Book

1. 1 Growth and Development of a
Cotton Plant
Derrick M. Oosterhuls
Crop Physiologist
Department of Agronomy
University of Arkansas
Fayetteville, Arkansas
A better understanding of cotton (Gossypium hirsutum L) growth and
development in commercial production is important in the continuing
efforts of growers to produce lint and seed yield more efficiently and
profitably. The cotton plant has perhaps the most complex structure of
any major field crop. Its lndeterminant growth habit and sympodial fruiting
branch cause It to develop a four dimensional occupation of space and
time which is difficult to analyze (Mauney, 1986). Associated with this
complex growth habit Is an extreme sensitivity to adverse environmental
conditions which is reflected in excess fruit abscission.
MAINSTAGES OF GROWTH ANDDEVELOPMENT
Plant development in cotton proceeds through a number of stages,
which for practical reasons (In relation to production management), may
be divided into five main growth stages: (1) germination and emergence,
(2) seedling establishment, (3) leaf area and canopy development, (4)
flowering and boil development, and (5) maturation. However, the transi-
tions between these successive stages are subtle and not always clearly
distinguishable. Furthermore, each stage may have different physiological
processes operating with specific requirements. If growers are aware of
these stage-dependent differences in cotton growth and requirements, then
many problems in crop management can be avoided resulting in increased
yields and profits. Understanding each of the above stages involves a
knowledge of the morphology and the physiology of the plant. This paper
describes the growth and development of a typical Mid-South cotton plant
from seed germination to boil maturation, and briefly summarizes the main
Copyright 0 1990ASA-CSSA-8SSA, 677 South Segoe Road, Madison,WI 53711,
USA. Nitrogen Nutrition in Cotton: Practical Issues.
1
Published 1990

2. 2 OOSTERHUIS
physiological events, growth requirements and possible production
problems of each stage.
The structure and developmental patterns of the cotton plant have
been covered In publications by Baranov and MaJtzev (1937), Brownand
Ware (1958), Dennls and Brlggs (1969), Evans (1896), Hector (1936),
Mauney (1984), McMichael (1990), and Tharp (1960). The physiology of
the cottonplanthasbeen covered by CamsandMauney (1968) andEaton
(1955), and more recently, In the treatise edited by Mauney and Stewart
(1986). These publications were used In preparing the following descrip-
tion of the growthand development of the cotton plant. A staging system
devised by Elsner et al. (1979) is useful in describing the growthstages of
the cotton plant.
GERMINATION AND EMERGENCE
The Seed
The acid~e1inted seed Is an ovoid, somewhat pointed, dark brown
structure. The cotton seed is surrounded by a seed coat (testa) and
contains an embryoand two well~eveloped folded cotyledons (Fig. 1-1).
,--CHALAZAL END
FOLDEDCOTYLEDON
GLAND CONTAINING
GOSSYPOL
EPICOTYL
~~-HYPOCOTYL
Fig. 1-1. Cross-section of a typical cotton seed showing the various
structures ofthe embryo. (Redrawn from Baranovand Maltzev,
1937.)

3. GROWTH OF THE COTTON PLANT 3
The epidermal layerof the seed coat bears the flbersor lint. The embryo
consists of a radicle, a hypocotyl, and a poorly developed eplcotyl. The
cotyledons, or seed leaves, eventually form the first green leaves but
initiallythey contain stored food whichsupplies the energy for germination
and earlydevelopment. There are usually about 3500-4000 acld-delinted
seed per pound.
Germination
Germination begins with the entry of moisture within the first few
hoursthroughthe chalaza andlater, throughall partsof the seed coat Into
the embryo(Fig. 1-1). Oxygen uptake occursand respiration Increases as
the stored food is utilized for energy and to build new cells and tissues.
The seed/embryo swells as water Is absorbed causing the seed coat to
split. Under favorable conditions for germination, the radicle emerges
throughthe pointed mycropylar end of the seed Intwo to threedays (Fig.
1-2). Theradicle becomes the primary root that growsdownward into the
soil. Thetissues between the radicle andcotyledons (thehypocotyl) grow
rapidly, arching nearthe cotyledons (Fig. 1-2). Withcontinued expansion
ofthe hypocotyl, thecotyledons andepicotyl arepulled upthroughthe soil
surface (Fig. 1-2). 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 hypocotyl and cotyledons.
HYPOCOTYL--, COTYLEDONS
~ ~ SEED I
~ ~ COTYLEDONS COAT
SEED RADICLE COVERED
SURROUNDED EMERGING ' BY SEED
BY SEED COAT COAT RADICLE
COTYLEDON
/UNFOLDED
TAPROOT
Fig. 1-2. Germination and seedling emergence.
Seed germination is favored by high soH oxygen, adequate soil
moisture and soli temperatures above65°F. Germination of cotton seed
can beginwhen the mean daily soli temperature Is 60°F or slightly below
at seeding depths, but growth will be slowat thesetemperatures. Under

