module 4-the botany of wheat- anatomy, growth and development and physiology
TRANSCRIPT
-
8/13/2019 Module 4-The Botany of Wheat- Anatomy, Growth and Development and Physiology
1/20
- 1 -
Module 4The Botany of Wheat: anatomy,
growth and development and
physiology
I. IntroductionII. Zadoks Decimal Growth Stages
a.Germinationb.Seedling growthc. Tilleringd.Stem Elongatione. Bootingf. Ear Emergenceg.Floweringh.Milk Development
i. Dough Developmentj. Ripening
III. RootsIV. Vernalization and PhotoperiodV. Physiological Processes Driving Growth and
Development
-
8/13/2019 Module 4-The Botany of Wheat- Anatomy, Growth and Development and Physiology
2/20
- 2 -
I. Introduction.
The anatomy of and the growth and development of wheat has been studied
extensively and will be presented here in review. Understanding of how the wheat plant
grows and develops along with the capacity to utilize the terminology that accurately
describes the plant and its life cycle is important. Firstly, an acquaintance with thesesubjects aid in communication with others that may be cooperating to some extent with
the breeding objectives. Secondly, proficiency in these topics will enable the breeder toidentify traits that may be associated with improved performance.
Physiological criteria are commonly used in breeding programs, a widespread
example being the selection for semi-dwarf cultivars. Selection for reduced height has
improved lodging resistance, and increased the harvest index (HI) of wheat by increasing
the partitioning of biomass to the ear and developing grain. The physiological bases
behind superior performance are just beginning to be uncovered. A number of these
traits have a strong association to performance and have high heritability and can
therefore be used in the selection process. These physiological traits will be discussed in
the modules: Breeding for Increased Yield Potential, and Breeding for Drought Stress.An understanding of the wheat plants growth and development will aid the
breeder in selection for disease resistance. Different diseases will attack the wheat plant
at different developmental stages, and knowing the proper time to rate disease is
paramount in applying accurate selection pressure. Selection for wheat plantdevelopment can also aid in developing a cultivar that can better compete with weeds by
shading them out more quickly, or avoid seasonal environmental stress by earliness of
maturity. In short: an understanding of the wheat plants anatomy, growth anddevelopment and physiology can give the breeder a wider range of tools to aid in the
science of breeding, and a better intuition to aid in the art of plant breeding.
In seed time learn, in harvest teach, in winterenjoy.
William Blake
Whatever kind of seed is sown in a field,prepared in due season, a plant of that same
kind, marked with the peculiar qualities of theseed, springs up in it.
Guru Nanak
-
8/13/2019 Module 4-The Botany of Wheat- Anatomy, Growth and Development and Physiology
3/20
- 3 -
II. Zodaks Decimal Growth Stages.
There are several scales or developmental codes for wheat that describe visible
growth stages. Haunss scale (Haun, 1973) is useful in defining vegetative growth stages,and Feekes scale (Large, 1954) provides a good description for both vegetative and
reproductive stages. However, Zadoks scale is the most comprehensive and easiest to
use. It describes all stages of the cereal growth cycle, incorporating characteristics not
considered in other scales. Growth is a complex process with different organs
developing, growing and dying in overlapping sequences and it is easier to think of it as a
series of growth stages as in the Zadoks scale. This has 10 main growth stages, labeled 0
to 9, which describe the crop; and each main growth stage can be further described using
a second digit, labeled 0 to 9 (Table 1). After emergence, all developmental stages arebased on observations on the main shoot. After stage 40 the stages of the main shoot and
tillers become similar, and the stages are determined by viewing the whole plant. Stages
70 to 93 are determined by the development stage of individual kernels or grain in themiddle of average spikes. Although Zadoks growth stages parallel growth and
development along time they do not necessarily follow a sequence per se as decimalstages 1 and 2 as well as 2 and 3 occur in parallel, and a plant in a single moment could
be described by two or three stages (see figure 2)
II. a) Germination: Zodaks 0
Imbibition describes the process of the seed taking up moisture from the soil in
order to break dormancy and begin germination. The minimum water content required in
the grain for wheat germination is 35 to 45 percent by weight (Evans, 1975 WGP).
Germination may occur between 4 and 37C (optimal germination temperatures range
from 12 to 25C). Upon hydration the scutellum (see figure 1) begins to mobilize itsown starchy reserves along with secreting enzymes that break down the starchy
endosperm. The digested endosperm is absorbed by the scutellum, and nutrients are
conveyed to the growing embryo. The radicle will first emerge and begin to growdownwards along with about four other seminal roots. The coleoptile emerges shortly
after the radicle. The coleoptile forms a sheathing structure through which developing
leaves grow. The coleoptile increases in length until it emerges through the soil surface,where it ceases to grow. On average the coleoptile reaches 5cm in length. Selection at
this stage will most likely be by natural means as those genotypes that do not contain
suitable vigor will either not germinate or will not reach the soil surface. CIMMYT
places selection pressure at this stage for drought tolerance by deep planting. Soil
moisture, in most cases, will be greater at depth; and deep planting is a way to exposeseeds to moisture that may not be available closer to the surface (a possible problem is
water-stressed environments).
