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Chapter 10
Growth Stages of Wheat
INTRODUCTION
A sound understanding of plant growth and development
is an essential element of efficient, economic wheat management
systems. The impact of frost, heat, drought, diseases, insects,
and weeds can be more accurately predicted with a clear picture
of the relationships between growth stage and plant response to
stress. The optimum timing of fertilizer, irrigation, herbicide,
insecticide, and fungicide applications are also best determined
by crop growth stage rather than calendar date.
The ten major growth stages that the wheat plant
progresses through during its life cycle are all familiar to farmers:
-
Germination
-
Seedling
-
Tillering
-
Stem elongation or Jointing
-
Booting
-
Heading
-
Flowering or Anthesis
-
Milk
-
Dough
-
Ripening
Measures of Growth and Development
Thermal time
[ Heat Units | Thermal Time Requirements for Wheat Production ]
Wheat Growth and Development
[ The Wheat Kernel | Germination | Seedling Stage ]
[ Tillering Stage | Stem Elongation or Stem Jointing Stage ]
[ Booting Stage | Heading Stage | Flowering or Anthesis Stage ]
[ Milk Stage | Dough Development Stage | Ripening Stage ]
Critical Growth Stages
Several systems have been developed to provide
numerical designations for growth and developmental stages. Among
these, the Feekes, Zadoks, and Haun scales
are used the most frequently (Table 1 ).
The Haun scale growth stages key on rate
of development of the main shoot (Table 1 ).
In the early stages, the description of each new leaf is related
to the previous leaf that was produced. For example, a seedling
with one fully extended leaf and a second leaf that is half as long
as the fully extended leaf is at Haun stage 1.5. Similarly, a plant
with five fully extended leaves on the main shoot and an emerging
sixth main shoot leaf that is 30% as long as the fifth leaf is at
Haun stage 5.3. The booting stage numerical designation starts at
one more than the number of leaves produced by the main shoot, i.e.,
the flag leaf number plus one. This can cause confusion because
the flag number is not a constant for all cultivars. For this reason,
the Haun scale has been used mainly to describe the growth stages
before the booting stage.
________________________________________________________________ Haun Feekes Zadoks scale scale scale General Description ________________________________________________________________ Germination 00 Dry seed 01 Water uptake (imbibition) started 03 Imbibition complete 05 Radicle emerged from seed 07 Coleoptile emerged from seed 0.0 09 Leaf just at coleoptile tip Seedling developmet 1 10 First leaf emerged 1.+ 11 First leaf unfolded 1.+ 12 2 leaves unfolded 2.+ 13 3 leaves unfolded 3.+ 14 4 leaves unfolded 4.+ 15 5 leaves unfolded 5.+ 16 6 leaves unfolded 6.+ 17 7 leaves unfolded 7.+ 18 8 leaves unfolded 8.+ 19 9 or more leaves unfolded Tillering 20 Main shoot only 2 21 Main shoot and 1 tiller 22 Main shoot and 2 tillers 23 Main shoot and 3 tillers 24 Main shoot and 4 tillers 25 Main shoot and 5 tillers 3 26 Main shoot and 6 tillers 27 Main shoot and 7 tillers 28 Main shoot and 8 tillers 29 Main shoot and 9 or more tillers Stem elongation or jointing 4-5 30 Pseudo stem erection 6 31 1st node detectable 7 32 2nd node detectable 33 3rd node detectable 34 4th node detectable 35 5th node detectable 36 6th node detectable 8 37 Flag leaf just visible 9 39 Flag leaf ligule/collar just visible Booting 40 ---- 8-9 note 41 Flag leaf sheath extending 9.2 10 45 Boot just swollen 47 Flag leaf sheath opening 10.1 49 First awns visible Heading 10.2 10.1 50 First spikelet of head visible 10.2 53 1/4 of head emerged 10.5 10.3 55 1/2 of head emerged 10.7 10.4 57 3/4 of head emerged 11.0 10.5 59 Emergence of head complete Flowering of Anthesis 11.4 10.51 60 Beginning of flowering 11.5 65 Flowering half complete 11.6 69 Flowering complete Milk 70 ---- 12.1 10.54 71 Kernel watery 13.0 73 Early milk 11.1 75 Medium milk 77 Late milk Dough 80 ---- 14.0 83 Early dough 11.2 85 Soft dough 15.0 87 Hard dough Ripening 90 ---- 11.3 91 Kernel hard (difficult to separate by fingernail) 16.0 11.4 92 Kernel hard 93 Kernel loosening in daytime 94 Overripe, straw dead and collapsing 95 Seed dormant 96 50% of viable seed germinates 97 Seed not dormant 98 Secondary dormancy 99 Secondary dormancy lost ________________________________________________________________
* Note:
Remember, Haun scale values from the booting to ripening stages
are dependent on the number of leaves produced on the main stem.