4. 4 OOSTERHUIS
favorable conditions, seedlings emerge In 5 to 10 days. Planting In
Arkansas should not start untK the mlcl-momlng soli temperature In the
knocked-down beds Is 68°F at 2 In. (5 cm) depth for three consecutive
days and there Is a favorable 5-day weather forecast (Bonner, 1990).
Between 60°F and 89°F the cotton plant doubles Itsgrowth rate for every
18°F Increase In temperature. Possible problems that hinder germination
include poor seed quality, drought, flooding, solicrusting, salinity, herbicide
reslclue, and cool temperatures.
EARLY SEEDUNG ESTABUSHMENT AND ROOT DEVELOPMENT
The cotyledons are carried a few inches above the soU by the
elongating hypocotyl before unfolding and expanding. After emergence
and exposure to the light, the cotyledons become green due to chlorophyll
and are capable of photosynthesis and food manufacture. Much of the
early development of the cotton plant is focused on the development of a
substantial root system while growth of the above ground portion (first true
leaves) is relatively slow prior to canopy development.
Roots
The radicle, or primary root develops rapidly and Is the first organ to
emerge from the seed coat during germination. The primary root, or
taproot, penetrates the soil rapidly and may reach a depth of up to lOin.
or more by the time the cotyledons unfold. Root development during the
early vegetative stage may proceed at the rate of 0.5 to 2.0 in. per day,
depending on conditions, such that the roots may be 3 feet deep when the
above ground portion of the plant Is only about 14 in. (Fig. 1-3). The
taproot may penetrate the soil from depths of less than 3 feet to as much
as 9 feet while the lateral roots remain fairly shallow, less than 3 feet
(McMlchael, 1990). On deep alluvial and Irrigated soils In California, roots
reached a depth of 3 to 4 feet when the young plants are only 8 to lOin.
high, with a final depth at maturity of 9 feet (Hubbard and Herbert, 1933).
In Arkansas, high water tables and compacted areas result In much
shallower root systems.
Numerous lateral roots proliferate outward from the taproot forming
a mat of roots which often extend horizontally several feet. The bulk of the
root system is located In the upper 3 feet, but this Is dependent upon soil
moisture, soU physical structure, and vlgor of the plant Itself. Root distribu-
tion within a soU profile (root length density) Is usually about 24 In. of root
per cubic inch of soil but can vary considerably with soil and plant

5. GROWTH OFTHECOTTON PLANT 5
conditions. Thetotal root dry weight comprises approximately 10-20% of
the total dry weight produced by the plant during the growing season.
However, the total root length producedduring the same timeperiod may
be several hundred yards. The total root length continuesto Increase as
the plant develops untM the maximum plant height Is achieved and fruit
begins to form. Root length then begins to decline as older roots die.
Furthermore, root activity beginsto decUne as the boil load developsand
carbohydrates are Increasingly directed towards the fruit. This occurs In
August in the MId-8outh.
1
4 inches
36 inches
50 days
Fig. 1-3. Root development of the cotton plant.
Main Stem and Branches
The cotton plant has a very prominent main stem, or primary axis.
which results from the elongation and development of the terminal bud or
apical meristem. The main stem consists of a series of nodes and