II. b) Seedling Growth: Zodaks 1
Zodaks 1 describes leaf emergence. Leaf primordium appears first as a bump on
the flank of the shoot apex. The leaf primordia grow laterally and acropetally becoming a
cowl shaped sheath. The sheath-like leaf grows upward as a conical cap over the shoot
apex and younger leaves develop within in the same fashion. When the leaf is about 20mm long, the ligule develops separating the leaf blade (lamina) from the sheath which
-
8/13/2019 Module 4-The Botany of Wheat- Anatomy, Growth and Development and Physiology
4/20
- 4 -
Figure 1
-
8/13/2019 Module 4-The Botany of Wheat- Anatomy, Growth and Development and Physiology
5/20
- 5 -
will remain wrapped around the stem. The ligule is a thin membranious outgrowth
generally regarded as a protective structure which prevents rain, dust, insects and otherforeign material from entering and accumulating within the sheath; protecting younger,
underdeveloped tissues from damage. Associated with the ligule and located in the sameposition are the auricles which partly wrap around the stem (fig. 2). Leaf shape and size
change with leaf position. The first leaf on the main shoot has parallel sides to within 1
cm of the tip making it relatively blunt ended. The leaves above the first have more or
less parallel sides for about two-thirds of their length above which they taper to a sharp
point. The last leaf produced upon the culm, the flag leaf, tapers from about the lower
third, giving the leaf an elongated ovate shape. As the life cycle of wheat progresses,
lower plant leaves die due to shading, drought, disease, or normal maturity. The flag leaf
is the last to remain green and accounts for 80 percent of the carbon dioxide assimilationof grains. Figure 3 demonstrates Zadoks notation in relation to leaf development.
Studies on historical sets of cultivars suggest that leaf shape and orientation have
contributed to genetic progress in yield and this will be discussed in the module:Breeding for Increased Yield Potential and Yield Stability. Also certain metabolic
processes that occur in the leaf will be discussed later along with their potential use asindirect selection criteria for yield and drought tolerance will be covered in the modules:
Breeding for Increased Yield Potential and Yield Stability, and Breeding for Drought
Tolerance.
FAO Irrigated Wheat. Howard M Rawson and
Helena Gmez Macpherson
Fig. 2 Vegetative Structures Of Wheat
Flower Spike
Culm
Node
Internode
Sheath
Tiller
Stem section and leaf sheath
Hollow Stem
Stem (culm)
Leaf Blade
Ligule
Auricle
Sheath
Figure 3 Zodaks 1
-
8/13/2019 Module 4-The Botany of Wheat- Anatomy, Growth and Development and Physiology
6/20
- 6 -
Table 1 - Decimal code used to quantify the growth stages in cereals
Code Description Code Description
0 Germination
0.0 Dry seed 38 Flag leaf ligule just visible
0.1 Start of imbibition 39 Flag leaf ligule just visible
0.2 Imbibition complete 4.0 Booting
0.3 Radicle emerged from seed 41 Flag leaf sheath extending
0.4 Coleoptile emerged from seed 43 Boots just visible and swollen
0.5 Leaf just at coleoptile tip 45 Boots swollen
1.0 Seedling growth 47 Flag leaf sheath opening
10 First leaf through coleoptile 49 First awns visible
11 1 leaf unfolded 5.0 Ear emergence
12 2 leaves unfolded 51 First spikelet of ear just visible
13 3 leaves unfolded 53 One-fourth of ear visible
14 4 leaves unfolded 55 One-half of ear emerged
15 5 leaves unfolded 57 Three-fourths of ear emerged
16 6 leaves unfolded 59 Emergence of ear complete
17 7 leaves unfolded 6.0 Flowering
18 8 leaves unfolded 61 Beginning of flowering
19 9 leaves or more unfolded 65 Flowering half-way complete
2.0 Tillering 69 Flowering complete
20 Main shoot only 7.0 Milk development21 Main shoot and 1 tiller 71 Seed water ripe
22 Main shoot and 2 tillers 73 Early milk
23 Main shoot and 3 tillers 75 Medium milk
24 Main shoot and 4 tillers 77 Late milk
25 Main shoot and 5 tillers 8.0 Dough development
26 Main shoot and 6 tillers 83 Early dough (fingernail impression not held)
27 Main shoot and 7 tillers 85 Soft doughc
28 Main shoot and 8 tillers 87 Hard dough
29 Main shoot and 9 or more tillers 9.0 Ripening
3.0 Stem elongation 91 Seed hard (difficult to divide with thumbnail)
30 Pseudo-stem erectiona 92 Seed hard (cannot dent with thumbnail)
31 1snode detectable 93 Seed loosening in daytime
32 2nd
node detectable 94 Seed over-ripe; straw dead and collapsing
33 3rd
node detectable 95 Seed dormant
34 4* node detectable 96 Viable seed giving 50% germination
35 5thnode detectable 97 Seed not dormant
36 6thnode detectable 98 Secondary dormancy induced
37 Flag leaf just visible 99 Secondary dormancy lost
(a) Winter cereals only. (b)An increase in the solids of the liquid endosperm is notable when crushing the seed between fingers.(c) Fingernail impression held; head loosing chlorophyll. Source: Zadoks et al., 1974.
