The cotton plant has perhaps the most complex structure of all major field crops. Its indeterminate growth habit and extreme sensitivity to adverse environmental conditions is unique. The growth of the cotton plant is very predictable under favorable moisture and temperature conditions. Growth follows a well-defined and consistent pattern expressed in days. Another useful and more precise way to assess crop development relies on using daily temperatures during the season to monitor progress (Table 1). The heat unit concept utilizes accumulated hours above a critical temperature rather than calendar days in describing growth and development. The growing degree days (DD) concept is based on a developmental threshold above which the crop grows. Below that temperature is where little or no development occurs. For cotton, the threshold temperature is 60˚F; therefore, the degree days are referred to as “DD60’s”. The basic formula for calculating heat units involves averaging the maximum and minimum temperatures for each day and subtracting the threshold temperature. Calculation of the accumulated heat units and knowledge of the heat unit requirement for any particular growth stage can be used to explain and predict the occurrence of events or duration of stages in crop development (Kerby et al., 1987; Landivar and Benedict, 1996; Oosterhuis, 1990).
Table 1. The average number of days and heat units required for various growth stages of cotton in the Mid-South.
Growth Stage
Days
Heat Units – DD60s
Planting to Emergence
4 to 9
50 to 60
Emergence to First Square
27 to 38
425 to 475
Square to Flower
20 to 25
300 to 350
Planting to First Flower
60 to 70
775 to 850
Flower to Open Boll
45 to 65
850 to 950
Planting to Harvest Ready
130 to 160
2200 to 2600
Modified from Oosterhuis, 1990
Stages of Growth
The developmental phases for cotton can be divided into five main growth stages: (1) germination and emergence (2) seedling establishment (3) leaf area and canopy development (4) flowering and boll development and (5) maturation (Fig. 1). The transitions between these stages are not always sharp and clear. Each stage may also have different physiological processes operating within specific requirements. If producers are aware of these stage-dependent differences in cotton growth and requirements, then many problems in crop management can be avoided, which will result in higher yields and profits.
Figure 1. Seasonal development of cotton in the Mid-South with a May 1 planting date, showing typical production patterns of squares, bolls and open bolls (Oosterhuis, 1990, with permission ASA).
Root Development
Under favorable germination conditions, the radicle (root) emerges within two to three days. The radicle becomes the taproot that grows downward into the soil. The taproot penetrates the soil rapidly after germination and may reach a depth of up to 10 inches or more by the time the cotyledons unfurl (5 to 7 days, 50 DD60s) (Fig. 2). Root development during the early vegetative stage may proceed at the rate of 0.5 to 2.0 inches per day, depending on soil temperature and moisture conditions (Huck, 1970; McMichael, 1986).
Figure 2. Stages of germination and seedling emergence (Oosterhuis, 1990, with permission ASA).
The roots may be 3 feet deep in some soils when the above ground portion of the plant is only about 14 inches (Fig. 3). The taproot may penetrate the soil from less than 1.5 feet to as much as 9 feet while the lateral roots remain fairly shallow, less than 3 feet (McMichael and Quisenberry, 1993). On deep alluvial and irrigated soils in California, roots reach a depth of 3 to 4 feet when the young plants are only 8 to 10 inches high, with a final depth at maturity of 9 feet (Grimes et al., 1972). The bulk of the root system is located in the upper 3 feet, but this is dependent upon the soil moisture, soil physical structure and vigor of the individual plant (Taylor and Ratliff, 1969; Pearson et al., 1970; Taylor and Gardner, 1983). The total root length continues to increase as the plant develops until the maximum plant height is achieved and fruit begins to form. Total root length begins to decline as older roots die. Furthermore, root activity begins to decline as the boll load develops and carbohydrates are increasingly directed toward developing the fruit (McMichael, 1986).
Figure 3. Early-season root development of cotton (Oosterhuis, 1990, with permission ASA).
