The shoot system of the cotton plant is made up of a main axis stem, leaves, buds, branches, floral buds, flowers and bolls. The prominent main stem, or primary axis, results from the elongation and development of the terminal bud or shoot apical meristem. The main stem consists of a series of nodes and internodes. The main stem has an erect, indeterminate mono-podial growth habit. The number of nodes, the length of the internodes and the number and location of branches is determined by genetic and environmental factors including cultural practices. Cotton shoot apical meristems are quite typical when compared to other dicotyledonous plants. The portion of the axis above the cotyledons, the epicotyl, is extremely reduced and it comprises a single shortened internode and one node. Thus, all subsequent leaves, nodes, internodes and axillary buds and branches develop following germination.
Node of Plant
The main stem of the cotton plant comprises many nodes, each capable of producing a branch. Except under very rare conditions, each node above numbers 5-7 produces a fruiting branch. The distance between two nodes is called internodal length. The internodal length is a genotypic character but depends highly on growing conditions. The three main factors responsible for longer internodal length are frequent Irrigation and rain, Excess supply of nitrogen, Inability of the plant to retain fruit (due to heat sterility, insects and other factors).
The branches that do not bear fruit directly are the monopodial branches. Monopodial branches are also called vegetative branches and are always formed at the base of the cotton plant.
Sympodial branches bear fruit directly, so they are called fruiting branches. The secondary branches on monopodial branches are also sympodial and bear fruit directly. Once a sympodial branch has formed on the main stem, one or two branches are formed on every subsequent node until the plant is physiologically exhausted and growth terminates. Once a sympodial branch has formed at a main stem node, the plant is no longer able to produce monopodial branches above that node.
Leaves, Flower and Bolls
The cotton plant has two kinds of leaves: cotyledonary leaves and true leaves. Cotyledonary leaves emerge before the true leaves. The cotton seed has two well-developed cotyledons. The two cotyledons always form the first green leaves. Cotyledonary leaves have a rounded shape and have a short life, shortest among all leaves on the plant. Cotyledonary leaves are almost always two in number. On very rare occasions an abnormal embryo will have three cotyledons. Cotyledonary leaves appear to be attached to the stem directly opposite each other but in fact one is placed slightly above the other. One Cotyledonary leaf falls off before the other with a margin of three to ten days. Cotyledonary leaves have a maximum life of 40 days. They are thicker than true leaves and do not have pointed edges like true leaves. The main stem emerges from the middle of the Cotyledonary leaves. True leaves have pointed edges from the beginning. By the time the first true leaf unfolds, 6-7 other leaves have already been formed. Every leaf is located at a three-eighth turn from the last leaf. A full-grown leaf is 12-15 cm in length and width. Leaf colour in cotton plants may be various shades of green or dark red. The upper surface of the leaf has 2-3 times more stomata than the lower surface, with 100-130 stomata/cm2 on the upper surface and 40-50 stomata/cm2 on the lower surface
Flower of cotton plant has a complete flower, surrounded by bracteoles, with a well-developed calyx, Corolla, gynoecium (female flower parts) and androecium (male flower parts). The cotton flower has three bracteoles inserted above the nectaries around the flower base. Bracteole size depends on genotype and ranges from 1-3 inches in length and width among cultivars. Bracteoles have many deep cuts (teeth), are green in colour, and do not change their colour significantly until the boll is ready to open. The calyx is cup shaped with five teeth indicating five sepals united into one. The Corolla has five large petals tapering toward the bottom. Generally, petals are showy, white, white-creamy in G. hirsutum. The petals of G. barbadense are usually bright yellow, Petals sometimes have a dark rose-colored spot at the base in G. hirsutum and almost always in G. barbadense. Petals are closed in a whirl until the day of anthesis. Stigma, style and ovary form the female part of the flower in cotton. The ovary comprises 3-5 carpels, forming the corresponding number of locules in the boll. Each carpel has a number of ovules (depending on genotype) arranged in two vertical rows. Stamens, which form the male part of the flower, are numerous. The lower parts of the stamen filaments are united in a tube the upper parts contain anthers bearing pollen grains. Pollen grains are round and have spikes. Their colour is creamy or creamy yellowish in G. hirsutum.
Boll of cotton, a fertilized flower takes about fifty days to become an open boll, Boll period is the duration from open bloom or flower to open boll. On average, a boll develops to full size 25-30 days after flowering by then, seeds have also grown to full size. Heat units per day have a significant effect on the time taken to mature. The boll period increases with a decrease in temperature, Night temperature is more important than day temperature, Boll weight is the weight of seed cotton picked from a single, naturally open boll. Seeds from a four-locule boll in a plant having five- and four-locule bolls may produce four- and five-locule bolls.
This article is collectively authored by Muhammad Nazim1, Zahoor Ahmad2*, Muqarrab Ali1 and Afnan Sehar3-1 Department of Agronomy, Muhammad Nawaz Sharif, University of Agriculture Multan, Pakistan, 2 Cholistan Institute of Desert Studies, The Islamia University of Bahawalpur, Pakistan, 3Department of Plant Pathology, University of Poonch Rawalakot, Azad Kashmir, Pakistan.