-
8/13/2019 Module 4-The Botany of Wheat- Anatomy, Growth and Development and Physiology
7/20
- 7 -
II c) Tillering: Zadoks 2The wheat plant has the ability to tiller (produce lateral branches). The number of
tillers that will be generated depend on a number of factors including: genetic,population density, sowing date, and the availability of water and nutrients. Tillering
normally starts when leaf 3 is fully expanded and leaf 4 is emerging on the main shoot.
The main shoot bears primary tillers in the axils of its leaves and can be described in
relation to the leaf number (TC emerging from the axil of the coleoptile, T1 emerges
from the axil of leaf 1, T2 emerges from the axil of leaf 2 etc.). Each primary tiller has
the potential to bear a number of secondary tillers and can be described in reference to the
leaf number of the primary tiller (T11 is the tiller borne in the axil of leaf 1 of tiller 1).
Of the tillers that emerge, only a proportion will survive to produce seed, the rest dyingwithout producing an ear, possibly due to competition for resources. Generally about
eight tiller buds will form, but only three or four will develop into full size tillers that
produce seed. Tiller appearance generally ends just before stem elongation begins.Figure 4 diagrams the nomenclature used for describing tillers along with drawings of
plants typical of Zadoks stage 2. Tillering is under genetic control and varies amongcultivars. Selection based on number of fertile tillers may be the source of genetic gains
for different cropping systems (basin or raised bed). Recent studies also show that a tiller
inhibition gene (tin) may prove useful in regions that are regularly subjected to terminal
drought (Duggan 2005a, Duggan 2005b) and will be discussed in the module breeding fordrought resistance.
Fig. 4 Tillering a) Nomenclature for leaves and tillers. b) drawing of wheat plantat Zadocs stages 1 and 2
Source: Kirby and Appleyard, 1985.(Courtesy of Kluwer AcademicPublishers)
a b
FAO Irrigated Wheat. Howard M Rawson andHelena Gmez Macperson
-
8/13/2019 Module 4-The Botany of Wheat- Anatomy, Growth and Development and Physiology
8/20
- 8 -
II d) Stem Elongation: Zadoks 3
Stem elongation usually begins between the late double ridge and terminalspikelet stage (discussed below). The elongation of the stem coincides with the growth
of leaves, tillers, roots, and the inflorescence, which raises questions about possiblecompetition for the assimilate supplies. Lower internodes of the stem remain short. A
spring wheat with 9 leaves will begin elongation with the 4thinternode, while a winter
wheat that has more leaves will begin elongation at a higher numbered internode. When
an internode has reached about half its length the internode above will begin to elongate.
Each succeeding stem internode is progressively longer and the sequence of elongation
will continue until anthesis. The peduncle, topped by the ear, is the final stem segment to
elongate and can account for as much as half of the total stem length. The height of
wheat ranges from 30 to 150 cm and is influenced by genotype and environment.Reduced height genes (Rht) affect internode length and have had a larger impact on
modern wheat production than any other physiological trait to date.
Until the time of stem elongation the shoot apical meristem had been the initiationsite for vegetative growth. Just before stem elongation begins the shoot apex begins to
initiate spikelet primordial which marks the commencement of reproductive growth forthe wheat plant. Each of the primordial initiated on what will become the ear has two
parts (the double ridge, see fig 5). The lower, smaller ridge is a leaf primordia, the
further development of which is more or less completely suppressed. The upper larger
ridge eventually differentiates to become the spikelet. The double ridge stage occurswhen from 40 to 80 percent of the spikelets have been initiated. After about 20 to 30
spikelet primordia have been initiated, the final number of spikelets is determined by the
formation of a terminal spikelet. Each spikelet has from 8 to 12 floret primordia in the
central part of the spike, while basal and distal spikelets have from 6 to 8 florets.