Vegetative Development
Under favorable conditions for germination, cotton seedlings emerge five to ten days after planting or after 50 to 60 DD60s are accumulated. The fully expanded cotyledons are 1 to 2 inches above the soil surface and are arranged directly opposite the main stem. The cotton plant has a very prominent main stem, which results from the elongation and development of the terminal bud or apical meristem. The main stem consists of a series of nodes and internodes and has an indeterminate growth habit (Fig. 4). Much of the early development of the cotton plant is directed by the development of a substantial root system while growth of the first true leaves is relatively slow. The number of nodes and the length of the internodes are influenced by genetics and environmental factors such as climate, soil moisture, nutrients, disease and insects. The appearance of a new node for relatively non-stressed cotton occurs after an additional accumulation of 50 to 60 DD60s (Kerby et al., 1987; Oosterhuis, 1990).
Figure 4. The basic structure of a cotton plant includes the main stem, which is made up of a series of nodes and internodes, and two types of branches, vegetative and fruiting branches (NCC, 1996).
The developmental rate of a new node is significantly slower when the plant is water stressed. Typically this produces shorter stature plants. Nodes give rise to main stem leaves and branches. Main stem leaves and branches are spirally arranged on the stem in a three-eighths phyllotaxy above the cotyledonary node. Two types of branches are produced: monopodial are the vegetative branches and sympodial are the fruiting branches. Monopodial branches are structurally similar to the main stem. Growth is from a single terminal bud and tends to grow in an upright position. Sympodial branches are produced by the main stem and monopodial branches and grow at an acute angle to the main stem. Every sympodial branch has a main stem leaf associated with the branch. As the branch extends from the main stem, each new fruiting node has an extending leaf and a fruiting structure or square at each node. Elongation of the internode behind the flower bud and leaf causes them to extend away from the main stem. The development of this branch terminates in a square, but a second leaf and square develop in the axil of the first leaf and similarly extend away from the first leaf and square by internode elongation. Repetition of this process produces several squares and leaves resulting in the typical zigzag appearance of the fruiting branch. The flowers are opposite the leaves on the sympodial branches and develop more rapidly than monopodial branches.
Final plant height is also a function of the extension of main stem nodes. Within cotton varieties, the seasonal total numbers of main stem nodes is strongly influenced by determinacy and growing environment. Cotton breeding and selection for earliness has favored shorter statured, more determinant cotton varieties. However, management factors such as excessive nitrogen fertilizer and excessive square loss from insect feeding can cause even moderate stature plants to grow excessively tall and rank (Siebert et al. 2006).
Reproductive Development
Signs of reproductive growth begin to appear about four to five weeks after planting with the formation of the floral buds or squares in the terminal of the plant (Table 1). Cotton has a distinctive and predictable fruiting pattern. Once fruiting begins, fruiting branches tend to be produced at each successive main-stem node. The first fruiting branch is often produced at the sixth or seventh node on the main stem. Approximately three days elapse between fruit on a given fruiting branch and the same relative position on the next higher branch. The time interval for the development of two successive fruiting forms on the same sympodial branch is approximately six days (Fig. 5). Squaring is followed about three weeks later by flowering and the start of boll development. The time requirement for a square to develop into a white flower is not influenced significantly by external conditions or plant stress. Throughout the remainder of the season, the cotton plant, due to its indeterminate growth habit, will continue adding vegetative growth at the same time as the reproductive development. The occurrence of the first position white flower moves closer to the terminal of the plant as the developing bolls become the major sink for photosynthate, which in turn also results in the slowing of new node or square development (Robertson et al., 2007a).
Table 2. Timing of various events during square development relative to the flowering date of an individual fruiting structure.
Days
Before
Flower
Size of Bud
Comments
40
Microscopic
Square initiation can occur as early as 2nd true leaf expansion. Hot weather induces four-bract squares, cool weather delays square initiation.
32
Microscopic
Lock numbers determined. Carbohydrate stress decreases number from 5 to 4.
23
2 mm PHS
Ovule number determined. Carbohydrate stress decreases potential seed number.
22
2 mm PHS
Pollen cells divide.
19
3 mm MHS
Pollen viability reduced by high nighttime temperatures.
5
13 mm
Squares start expanding rapidly
3
17 mm
Fibers begin to form
0
Flower opens White flower
Pollen sheds and fibers start to elongate. Extremes of humidity or water disrupts pollen function.