The example given here is for a plant with eight leaves on the
main stem. The Feekes scale recognizes eleven major
growth stages starting with seedling emergence and ending with grain
ripening (Table 1). The Feekes
scale is frequently used to identify optimum stages for chemical
treatments, such as fungicide applications, that focus on the plant
development period from the start of stem elongation (Feekes stage
6) to the completion of flowering (Feekes stage 10.53).
The Zadoks scale provides the most complete
description of wheat plant growth stages ( Table
1 , Figure 1).
It uses code based on ten major stages that can be subdivided, making
it particularly suited for computerization. When using the Zadoks
scale, the main growth stages, e.g., seedling versus tillering,
should be identified before proceeding to a description of the secondary
stages, e.g., seedling leaf number or tiller number.
Crop growth and development is often described
in terms of time, e.g., 60-day barley, frost-free days, heading
date, etc. However, a consideration of temperature is also important
in these discussions because temperature determines the rate of
growth and development. The time/temperature relationship that governs
plant growth and development is known as thermal time and
it is measured in heat units or growing-degree days. Heat units
have been chosen as the measure of thermal time in this discussion
to avoid confusion between growing-degree days and calendar days.
Heat Units
The thermal time required for crop production is
determined by adding the daily heat units together for the period
between planting and harvest. When the centigrade temperature scale
is used, the heat units generated each day is determined by adding
the minimum and maximum daily temperatures together and dividing
by two. For example, a day with minimum and maximum temperatures
of 10 and 20°C respectively, would generate 15 heat units [(20
+ 10) / 2 = 15]. Days with average daily temperatures below
0°C do not contribute to the heat unit total.
Thermal Time Requirements For Wheat Production
The heat unit requirements to produce a mature
crop are approximately 1550 for spring and 2200 for winter wheat.
Translated into calendar days, this means that it would take 103
(103 x 15 = 1545) days to produce a spring and 147 (147 x
15 = 2205) days to produce a winter wheat crop if the average
daily temperature was a constant 15°C. As we all know, there
are large variations in temperature from day to day and growing
season to growing season. The use of thermal time rather than calendar
time takes this variability into consideration and provides an explanation
for differences in crop maturity when observations from different
years are compared. For example, we harvested Norstar winter wheat
on July 20 in 1988 and August 24 in 1993 at Saskatoon. The 1988
growing season was much warmer with the result that the thermal
time requirements to produce a mature Norstar crop were met five
weeks earlier in 1988 than in 1993.
Development of the roots, leaves, tillers, and
spikelets on the head of the wheat plant takes place in an orderly,
predictable pattern that is dependent upon thermal time. It takes
approximately 105 heat units for a wheat plant to germinate and
emerge from a seeding depth of less than one inch (2.54 cm). The
appearance of each successive leaf on the main shoot and tillers
then proceeds at a constant rate that is determined by cultivar,
sowing date, and latitude. Most wheat cultivars require between
80 to 100 heat units to produce each leaf on the main shoot. After
the requirements for leaf development have been met, another 650
heat units are normally required to complete the heading and maturation
stages.
See Table 1
for Haun, Feekes, and Zadoks scale numerical designation for the
following growth and developmental stages.
The mature wheat kernel (caryopsis) is composed
of approximately 83 percent endosperm, 14.5 per cent bran, and 2.5
percent embryo. Once germination starts, the endosperm provides
the developing plant with an energy source until its roots are established
and newly expanded leaves allow it to harvest energy from the sun.
The embryo of the mature wheat kernel has already undergone the
first stages of plant development before the kernel is separated
from the parent plant. In a mature kernel the embryo includes the
coleoptile, which protects the first leaf as it pushes its
way through the soil to the surface during germination, the radicle,
which becomes the first root, and primodia, which develop
into the first three leaves and seminal roots.
Germination starts with the uptake of water (imbibition)
by a wheat kernel that has lost its post-harvest dormancy. Plant
development is resumed once the embryo is fully imbibed. With the
resumption of growth, the radicle and coleoptile emerge from the
seed ( Figure 2 ). The
first three seminal roots are produced and then the coleoptile elongates
pushing the growing point toward the soil surface.
The seedling stage begins with the appearance of
the first leaf ( Figure 3 )
and ends with the emergence of the first tiller. Up to six seminal
roots and three leaves support the plant at this stage. The crown
of the plant usually becomes noticeably distinct after the third
leaf has emerged ( Figure 4 ).
Crown formation is soon followed by the appearance
of tillers and development of a secondary or crown root system (
Figure 5 ). The crown root system provides
the plant with most of its nutrients and water during the growing
season.