a
b c
d
e f
Source: Adapted fromKirby and Appleyard,1987.(Courtesy of Arable UnitRASE)
Fig. 5 - Successive stages of shoot apex development from a vegetative apex (a) todouble ridge (c) to terminal spikelet stage (f)
dome
leafprimordia
Site of spikelet ridge
Lower leaf ridge
leaf primordium
Axillary spikelet ridge
Lower leaf ridge
Spikelet meristem
Glume Primordia
Spikelet meristem
Floret
Lemma
Glume
Lemma, Floret
Lower glume
Terminal Spikelet
Spikelet meristem
Lemma, Floret 3
Stamenlemma
-
8/13/2019 Module 4-The Botany of Wheat- Anatomy, Growth and Development and Physiology
9/20
- 9 -
Spikelet development on the microscopic head is usually completed by the time the first
node is 1 cm above the soil surface. The terminal spikelet is produced at about Zadoksstage 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 thelast (flag) leaf.
II e) Booting: Zadoks 4The developing head within the sheath of the flag leaf becomes visibly enlarged
during the booting stage (figure 5). The booting stage ends when the first awns emerge
from the flag leaf sheath and the head starts to force the sheath open. Spike growth
occurs after the terminal spikelet is formed and stem elongation has begun. Spike growth
is slow in its early stage and increases greatly about the time the ligule of the flag leaf
becomes visible (Krumm et al., 1990FAOBW44). This time of rapid spike growth is
important in determining yield at harvest as an environmental stress can decrease the
supply of assimilates and contribute to floret death (see figure 6). Floret abortion, whichstarts in the boot stage and finishes at anthesis, occurs when stem and peduncle are at
maximum growth rate (Siddique et al., 1989FAOBW44). Meiosis in wheat, which
originates the pollen in the anthers and the embryo sac in the carpel, coincides with the
boot stage. This stage is very sensitive to environmental stresses. In wheat meiosis starts
in the middle of the spike, continuing later above and below this zone (Zadoks et al.,
1974 FAOBW 44).
Fig. 5 Wheat inflorescence at Zadoks stages: 4 Booting, 5 Ear Emergence, and 6 Flowering.
FAO Irrigated Wheat. Howard M Rawson and Helena Gomez Macpherson
-
8/13/2019 Module 4-The Botany of Wheat- Anatomy, Growth and Development and Physiology
10/20
- 10 -
Fig. 6 External and Internal Stages and When Yield is Formed
Figure From: FAO Irrigated Wheat. Howard M Rawson and Helena Gmez Macpherson
-
8/13/2019 Module 4-The Botany of Wheat- Anatomy, Growth and Development and Physiology
11/20
- 11 -
Possibilities for selection of a trait to increase performance are related to the phenology
surrounding spike growth. Phenologyrefers to the study of cyclic events in nature and inthis case it is addressing the length of time for spike development. This will be further
discussed in the module: Breeding for Yield and Yield Potential. The duration of spikegrowth has been shown to be associated with fertility and is under genetic control which
varies among cultivars (Miralles and Slafer 2007).
II f) Ear Emergence: Zadoks 5
As the stem continues to elongate, the still growing spike begins to emerge from
the flag leaf sheath (figure 5 heading). The time required for the spike to completely
emerge from the sheath is highly dependent on environmental conditions and can range
from one to five days. Throughout the preheading period, differences in the duration ofthe various developmental phases among shoots on the same plant help synchronize
development. This means a difference of several weeks between emergence of the main
shoot and a tiller is reduced to a difference of only a few days by the time the headsemerge from the flag leaf sheaths. At this stage final developmental events take place in
the ovary, and pollen mother cells leading to anthesis.
II g) Flowering: Zadoks 6
The wheat inflorescence is a spike bearing spikelets at the nodes (see figure 7).
There is one spikelet per rachis node. Each spikelet is surrounded by a pair of glumesand contains two to six florets. Found in each floret, enclosed by the palea and the
lemma, are three anthers, the stigma, and the ovary. Anthesis, or the shedding of pollen
occurs about three to ten days after the ear emerges from the flag leaf sheath (see figure 5
flowering). The lodicules (see figure 7) swell causing the palea and the lemma to open.
The stamen filaments elongate and the anthers dehisce, or shed pollen. The wholeprocess is complete within about five minutes (Percival, 1921 FAOBW 34). Anthesis
begins in the early morning and continues throughout the day. For a single spike this
process takes four to seven days. Anthesis begins in floret one of the spikelets in theupper two thirds of the spike. The following day anthesis progresses to the first floret of
the basal spikelet and to the second floret of the upper spikelets. The progression
continues ending with anthesis occurring in the last fertile floret of the basal spikelets(Evans et al., 1972FAOBW34). Within a single plant, anthesis occurs first in the main
shoot with the anthesis of tiller spikes commencing within three or four days. Within ten
days or less, depending on environmental conditions, a single wheat plant completes
anthesis.