6. 6 OOSTERHUIS
intemodes and has an Indeterminant growth habit. The number of nodes
and the length of the Intemodes Is determined by genetic and environ-
mental factors such as climate. soil moisture, nutrients, disease, and
Insects. The main stem Is erect. rnonopodialln growth, and supports true
leaves and branches.
Two types of branches are produced - vegetative or monopodlal and
fruiting or sympodial branches (Fig. 1-4). Branches develop from a bud
located at a node In a leafaxH. Vegetative branches are structurally like
the main stem; growth Is from a single terminal bud (monopodial) , and
they bear flowers and fruit only after rebranchlng. They normaJly arise from
the main stem near the ground and tend to grow In an upright position.
The number of vegetative branches produced depends primarily on the
environment and plant spacing. Wide spacing and adequate water and
nutrients increase vegetative branching. Only about one monopodlal
branch per plant develops under normal Mid-South cotton growing
conditions with average plant density. However. damage to the terminal
bud on the main stem will increase the number of vegetative branches that
develop.
TERMINAL
BUD -....
13
PRIMARY SYMPODIAL BRANOf
I (FRUITING BRANCH)
12
I
MONOPODIAL BRANCH
(VEGETATIVE)
!I
I 4 _MAIN STEM (MONOPOOIAL)
SECONDARY SYMPODIAL 2 1
ABORTED NODES
BRANOf I
o COTYLEDON NODE (PAIRED)
• MON0POlltAL NODES (VEGETATIVE)
o SYMPODIAL NODES (FRUITING)
Ag. 1-4. Structural development of the cotton plant showing main stem,
vegetative (monopodlal) and fruiting (sympodial) branches.

7. GROWTH OFTHECOTTON PLANT 7
Thefruiting branches are produced by the mainstemand vegetative
branches and they grow at an acute angle to the main stem (Fig. 1-4).
When a fruitingbranchdevelops fromthe mainstem, a prophyll, a trueleaf
and a square are formed at the same node. Elongation of the Internode
behind the flower bud (square) and leaf causes these organs to be
extended away from the main stem. The development of this branch
terminates In a square, but a second leafand square developInthe axl of
the first leaf and simUarly extend away from the first leaf and square by
Internode elongation. Repetition ofthisprocess produces several squares,
leaves and Internodes (each Internode Isa new branch), and results Inthe
typical zig-zag appearance of the fruiting branch (Fig. 1-4). The flowers
thus appearto be borne opposite the leaves on the fruiting branches, but
they are actually terminal structures, with the continuation of the branch
broughtabout by lateral bud development. Thefruiting branches develop
more rapidly than vegetative branches.
All flowers and boils produced on a cotton plant originate on
sympodlal/frultlng branches arising from the main stem (or from mono-
podial branches). Once fruiting has begun, fruiting branches tend to be
produced at eachsuccessive main-stem node. Thefirst fruiting branchis
normally produced at the sixth or seventh node on the main stem (not
countingthe cotyledonary node) butthis Islargelyaffected by temperature
and plant density.
Leaves
There arethreetypesof leaves: cotyledons, prophylls, andtrueleaves
(Mauney, 1984). The kidney-shaped cotyledons are usually about 2 in.
wide. The prophylls are the first leaves developed on a branch and are
usually small (about 0.2 in. long), inconspicuous, without a petiole and
resemble a stipule. The true leaves vary in sizeand shape from entire to
deeply lobed (Fig. 1-5), although cultivars grown in the Cotton Belt are
usually three- to five-lobed and about 4 to 6 In. wide. Thefirst true leaves
formed are usually heart shaped. Cotton leaves generally have a thick
waxy cuticle, with numerous stomata, and epidermal and glandular hairs
on the surface (Plate 1-1). Stomata, which are more numerous on the
lower (abaxial) surface, permitgas exchange; the entry of carbondioxide
for photosynthesis andthe lossofwaterbytranspiration for nutrient uptake
and evaporative cooling.

8. 8
B
OOSTERHUIS
o
Fig. 1-5. Cotton leaves. (A) Cotyledon leaves, (B) first true main-stem
leaf, (C) sympodial leaf of a fruiting branch, and (0) main-stem
leaf.
Plate 1-1. Scanning electron micrograph of a cotton leaf abaxial surface
showing stomata, a glandular hair, and the waxy cuticle
covering the undertying epidermal cells (X200).

9. GROWTH OFTHE COTTON PLANT 9
True leaves are developed from primordia on the terminal growing
point initially located between the two cotyledons, and later from axUlary
buds on the main stem. Their growth Is relatively slow at first compared
to root growth, such that at one month after planting only about 4 or 5 true
leaves are unfolded and visible (Fig. 1~). However, by the time the first
true leaf unfolds, the plant has already developed 6 or 7 more leaf Initials
In the apical merlstern. The leaves, like the branches, are spirally arranged
on the stern In a three-eighths phyllotaxy I.e., each new leaf Is a 3/8
clockwise or counter-clockwise turn above the last.
A B
Fig. 1~. Seedling establishment showing the development of the true
leaves on the main stem three (A) and fIVe (6) weeks after
planting.
True leaves can be further divided Into main-stem leaves which arise
from the main stem, and sympodialleaves which arise from the sympodial
(fruiting) branches (Fig. 1-7). The main-stem leaves are mainly associated
with the development of the main stem and roots, as well as the boils that
develop at the first node position along the fruiting branches. On the other
hand, the sympodial leaves are almost exclusively associated with boil
development. Main-stem leaves are the only true leaves present during the
first 5 or 6 weeks after planting, after which the sympodlal leaf area
increases rapidly and exceeds the main-stem leaf area soon after first
flower.
The average life of a leaf is about 65 days, although Its photosynthetic
activity peaks at about 20 days after unfolding and thereafter declines

10. 10 OOSTERHUIS
steadUy. Themature leafpetiole, whichIsoftenanalyzed to estimate plant
nutrient status, Is about as long as the leaf bladeIswide.
Possible problems dUring seedling establishment In the MId-South
Include: low temperatures, low seedling vIgor, seedling diseases, Inade-
quate soil moisture, presence of a soli hardpan preventing root growth,
weed competition, ImproperdosageorIncorporation of herbicides, fertHlzer
salt damage, waterlogged soli ~ow oxygen), sand blasting from wind,and
thrips Infestation. These factorswUllengthen the timefrom emergence to
true leafdevelopment.
MONOPODIAL
(VEGETATIVE)
BRANCH
4
~
I
o
• MONOPODIAL NODES (VEGETATIVE)
o SYMPODIAL NODES (FRUITING)
Fig. 1-7. Development of main-stem leaves and sympodlalleaves along
a fruiting branch.
LEAFAREAAND CANOPY DEVELOPMENT
Vegetative development proceeds withthe growth of the true leaves
on the main stem (Fig. 1-6) and the development of the monopocllal and
sympocllal branches (Fig. 1-4) to support subsequent sympocllal leaf

11. GROWTH OF THE COTTON PLANT 11
development (Fig. 1-7)for laterfruit development. Much of the emphasis
during this stageisthe establishment ofa suitable crop canopyto intercept
incoming solar radiation and an adequate branchingstructureto support
the leaves and eventual fruit load.
Leafareadevelopment continues to increase and typically follows a
sigmoid growth curve (Fig. 1-8). Thetotal amountofleafarea, commonly
referred to as the 'eaf area index"(LAJ), Isthe amountofleafarea per unit
area of land. Total leafareaor LAJ increases s10wtyat first during the first
6 to 7 weeks (Plate 1-2), and then more rapidly during early fruiting and
canopy closure. Duringthe earlyvegetative period much ofthe emphasis
is on root development, whereas during the later vegetative period the
emphasis changesto squareand flowerdevelopment. It is imperative that
leaf areadevelopment during this period not be delayed so as to achieve
maximumutilization of incomingsolarradiation for photosynthesis as soon
as possible. Canopy closure (Plate 1-3), when the foliage just meets
between the rows approximately 75 days after planting, improves weed
control, and decreases soU evaporative lossesby confiningevaporation to
that occurring by transpiration through the plant and thereby, allowing
more plant control over the water loss.
5.0
140
TOTAL
100''10
120
60 80 100
DAYS FROM PLANTING
.,....Lr.-·-o-·-·-·-O
......... , SYMPODIAL LEAVES
,.,•.t:.:T'. 60-/.
T......
,p- - - ~
;-<>- - - - - -o- -- - -- -<>-- - - -o- - - -o
" / MAIN-STEM LEAVES
/ ; ~%
",d' ,d'
",-'
0'"
40
4.0
~
Ne
~3.0
~
~
~2.0
a::
et
I.L.
et
~ 1.0
Fig. 1-8. Leafarea development during the season.
Main-stem leaf area reaches a maximum prior to first flower and
remains relatively constant thereafter, whereas sympodlaJ leaf area
continuesto increase and eventually constituteabout 60%of the total leaf
area. Possible production problemsduring this stage are inadequate

12. 12 OOSTERHUIS
Plate 1-2. Leaf area and canopy development: ear1y season growth.
Plate 1-3. Leaf area and canopy development: canopy closure.

13. GROWTH OFTHE COTTON PLANT 13
temperature or soilmoisture, Insectdamage, and maintenance of sufficient
but not excessive vegetative growth.
FLOWERING AND BOLL DEVELOPMENT
Reproductive growth commences about 4 to 5 weeks after planting
with the formation of the floral buds in the apical part of the plant (Fig. 1-
98). This Isfollowed a few weeks later by flowering and the start of boil
development (Fig. 1-9b). Thecotton plant, due to ItsIndeterminate growth
habit,continuessomevegetative growthat the sametimeasthe reproduc-
tivedevelopment, throughoutthe remainder ofthe season. Thephysiology
of the plantduring this stageis mainlyassociated with photosynthesis and
carbon partitioning to the developing fruit.
B
Fig. 1-9. (a) Earlyreproductive growth showingthe formationof squares
In the apical part of the plant. (b) A full-grown plant with all
stagesof boil development present.

14. 14
Squares
OOSTERHUIS
The floral buds appear first as small, green, pyramidal structures,
known as squares (Fig. 1-10). The square Is composed eX 3 large green
braets (eplcalyx) which completelyencloseand protect the growing flower
parts. Initially, the braets are all one can see of the square. Braets
contribute about 10% eX boil photosynthetic requirements. The first
squares are usually visible about 5 weeks after planting, and the first
flowers about 3 weeks later. New squares appear In the terminal of the
plant at approximately3-day Intervals and appear oneach fruiting branch
at approximately EkJay Intervals. The length of time for a square to
develop from when It first begins to differentiate in the apex untU anthesis
is approximately6 to 7 weeks.
lARGE Ball
~
W
l-
V)
Z
Ag. 1-10. Cotton fruiting forms,
The Flower
WHITE
FLOWER
The structureof a typical cotton flower Is shownIn Fig. 1-11. On the
outside of the flower are the three large green breets. ImmediatelyInside
the braets Is the fused calyx ring (sepals) which tightly enclosesthe base
of the fIVe conspicuous petals which collectively form the corolla. The

