Module 4-The Botany of Wheat- Anatomy, Growth and Development and Physiology

8/13/2019 Module 4-The Botany of Wheat- Anatomy, Growth and Development and Physiology

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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


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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


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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


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Figure 1


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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


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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.


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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


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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


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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


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Fig. 6 External and Internal Stages and When Yield is Formed

Figure From: FAO Irrigated Wheat. Howard M Rawson and Helena Gmez Macpherson


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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).


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Fig. 7 A wheat Spikelet


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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


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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


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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


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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.


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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


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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.


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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?


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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

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Module 4-The Botany of Wheat- Anatomy, Growth and Development and Physiology

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