Cultivars differ in the degree to which the lemma and palea are separated. Mostoften, individual florets are self pollinated. However, while the flower is open, foreign
pollen may enter, resulting in about one or two percent cross-pollination. Lodicules lose
their turgor and the florets close within an hour of opening. The lodicules degenerate
after the first opening, but the ovary will swell and the floret may open again. The stigma
of an unfertilized floret will remain receptive for up to five days after the time of anthesis
(emasculated or male sterile floret).
-
8/13/2019 Module 4-The Botany of Wheat- Anatomy, Growth and Development and Physiology
12/20
- 12 -
Fig. 7 A wheat Spikelet
-
8/13/2019 Module 4-The Botany of Wheat- Anatomy, Growth and Development and Physiology
13/20
- 13 -
II h) Milk Development: Zadoks 7
The pollen grain contains two nuclei, one generative and one vegetative. Withsuccessful pollination, the vegetative nuclei will begin developing a pollen tube which
will create a path through the style to the ovule and entering the embryo sac through themicropyle. The generative cell follows the path created by the tube cell and along the
way replicates and divides to produce two sperm cells. One sperm cell unites with the
egg. This union produces a zygote which will become the embryo. The second sperm
cell will unite with the central cell which will become the 3n endosperm which will
eventually be the source of wheat flour (figure 8a).
The seed increases in size about three fold in the four days after fertilization as the
tissues surrounding the embryo swell. The growth is caused by an expansion of the cells
rather than cell multiplication. In these first few days development of the zygote is slow,cell division occurs producing a globular shaped embryo (figure 8b). Also in the first four
days, the 3n nuclei replicate and divide fueled by the antipodal cells. This phase of free
nuclear division will, in part, determine the final number of cells in the endosperm.The time between four and ten days after fertilization is called water-ripe and is
described by Zadoks stage 71. At this time the inside of the grain has little structure andwhen opened appears to contain only water. At this stage the nuclei continue to divide
rapidly and cell walls begin to form. By 7 days after fertilization the embryo begins to
show signs of differentiation.
Eleven days after fertilization marks the medium milk stage, Zadoks 75, and thefirst stage of grain filling. By 16 days after fertilization lipid and protein bodies can be
found in the endosperm as well as A-type starch grains. The cell layers that surround the
embryo sac continue to change. The cell walls thicken and the aleurone becomes
recognizable. The embryo is developing quickly and takes on an elongated shape. At 16
days after fertilization the scutellum is clearly defined and the embryo will begin to usethe endosperm starch reserves for its development.
II i) Dough Development: Zadoks 8At 21 days after fertilization the outside of the grain begins to turn from green to
yellow and marks the soft dough stage, Zadoks 85. Cell division of the endosperm has
ceased and A-and B-type starch granules that formed during the medium milk stage arepacked into cellular compartments. The inside of the grain is still moist but the contents
are now semi-solid. The embryo, continuing to feed off of close-by endosperm, has
grown to half its size.
Twenty-one to thirty days after fertilization the caryopsis enters the hard-dough
stage, Zadoks 87. The grain takes on a golden color as protein and starch accumulationceases followed by the death of the endosperm cells. The embryo is now fully
developed, but will continue to receive storage reserves until the grain begins to dry and a
state of dormancy in initiated.
II j) Ripening: Zadoks 9
Ripening or dry down occurs between days 30 and 40 after fertilization. The
water content of the grain drops at a continuous rate, but there is a wide degree of
variability of the time for complete dry down between different grains in the ear anddifferent ears in the crop. The moisture of the grain must be watched closely in order to
-
8/13/2019 Module 4-The Botany of Wheat- Anatomy, Growth and Development and Physiology
14/20
- 14 -
harvest at the optimum time. Further air drying is often used after harvest to better
control the final moisture content of the grain. If the process of desiccation is notcontrolled properly, the grain may begin germination destroying the quality of the grain.
Pre-harvest sprouting is dependent on both environmental and genetic factors and is amajor problem that occurs most often in white wheat exposed to moisture at harvest time.
Grain moisture at harvest is generally 20% and must be further dried to about 14% for
proper storage.
III. Roots
So far described have been the aerial parts of the wheat plant, and a few words
must be said about the development of the root system. The root system of wheat is, of
course, important for its performance in all settings. It is of critical importance in
developing cultivars that can perform well in marginal and water-stressed environmentsas their ability to reach water and or nutrients at greater depths is a key issue (see chapter:
Breeding for Drought Tolerance). The wheat plant has two types of roots, the seminal
(seed) roots and roots that initiate after germination, the nodal (crown or adventitious)roots. About six root primordia are present in the embryo. At germination, the primary
root bursts through the coleorhiza, followed by the emergence of four or five lateral
seminal roots. These form the seminal root system, which may grow to 2 m in depth and
support the plant until the nodal roots appear. Nodal roots are associated with tiller
development and are usually first seen when the fourth leaf emerges and tillering starts.