15. GROWTH OFTHECOTTON PLANT 15
stamina! column, composed of numerous stamens eachwith a twcHobed
anther, surrounds the style which together with the stigma and rNary
constitutes the female part of the flower. The rNary Is superior and
composed of three to fIVe unitedcarpelseach with several ovules In axile
p1acentatlon Inthe locule. Thecotton flower Iswhite onthe day It unfolds,
but the petals turn pink-red the following day and absclse at their base.
Theyusuallyfall off within a fewdays but can remain trapped by the braets
and boil as dried vestiges.
Int-..---- COROLLA (PETALS)
Alf
(-7-j'----ft:I-+--r-- STIGMA
/r--7f.+--Hft-,.....-- FILAMENT
~---i.~Ht-~
-- ANTHER
';-/+--BRACT (EPICALYX)
1I11~'frI----7f':,L-.I:of-h'JlJ--:.~ STAMINAL COLUMN
1lI1ft-~~""So-"~=-r-- STYLE
~§~t#7~~:----
OVARY
VM"'7""~~"""",--CALYX
~""':;"-I+--- OVULES IN LOCULE
1 - - - - - PEDICEL
Fig. 1-11. Cross section of an open cotton flower.
Flowering Pattern
Cotton has a distinctive and predictable flowering pattern. The first
flowers to open are low onthe plant usually on main-stem nodes 6 or 7
and on the first position along a fruiting branch. About 3 days elapse
between the opening of a flower on a given fruiting branch and the
openingof a flowerat the samerelative position on the next higherfruiting
branch (Fig. 1-12). On the other hand the time interval for the develop-
ment of two successive flowerson the same branch Isabout 6 days. The

16. 16 OOSTERHUIS
order Isthus spirallyoutwardand upward.These floweringIntervals are not
constant and vary slightly with genotype and environment; hoYiever, they
do provide a useful guide for assessing plant development. Rowers wNI
continueto be produced untY defoliationor frost If the plant has not gone
Into cutout and Is growing actively.
/
MONOPOOIAL
,VEGETATIVE)
BRANCH
SYMPOOIAL
,FRUITING) BRANCH
• EXPECTED fLOWERING DATE lOAYS FROM FIRST FLDWER)
Fig. 1-12. Cotton flowering pattern.
The shedding or abscission of squares and young boils Is a natural
occurrence In cotton that Is accentuated by adverse environmental
conditions Including extended overcast weather, extreme temperatures,
water stress, nutrient deficiencies, and Insect damage. Rowers do not
shed. A cotton plant commonly sheds about 60% of Its squares and
young 5- to ~y-old boils. A typical patternof squareproduction and the
ensuing boils that develop is shown in Fig. 1-13.

17. GROWTH OF THECOTTON PLANT 17
-EARLY SEASON---MID SEASON ---lATE SEASON--
FLOWERING AND sou, ANDFIBER
sou, DEVELOPMENT MATURATION
",C?,..'
1"'",••'
~,::,. : ,
60 80 100 120 140 160
I I I
JULY I AUG I SEPT I OCT I
DAYS AFTER PLANTING
.......... - ....,
"
/
,/
,/
t'/ "
Ul .f!!/ .' ::;.:-
~ ~/ »>
~ "/ ".---- s
~ / ' • ·~o....
ID/ .......·o~€.~
I
(f)
I!J
ij(
I!J
J
I!J
~
....
q-
20 40
I
JUNE I
I!J Ul
U Z
z 8
~ I!J
a: J
I!J ~
:lE 0
I!J U
o
I
MAY I
200
400
..... SEEDLING/ROOT l EAF AREA
o 1000 ESTABLISHMENT EXPANSION AND
o CANOP
Y CLOSURE
o
~ 800
W
0::: 600
~
en
0:::
W
CD
:E
::>
z
Fig. 1-13. Seasonal development of cotton In the Mid-South showing the
production pattern of squares, boils and open boils.
Pollination and Fertilization
The flower opens as a white flower at dawn and pollination, the
transfer of pollen from the anthers to the stigma. occurs within a few hours.
Cotton flowers are self-pollinated although some Insect pollination can
occur. The stigma becomes receptive shortly after the flower opens. The
anthers split open releasing the pollen grains which adhere to the surface
of the stigma and germinate by extending a germ tube down the style to
an ovule in the ovary where fertilization occurs In as little as 12 hours. The
fertilized ovule develops into a seed If the young boil is not abscised.
Unfertilized. or aborted ovules, called motes. are often found in mature
boils and cause difficulty in the spinning of the cotton flbers If not removed
during ginning. Note that the pollen cells undergo meiosis about three
weeks prior to anthesis, and subsequent pollen viability, therefore, is
particularly sensitive to environmental stresses at this stage.