The distance between the wheat kernel and the crown
is determined by the length of the subcrown internode ( Figure
4 ). The subcrown internode can elongate several inches
and, depending on soil temperature, usually positions the crown
within 1.2 inches (3.0 cm) of the soil surface. At Saskatoon, crown
depths of 0.7 inches (1.8 cm) and 1.2 inches (3.0 cm) have been
recorded when soil temperatures were 18 and 11°C, respectively,
at the time of seeding. The recommended planting depth for winter
wheat is less than one inch (2.54 cm). Consequently, a clearly defined
subcrown internode is not usually found on seedlings of properly
managed winter wheat.
The roots, leaves, tillers, and spikelets on the
head of the wheat plant develop from primodia at nodes. While the
first tiller is not produced until the third leaf has fully emerged,
the appearance of later tillers is usually synchronized with the
emergence of each subsequent new leaf that develops on the main
shoot. For example, emergence of the fifth leaf is normally accompanied
by the appearance of the second crown tiller ( Figure
5 ) which originates from an auxiliary bud (primodium)
located in the node at the base of the second leaf (leaf axil).
Similarly, a tiller can start producing its own subtillers once
it has three fully developed leaves.
Each tiller that is produced represents the potential
for a wheat plant to develop an additional stem complete with its
own leaves, roots, and head. Root and shoot development of the plant
is synchronized so that the number of crown roots is related to
the number of leaves produced. However, root production by a tiller
is usually delayed until its third leaf has emerged. Consequently,
tillers that do not produce at least three leaves are not competitive
and usually die off once the stem elongation stage starts.
Coleoptile tillers can develop when environmental
conditions are favorable ( Figure 5 ).
The development of coleoptile tillers is not closely synchronized
with the development of the rest of the plant, but their appearance
often coincides with the emergence of the third leaf on the main
shoot. They develop from a node at the base of the coleoptile and
are separated from the main shoot by the subcrown internode. However,
when wheat is seeded shallow the subcrown internode does not elongate
and the coleoptile tiller will originate from a position adjacent
to the crown of the main shoot.
A major change in the development of the wheat
plant occurs at the end of the tillering stage. At this time, the
growing points of the main shoot and tillers stop initiating new
leaves and start producing reproductive structures. Conversion of
the growing point signals the end of the vegetative and the start
of the reproductive period.
Early maturing spring wheat cultivars change from
the vegetative to the reproductive phase after seven to eight leaves
have been initiated on the main shoot. However, many commercial
wheat cultivars have a vernalization (growth at low temperature
- see Chapter 7 )
or photoperiod (growth under long day length) requirement that extends
the vegetative period allowing for the production of more main shoot
leaves and a larger number of tillers. An extended vegetative period
due to a vernalization requirement is the main reason why more heat
units are needed to produce a winter than a spring wheat crop.
The nodes from which leaves develop are telescoped
at the crown during the tillering stage. Once jointing starts, the
internode region elongates, moving the nodes and the growing point
upward from the crown to produce a long stiff stem that will carry
the head. Appearance of the first node (Zadoks stage 31) can usually
be detected without dissecting the plant by pressing the base of
the main (largest) stem between your fingers.
Each successive tiller on a wheat plant normally
has one less leaf than its predecessor. This synchronizes the start
of the stem elongation stages of the main stem and tillers.
Synchronization of growth and development at this stage ensures there will be no more than a few days difference in the maturity of all heads on the plant.
Synchronization of growth and development at this stage ensures there will be no more than a few days difference in the maturity of all heads on the plant.
Spikelet development on the microscopic head is
usually completed by the time the first node is 0.4 inches (1 cm)
above the soil surface. The terminal spikelet is produced at about
Zadoks stage 31. A rapid loss of younger, poorly developed tillers
also normally starts at this stage.
The stem elongation or jointing stage comes to
an end with the appearance of the last (flag) leaf.
The developing head within the sheath of the flag
leaf becomes visibly enlarged during the booting stage. The booting
stage ends when the first awns emerge from the flag leaf sheath
and the head starts to force the sheath open.
The heading stage extends from the time of emergence
of the tip of the head from the flag leaf sheath to when the head
has completely emerged but has not yet started to flower.
The flowering or anthesis stage lasts from the
beginning to the end of the flowering period. Pollination and fertilization
occur during this period. All heads of a properly synchronized wheat
plant flower within a few days and the embryo and endosperm begin
to form immediately after fertilization.
Early kernel formation occurs during the milk stage.
The developing endosperm starts as a milky fluid that increases
in solids as the milk stage progresses. Kernel size increases rapidly
during this stage.
Kernel formation is completed during the dough
development stage. The kernel accumulates most of its dry weight
during dough development. The transport of nutrients from the leaves,
stems, and spike to the developing seed is completed by the end
of the hard dough stage. The developing kernel is physiologically
mature at the hard dough stage even though it still contains approximately
30 percent water.
The seed loses moisture, and any dormancy it may
have had, during the ripening stage.
