission. Increased enzymatic activity of pectinase and cellulase dissolve the middle lamella and weaken the call wall thus causing flower and fruit drop. The ABA increases ethylene production, which causes increase in cellulase.
The cytokinins delay or inhibit fruit senescence and abscission. The balance between production of auxin, ethylene and ABA probably regulates growth, flowering, fruiting and abscission. The consensus is that increased endogenous production of ethylene is responsible for boll shedding.
Photosynthesis and LAI:
Net photosynthetic rates are in the order of 130 mg CO2 cm-2 s-1. Radiation use efficiency ranges from 1.2 to 1.4 g MJ-1. Leaf area may not be a limitation since the peak LAI of 3-5 have been consistently achieved from flowering to boll formation. Ceiling of LAI seems to be 6. Dry matter production ranges from 5 to 10 t ha-1 with a mean of 7.5 t ha-1 at maturity.
Yield Components:
Yield components of cotton are bolls per plants, seeds per boll, boll weight and lint percentage. The number of bolls per plant is the most important yield variable.
Yield oil seed cotton can be estimated just before boll bursting as indicated below:
Fruiting efficiency or fruiting index indicating relative fruitfulness is given by:
