W287 Cotton Growth and Development

Christopher L. Main, Associate Professor Extension Cotton and Small Grains Specialist, Sciences

Heat Units or DD60’s Cotton growth milestones are often temperature at which cotton growth given in terms of days after planting occurs. The formula for calculating or between growth stages, but heat units per day is as follows: the development rate of cotton is strongly influenced by temperature. (°FMax + °FMin ) A cotton crop grows more slowly DD60 = – 60 on cool days than on warm days, so 2 temperature measurements during the cropping season help estimate Calculating the accumulated heat when a crop reaches a specific units of a crop over time can then developmental stage. Heat units, be used to estimate the growth of

or DD60's, are an estimation of this the cotton during the season. Table accumulated temperature effect 1 demonstrates how to calculate during a day, based on the average accumulated heat units over a five- of the maximum and minimum daily day period. Table 2 provides basic temperatures in degrees Fahrenheit benchmarks in heat unit accumulation (°F and °F , respectively). The max min for different developmental stages of number 60 is subtracted from this cotton growth. average, because 60 degrees F is generally accepted as the lowest

otton has many unique Cqualities as a commercial crop. Understanding the history, Table 1. Calculation of daily and accumulated heat units growth habit and development of based on daily high and low temperatures. cotton allows for more efficient and profitable production system.

Cotton is a perennial plant adapted Daily High Daily Low Average Daily Heat Accumu- to tropical and subtropical areas of Tempera- Tempera- Daily Units Day lated the world. It has an indeterminant ture ture Temperature (°F +°F )/2 max min Heat growth habit in which both (°F ) (°F ) (°F +ºF )/2 – 60 vegetative and reproductive max min max min Units growth occur at the same time. The 1 89 60 74.5 14.5 14.5 indeterminent habit allows cotton to endure environmental stresses 2 91 59 75 15 29.5 and continue to produce fibers. 3 90 64 77 17 46.5 There are two main commercial species of cotton grown in the 4 85 68 76.5 16.5 63 United States: Gossypium hirsutum 5 82 70 76 16 79 L., or upland cotton, and Gossypium barbadense L., or pima cotton. Table 2. Typical heat units required for various cotton growth stages.

Growth stage Heat units Planting to establish- 50-60 ment Nodes up the main stem 45-65 Emergence to first square 425-475 Square to white 300-350 Planting to first flower 775-850 White flower to open boll 850 Figure 2. Seedling cotton. Planting to harvest 2300

Seed and successful Hypocotyl Terminal Germination and Emergence emergence is favored by adequate 'Crook Stage' oxygen, moisture and warm soil Emergence temperatures (above 65 degrees Cotton contain all the F). Problems with germination essential ingredients for are typically related to excessively germination except one, water. dry or wet soil conditions, soil Cotton begin to germinate crusting, salinity, residue, Lateral cool temperatures and poor seed when seed weight increases by 50 percent, due to water Radicle quality. In Tennessee we typically entering the seed. As the seed Emergence have marginal soil temperatures swells, the seed coat splits, Tap when planting begins, since we have a short growing season. Also, allowing the radical to emerge. Figure 1. Cotton seedling establishment. This process takes about three the high percentage of no-till days if adequate soil moisture production acres in Tennessee is present. Tissues between ‘crook’ comes from the appearance present problems with maintaining the elongating radical and the of the curvature of the hypocotyl as adequate seed-to-soil contact cotyledon (seed leaves) it pulls the from beneath across various soil types, moisture swell rapidly. As the hypocotyl the soil surface. At this stage, cotton conditions, soil residue and extends to the soil surface, the is extremely susceptible to soil compaction zones. seedling is said to be in the crusting, preventing the cotyledons ‘crook’ stage (Figure 2). The term from emerging.

Cotton Planting Forecast Table 3. Probability of cotton stand establishment based on DD60 To aid in making planting decisions, accumulation. a cotton planting forecast can be calculated. A planting forecast Predicted DD60 accumulation will consider the predicted Outlook for stand establishment 5 days after planting temperatures, DD60 accumulation, rainfall and potential for drying <10 Very poor winds. Forecasts are subject to the same inaccuracies associated with 11-15 Poor weather prediction. This information 16-25 Marginal should be used along with good 25-50 Good judgment for making a planting decision (Table 3). >50 Very good the fruit load on the plant increases Cotyledon First true Main stem leaf and ages, the carbohydrate demand increases, and the development rate of new leaves declines. Premature aging of the cotton leaf canopy due to water stress, low fertility and other stresses further reduces the Figure 3. Cotton leaf shapes. photosynthetic capacity of the crop.