Compared with the seminal roots, they are thicker and emerge more or less horizontally;when they first appear they are white and shiny (the white root stage). Nodal roots
occur on the lower three to seven nodes (depending on environmental conditions andfinal number of leaves on the shoot). The uppermost node, on which roots occur, at the
base of the culm, may be above soil level, and the roots may not penetrate the soil but
appear as short pegs protruding from the stem. At maturity, the root system extends to
between 1 and 2 m deep or more depending on soil conditions. Most roots occur in the
top 30 cm of soil (Kirby E.J.M., 2002 from Botany of wheat plant FAOBW p22).
IV. Vernalization and Photoperiod(Taken from E. Acevedo et al.2002 p.42FAOBW)
Vernalization
Wheats, which are responsive to vernalization, flower after the completion of a
cold period. The double ridge stage is not reached until chilling requirements are met, andthe vegetative phase is prolonged generating a higher number of leaves in the main shoot;
the phyllochron, however, is not affected (Mossad et al., 1995). Two major flowering
types of wheat are differentiated by their response to vernalization (Flood and Halloran,1986):
Spring-type wheat has a very mild response or no response at all tovernalization, and frost resistance is low.
Winter-type wheats have a strong response to vernalization and require aperiod of cold weather to flower. In the early stages of growth, they are very
resistant to frost (-20C), but frost resistance is gradually lost towards heading
-
8/13/2019 Module 4-The Botany of Wheat- Anatomy, Growth and Development and Physiology
15/20
- 15 -
Pollen Grain
Vegetative
nucleus Generativenucleus
Cytoplasm
Vegetative
nucleus Generativenucleus
Cytoplasm
OvuleAntipodals
Central Cell with 2polar nuclei
SynergidsEgg Cell
micropyle
funiculus
Integulment
Antipodals
Central Cell with 2polar nuclei
SynergidsEgg Cell
micropyle
funiculus
Integulment
Photos: 'WHEAT:THE BIG PICTURE'
Fig. 8 Grain Development
A
B
C
A) Diagram of pollen and ovule
B) From top to bottom embryodevelopment at 2, 7, 15, and 26days after anthesis.ce = cellular endospermem = embryonu = nucellussc = scutellumsp = shoot polerp= root poll
C) Grain at 6, 8, and 10 days afterfertilization
-
8/13/2019 Module 4-The Botany of Wheat- Anatomy, Growth and Development and Physiology
16/20
- 16 -
and flowering. The vernalization requirements of winter types may be fully
substituted by short days at nonvernalizing temperatures between 21 and16C (Evans, 1987).
Flood and Halloran (1986) point out that vernalization may occur at three stagesof the growing cycle of the wheat plant: during germination, during vegetative plant
growth (GS1) and during seed formation in the mother plant. The effectiveness of low
temperatures to accomplish vernalization decreases with increasing plant age, being
almost nil after three months (Chujo, 1966; Leopold and Kriederman, 1975).
Vernalization occurs at temperatures between 0 and 12C (Ahrens and Loomis, 1963;
Trione and Metzger, 1970). Spring genotypes usually require temperatures between 7
and 18C for 5 to 15 days for floral induction, while winter types require temperatures
between 0 and 7C for 30 to 60 days (Evans et al., 1975). Manupeerapan et al. (1992)observed that vernalization in winter genotypes stimulated cell division, overcoming an
inhibitory process that occurs at high temperatures.
Photoperiod
After vernalization is completed, genotypes, which are sensitive to photoperiod,require a certain day-length to flower. Sensitivity to photoperiod differs among
genotypes. Most cultivated wheats, however, are quantitative long-day plants. They
flower faster as the day-length increases, but they do not require a particular length of day
to induce flowering (Evans et al., 1975; Major and Kiniry, 1991).Stefany (1993) observed a period of insensitivity to day-length in wheat, which
starts with germination. During this period, the plant develops foliar primordia only. This
may be considered a juvenile phase, which is longer in winter wheat.
The photoperiod is sensed by mature leaves and not by apical meristems (Barcell
et al., 1992; Bernier et al., 1993). A single leaf is usually enough to sense the photoperiodfor floral induction. Once the photoperiod insensitive period ends, floral induction starts
and the reproductive stage begins (double ridge). The shorter the length of the day, the
longer the inductive phase is (Major, 1980; Boyd, 1986), the longer the phyllochron (Caoand Moss, 1989a, 1989b; Mossad et al., 1995) and the bigger the flag leaf (Mossad et al.,
1995). On the contrary, longer days advance floral induction (Evans et al., 1975).