Modified from Stewart, 1986
Figure 5. Expected flowering interval in days beyond first flower illustrating the three day age difference in flowering dates of the same fruiting position on the next higher fruiting branch and the six day difference in age between fruiting positions on the same branch (Oosterhuis, 1990, with permission ASA).
The boll develops rapidly after fertilization and reaches its full size within three weeks (Fig. 6). An additional four to five weeks are required for boll maturation. Seeds attain their full size about three weeks after fertilization, but do not reach maturity until shortly before the boll opens. Fibers attain their full length in about 25 days after fertilization with the maximum growth rate occurring during the first 10 to 15 days of this period. Thickening of the fiber begins at about 16 days after fertilization and continues until the boll is mature.
Figure 6. Although bolls are full size 21 days after flowering, fiber and seed development requires an additional 28 to 35 days (NCC, 1996).
Fiber thickening occurs by the daily deposition of consecutive layers of cellulose on the inner wall of the fiber in a spiral fashion. The degree of thickening and the angle of the spirals affect fiber strength and maturity. Fiber elongation and maturity can be impacted by numerous factors from fertilization to maturity (Table 3). Until the boll opens, the fiber is a living cell, but upon opening the fiber is exposed to the air and soon dries out and becomes twisted (Seagull, 2001). In addition to the long fibers, most commercial cultivars (excluding Gossypium barbadense) have very short white or colored fibers on the seed called linters or fuzz fibers.
Days
After
Flower
Event
Primary Factors Influencing the Event
-2 to 12
Fiber density on seed surface
Temperature and carbohydrate status
0
Pollen shed
Temperature and relative humidity
0 to 3
Rate of fiber initiation
Temperature and potassium status
0 to 3
Pollen tube growth and seed fertilization
Temperature and relative humidity
1 to 14
Boll abscission
Plant water and carbohydrate status
3 to 25
Fiber length and seed number
Temperature and potassium status
15 to 45
Fiber cellulose (fiber thickening)
Temperature
25 to 50
Protein and oil accumulation
Temperature, plant water, nitrogen and potassium status
49 to 50
Boll opening
Temperature and relative humidity
Table 3. Timing of various events during boll development relative to flowering and primary factors influencing the event.
Cotton quality is defined by the length, maturity, strength and micronaire of the fiber. These qualities are determined by the genetic makeup of specific plant varieties, the climatic conditions experienced by the crop, and the management of the crop through production and harvest (Table 4). For example, bolls maturing late in the season, when temperatures are lower, require a longer period for fiber growth and development and usually produce less lint often of lower quality.
Fiber Quality Parameter
Genetics
%
Environment
%
Staple
82
18
Micronaire
41
59
Color
21
79
Strength
90
10
Table 4. The degree of variability in fiber quality parameters as influenced by the genetic makeup of the variety and the environment (weather and management) during the growing season (NCC, 1996).
The relative importance of the fruiting positions oriented from the main stem along a sympodial branch varies, i.e., the first, second and third sympodial positions contribute about 60, 30, and 10 percent of the total seed cotton yield, respectively (Bednarz et al., 2000; Jenkins et al., 1990). The lint quality tends to also decrease away from the main stem. The likely production problems occurring during the maturation stage include low temperatures and slow upper-canopy boll development, which can increase boll rot, delay harvesting, reduce the efficacy of defoliants and boll openers, and lower quality lint.
The growth and development of the cotton plant follows a typical sigmoid curve with a relatively slow start during emergence and root growth, followed by an exponential increase in growth rate during canopy formation, flowering, boll development and slowing down during the boll maturation phase (Fig. 7). Both genotype and environment affect this pattern. Nevertheless, a general and predictable pattern of growth exists for the cotton plant (Hearn, 1994, Jones and Wells, 1997).
Figure 7. The growth and development of the cotton plant follows a typical sigmoid curve (NCC, 1996).
Understanding cotton growth and development is critical in order to implement sound management strategies for maximum yields and profits. Cotton is a perennial plant with an indeterminate growth habit and has a very dynamic growth response to environment and management. Site-specific management strategies need to be taken into consideration to optimize yields. Furthermore, management strategies should be flexible to allow for changing environmental conditions.