The branches from which fruiting arise are called fruiting Once the cotyledons unfold, branches, or sympodia, because each photosynthesis promotes fruiting branch contains multiple development of new plant tissues . Fruiting branches have a from the apical meristems located “zig-zag” growth habit, as opposed between the cotyledons. The next to the straight growth habit of the structure to develop is the first true- vegetative branches. The initial leaf, which is ovate in shape (Figure growth of a fruiting branch is 4). It typically develops 7 to14 terminated once a fruiting days after the seedling becomes forms. The fruiting branch, however, established. The emergence of initiates a new growing point, called the first true leaf shifts the plant’s an axillary meristem. The axillary energy supply to gain carbohydrates Figure 4. Stem arrangement. meristem is located at the base of a from photosynthesis instead of leaf that subtends the newly formed the stored carbohydrates in the fruiting bud. cotyledons. The first several true stem leaves and branches form at leaves to develop are important for The first fruiting branch will proper plant development and need points of attachment on the main stem called nodes. As a general generally arise at main-stem node protecting from insect damage, 5 or 6. A cotton plant will mainly as the carbohydrates created are rule, a new node is produced from the apical meristem an average of produce fruiting branches, but responsible for the development of several common environmental a deep, healthy root system. Cold every three days, although nodes develop more quickly earlier rather factors such as low population soil, insect injury, seedling disease, density, insect and disease pressure, nematodes, low soil pH, water stress, than later in the season. Leaves that arise directly from the main and overfertilization can cause hard pans and herbicide residues vegetative branches to form. can inhibit root development. Root stem are referred to as main stem leaves, while leaves that arise from Vegetative branches are produced development continues during after fruiting branches, and develop the early season. Peak root mass is the fruiting branch are referred to as subtending leaves. The fruit at nodes directly below the node at achieved during early flowering and which the first fruiting branch was then declines into the late season. produced by a branch will primarily receive carbohydrates produced developed. For instance, if the first Carbohydrates are redirected to fruiting branch is initiated at main- metabolic sinks associated with by the leaf subtending that fruit. However, the main stem leaf also stem node 5, a vegetative branch boll formation instead of root and may develop at main-stem node 4. vegetative growth at this time. supplies carbohydrate for fruit development New fruiting branches generally Vegetative Growth develop approximately every three Leaf and Canopy days, although recent studies Cotton has an indeterminate show that this developmental growth habit and can grow very Development rate varies. Squares are produced tall under favorable environmental As a cotton plant develops, at new positions on a fruiting conditions. Growth regulators, such new leaves appear and expand, branch approximately every six as mepiquat chloride, are generally increasing sunlight interception. days. The age of fruiting structures applied to cotton to slow internode Initially the carbohydrates produced on a cotton plant can be mapped elongation, especially for well- by the leaves are used to produce according to this time sequence. fertilized, irrigated cotton. Increased roots and more leaves. This vegetative growth promotes boll production of new leaves causes rot and fruit abscission and makes a the leaf area of the cotton plant to cotton crop difficult to harvest. increase rapidly. Once reproductive structures begin to develop, The first vegetative structures that carbohydrate supplies are slowly appear on the main stem are main shifted to the developing fruit. As stem leaves (Figure 3 and 4). Main Photosynthate Partitioning Most of the cotton plant’s carbohydrate energy is directed to root growth prior to the time reproductive growth begins. This is a function of source-to-sink relationships. Carbohydrates are transported from supply areas (leaves), called sources, to areas of growth or storage (roots, , bolls), called sinks. As bolls develop, Figure 5. Pinhead square, matchhead square, candle and white. they become carbohydrate sinks. Root and growth slow, boll development dominates plant growth and roots continue to supply water and nutrients to the shoot. The first visible structures of the attached to the developing boll The branches on a cotton plant can square are the leaf-like bracts, or for a longer period of time. This is be classified as either vegetative epicalyx. Three bracts surround referred to as a bloom tag. branches (monopodia) or fruiting the flower bud in a pyramid-like branches (sympodia). Because shape. The cotton plant produces The development of the cotton vegetative branches have only one perfect , meaning the flower plant, in terms of leaf number, meristem, they grow straight and contains both male and female node number and fruiting stage, erect, much like the main stem organs. The first square is typically is discussed in previous sections. (Figure 4). Vegetative branches can visible on node 5 to node 7 about During the flowering period, the also produce fruiting branches. A 35 days after planting. Anthesis, stage of cotton development can flower bud, called a square, begins or flowering, occurs approximately also be discussed in terms of Nodes to form at the initiation of the 21 to 28 days after the first square Above White Flower (NAWF). This fruiting branch. The first square appears. A cotton plant typically is a measurement documenting the produced on a fruiting branch is blooms or flowers for about number of nodes separating the referred to as