18. 18
Vegetative· Reproductive Balance
OOSTERHUIS
Since cotton plants continue to grow vegetatively after fruiting Is
Initiated, the vegetative to fruiting balance of the plant Iscritical. Excess
vegetative growth can delay maturity and Increase problems with Insects
and boil rot. Excess fruiting, on the other hand, may cause early cutout
with associated fruit shed and lessen yield potential.
Possible production problems during the reproductive stage are
associated with cloudy weather, excessivetemperatures,lnsects, water and
nutrient deficiencies, all of which result In excessive shedding of squares
and young boils. Verticillium wilt may also cause problems and yield
reductions.
MATURATION
During the maturation phase, boil development proceeds with the
formation of seeds and lint, leading up to boil opening, defoliation, and
harvesting.
The Boil
The boil, or fruit, of the cotton plant varies In form and size but Is
generally a spherical or ovoid leathery capsule, light green In coIor, and
with a few pigment glands (Fig. 1-10). The boil grows rapidly after fertiliza-
tion following a sigmoid curve; the most rapid growth occurs after 7-18
days and full size Is reached In about 20 to 25 days. Maturation of the boil
from anthesls to the time of opening (carpel dehiscence) usually takes
about 50 days but this varies with genotype and environmental conditions.
This period can be substantially lengthened if low temperatures preva" I.e.,
during September. At maturity the capsules (burs) crack or spilt along
their sutures, and the mature white seedcotton within expands greatly,
pushing out beyond the capsule, forming 8 white ftufty mass divided Into
locks (Plate 1-4). About 300 boils are required to produce a pound of lint
and about 145,000 boils per bale of lint.

19. GROWTH OF THECOTTON PLANT 19
Plate 1-4. Cotton fleld readyto harvest. Inset showsa maturecotton boil
with lint exposed.
Fiber Growth and Development
Seeds attaintheir full sizeabout3 weeksafterfertHization, but do not
reach maturity untH the boil opens. The epidermal layer of the seed coat
gives rise to the lint floors, each floor being an extension of a single
epidermal cell. These are visible as swelling of the epidermal cells at the
time of anthesls
. Therearetwo stages Inthe development of cotton flbers;
elongation and secondarythickening.
Floorsattaintheirfull lengthInabout 25daysafterfertUizatlon withthe
maximum growth rate occurring at 10-15 days. Thickening of the flber
beginsat about 16daysand continuesuntK the boil Is mature. Thickening
occurs by deposition of consecutive layers of cellulose on the Inner wall
of the flber In a spiralfashion. The degree of thickeningand the angle of
the spiralsaffectflber strength and maturity. UntH the boil opens, the flber
is a living cell, but upon opening, the fiOOr Is exposed to the air and soon
dries out and becomes twisted. In addition to the long floors, most
commercial cultivars (excluding Gossypium barbadense) havevery short
white or colored flbers on the seed called Iintersor fuzz fibers.

20. 20 OOSTERHUIS
Cotton fiber quality as defined by length, maturity, strength, and
mlcronalre, Is primarilydetermined by the genetic makeupcl the plant but
Is also Influenced by climatic conditions experienced by the crop. For
example, boils maturing late In the season when temperatures are lower,
require a longer perkx:t for fiber growth and development and usually
produce less lint and of lower quality.
Boil Distribution on the Plant
Thedistributionof the boilsonthe plantvaries dueto abscission from
physiological and environmental causes. Plantdiagrams areusedto "map"
the positionsof boils (Fig. 1-14) andare useful management tools to follow
fruit development and assess the success of production Inputs. A large
percentage of the total yield Is derived from the central portion of the
canopy, approximately between main-stem nodes 6 and 13, which
coincideswith the distributionof leaf areawithinthe canopy. Fewerboils
are produced above these nodes and they tend to take longer to mature
and be smaller In size.
.... Ablel,loft
l:J. Squa,.
~ Flow.,
• Boil
o Open Boil
Fig. 1-14. Typical boil location on
a plant yielding 1,000 Ib
lint per acre. (From
Mauney, 1984.)