Successful adaptation of a crop species is dependent
upon the programming of critical growth stages so that the plant
can capitalize on favorable weather periods during the growing season.
Plants have evolved a variety of adaptive mechanisms that allow
them to optimize growth and development while coping with environmental
stresses. Plant breeders have selected and recombined the variability
that exists in natural populations to produce cultivars with increased
production potential and adaptation to a wide range of environments.
An understanding of how plants respond to environmental stresses
at different growth stages can assist in the assessment of crop
condition and production potential throughout the growing season.
Winter wheat plants must survive the many stresses
of winter (see Chapter 12
). Roots and leaves that develop in the fall are often killed off
during the overwintering period. However, as long as the crown remains
alive, new roots and leaves can be regenerated. Therefore, plants
that enter the winter with well developed crowns have the best chance
of winter survival.
Grain yield can be expressed as the product of three
variables (yield components):
Grain yield =
(number of heads) x
(kernels per head) x
(kernel weight).
The impact of each yield component on final grain
yield is determined at different stages during the growing season.
The number of viable seeds planted and the number
of tillers produced per plant sets the upper limit on the number
of heads that can be produced by a wheat crop. Tiller production
is favored by moist, warm weather and good soil fertility, especially
nitrogen fertility, prior to the stem elongation stage.
Tillers produced during the tillering stage must
survive to maturity to contribute to grain yield. The developing
head and elongating stem start making large demands on the plants'
resources once stem elongation starts and younger, poorly developed
tillers that are unable to compete are quickly lost. Tiller mortality
level is especially dependent on environmental conditions immediately
after terminal spikelet formation (Zadoks stage 31).
Drought and heat stress during the stem elongation
and booting stages increase the rate of tiller mortality by placing
added restrictions on resource availability. For example, nearly
250,000 tillers per acre per day were lost from winter wheat stands
subjected to extreme drought and heat stress in the late spring
of 1988 at Saskatoon. Only the main stem of each plant was left
to set seed after this period of extreme environmental stress. If
a drought is broken or a late application of nitrogen fertilizer
suddenly becomes available during this period, the developmental
synchrony of the plant may be disrupted producing a flush of later
maturing heads. A dry spring followed by cool damp weather during
the last half of June in Saskatchewan in 1993 produced many examples
of this type of maturity problem ( Figure 6
).
Environmental stress prior to flag leaf appearance
can result in a loss of spikelets on the developing head ( Figure
7 ). As many as twelve florets per spikelet can be initiated
under favorable conditions for development. However, later forming
florets abort and normally only two to four florets actually set
seed in each spikelet ( Figure 8 ). Floret initiation
starts in the lower central region and progresses toward the base
and tip of the head. Under extreme environmental stress, all of
the florets in the spikelets at the top and bottom of the head may
abort prior to flowering.
The number of tillers and florets initiated by the
wheat plant is usually far in excess of the number of heads and
kernels that can be supported through to maturity. As we have seen,
a downward adjustment in yield potential normally starts with tiller
loss at the beginning of stem elongation and continues with floret
abortion prior to flowering. Environmental conditions experienced
during these developmental stages determine the magnitude of the
loss in yield potential. The final adjustments in yield potential
are made during the grain filling period when kernel size is determined.
Figure 6.
Asynchronous tiller development
caused by a dry spring followed by cool, damp weather during the
booting stage. Note the large differences in stage of tiller maturity.
Figure 7.
Drought stress immediately before
flowering can cause floret abortion (blasting).
Figure 8. 
Under favorable conditions of
moisture and temperature, winter wheat can produce as many as five
kernels per spikelet. REFERENCES
Bauer, A., D. Smika, and A. Black. 1983. Correlation
of five wheat growth stage scales used in the Great Plains. USDA-ARS,
Peoria, Ill.
Cook, R.J. and R.J. Veseth, 1991. Wheat health management.
Amer. Phytopath. Soc., St. Paul, Minn.
Haun, J.R. 1973. Visual quantification of wheat
development. Agron. J. 65: 116-119.
Large, E.G. 1954. Growth stages in cereals: Illustration
of the Feeke's scale. Pl. Path. 3: 128-129.
Nelson, J.E., K.D. Kephart, A. Bauer, and J.E. Connor.
1988. Growth staging of wheat, barley, and wild oat. Montana State
Univ. Coop. Exten. Service, Bozeman, and Univ. Idaho Coop Exten.
Service, Moscow.
Zadoks, J.C., T.T. Chang, and C.F. Konzak. 1974.
A decimal code for growth stages of cereals. Weed Res. 14: 415-421.
D. Brian Fowler
Crop Development Centre
University of Saskatchewan, Saskatoon, Canada.
Copyright © 2002. D.Brian Fowler
All Rights Reserved.
Revised
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