a first-position square. four to six weeks, depending on uppermost first position bloom and As this square develops, the portion environmental conditions. Thus, the terminal of the plant. of the fruiting branch between until the cotton begins to produce When the cotton plant first begins the main stem and the square fruit, the stage of development to bloom, there will be typically also elongates. This portion of the is discussed in terms of leaves nine to 10 NAWF. As the season fruiting branch is also called the or nodes. Once flower buds are progresses, the number of NAWF internode, similar to the portion of present, the stage of cotton decreases. NAWF generally decreases the main stem between main-stem development is discussed in terms more quickly after bloom in early nodes. An axillary meristem also of square development and the maturing varieties than in mid- or develops adjacent to this square. number of nodes. Once blooms and full-season varieties. As the flowers The axillary meristem produces bolls are present, the stage of cotton develop into bolls, they become a second position square and development is discussed in terms of stronger sinks for carbohydrates subtending leaf. As many as four weeks of bloom. and their combined demand for squares may be produced in this carbohydrates increases. Eventually, fashion on a fruiting branch. Flowering is important to cotton production because pollinated the carbohydrate supply produced flowers form cotton bolls. The by the leaves will be used primarily Squaring through Boll bloom process takes several days, by developing bolls, leaving less and bloom age can be estimated available for the production of new Development by the bloom characteristics. vegetative growth. As flowering The cotton flower bud has several On the day a flower opens it is progresses up the plant, less top recognized developmental stages. A white. of that flower growth is produced, and the number pinhead square is the first stage at usually occurs within a few hours of NAWF will decrease. which the square can be identified. after the white flower opens. On The next stage of square growth the second day, the flower will As the flowering approaches is match-head square. Just prior to become pink, then turn red on the the top of the plant, the plant the time the flower opens, a candle third day. Approximately five to eventually puts all of its energy shape can be seen (Figure 5). This seven days after a flower appears into boll development and ceases period of square development prior it usually dries and falls from the flower development. This event is to bloom can be called squaring, plant, exposing the developing termed cutout. Cutout generally pre-bloom or pre-flower. boll. Occasionally a flower will stay occurs at 4 or 5 NAWF. Cutout occurs when carbohydrate supply approximately three weeks. During Yield Distribution equals demand and vegetative this time, the fibers produced growth ceases. At cutout, no more on the seed are elongating and The contribution of a single fruiting harvestable fruit is set. the maximum volume of the boll structure to the overall yield of the and seeds contained therein are cotton plant depends largely upon attained. its position on the plant. First- Fruit Shedding position bolls are typically heavier The filling phase of boll and produced in higher quantities A phenomenon often seen in a than bolls at any other position. In cotton field is square shedding. The development begins during the fourth week after flowering. Fiber cotton populations of three shedding of squares may be the per foot of row, first-position bolls result of several factors, including elongation ceases and secondary wall formation of the fiber begins. contribute from 66 to 75 percent water stress, shading (rank growth of the total yield of the plant, and or prolonged cloudy weather), Cellulose is deposited inside the elongated fiber, filling the void second-position bolls contribute 18 nutrient deficiencies (especially to 21 percent. N), high temperatures, high plant space. During the boll enlargement and fiber elongation phase, the populations, high percent fruit set Yield distribution research is an and insect damage. development of fiber is very sensitive to adverse environmental intensive, detailed process that conditions. Low water availability, involves counting and weighing Flowers and young bolls also may bolls from each fruiting position be shed from the plant due to the extremes in temperature and nutrient deficiencies (especially on many plants. First-position bolls same factors that lead to square tend to fill out more and be heavier shedding. Generally, though, the potassium) can reduce the final fiber length, strength and micronaire. than bolls from other positions, sensitivity of squares, flowers and so the majority of boll weight on bolls to shedding can be related to However, bolls at this stage will rarely shed. plants generally comes from the their age. Small squares and bolls first-position fruit between nodes are more likely to be shed than are 7 and 20. However, second-position more developed squares and bolls. The boll maturation phase begins as the boll reaches its full size and bolls or bolls from upper nodes will get larger (compensate) if extensive A boll begins to develop after maximum weight. During this phase, shedding of first-position fruit pollination occurs. Under optimum fiber and seed maturation take occurs. This may increase the time conditions, it requires approximately place and boll dehiscence occurs. to crop maturity and delay harvest. 50 days for a boll to “open” after The capsule walls of the boll dry, pollination occurs. Boll development causing the cells to shrink unevenly. can be characterized by three This shrinking causes the suture phases: enlargement, filling and between the carpel walls to split, maturation. The enlargement and the boll opens. phase of boll development lasts

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