The development of the inflorescence after induction occurs at a rate that is alsodependent on daylength in those genotypes sensitive to photoperiod (Stefany, 1993). The
shorter the day, the longer the phase is from double ridge to terminal spikelet (Figure
3.2), increasing the period to terminal spikelet and the number of spikelets per spike.
Changes in daylength after the terminal spikelet have no effect on floret initiation or
anthesis date. Wheat adaptation to a wide range of latitudes occurs at lower levels ofphotoperiod sensitivity such that flowering is not retarded significantly if the day-length
is shorter than optimal (Santibaez, 1994).
Vernalization and photoperiod constitute the basic processes of the adaptation of
wheat to various environments. Knowledge and genetic manipulation of them should
continue to provide significant tools for adaptation and yield.
-
8/13/2019 Module 4-The Botany of Wheat- Anatomy, Growth and Development and Physiology
17/20
- 17 -
V. Physiological Processes Driving Growth and Development(Taken from E. Acevedo et al. 2002 p45 to 48)
The net carbon dioxide (CO2) assimilation at the tissue level constitutes the basisfor growth. Many factors affect the net assimilation of CO2, among others, the growth
and development stage of the plant and environmental characteristics, such as light,
nitrogen, temperature, CO2and water status.
Four main basic processes are involved in photosynthesis: (i) a photochemical
process determining the quantum yield and depending on light intensity; (ii) a
biochemical process particularly linked to carboxylation; (iii) physico-chemical processes
of CO2transfer from the external air to the carboxylation sites; and (iv) the
photorespiration process in C3plants.At optimum temperature (20 to 25C), the maximum light saturation rates of
photosynthesis (Amax) at the leaf level in bread wheat are between 15 and 25 mol
CO/m2s (25 to 40 mg CO2/dm2h). Ninety percent of the light saturation rate is reached at1 000 mol quanta/m
2s of photosynthetically active radiation (PAR). Wild relatives of
wheat, however, may have substantially higher Amax than cultivated wheat (Austin,1990).
Much attention has been given to the question of how to increase total photo-
synthetic yield. Of the two photosynthetic parameters, quantum yield (rate of photo-
synthetic assimilation/incident light intensity) and Amax, a much greater improvement incanopy photosynthesis could be theoretically achieved by increasing the quantum yield.
Unfortunately, the quantum yield of the photosynthetic process itself is very constant
among genotypes (Austin, 1990). An improved discrimination of the enzyme ribulose 1,5
carboxylase oxygenase (rubisco) for CO2with respect to oxygen (O2) would increase the
quantum yield of the overall process by decreasing photorespiration (normally 25 percentof the energy produced by photosynthesis), but not much variation in the discrimination
of rubisco has been found between species (Sommersville, 1986; Loomis and Amthor,
1996). Some scope appears to exist for selecting genotypes with a reduced maintenancerespiration, which normally uses 2 to 3 percent of the dry weight per day (Robson, 1982),
but its effect on radiation use efficiency would be low (Loomis and Amthor, 1996).
Amax varies significantly among species and cultivars. In wheat, it has been known forsome time that certain diploid ancestor species have higher Amax values than present
advanced lines of bread and durum wheats (Dunstone et al., 1973); however, little
progress has been made with respect to yield increases by this approach.
Canopy photosynthesisCanopy photosynthesis is closely related to the photosynthetically active (400 to
700 mm) absorbed radiation (PARA) by green tissue in the canopy (Fischer, 1983). The
PARAcan be calculated from the fraction of solar radiation at the top of the canopy,
which is transmitted to the ground (I/I0), such that:
(2) PARA= RS* 0.5 * 0.9 * (1 - I/I0)
where RSrefers to the total solar radiation (MJ/m2d); the factor 0.5 refers to the fraction
of total solar energy, which is photosynthetically active; (1 - I/I0) is the fraction of total
solar radiation flux, which is intercepted by the crop; and 0.9 * (1 - I/I 0) is the fraction of
-
8/13/2019 Module 4-The Botany of Wheat- Anatomy, Growth and Development and Physiology
18/20
- 18 -
radiation absorbed by the crop allowing for a 6 percent albedo and for inactive radiation
absorption (Loomis and Amthor, 1996).The I/I0essentially changes as the crop leaf area index (LAI) increases, and it is
not very dependent on other factors, such as cloudiness or time of day. It is measuredwith a PAR sensor since the attenuation of RSin the canopy differs from that of PAR.
The relationship between I/I0and LAI fits a negative exponential (similar to the Beer
Lambert Law), such that:
where e is the base of the natural logarithm and K is known as the canopy extinction
coefficient.