21. GROWTH OF THE COTTON PLANT 21
The relative importance of the fruiting positions (nodes) along a
fruiting branch varies, I.e., the first, second and third sympodlal positions
contributeabout 60%, 30% and 10% of the total seedcotton yield, respec-
tively. Furthermore, the lint qualitytendsto decrease awayfrom the main
stern.
The likely production problemsduring the maturation stage Indude
low temperatures and slow upper-canopy boil development, which can
Increase boil rot, delay harvesting, reduce the efficacy of defoliants and
boil openers, and reduce lint quality.
CROP DEVELOPMENT AND HEAT UNITS
The cotton plant is genetically programmed such that, under
favorable conditions, Its growth and development follow a rather well-
defined, consistent, and orderly calendar of events. However, another
useful way to assess crop development is by using daUy temperatures
during the season. The heat unit (growingdegree days or 00-60's)
concept utilizes temperature ratherthan calendar days in describing
growth and development of a crop. The concept is based on a devel-
opmental threshold (60°F in the Mid-South) above which the crop
grows and below which little or no development occurs. The basic
formula for calculating heat units involves adding the maximum and
minimumtemperatures for each day, dividing by 2, and subtractingthe
threshold temperature. Calculation of the accumulated heat units, and a
knowledge of the heat unit requirement for any particulargrowth stage
(Table 1-1), can be used to explainand predict the occurrence of events
or duration of stages in crop development, I.e., as a general physiologi-
cal time scale of development during the season.

22. 22 OOSTERHUIS
Table 1-1. The average number of heat units required for various
growth stages In cotton In the Mid-South.
Growth stage
Planting to seedling emergence
Nodes up the main stem
Emergence to first square
Squareto white flower
Planting to first flower
White flower to open boil
Plantingto harvest
Heat units
DD-60's
50-60
45-65
425-475
300-350
n5-850
850
2600
SUMMARY OF DEVELOPMENTAL EVENTS
The overall 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, and boil development), and finally by a
slowing down maturationphase (of boil maturation). Both genotype and
environment affect this pattern. Furthermore, each stage may have
different physiological processes operating with specific reqUirements.
Nevertheless a general and predictable pattern of growth exists for the
cotton plant (Fig. 1-13). A knowledge of these growth patternsas well
as the stage-dependent differences in cotton growth and requirements
will aid greatly In crop management decisions. The following table
briefly summarizes some of the main growth stages, the averagetime of
occurrence in the Mid-South, and the main physiological eventsof each
stage.

23. GROWTH OFTHECOTION PLANT 23
Table 1-2. The average time afterplanting to selected developmental
stages, for the Mid-South, and the main growth event occur-
ring.
Developmental stage Days
afterp1antlngt
Growth event
Germination/radicle appearance 3 Imblbltlon and respiration
Seedling emergence 6 Hypocotyl hook extension
Cotyledons unfolding 7 Food storage and photosyn-
thesis
Rootdepth of 6-12 inches 10 Establishment of root system
Firsttrue leaf unfolding 14 Photosynthesis and sugar
production
First "pin-head" square 35 Earty reproductive develop-
ment
Second "pin-head" square 38 Earty reproductive develop-
ment
Firstwhiteflower 65 Pollination and fertilization
Canopy closure 75 Maximum light interception
Peak flower 93 Increase In boil requirements
First full size boil 95 Maximum boil size, fiber
growth
First open boil 110 Carpels dehisce
t Influenced by environment and genotype.
CONCLUSIONS
Understanding cotton growthand development is critical in orderto
implement sound management strategies for maximum yields and profits.
Cotton is a perennial plantwith an indeterminant growth habit, and hasa
very dynamic growth response to environment and management. Plant
development proceeds through various developmental phases Including
germination andemergence, seedling establishment, leafareaandcanopy
development, flowering and boil development, and maturation. Manage-
ment strategies need to take this plant development and associated
requirements into consideration to optimize yields. Furthermore, manage-
ment practices should be flexible to cater for changing environmental
conditions.

24. 24
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OOSTERHUIS
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