The canopy extinction coefficient in wheat crops ranges from 0.3 to 0.7 and is
highly dependent on leaf angle (low K for erect leaves). From equation 3, it can be
calculated that 95 percent PAR interception requires a LAI as high as 7.5 for erect leavesbut a LAI of only about 4.0 for more horizontal leaves.
The total canopy net photosynthesis is linearly related to PARAand so is crop growth rate
(CGR, g/m2d), which is the net accumulation of dry weight, such that:
(4) CGR = RUE * PARA
where RUE is the radiation use efficiency (g/m2d).
Final yield is therefore the product of cumulative seasonal radiation absorption,
RUE and the portion of total biomass that goes to yield (harvest index).Potential radiation use efficiency in strong light depends on several factors:
adequate water to allow high stomatal conductance and transport of CO2into leaves; leaf
arrangement relatively vertical to the radiation beam; good leaf nutrition to support large
photosynthetic capacity; an active Benson-Calvin cycle to incorporate CO2; andappropriate canopy ventilation supplying CO2and dissipation of heat (dissipation of
excess energy due to light saturation). Due to environmental constraints, a quantum
requirement of 10 mol quanta/mol CO2under light-limited conditions may increase to 20
and 30 mol quanta/mol CO2under field conditions with a decrease in RUE from 8.2 to
3.7 and 2.2 g/MJ PAR (Loomis and Amthor, 1996). Practical estimates of maximumRUE by these authors were 3.8 g/MJ PAR, which would occur with long cool days and
moderate radiation (20 MJ/m2d). Warm temperature, the small concentration of CO2
relative to O2and light saturation limit attainment of a greater RUE. Measured values of
RUE in a wheat crop are close to 3.0 g/MJ PARAwhen roots are included (Fischer,
1983).
The RUE varies as Amax changes. Increases in the nitrogen of the canopyincrease Amax and RUE. Frost at night and temperatures below 15C during the daytimecan reduce Amax. Water stress has a small effect on RUE, but radiation intensity beyonda given value may reduce RUE. The RUE declines during grainfilling probably due to
sink limitation and/or leaf senescence (Fischer, 1983). Most studies show no difference in
CGR between genotypes, even when Amax varies (Austin et al., 1986), but a higher CGR
at anthesis was related to higher yield in Australian modern wheat cultivars grown under
water stress (Karimi and Siddique, 1991).
A number of possibilities for utilizing the variability found for these physiological
processes will be discussed further in the chapters: Breeding for Yield, and Breeding for
Drought resistance.
-
8/13/2019 Module 4-The Botany of Wheat- Anatomy, Growth and Development and Physiology
19/20
- 19 -
Questions to Test Understanding:
1. Explain the Zodoks decimal growth stages.
2. What are the disadvantages of tilling?
3. Define anthesis.
4. What takes place during the medium milk stage?
5. When does vernalization occur?
6. How does photoperiod affect inflorescence?
-
8/13/2019 Module 4-The Botany of Wheat- Anatomy, Growth and Development and Physiology
20/20
- 20 -
References:Ahrens and Loomis, 1963
Austin, 1990
Austin et al., 1986
Barcell et al., 1992
Boyd, 1986Bernier et al., 1993
Cao and Moss, 1989a, 1989b
Chujo, 1966Duggan, B.L., R.A. Richards, A.F. van Herwaarden, and N.A. Fettell. 2005a. Agronomic evaluation of a
tiller inhibition gene (tin) in wheat. I. Effect on yield, yield components, and grain protein.
Australian Journal of Agricultural research 56: 169-178.
Duggan, B.L., R.A. Richards, and A.F. van Herwaarden. 2005b. Agronomic evaluation of a tiller
inhibition gene (tin) in wheat. II. Growth and partitioning of assimilate. Australian Journal of
Agricultural Research 56: 179-186.
Dunstone et al., 1973Taken from E. Acevedo et al.2002 p.42FAOBW
Evans, 1987Evans L.T., J. Bingham, P. Johnson, and J. Sutherlands. 1972. Effect of awns and drought on the supply
of photosynthateand its distribution within wheat ears.Annals of AppliedBiology 70:6776.
Evans et al., 1975
Fischer, 1983
Flood and Halloran (1986)
Haun, 1973
Karimi and Siddique, 1991Kirby E.J.M., 2002 from Botany of wheat plant FAOBW p22
Krumm et al., 1990FAOBW44Large, 1954
Leopold and Kriederman, 1975
Loomis and Amthor, 1996
Major, 1980
Manupeerapan et al. (1992
Miralles and Slafer 2007Mossad et al., 1995Percival, 1921 FAOBW 34
Robson, 1982
Santibaez, 1994
Siddique et al., 1989FAOBW44Sommersville, 1986
Stefany, 1993
Trione and Metzger, 1970
Zadoks et al., 1974 FAOBW 44